This resource will provide all the specific technical information required for this module. Related background information can be found in:
Gain, J., 1996, Engineering Workshop Practices, Thomas Nelson Australia Ltd., South Melbourne
Culley, R., (ed) 1989, Fitting and Machining TAFE Publications Unit RMIT Ltd., Collingwood, Victoria
This section covers the identification and selection of cutters and inserts by ISO coding, the selection of appropriate speeds and feeds to achieve specified surface finishes and the selection of appropriate work holding methods for specified CNC milling operations.
After completing this section you should be able to:
There are three commonly used cutting materials for milling applications. They are:
Po1ycrystalline materials — CBN (cubic boron nitride) and diamond —not commonly used for turning applications are even more rarely used for milling applications.
Looking at the three groups in more detail:
HSS offers high toughness, relatively low purchase cost and suitability to resharpening as advantages but the disadvantages are low machining rates (due to low cutting speeds dictated by low cutting temperature ability), short cutting edge life and the expense incurred in resharpening.
Some tooling companies manufacture the smaller range of tools in Titanium Nitride (TiN) coated HSS which offers improved wear resistance and higher cutting speed than the normal grades of HSS.
Suited to softer, ductile materials.
Cemented carbides, in general, have the combined advantages of good hardness and toughness with a high cutting temperature rating. Several grades are available to suit different materials or cutting conditions and coated grades are available for some applications.
The coatings may be single or combined with a depth as little as one micron (0.001 mm) and may be Titanium Carbide (TiC), Titanium Nitride (TiN) or Aluminium Oxide (Al₂O₃).
A different type of cemented carbide is Cermet (CERamics with METallic binders). These materials are titanium carbides rather than tungsten carbides and are made up of TiC/TiN, sometimes with the addition of Tantalum Nitride (TAN).
Suited to most materials.
Ceramics are made from cold pressed Aluminium Oxide (white), hot pressed Aluminium Oxide with titanium carbide added (black) and for tougher applications, Silicon Nitride is used.
Ceramics are not as tough as carbides but are harder and have the higher cutting temperature rating which means higher cutting speeds but lower feed rates than carbides. Suited to steel and cast iron materials.
The suitability of cutting tool materials to a particular workpiece material can best be found by referring to tooling manufacturers' catalogues.
A useful comparison between different grades or even between comparing manufacturers' products can be made by studying the I.S.O. classification charts.
These charts rate the cutting conditions of various materials into the three main groups ¬P, M and K with the P group covering long chipping steels, the M group covers all alloys that are more difficult to machine and the K group includes metals that produce short or flaky chips.
Each group is divided into sub-groups which rate the type of machining as heavy, medium or light, as well as the favourable or unfavourable conditions. Sub-group 01 covers light machining under favourable conditions such as small chip sections, vibration free machining, consistent cutting depth, etc. while sub-group 50 covers heavy machining under unfavourable conditions such as large chip sections, materials with variable hardness or hard skins, interrupted cutting, work subject to vibration, variable cut depth, etc.
The requirements of wear resistance against toughness can also be compared on this chart; a typical non-specific example is shown here in figure 1.1.
Figure 1.1
Although the ISO lists sixteen basic shapes for milling cutter inserts, by far the most commonly used are square, triangular, rhombic and round, with round being the strongest and triangular the weakest of these.
Some inserts are very similar to turning inserts in that they have a nose radius on each comer and either zero clearance or positive clearance on the cutting edge, but more dedicated geometrises have a parallel land behind the cutting comer in order to improve surface finish.
Special inserts known as wiper inserts that have an asymmetrical shape can be fitted to face mills to further improve surface finish with these cutters. This is achieved by fitting one wiper insert to the cutter and adjusting the axial position of the inserts so that the wiper is just below the others and therefore becomes a finishing cutting edge.
An appreciation of the different comer geometries can be found in figures 1.2 and figure 1.3.
Figure 1.2
Figure 1.3
Inserts may be single-sided or double-sided, with single-sided inserts being fitted to positive geometry cutters and double-sided inserts are fitted to negative geometry cutters which provide the necessary cutting clearances.
Double-sided inserts have twice the number of cutting edges and may be made with positive radial rake formed into the chip face. Quite often a negative land is employed on the cutting edge of inserts to strengthen the edge against impact stresses, especially for machining steel as shown in figure 1.4.
Figure 1.4
Most milling cutters hold the inserts in position using wedge clamps and screws, some large fine pitch cutters use quick-change spring actuated wedges and lighter duty end mills may use screw clamps.
With all types, when replacing inserts ensure:
The quick-change and screw clamp systems have in-built locations, but the more common wedge clamp system has an adjustable axial location system that requires setting or at least checking at regular intervals or whenever the insets are changed, depending on the usage.
If these locations allow the axial position of the inserts to vary, then the quality of the surface produced on the workpiece will suffer, and as well unequal cutting forces will be applied to each insert that may result in bearing chatter in the spindle of some machines when using face mills.
If a wiper insert is used, the location should be adjusted to allow this insert to be lowest by a small margin.
The ISO code for insert identification is shown here in part. For more detail regarding tolerances or imperial sizes refer to the manufacturers' catalogues.
A typical insert could be identified as: SPKN1504ED and briefly described as being a single-sided 15 mm square insert with parallel land.
The code breakdown is as follows:
Shape |
Cutting edge clearance |
Tolerance |
Type |
Size |
Thickness |
Entering angle |
Land clear |
S |
P |
K |
N |
15 |
04 |
ED |
|
1 |
2 |
3 |
4 |
5 |
6 |
7 |
|
Figure 1.5
In detail:
Figure 1.6
Figure 1.7
Also listed but rarely used: A = 3°, G = 30°, O = other
There are eleven tolerance classes with the widest tolerances in classes M and D, the finest tolerance in classes A, F and J and middling tolerance on the remaining six (C, H, E G, K, L)
As with anything else, finer tolerances usually mean higher unit cost, so fine tolerance classes should be chosen only when essential.
By far the most popular is type N, with type X usually designating wiper inserts.
Figure 1.8
The insert size is indicated by the length of the cutting edge or diameter of a round insert.
The thickness is taken from the bottom surface to the cutting edge.
Figure 1.9
Figure 1.10
Sometimes a code is used to indicate a chamfered cutting edge.
Sometimes a code is used to indicate right hand insert (for a tool designed to be used in a spindle rotating in the "normal" CW direction), left hand insert (spindle CCW), or neutral (either direction); the difference being applied to inserts with non-symmetrical geometry.
There are three basic groups of milling cutters:
Within each group are many types of tools to suit particular applications or materials, a general overview of each group follows:
Face mills are cutter designed to machine with a cut depth defined as an axial amount, therefore their primary purpose is to remove material from large flat surfaces.
Face mill size covers the range from about 050 mm to 0500 mm and have from three inserts to four dozen inserts, although fine pitch cutters designed to machine large areas of cast iron may have up to eighty inserts.
The most widely used geometry of face mills is either double positive, double negative or positive/negative.
These terms refer to the axial and radial rake of the insert face as presented to the workpiece.
Double negative tools are designed to use double sided square inserts which offer strength and eight cutting edges (sixteen if RH and LH cutters are available).
Double negative tools suit:
and have these drawbacks: The chip produced on ductile materials forms a clock spring shape that can jam in the chip pocket under certain circumstances.
Double positive tools must be single-sided inserts and possess these features:
Positive/negative tools can use the strong basic negative insert, but with clearance provided on the parallel land to accommodate the positive axial rake.
Another important aspect of face mill performance is determined by the entering angle, that is, the angle formed between the cutting edge and the machined surface.
An entering angle of90° creates the following:
Entering angles less than 90°create the following:
The usual entering angles are 90° , 75°, 60° and 45°.
There are probably more types of endmills available than any other milling cutter, from the well-known but inefficient HSS types through to the long shank inserted type.
The HSS types offer a good cutting length in relation to diameter as well as a wide range of sizes, particularly below 10 mm where other types are not available except for solid carbide versions.
Two and three tooth cutters are usually able to feed axially, as well as radially, as are ball nose cutters which are designed primarily for die sinking work. These cutter types are often known as slot drills. Four tooth endmills usually cannot feed axially.
Also includes in this group are tee slot and dovetail cutters, chamfering cutters and the like.
All endmills are suited to peripheral machining, that is, when the depth of cut can be described as being radial.
In an attempt to improve the efficiency of small endmills (up to 030), some manufacturers produce solid carbide versions which offer higher machining rates and rigidity for the increase in cost.
In larger sizes (ø20-ø50), helical carbide teeth are brazed onto steel cutter bodies to help reduce the cost and still retain the benefits of carbide cutting edges, and are suited to finishing operations.
Cutters with inserts range from ø16 (with one insert) to about ø60 (with 3 or 4 inserts) have limited axial cut depth but offer the economy of replaceable inserts as well as high machining rates.
Cutters with many inserts arranged in helical formations along the cutter are suited to rough machining long perpendicular faces at very high machining rates.
Most types of endmills have positive axial rake but the smaller inserted type may be of double negative or positive/negative geometry.
Side and face cutters are usually mounted on the arbor of a horizontal milling machine for machining shallow to deep through slots and steps. Also included in this group would be slitting cutters, angle cutters and the like.
HSS cutters still account for a fair proportion of the machining performed by this group of cutters, probably because they are fairly easily sharpened. The type of machining performed does not make up a large proportion of milling operations overall and the majority of these types of machines are not of the high power, high speed modem design.
But in a competitive world, milling with HSS as with any machining operation has little in its favour.
Inserted tooth side and face mills cover the range of sizes from ø80 to ø300 with slot producing widths from 6 mm to 26 mm and usually have positive geometry.
Half-width cutters allow economical machining of steps by having inserts on only one side of the cutter.
Note: The three groups of cutter types discussed here cover the vast majority of cutter types and applications. However, there is a market for specified cutters within specific industries for specific needs. In these cases, a specially designed tool may result in large savings in time and cost by being able to utilise expensive machines by reducing or eliminating operations and set-ups.
In an effort to keep the cost of these tools as low as possible, the manufacturers utilise computer aided design and CNC manufacturing techniques.
There are several logical steps to follow in order to select the best cutter for the application.
Is the operation face milling, square shoulder face milling, endmilling, side and face milling or otherwise?
Is the operation best performed on a vertical or horizontal machine and can more than one operation be performed on this machine?
These influence the cutter type geometry and insert grade.
Factors that may influence a decision include tolerances, finish, condition of workpiece prior to this operation, stability of the part and clamping required. Having identified the type of cutter required, reference to tooling manufacturers, catalogues is necessary to determine the exact cutter and insert number. The next stage is to investigate the spindle mounting.
These systems are adaptable to a wide variety of machines but there are also modular mounting systems and manually operated quick change systems available to cover practically any requirement.
CNC mills with manual tool changer may be fitted with a variety of mounting types, but CNC machining centres (that is, machines with automatic tool changers) usually have ISO taper spindle mounts and an internal gripping mechanism that securely holds the tool in place via a pull-stud fitted to the adaptor.
You may like to ask your teacher to demonstrate the tool changer on the machine at your college at this point.
Multi-inserted cutters are precision manufactured components and can only be expected to perform consistently if they are properly maintained.
Fortunately, most machine tool magazines are designed to protect this area ofthe tools under normal circumstances but such activities as blowing down the machine with compressed air is likely to force swarf particles into the tool magazine area. A particle of swarf on the mating taper will damage the spindle mating surface when the tool is loaded into the spindle and accuracy will then be impaired for ever after.
Pull studs should be periodically checked for security and wear.
There are many different workpiece materials and to achieve optimum cutting efficiency each one requires certain cutting data (speed, feed, depth) and tool geometry.
Workpiece materials may be metal or non-metal.
Generally, tool geometry affects the cutting action, the grade of insert affects the life of the cutting edge and the cutting data affects the requirements of the machine and workpiece, but there is an interaction between these areas that influences the results of each.
As a guide:
Two definitions of speed can be considered, one is the actual cutting speed of the tool inserts (expressed as m/min) and is dependent on the type ofworkpiece material and insert grade and the other is the spindle speed of the machine (expressed as RPM) and is dependent on the cutting speed and the diameter of the cutting tool.
Cutting speeds can be found by referring to tooling manufacturers' catalogues or charts. The values shown are usually suitable for ideal conditions and may be slightly optimistic under general machine shop condition.
If cutting speeds are found to be too high -shown mainly by excessive flank wear but also cratering, chipping or deformation -reducing the cutting speed by 10% to 30% could increase tool life by around 50% -100%.
There are many factors affecting the most suitable cutting speeds and feed rates for milling, such as:
Each of the above factors could be broken down into more factors if a detailed study of economic machining were to be considered.
The table below gives a general guide to speeds and feeds but more specific details must be obtained from the manufacturers' catalogue, from a card calculator or by using their technical services facility.
The speeds (m/min) shown here cover the range from roughing and finishing over several carbide grades from different manufacturers and the feed rates are expressed as average chip thickness.
Note: The feed rates for carbide endmills varies with the size of the tool.
Example:
The feed rate of a milling cutter can be described three ways:
The insert chip load must be of a suitable value. If the chip is too thin, the cutting edge may tend to rub as it engages into the material and more heat will be retained in the tool from the cutting process.
If the chip thickness is excessive, the chip pocket of the tool may not be sufficient to cope with the chip size or the cutting edges may not be strong enough to withstand the cutting forces.
The second mentioned feed rate (feed/rev) is a result of the feed/tooth multiplied by the number of inserts in the cutter and may be specified as a programmed feed rate on most CNC mills and machining centres.
The advantage in doing this is to relate feed more closely to chip load, as well as having the ability to maintain the desired chip if the spindle speed is varied during the program, or by the operator using the spindle override control.
Feed/rev is the factor most closely controlling the surface finish on the work. Inserts with parallel lands will generate a very smooth surface providing they are all set to exactly the same level. As this is virtually and economically impractical, fitting the cutter with one wiper insert (that is set lowest) and that has a wiper flat longer than the feed/rev will ensure a smooth surface on the work.
The third mentioned feed rate (feed/min) is found by multiplying the feed/rev by the RPM of the cutter.
Programming this feed is necessary on CNC machines that do not have a synchronised feed capability, or if feed is required at some stage when the spindle is not running.
It should be noted that chip thickness can vary considerably from the theoretical feed/tooth during milling because of the cutter positioning, entering angle, cutter diameter and radial cutting depth. Therefore, under some circumstances, it may be necessary to adjust feed rates to achieve the desired chip load.
It is often advantageous to express feed/tooth as a value representing average chip thickness, especially when calculating power requirements for certain operations.
The factors affecting actual chip thickness can be appreciated in the figure 1.11, 1.12, 1.13 and 1.14 below.
Figure 1.11 Symmetrical face milling
Figure 1.12
Figure 1.13 Asymmetrical face milling
Figure 1.14 Peripheral milling
To find average chip thickness (ct) for face milling and peripheral milling when 
is greater than 0.1-(note: in this case, W becomes d, and sin K is deleted).
for peripheral milling:
For example:
(i) A ø150 mm face mill with 75° entering angle is cutting 100 mm wide at a feed of 0.2 mm/tooth. The cutter is placed centrally.
First, Find A°,
For various entering angles, the ct is: 90° = 0.182 mm 60° = 0.158 mm 45° = 0.129 mm
(ii) For the same problem but with the cutter positioned off centre by 15 mm.
For various entering angles, the ct is: 90° = 0.174mm 60° = 0.150 mm 45° = 0.123 mm
(iii) A ø150 mm side and face mill is cutting 10 mm deep at 0.2 mm/tooth
(iv) The same cutter cutting 45 mm deep.
Note: For acceptable wear and chip formation when using carbide cutters, the average chip thickness should not be less than 0.1 mm, preferably in the range of 0.15 mm to 0.3 mm.
Because average chip thickness has a large effect on chip formation and power requirements, it is often necessary to calculate the feed/tooth (and therefore programmed feed) back from the ct or power values.
This is easily achieved by transposing the previous formulae.
For face milling or square shoulder face milling, the effects of different cutter positions should be considered:
The depth of cut usually refers to the amount of engagement of the cutter into the work, ie. the amount removed from the surface of the workpiece during face milling or the depth of the slot produced by side and face milling or endmilling.
In fact, there are two different depths of cut to consider:
Axial depth of cut (and entering angle) controls insert size on face mills but it is the insert size on standard inserted endmills that controls depth of cut. In both cases, engagement should be less than two-thirds of the insert. Radial depth of cut is limited by cutter size and mounting methods, although tooth pitching may have to be considered to allow for chip accommodation when deep slot cutting with side and face cutters.
Axial and radial depths are chosen in consideration of the machine power and workpiece rigidity.
Axial depth of cut has a direct influence on machine power requirements, all other factors being the same, whereas radial depth alterations change the average chip thickness, a fact that must be considered when calculating power requirements.
The power required for a certain machining operation depends on many factors:
Motor power can be found from the following formula which assumes a machine efficiency of about 80%.
Where:
The values for Ksm can be found from the following table.
NOTE:
Additional information regarding the above formula:
In V/min feed/min = RPM x No. teeth x feed/tooth (not ct)
NOTE:
(i) To find the depth of cut that can be taken by a mill with a 7.5kW motor when machining carbon steel 60 mm wide using a negative rake face mill 100 mm diameter with 6 teeth, 75° approach angle and a feed/tooth of 0.22 mm cutting speed is 180 m/min and a central position is chosen.
From the formulae:
A° is found to be: 73.74°0
ct is found to be: 0.198 mm (say 0.2)
Depth can be found from the formula,
V /min = w x d x feed/min
and volume/min must be found by transposing the power requirement formula. So, we have
If the machining centre being used has a dual range gearbox, low gear should be selected as it is unlikely that full power is available at only 573 RPM in high gear. If the machine does not have dual range gearbox, the power chart for the machine should be consulted to determine the power available at that speed and the depth or feed reduced accordingly.
Alternatively, a smaller cutter (say 80 mm with 4 teeth) with positive rake could be used. This would result in more power from a DC motor because of the higher RPM and less power required due to less teeth giving reduction in the removal rate, and the positive rake which improves the machine ability factor.
(ii) It is necessary to determine the power required to machine a slot 16 mm wide by 32 mm deep in low tensile grey cast iron. The side and face cutter is 160 mm diameter with 12 teeth and 3° axial rake. The cutting speed is 100 m/min and the feed is 0.25 mm per tooth.
(d) Feed/min = 200 x .25 x 12
= 600mm/min
(e) Estimate Ksm (use average values)
The Ksm for 3° will be more than the Ksm for 7° by an amount equal to 4° (the difference) times 1 ½% of 105. Therefore, the Ksm for 3° is close to 110.
A machine requiring 7 kW at only 200 RPM would have to have a geared spindle drive. As CNC machines usually have DC drive motors, the power chart for the machine would have to be consulted to determine if the power required was available at the speed in any of the gear ranges available.
Cutting fluids assist efficient CNC milling in several ways:
High speed machining is limited by cutting edge temperature, so if coolant can be directed in such a way as to stop cutting edge temperature from elevating too high, then slightly higher cutting speeds may be possible or conversely slightly improved tool life may be achieved.
If the design of the machine allows a collection of chips in one general area, localised heating and therefore uneven expansion of the machine can result in inaccuracies.
A plentiful spray of coolant may help reduce the chip temperature and so reduce any resulting inaccuracies.
When the cutting tool surface is lubricated, chip flow is assisted thereby offering slight improvement to tool life, surface finish and power requirements. A slight residue on the part can also assist corrosion resistance between manufacturing stages. Most non-ferrous metals require coolant to achieve satisfactory cutting and surface finish.
By flushing chips away from the cutting edge in situations where the chips could be re-cut, such as cavity milling, some drilling operations, etc.
On CNC mills, coolant is usually supplied through an adjustable hose or nozzle or through a multi-outlet high velocity spray system.
Some drilling tools have coolant supply holes machined in their flutes so that chips are forced back from the cutting edge. The machine must have central spindle supply for these tools or an adaptor must be used.
There are many coolants on the market, some having specific application areas relating to machining method or to material type while others fall into the more general category and are suitable for most machining applications, particularly related to ferrous metals.
Fixtures can be as simple as a vice, self-centring chuck, the usual array of clamp, bolts, stops and setup blocks found in almost any machine shop or they can be sophisticated purpose built devices to suit a job or particular operation.
A horizontal spindle machine lends itself to the fitting of a rotary or indexing table so that face machining can be performed on three faces in one set up, whereas a vertical spindle machine has access to one face. There are, of course, exceptions to this statement, but this is how respective machines are generally configured.
Swarf clearance should be considered, as stopping a CNC machine to clear swarf will seriously reduce the advantages of almost continuous cutting that CNC provides. It is this area that the horizontal machine offers the advantage of swarf falling away to be (usually) collected by a swarf conveyor that clears it from the machine.
Clamping positions should be designed to avoid the possibility of misplacement by the operator, as misplacement could cause a collision with the cutter. At the same time, there is a need for speedy set-ups and ease of handling in order to fully utilise the CNC advantages with consideration given to expected cutting forces when evaluating the position and the method of clamping so that the component is not distorted when removed from the fixture.
The fixture should be located on the machine in such a way that the datum surfaces are accurately aligned with the axis motions of the machine. While some fixtures may be located in the tee slots oft he machine table, it is more generally accepted to bolt a riser table onto the machine and use the CNC machine itself to machine the various slots, dowel holes, locating faces and bolt holes into the riser table surface, that will allow accurate location and suitable clamping for all the fixtures to be used on that machine.
On a horizontal machine the equivalent to a riser table is an angle plate or double-sided angle plate or a turret (4 sides), or a fixture known as a window frame which is particularly suited to holding smaller parts of a 'plate-like' shape and allowing access to both sides.
There may be variations to these basic designs but some examples as shown below.
Figure 1.15 Double-sided angle plate
Figure 1.16 Double-sided angle plate. Designed to hold up to four vices, or a combination of vices and other fixtures
Figure 1.17 A four-sided turret |
Figure 1.18 Dovetail fixture allows access to almost all |
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|
Figure 1.21 Second stage completed. Profile machined in fixture held on side of window frame
1. List the three most commonly used cutting materials in order of:
2. In the I. S. O. rating of metal cutting conditions, describe briefly the following:
3. Arrange the four most common milling insert shapes into order of edge strength with the strongest first.
4. Describe the geometric feature designed to improve surface finish with some inserts.
5. In relation to wiper inserts, describe:
6.
7. Milling cutters using the common wedge type insert clamping system should have the axial position of the inserts checked when they are replaced. Describe why this is necessary.
8. With guidance from your teacher, remove and replace inserts on milling tools with at least two different clamping methods.
Note the following:
Insert identification code (estimate from visual appearance and measured sizes)
Insert identification code (estimate from visual appearance and measured sizes)
9. In relation to face mill geometry, describe the meaning oft he following terms:
10. List the most suitable face mill geometry for the following applications:
11. Which milling tool would be most suitable for the following operations:
12. List the four areas of consideration when selecting the best cutter for a particular application.
13. Why should adaptor mounts, drive keys, clamps, screws, wedges, etc, be properly maintained and kept in good order?
14. List the factors having an effect on the following:
15. Beside each of the following machining situations, list the cutter geometry that would be most suitable.
16. Using the reference book or manufacturers charts, note a suitable value for cutting speed and feed rate per tooth for the following conditions.
17. If a CNC mill or machining centre has a synchronised feed capability, which method of programming feed rate would be most desirable?
18. What consideration should be given to the choice of face mill if a centralised cutting position is chosen?
19. If down cut (climb) milling is chosen for a peripheral milling operation, what may be the likely outcome?
20. On a 12 mm insert in a face mill, what would be a recommended cutting length or engagement?
21. Which three factors affect the machineability factor?
22. Find the power required to machine to the following data.
Facemill —80 mm diameter, positive/negative geometry, 4 inserts.
Machining — Carbon steel, 65 rum x 3 cut, ct of0.15 mm, CS of 150 m/min.
23. Which type of machine (horizontal or vertical spindle) would be most suitable for the following?
1. From the selection of indexable milling inserts given to you by your teacher, identify each one by its ISO code and fill in the table.
Indexable milling inserts |
|
Insert number |
ISO code |
1 |
|
2 |
|
3 |
|
4 |
|
5 |
|
2. From the selection of indexable milling cutters given to you by your teacher, identify each one by its ISO code and fill in the table.
Indexable milling cutters |
|
Insert number |
ISO code |
1 |
|
2 |
|
3 |
|
4 |
|
5 |
|
3. Select one of the indexable insert milling cutters and under the supervision of your teacher select the correct replacement insert and change one insert in the cutter.
Cutter ISO code identified |
|
|
Insert ISO code identified |
|
|
Correct clamp key selected |
|
|
Pocket checked for damage |
|
|
Pocket and insert cleaned |
|
|
Clamp screw lubricated and tightened to specifications |
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|
4. Select a suitable indexable milling cutter and insert grade and calculate the recommended spindle speed and feed rate in mm/min for the following operations.
Operation |
Cutter code |
Insert grade |
Spindle speed |
Feed mm/min |
Face milling a grey cast iron workpiece HB160, 110 mm wide by 200mm long by 4mm depth of cut |
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|
|
Roughing out a square pocket 100mm by 100mm by 10mm deep with 12mm radius corners in low alloy steel HB180 |
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|
|
|
This section covers the preparation of job plans for CNC milling operations including the calculation of intersection points, logical sequence of operations, calculating speeds and feeds, workpiece checks and machine checks.
After completing this section you should be able to:
Before any machining can be performed on one or many workpieces, it is essential to plan the exact sequence of operations required in order to produce the part to the required accuracy and as efficiently as possible.
Thorough operational planning ensures: The part being made will meet the required standards of quality and accuracy.
In order to produce a job plan, it is necessary to have full information about the part to be made, and about the materials, processes and equipment that are available for use.
The main concern is to have a properly detailed and dimensioned drawing of the part and sometimes a sample part can be useful. A detailed drawing should provide:
After determining the general form and function of the part, the drawing details, notes and specified tolerances should be studied in order to determine which are the functional surfaces, functional dimensions and datum, as these will affect the fixturing, location, machining and inspection of the part. (Functional surfaces are those that on other components. function dimensions locate or give the size of functional surfaces and datum surfaces are in close contact with surfaces are those surfaces from which other surfaces or features on the part are dimensioned).
The information required can be found by first listing the equipment available in the workshop, or that can be obtained, and then referring to the appropriate handbooks, manuals, catalogues and charts that determine the capacities of the various machines, tooling, fixturing and handling devices and whether consistent accuracy can be expected from the first to the last part.
1. From the drawing, determine these facts:
2. Identify the machining processes. This directly influences the tooling, fixturing and machines used.
3. Consider all of the operations. Alternative methods may be required if there are limitations with the equipment available.
4. Determine the best sequence of operations.
An Operation Sheet is a document that presents the order of machine operations and tooling required to produce the part on a specific machine or type of machine.
From this sheet, the number and order of machine operations and setting details can be determined:
The Operation Sheet is required by:
An Operation Sheet should contain information on the following:
NOTE: The actual design of an Operation Sheet will vary depending on the needs of a company, but it should contain as much of the information listed above as is required. A typical design is shown on the next page.
Operators setup sheet
The component drawing may clearly show functional surfaces, dimensions and datum, but may not give all the coordinate positions necessary for programming the machining operations.
Calculations are often required to determine these coordinates, with most problems being solved from right-angled triangle geometry and simple trigonometry, Pythagoras Theorem and ratios of proportional triangles.
A right-angled triangle can be looked on as a method of defining the relationship of one point to another in either cartesian or polar coordinates.
If the base and height are both known, then the relationship of the points at each end of the hypotenuse are defined. When the base and height are represented as being parallel to CNC machine axes, an angled motion along the hypotenuse is possible by linear interpolation.
If the hypotenuse length and the angle formed between the base and hypotenuse are both known, then the two points are defined in relation to each other by polar coordinates. Not all CNC machines have polar coordinate programming capabilities, but all can be programmed by the cartesian coordinate system, so it is essential to be able to find and solve triangles in order to define coordinate positions.
The possible situations where calculations are needed are:
Tackle one coordinate at a time, working from known information in order to create a dimensional link to the unknown coordinate. Where circles are involved, geometric construction could begin this way:
• Draw a line from the circle centre to the tangent point,
Figure 2.1 Figure 2.2
• Draw a line from centre to centre of blending circles.
Figure 2.3 Figure 2.4
In the first case, two lines at 90° now exist -draw a third to complete the triangle, then:
In either case, draw lines parallel to the represented machine axes, connecting to form triangles from previous or existing lines.
In other cases, or for further geometric development:
Look for: Two triangles sharing a common side and similar triangles (same angles, different sizes).
Keep geometric construction simple and logical -only draw in lines that create or help create a triangle (preferably right-angled triangle) with at least one known length or angle.
Consider the workpiece shown below as NM12.1. (Figure 2.5) Some of the points to be considered when planning this job are as follows:
The part shown is an example for your reference.
The machining operations are:
Figure 2.5 NM12.1
Fixturing is by machine vice with simple location, ie. parallel strips and aligning the left edge of the part with the left side of the vice.
If more than just a few parts were required, then machined vice jaws incorporating location and support would be preferable.
Machining the cavity creates quite a few options in regard to the tools used. If it is not required to have high precision finish or size tolerance, a semi-finishing operation preceded by a roughing operation to remove the bulk of the metal may be most suitable, all performed by the same tool.
If a high precision is required, then a large cutter for roughing and a helical flute carbide cutter for finishing would be suitable. Cycle time may be longer in spite of the high speed finishing operation due to the time required to perform the tool change.
If the first option is taken, then there is a further choice of using a slot drill to complete all the cavity machining before drilling the holes. This would speed up the drilling operation as the holes would then be shorter.
Or the holes can be drilled first, clearing away a useful amount of metal and allowing an endmill to be used to machine the cavity. An endmill can machine at a higher rate than a slot drill due to the extra teeth, although it is never twice the rate.
The operations in sequence are:
The program following is presented as an example only and may not represent the most suitable sequence for all machines.
NOTE: The sections of the program most likely to alter from one machine (or control) to another have been left blank and can be filled in under instruction from your teacher.
Before and after machining, a range of checks should be carried out on both the material and the machine to ensure that all is in readiness for the machining operation to be performed safely and in the most efficient manner with regards to the quality of the part being produced and care of the machine tool itself.
These checks will include the following points.
1. List four facts that need to be determined in the planning procedures.
2. List six items of information that should be completed on a job operation sheet.
3. Show the appropriate geometric construction lines to produce right angles triangles for the points on the path below.
4. What term would describe the process of ensuring a new component could be produced economically, efficiently, accurately and promptly by a workshop.
5. List the four areas of which full information is required in order to produce a job plan.
6. The planning procedure should follow a clear, logical progression. List the four areas of consideration in their logical sequence.
7. What is an operators set up sheet?
8. If you were a programmer designing an operators set up sheet, what information would you include on it to make it a useful document for the machine setter or operator, production management and the tool store?
9. List two workpiece checks that should be carried out before machining.
10. List two workpiece checks that should be carried out after machining.
11. List two machine checks that should be carried out before machining.
12. List two machine checks that should be carried out after machining.
1. Calculate the co-ordinates and complete the table below for the job NM12.2
|
A |
B |
C |
D |
E |
F |
G |
H |
X |
|
5.0 |
5.0 |
25.0 |
5.0 |
|
80.0 |
|
Y |
5.0 |
20.0 |
|
95.0 |
|
|
|
5.0 |
2. Make out a job operation plan that will complete the job in the most efficient manner and fill in the following table showing the order of operations, the cutters required and the speeds and feeds for each cutter for job NM12.2
This section covers the writing and editing of CNC milling programs producing straight and circular tool movements for external, internal and pocketing features, the use of drilling and tapping cycles and the preparation of an operators setting up sheet.
After completing this section, you will be able to:
A program usually is first written by hand in pencil, the completed and correct version being known as the manuscript.
This is copied through the keyboard of the machine or the program preparation equipment from which a printed copy (print out of hard copy) of the program can be obtained.
The print out should be used for verification in conjunction with program trialling.
Writing a program can only be attempted if all the required information is known, as found on a comprehensive Operation Sheet for the particular part.
Set up sheets can be provided to assist the setter/operator in positioning and locating the part on the machine as the programmer envisaged it.
You have already considered the drawing NM12.1 as the sample for developing a job plan. You will now use that drawing and the job plan developed to write the program.
Contouring can be described as peripheral milling of surfaces involving more than a single straight motion.
Peripheral milling is machining the component surfaces with the radius of the tool, that is the metal removed from the workpiece is taken at right angles to the axis of tool rotation.
The most common examples of peripheral milling are shoulder or finish cut endmilling and side and face milling.
Peripheral milling, particularly endmilling, involves compensating for the cutter size because the path followed by the machine must be reckoned to be at the cutter centre. Compensation can be effected by displacing the machine path by a distance equal to the cutter radius. The most effective way of doing this is by automatic cutter radius compensation (CRC).
Automatic CRC allows the machine control unit (MCU) to make the necessary machine path displacement, thereby simplifying the programmers task and allowing easy and fine adjustments to be made to the displacement, so component tolerances can be maintained during a production run.
Automatic CRC can only be effective if used correctly, and to use it correctly, the MCU requires the following information:
The cutter radius size is usually entered by the operator or machine setter into an appropriate offset register in the control, where it is accessed by the control during the start-up motion.
Right or left hand compensation is commanded by an appropriate preparatory function (G code).
Right hand means the cutter is always to the right of the workpiece as viewed along the direction of travel (programmed path).
The commanding codes are:
These codes are modal and should be commanded in G01 mode. The start-up motion usually contains all the commands required to activate CRC, as well as the workpiece coordinate from which all commanded motions are automatically displaced.
Consideration must be given to the block within which the start-up motion takes place ¬the safe approach zone. The safe approach zone is shown in figure 3.1, 3.2 and 3.3. The command point is the coordinate specified in the start-up block.
It must be understood that while the activation commands may be different from one machine to another, the relative position the tool takes on completion of the start-up block will always be the same regardless of the machine or even the type of machine.
The position the tool takes on completion of the start-up motion will be tangent to the coordinate commanded, relative to the motion command on the next block. (The tool will be beside the command point)
This can be seen also in the previous figure where the tool is shown in two different correct positions, due to the effects of right and left hand compensation.
The effects of the G40 (cancelling) code can also be considered to be the same on all machines, that is, the tool will be left 'beside' the coordinate position as commanded in the block before the block containing the G40 code, as shown in figure 3.4.
Figure 3.4 Tool positions prior to G40 command
There will be a discrepancy between the position of the tool and of the coordinate position command in the program, which will usually be rectified by the machine during the next motion command after the G40 block, therefore that motion must be to a position away from the work, a distance of at least cutter radius. This position could be anywhere in a safe departure zone as shown in figure 3.5 and could be included in the G40 block.
Figure 3.5 Safe departure zones
The relevant methods of programming are included here for FANUC controls, ANCA controls and SIEMENS controls. Assume a 50 mm square is to be machined by up-cut milling, starting at X0 Y0 with the work placed in cartesian coordinate quadrant one.
(i) FANUC G42 G01 X0 Y0 D01 F.25
where:
G42 calls compensation tool to right of workpiece.
G01 commands linear interpolation (must follow G42 in the block).
X0 Y0 co-ordinate for point one on the job
D01 is the offset number the control will access for the cutter radius size.
F.25 is the feed rate (0.25 mm/rev)
(ii) SIEMENS G42 G01 X0 Y0 D01 F.25
where: all as above.
(iii) ANCA G42 G01 X0 Y0 H01 J-10 F.25
where: all as above, except
H01 is the offset number
J-10 is the vector the tool centre will be on when the tool is tangent to X0 Y0.
NOTE: The ANCA vector description (using I and/or J values) will place the tool centre in relation to the coordinate commanded in the start-up block.
Some older FANUC controls used vectors in the start-up block, but in that case they describe the direction of the next motion, not the tool centre position.
Also, the ANCA controls have the option of describing cutter radius two other ways — directly, by describing the radius in place of the offset (R10. = 10 mm radius), or not at all, in which case the last radius used becomes effective again. The method shown in the example is called the indirect method.
If down-cut milling was required for the same job, then G41 would be used instead of G42 for the FANUC and SIEMENS, the ANCA is as follows:
G41 G01 X0 Y0 H01 I-10. F.2S
where:
the vector describing which direction the cutter centre will be is altered.
NOTE: The vector values are unimportant if I or J are used alone, but must be the correct ratios of the angle to be cut if used together.
Of course, the position of safe approach may need to be altered from the first example for all controls.
Figure 3.6 Tool motion for both directions
Internal machining on milling machines covers several techniques including drilling, boring and tapping.
Drilling tools are made in many styles, each offering particular advantages.
Still the most common type of drilling tool. Can be made from HSS, solid carbide or carbide tipped; the last two offer improved performance over HSS, but are more costly to sharpen, due, in some cases to a complicated tip geometry. HSS with Tin (Titanium Nitride) coating gives higher performance than plain HSS, but once sharpened cannot give full performance. Even plain HSS drills are not cost effective to use, due to low performance and the cost of resharpening. The major advantage of twist drills is the range of sizes (particularly HSS and particularly below 5 mm).
The name ‘short hole drill’ is derived from the diameter to length ratio of the hole they drill.
Very high machining rates are obtainable due to inserted tooth design, but they require more power than equivalent size twist drill and also a central flushing coolant delivery to force chips out.
Available in five or six sizes from ø18 to ø065.
Economical to use with cost savings of 50% to 90% per hole over HSS twist drills.
Standard styles or geometrises cannot drill stacked work pieces. Some manufacturers supply a different insert configuration that drills the centre out, and these can be used to drill stacked workpieces.
For diameters above 50 mm with a length up to about twice the diameter, trepanning tools give the highest machining rates. Trepanning tools are tubular with an inserted tool on each side. One produces the outside diameter, the other the inside diameter. Power requirements are fairly high. The tools are quite expensive but compared to the variety of tools required to produce large holes by more conventional means, these tools become cost effective.
One case study reduced the number of operations required to machine a ø200 x 300 deep hole in stainless steel from three to one, and the machining time from 8 hours to 9 minutes.
There are at least a dozen other techniques and tooling available to machine holes to any requirement of size, proportion, speed accuracy and surface finish. Most are suitable for milling machines.
Tapping is the usual method to produce small internal threads.
These taps are designed as a general purpose tool for hand or light machine tapping. These taps are manufactured in sets of three.
Figure 3.7 Hand tap
A set of hand taps comprises:
Pipe taps are used to produce either straight or taper pipe threads. They are usually suited for hand as well as machine tapping.
Figure 3.8 Pipe tap
For machine tapping only. Spiral pointed taps have straight flutes with angular cutting faces at the point. These faces cut with a shearing action which propel the chip ahead of the tap. The flutes are clear of chips so allow an improved coolant flow to the cutting edges. Generally, these taps have a stronger cross section than hand taps because of the smaller flutes.
Spiral pointed taps are designed for through hold tapping, or for blind holes as long as there is ample clearance at the bottom of the hold for chips to accumulate.
Figure 3.9 Gun tap
For machine tapping only. Spiral fluted taps are used primarily on blind holes as the spiral helix angle forces the chip back along the tap and out of the hole, reducing the chance of chips clogging the cutting edge of the tap. These taps are used for tapping soft and ductile materials (materials that produce long stringy chips), as well as materials that present machining difficulties.
Figure 3.10 Spiral flute tap
Fluteless taps do not remove material when 'cutting' a thread. These taps have no flutes and are not round, but are made with three or four lobes spaced around its periphery. the thread is 'formed' by these lobes pressing and pushing the material, forming a high degree of finish. They are used on ductile materials such as copper, brass, die casting aluminium, and leaded alloy steels.
Figure 3.11 Fluteless tap
Note: A larger tapping size drill than used for conventional taps is required. For example, a tapping size drill of 7.4 diameter will provide a 65% thread when tapping ductile materials with a M8 x 1.25 fluteless tap.
Tapping drill sizes should be chosen to suit the application, as a small size will give a high percentage of full thread height but will require high torque exerted through the tap to machine it.
Conversely, a large tapping drill size will reduce the effective thread height and reduce thread strength while allowing easy machining.
Depending on material and application, a thread height of 65% to 85% usually offers a good compromise of strength and ease of machining. Maximum thread strength is achieved from around 70% thread height.
Choosing a drill size equal to the nominal thread size less the thread pitch will give an effective thread height of 82%.
A thread height of about 73% can be achieved by drilling a hole larger by one tenth of the thread pitch than shown above, that is, a drill size equal to nominal size less 90% of pitch.
The torque required to drive a tap will vary depending on the size of the drilled hole, the material, the type of tap, the sharpness of the cutting edges and whether it is a fine pitch.
As a guide, dull taps and thread heights of 85% require as much as 50% more power as threads cut to 65%-70% thread height, whereas spiral flutes and fine pitch taps may require 25% or so less power.
Tapping heads are made to suit many requirements in both conventional and CNC machines. Some features available are:
Combinations of most of the above are available in some models.
Some newer CNC machines have a 'rigid tapping' capability where the feed motion precisely tracks the spindle motion during slow down and reversal to allow high precision of threaded hole depth at very high tapping speeds -in fact, dedicated CNC drills/mills with this feature can tap at 4000 RPM and maintain depth accuracies in the order of ±0.1 0 mm or better.
Cutting speeds for HSS machine taps is generally about one third of that required for HSS drilling.
Boring operations performed on CNC mills covers rough and finishing operations on through holes, blind holes, counter bores and recesses.
Boring tools consist mainly of these types:
Chip formation may depend on the type of operation more than most other machining operations.
Through holes, counter bores and recesses are amenable to short, tightly wound chips, but when blind hole boring on vertical spindle machines a longer helical coil formation may be desirable to help clear the swarf out of the hole, or perhaps an air blast or strong coolant flow direction to the hole may clear short chips during machining.
CNC drills, mills and machining centres have in-built cycles to assist programming most internal machining operations including drilling, tapping and boring. These cycles are quite similar in operation on all machines although the programming format may differ considerably from one control to another.
Canned cycles can be defined as a predetermined sequence of motions activated in a single block by a preparatory function with tertiary motions controlled by programmable parameters.
Basically, cycles start by rapid traversing to a reference level clear of the work face and returning after the machining motions to the same reference level or to a high (retract) level. Cycles are activated by G codes and remain active (modal) until cancelled, to allow repeating the cycle at any subsequent position.
There are three or four drilling cycles covering:
The first is sometimes called a spot drilling cycle.
The second is useful for blind holes, spot facing, counter boring, chamfering, etc.
The third is sometimes known as a peck drilling or pecking cycle and is designed for use with HSS twist drills or materials that present chipping problems.
The fourth is similar to the third but is more suited to machining deep holes.
The preparatory codes and tertiary motion commands for these cycles and others is detailed towards the end of this section.
During execution of the tapping cycle, the feed override control is usually made inoperative for fairly obvious reasons.
The depth coordinate is usually reached before spindle rotation is changed, so run-on should be considered when tapping blind holes.
If the machine has a dual or multi-range spindle gear box fitted, tapping should be performed in the highest range possible. This action causes the spindle motor to run at a lower speed for a given spindle speed, thereby allowing more rapid reversal and therefore less run-on.
Left hand tapping can be performed on most machines. The feed rate used should equal the lead (pitch for standard threads) of the thread, although it may be desirable to program a feed of about 95% in some cases, when using compression/tension tap holders.
The cycle motions usually are:
Most controls use the same G codes to activate the respective cycles, although this is not always the case.
The code used to cancel any of the cycles is usually G80, although ANCA controls do not. Their code is G00 and other from that modal group.
Cycles are not tied to a particular tool or operation, it is with the programmers discretion as to how they are used.
The next few pages are dedicated to the program formats for these cycles relative to the controls mentioned there.
Cycles can be commanded with the control in Absolute mode (G90) or Incremental mode (G91) if the repeat function is desired. As more controls execute cycles only in Absolute mode, the FANUC Incremental programming function will be ignored for reasons of simplicity and consistency.
The return point (on retract) level can be specified as:
G98 — return to initial level. (That is, the Absolute value of the Z axis at the time when the canned cycle was commanded)
G99 — return to reference level.
The hole machining motions are controlled by the programming parameters (or variables) dedicated to each cycle, as listed below:
X — coordinate position of X-axis where cycle will be performed.
Y — coordinate position of Y-axis where cycle will be performed.
Z — depth coordinate of hole. (Absolute in G90 mode)
R — reference level coordinate (Absolute in G90 mode)
Q — specifies depth of cut for each step (or peck) in G73 and G83, also X-axis shift in G76 cycle and G87 cycle. Q value is always incremental.
P — dwell time at Z position. Expressed as seconds or part thereof.
F — cutting feed rate (remains effective after G80).
NOTE:
If X and Y are included, the machine will rapid traverse at the initial level to the specified X-Y coordinate, then rapid to the R level before feed cutting.
Spindle start must be executed before commanding a cycle.
Once active, a cycle can be performed again at a new position just by commanding the X-Y coordinates or incremental shifts. A change of retract position can be effected by including the alternative code on the new position block. The change takes place after the cycle is performed at the new position.
Z,R,F ,P and Q are modal and can be carried to a different cycle by including only the cycle code on a new position block.
NOTE: Letter addresses in brackets are either optional (mayor may not be included) or alternative (either G98 or G99). All others must be included for the first use.
Drilling cycle —feed to Z, rapid retract
G98 (G99) G81 (XY) Z R F
Drilling cycle —-with dwell
G98 (G99) G82 (XY) Z R P F
Drilling cycle —peck drilling
G98 (G99) G73 (XY) Z R Q F
Drilling cycle —deep hole drilling
G98 (G99) G83 (XY) Z R Q F
Tapping cycle —right hand
G98 (G99) G84 (XY) Z R F
Boring cycle —reaming operation
G98 (G99) G85 (XY) Z R F
Boring cycle —spindle stop, rapid retract
G98 (G99) G86 (XY) Z R F
Boring cycle —feed to Z, spindle orient, X shift, rapid retract, X shift back, spindle restart
G98 (G99) G76 (XY) Z R Q F
Boring cycle —back bore or bottom chamfer
G98 (G87) (XY) Z R QF
The motions of this cycle are: spindle orient, X shift, rapid to R (below Z), X shift back, spindle start, feed up to Z, spindle orient, X shift, rapid retract to initial level, X shift back, spindle start.
Boring cycle — dwell, then manual retract
G98 (G99) G88 (XY) Z R P F
Boring cycle —reaming operation with dwell
G98 (G99) G89 (XY) Z R P F
This concludes the FANUC cycle programming format.
The control must be in Absolute mode before commanding any cycle.
The retract levels can be specified by:
G98 — return to initial level. (That is, the Absolute value of the Z axis at the time when the canned cycle was commanded)
G99 — return to reference level.
The hole machining motions are controlled by the programming parameters (or variables) dedicated to each cycle, as listed below:
X — coordinate position of X-axis where cycle will be performed
Y — coordinate position of Y-axis where cycle will be performed
Z — depth coordinate of hole
R — reference level coordinate
Q — specifies depth of cut for each step (or peck) in G73 and G83, also X-axis shift in G76 cycle. Q value is always incremental
P — dwell time at Z position. Expressed as seconds or part thereof.
F — feed rate for cutting.
NOTE: If X and Y are included, the machine will rapid traverse at the initial level to the specified X-Y position, then rapid to the R level before feed cutting. Once active, a cycle can be performed again at a new position just by commanding the X-Y coordinates or incremental shifts. DO NOT include a G00 code, as this will cancel the cycle Z,R,P,Q and F remain modal and can be carried to a different cycle by including the cycle code on the new position block. Also, a change of retract position can be effected by including the alternative code on the new position block. The change takes place after the cycle is performed at the new position.
Cycles can be cancelled by one of the following G codes:
G00, G01, G02, G03.
NOTE: Letter addresses in brackets are either optional (X -Y may or may not be included) or alternative (either G98 or G99). All others must be included for the first use.
Drilling cycle — feed to Z, rapid retract.
G98 (G99) G81 (XY) ZRF
Drilling cycle — with dwell.
G98 (G99) G82 (XY) ZRPF
Drilling cycle —peck drilling.
G98 (G99) G73 (XY) ZRQF
Tapping cycle —right hand only.
G98 (G99) G84 (XY) ZRF
Boring cycle —reaming operation.
G98 (G99) G85 (XY) ZRF
Boring cycle —feed to Z, spindle stop, rapid retract.
G98 (G99) G86 (XY) Z R F
Boring cycle —reaming operation with dwell.
G98 (G99) G86 (XY) ZRPF
Boring cycle — feed to Z, spindle orient, X axis shift, rapid retract, X shift back, spindle restart.
G98 (G99) G76 (XY) ZRQF
This concludes the ANCA cycle programming format.
NOTE: (The information below is for the SIEMENS cycle module UMS4).
The control must be in Absolute mode before commanding a cycle.
Cycles can be commanded by G code (where they remain active until cancelled), or by L address (where they are not modal).
The hole machining motions are controlled by R parameter programming (variables) which remain stored until changed. They cannot be cancelled or cleared, only changed to a value of zero, although this is not necessary unless the value must be zero. R parameters may be set at the beginning of the program, or at any time up to, and included on the block commanding the cycle. R parameters having the same value for different cycles need not be re-commanded for the next cycle, although program writing may be clearer, if they are.
The uses for R parameters in the cycles are:
R0 — dwell at start of cycle
R1 — first drilling depth in peck cycles (incremental unsigned)
R2 — reference level (Absolute)
R3 — Z depth coordinate value (Absolute)
R4 — dwell on down feed (Z and peck)
R5 — other drilling depth (incremental -unsigned)
R6 — spindle reversal
R7 — spindle restart
R9 — feed in tapping cycle
RI10 — retract level (Absolute)
R11 — peck type for G83 cycle. 0 = peck, 1 = deep hole drilling, also for G84 cycle, where 3 = Z axis, 2 = Y axis, 1 = X axis.
RI2 — X-axis shift for G86 cycle (incremental-signed)
R13 — Y-axis shift for G86 cycle (maybe altered to Z axis)
RI6 — down feed for reaming
Rl7 -up feed for reaming.
The format for correct use of R parameters is as follows, eg:
R = 0 (dwell is set to zero seconds) R
R1 = (first drilling feed motion is 7 mm)
R3 = -20 (Z-20 mm)
R6 = 04 (same as M04)
R7 = 03 (same as M03)
NOTE: All R parameters for any given cycle must be set before executing that cycle.
NOTE: If X and Y coordinates are included in the cycle command block, the machine will rapid traverse to that position at the present Z position. If the cycle is still active (commanded by G code), it can be performed again at new positions by commanding new X-Y coordinates or incremental shifts. Any altered R parameters included on this block are also effective for that (and subsequent) cycle operation.
If the parameters are all included in the same block as the cycle command, it is best to place the G (or L) code last on the block line.
Drilling cycle — feed to Z, rapid retract.
R2= R3 = RIO = F G8I (L81)
Drilling cycle — with dwell.
R2= R3 = R4 RIO F G82 L82
Drilling cycle — peck drilling.
R0 = RI = R2 R3 = R4 = R5 = R10 = R11 = 0 F G83 (L83)
Drilling cycle —deep hole drilling.
R0 = R1 = R2 = R3 = R4 = R5 = R10 = R11 F G83 (L83)
Tapping cycle — right hand.
R2 = R3 = R6 = 04 R7 = 03 R9 = R10 R11 = 3 G84 (L84)
Tapping cycle — left hand.
R2 = R3 = R6 06 R7 = 04 R9 = R10
Boring cycle — reaming operation.
R2= R3= R4=R10= R16= R17= G85 (L85)
Boring cycle —spindle start, feed to Z, spindle orient, axis shift, rapid retract, axis shift back.
R2= R3= R4= R7= R10= R12= R13 = F G86 (L86)
Boring cycle — manual intervention.
R2 = R3 = R10 = F G87 (L87)
Boring cycle —manual intervention.
R2 = R3 = R10 = F G88 (L88)
Boring cycle —manual intervention.
R2 = R3 = R4 = R10 = F G89 (L89)
This concludes the SIEMENS cycle programming format.
1. Describe the function of the following Cutter Radius Compensation (CRC) codes.
2. Label each diagram with the correct Cutter Radius Compensation (CRC) code.
3. Circle the diagram that correctly depicts the position achieved as a result of the start-up block. G41 (G42) X_Y_D_ (H)
4. Circle the diagram that correctly depicts the position resulting from a G40 in the next program block.
5. Circle the diagram that depicts the correct safe approach zone.
6. List four materials that twist drills are made from.
7. Mark the following with an A (advantage) or D (disadvantage), and T (true) or F (false).
Short hole drills:
8. Name the four types of machine taps.
9. Which of the four machine taps is most suitable for tapping each of the following.
10. Tapping size drills are usually: (TICK CORRECT ITEM)
11. What is the correct hole size to drill to achieve about 73% of full thread height for an M10 x 1.5 tap. (Tick the correct item).
12. Circle the correct cycle most suitable for the following operations on the machine you will be programming.
GSO GS1 GS2 G73 GS3
GSO GS1 GS2 G73 GS3
G73 GS2 GS3 GS4
GS5 GS6 GS7 GS9
1. Write a program by hand to machine the component to drawing specifications.
2. Type the program into a program editor and edit the program as required
3. Prepare an operators set up sheet using the following Operators Setup Sheet
4. Hand in your completed work to your teacher for comment and correction.
Operators set up sheet
This section covers the preparation of a CNC milling machine to produce a component including tool set up and offsets, program entry, setting work holding devices, establishing tool and workplace datums and tool change positions while maintaining workplace safety standards.
After completing this section you should be able to:
Do not work on the machine unless your teacher has cleared you to do so. Take care when mounting sharp tools in the magazine.
Ensure tools will clear other machine parts when mounted in magazine. Ensure setting tool is clear of workpiece and holding device before moving table after setting datum.
Note: Operation of the machine tool is only to be done under the teacher's supervision.
The setting up of the CNC milling machine involves the following procedures:
Programs may be entered into the machine controller by several methods:
Once the program is entered into the machine control, editing is carried out via the control's keyboard.
The procedures for transfer via disc or direct line vary from one machine to another but the basic principle is to prepare the machine to receive the information and then to prepare the computer to send the information and finally to actually send.
A trap that can be fallen into when transferring information via computer is the fact that the baud rate on both the receiver and the sending machines must be the same.
The correct procedures for transferring data to and from the CNC machine controller will be demonstrated to you by your teacher.
A machine vice can be used in conjunction with parallel strips, although it is preferable to machine soft jaws in order to achieve workpiece support with no loose pieces involved in the set up. For greatest accuracy, soft jaws should be machined in position on the machine.
Similarly, fixtures are most accurate if their locating surfaces are machined in position with the fixture accurately positioned on a riser table or some form of adaptor.
If any fixture can only be located in one certain position on the machine, then the machine position that locates the program origin can be stamped onto the fixture to eliminate setting and checking each time the fixture is used.
Fixtures should at least be aligned accurately in one plane via tee slots.
Simple fixtures designed to hold a part for a once-only run of a single part or a few parts may be simply bolted or clamped.
Where possible, the tools should be placed in the tool changer adjacent to each other and in the sequence desired for the machining operations. There may be some limitation to this ideal if the tools are heavy or have a large diameter.
A few heavy tools placed together can unbalance or subject the tool changer to undue stress and a large tool may overlap the adjacent tool pockets, forcing them to be left empty. Also, if jobbing work is frequently performed rather than long run production work, it may be desirable to mount a series of 'standard' tools in certain positions in the tool changer magazine, to reduce the set up time on job change overs.
The actual placing of the tools in the magazine is usually done by hand, but some machines may have to transfer the tool from the spindle to the magazine using the auto changer.
Your teacher can demonstrate the correct method to you.
The tools used on CNC milling machines usually require a tool length offset and some tools also require a radius offset. The values for these various offsets must be determined and entered into the respective offset register in the control unit.
There are a few variations to the way tool length offsets can be used, such as:
Your teacher can detail the technique used at your college.
Tool changing should take place at a position that removes the risk of any tool colliding with any part of the machine or fixturing during the tool change sequence.
On some machines, the position may vary, on others the position may be fixed. Your teacher can supply this information for the machine at your college.
As with tool offsets, there are a few variations to the method of locating the three dimensional workpiece zero (program origin) within the total motions of the machine, the differences mainly depend on the control executive program and are basically:
Again, your teacher can assist you in this regard.
Dry running a CNC program is a procedure that allows the program to be run with or without tool movements in order to check that the program will actually run on the machine.
The initial dry runs are usually done without tool or axis movement so as to verify that the program can be understood by the machine. During a dry run certain alarms may come up because the machine cannot carry out the commands given to it by the program. These alarms must be cleared and the program edited to remove the cause of the alarm.
After the program is proved to run correctly on the machine, a dry run may be made with the Z axis set a safe distance above the Z zero so that the tools will not come in contact with the workpiece and all tools and axis movements used.
Your teacher will assist you to carry out the dry run.
Note: This exercise should only be carried out under the supervision of your teacher.
Instructions
This section covers the use of a CNC milling machine to produce a component to AS 1654 including the providing and editing of the program on the machine, the use of the CNC milling machine in MDI, jog and automatic modes and the performance of safe procedures for tool failure and program restart. The section also includes the comparison and reporting on the completed component and the instructing of a machine operator on the requirements to run the job while maintaining workplace safety standards.
After completing this section you should be able to:
There are two types of surface finish obtained from milling operations:
Peripheral surfaces generally are easier to obtain a good surface finish on because of the sliding effect as the tooth engages or disengages the act at the thin end of the chip (depending on whether up cut or down cut milling is used.)
Axial surfaces are produced by teeth with sharp comers (HSS endmills, etc), or radiused comers (inserts), or by milling inserts with parallel lands -in which case the finish may be stepped because of uneven insert height.
In this case, one wiper insert can be fitted to the cutter and set to be the lowest insert, thereby acting as a finishing tool built-in. The effects can be seen back in Fig.l.2 and 1.3. The length of the flat (wiper) is usually about 10 mm for most insert shapes, enough to cover the feed distance for one revolution of most cutters except perhaps large diameter fine pitch face mills.
Surface finish is also dependent on:
The sizes produced by milling are a result of the correct positioning of the cutter, which in tum is under the direct influence of machine offsets.
The height of faces is controlled by tool length offsets (Z-axis), increasing the offset value results in a lower machined surface.
The size of peripherally milled surfaced is controlled by cutter radius offset in conjunction with cutter radius compensation. Increasing the radius offset will increase the external size of the part for a given tool and decrease the internal size of the part for the same tool.
You will be required to measure the first part from your program and report on the measurement obtained. You will then make any necessary adjustments to offsets to compensate for inaccuracies found.
Note: Operation of the machine tool is only to be done under the teacher's supervision.
Most controls offer similar functions and uses, even though the panels differ in appearance. For you to operate the machine correctly and confidently, it is important that you understand the functions of, and uses for all the buttons and switches on the control panel.
The most frequently used controls are:
This selects
This initiates program operation, either complete by one press or a block at a time depending on the setting of the single block switch.
A red or an orange coloured button immediately adjacent to the cycle start button (usually green). Pressing the feed hold button will halt slide motion only, pressing cycle start will resume the operation of the program.
Multi-position switches used to vary the programmed rates. The feed override usually overrides rapid traverse motion, but some machines have a separate switch for this.
A large red button placed conspicuously on the control panel, it may also be duplicated elsewhere on the machine. Pressing this cuts power to the machine (and on some machines to the control as well), thereby effecting a halt to all machine motions. This button should only be pressed in an emergency situation where personal injury or machine or tool damage is likely, and not for clearing alarms or other minor problems.
Dry run, block delete, optional stop, Z-axis inhibit, axis jog, axis zero, homing (zero return), keyboard, handwheel and 'soft' key which are buttons placed under the VDU (screen) and have a variety of functions as indicated on the bottom of the screen above them. They are so named because their function is controlled by software, they are not ‘wired-in’ for just one function.
Simple shapes resulting from motion of one axis at a time can be machined by manual control of the machine through the handwheel and axis selector switch. Actual positions can be displayed relative to the start position to easily allow accurate distances to be traversed.
More consistent motion may be obtained through the MDI function which then places the machine under programmed conditions. Some machines will allow several blocks of MDI to be entered resulting in a 'one shot' program with most functions available but usually excluding canned cycles.
Other machines will only accept one MDI block at a time, still without canned cycles and the like.
Another disadvantage of MDI is that each command must be correct, no trialing is possible and pressing the feed hold will defeat the MDI block (or program).
The third method used for machining is the execution of a properly prepared program, carefully trialled and checked.
Program proving (or trialling) is the careful and concentrated execution of the program by various means in order to eliminate all errors that could cause machine or tool damage or spoiling the workpiece before actually machining the part under automatic cycle conditions.
The technique used may differ from one machine to another even from one person to another, but the result of trialling must be a knowledge that the program is as close to 100% operational as can be ascertained.
Use such methods as:
If the program results in many alarms during the graphic display, it is probably wise to move off the machine and carefully check the print out, correcting faults at the program preparation area, rather than perform involved editing at the machine.
Any editing, be it simple or involved will alter the execution of the program and therefore program re-trialling may be necessary before safe and sure execution of the program is proved.
There occasionally comes a time during machining when something goes wrong; perhaps a cutter chips or breaks, an insert comes loose, the part moves during heavy machining the coolant falters to a dribble, there is a general failure etc.
In any situation like these, it is advisable to stop the machining process temporarily, if not totally and immediately.
In other situations, such as cutter dulling, the machining process can be nursed to completion by using the override controls. It should be noted there, that if the spindle is slowed with a 094 command active (feed/minute) the effective chip load on the cutter increases, but if a 095 command (feed/revolution) is active, the feed slows proportionally. In either case, feed can be overridden independently of the spindle.
Some controls have only one override control for all axis motions, and will usually be effective in all modes i.e. cycle, single block, dry run, jog (or manual), feed and rapid traverse.
Other controls will override rapid motions only if the single block switch is on, while others again have separate override controls for feed and rapid motions and may also have a jog rate switch marked in feed rate per minute values, which can be advantageous if machining in the jog mode.
The hand wheel (manual pulse generator, or MPO) works independently of any override controls, is effective on only one axis at a time and can be switched to different sensitivities, usually 0.001 mm, 0.01 mm and 0.1 mm, to allow precise positioning.
The effect of pressing the emergency stop button may be different between one control and another.
The emergency stop on some controls switches everything off, including the control itself, but most will cause all motions and miscellaneous functions to cease.
Some machines may require re-homing after an emergency stop, but most newer machines won't.
After an emergency stop, power must be connected again to the machine via the reset button before the machine can be moved, and if the reason for the emergency stop was tool breakage, the motion directions must be chosen with a view to minimising any further damage to the tool or work piece.
The cause of the damage should be determined before the program is restarted, in case the cause was program related.
After reset and damage repair, the block search function can be used to restart the program at any point providing that block has a sequence number.
The use and effect of block search and program restart will be different on different controls; some process the program while searching, which slows up the procedure on long programs, but results in all modal conditions being set and active at the block searched to.
Other controls ‘jump’ straight to the block required with only the default modal conditions active. In this case all necessary modal conditions must be entered through the MDI function before restarting the program.
Possibly the most successful way to restart a program is to block search to a tool change command -from there all necessary modal conditions will be commanded in the program for that tool. Another advantage is that the wrong tool cannot be in the spindle for the sequence required, although any machining already completed will be re-executed, and that may cause a problem on parts having a fine tolerance, or where tapped holes will be re-tapped.
Some controls allow subsequent block searching (once the tool is in position and running) to bypass machined sections without a loss of modal conditions.
A program can be written with easy and accurate restarting in mind by judicious use of sequence number and perhaps skip block commands.
1. Name the two types of surface pattern obtained by milling operations. With which of these is it easiest to obtain a smooth surface?
2. If a face milled surface is cut undersize (too deep), would the tool length offset need to be adjusted to: (a) a greater value (b) b lesser value?
3. When peripheral milling an external contour in conjunction with CRC, how could the component sizes be reduced if they were too large?
4. Describe three methods of machine operation that can be used for machining a component, listing also a limitation or advantage for each.
Note: This exercise must be done under the supervision of your teacher.
Instructions
Refer to the drawing NM12.2 and prepare the material for machining.
Referring to the drawing of the part that you have machined, measure at least five features specified by your teacher and fill in the chart below.
Under supervision of your teacher modify the tool offsets necessary to correct any errors in the part dimensions.
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Tool number |
Drawing size |
Measured size |
Offset correction radius |
Offset correction length |
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2 |
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3 |
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4 |
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5 |
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1. a) 1.High speed steel. 2. Cemented carbides. 3. Ceramics
b) 1. Ceramics. 2. Cemented carbides. 3.High speed steel
2. (a) High wear resistance low toughness. (b) High toughness low wear resistance. (c) Machining of steels in unfavourable conditions
3. 1. Round (R) 2. Square (S) 3. 80 rhomboid (C) 4. Triangular (T)
4. A cutting edge parallel to the surface being machined known as a wiper insert
5. (a) A face milling tool. (b) One. (c) Usually set slightly lower than the other inserts.
6. (a) single sided inserts. (b) double sided inserts
7. So that the inserts are set to take equal chip thickness per tooth.
8. Note the following to be checked as completed by your teacher
(a) Clamping method
Cutting edge condition
Clamp condition
Screw condition
Insert identification code
(b) Clamping method
Cutting edge condition
Clamp condition
Screw condition
Insert identification code
9. Double positive: A positive cutting rake angle in both the axial and radial planes of the cutter.
Double negative: A negative cutting rake in both the axial and radial planes of the cutter
Positive/negative: A positive cutting rake in the axial plane and a negative cutting rake in the radial plane
10. (a) Positive/negative (b) Positive/positive (c) Negative/negative (d) Positive/positive (e) Negative/negative
11. Which milling tool would be most suitable for the following operations:
12. The type of operation. The type of machine. The workpiece material. The part specifications.
13. So that the mounts will locate accurately in the machine spindle and so that inserts will be located accurately and securely in the cutter body. Also, damage may be caused to mating surfaces resulting in impaired accuracy.
14. (a) Tool geometry (b) Insert grade (c) Cutting data
15. (a) Positive / negative (b) Double negative (c) Double positive (d) Positive / negative (e) Double positive
16. These answers should be checked by your teacher from the reference book used.
17. Feed per minute
18. That there are at least two teeth in contact with the work at anyone time
19. Initial shock load is taken on the outer most point of the insert which may lead to mechanical insert failure especially on roughing cuts
20. 8mm
21. The type of material. The axial rake. The average chip thickness.
22. a) Spindle speed = 597 RPM
b) f/min = 358mm/min
c) V/min = 69 810mm₃/min
d) power = 3.28kW - 3.91kW
23. (a) Vertical (b) Horizontal (c) Vertical (d) Horizontal
1. Material. Material condition. Functional surfaces. Functional dimensions.
2. Machine tool. Fixturing. Cutting sequence. Cutting tool. Sequence of operations. Inspection requirements.
3.
4. Job planning
5. Drawings. Processes. Operations. Sequence.
6. Determine all the information from the drawing. Identify the machining processes. Consider all the options. Determine the best sequence of operations
7. An operators set up sheet is a document that presents the order of operations and tooling required to produce the part on a specific machine or type of machine.
8. The machine tool. The cutting tools. The fixturing. Sequence of operations. Cutting sequence. Inspection requirements.
9. Correct workpiece specifications. Pre-machined datums. Correct roughing size. Castings checked for size.
10. Size correct. Surface finish levels correct
11. Lubrications systems levels correct. Machined referred
12. Return to safe load position. Program returns to start
1. G40 Cancels automatic cutter compensation
G41 Activates automatic cutter compensation to left
G42 Activates automatic cutter compensation to right
2.
3.
4.
5. b
6. List four (4) materials that twist drills are made from. (1) HSS (2) Solid carbide (3) Carbide tipped (4) Titanium nitride coated
7.
8. Hand (light machine use only). Gun. Spiral. Fluteless.
9. (a) Spiral (b) Gun (c) Fluteless (d) Gun
10. (a) HSS twist drills
11. (c) 8.65 mm
12. (a) G81 (b) G73 (c) G84 (d) G85
1. Circular pattern. Linear pattern. (b) Circular pattern
2. (b) A lesser value
3. By increasing the cutter radius compensation in the control
4. (1) Manual control limited control over feed motion.
(2) MDI. Consistent motion control using a ‘one shot’ program that cannot be saved and repeated.
(3) Automatic execution of properly prepared program allowing machine to perform at optimal rate.
Source: http://lrr.cli.det.nsw.edu.au/LRRDownloads/12570/1/12570.doc
Web site to visit: http://lrr.cli.det.nsw.edu.au
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