Drilling is a very common operation performed during the fabrication of parts, assembly and installation. This section details the safe and effective use of drills and drill bits, including types of drills, terminology, drilling techniques and safety.
Care is necessary in the sharpening of a drill, the selection of suitable speed and feed, and the method of cooling the drill.
At the completion of this section you should be able to:

• identify various types of drills used in the electrotechnology industry
• demonstrate the correct method in sharpening twist drills
• demonstrate the correct procedure for drilling metallic and non-metallic components
• demonstrate the correct and safe use of a bench drill

Metal twist drills

The twist drill is a rotary end-cutting tool with cutting lips and usually helical flutes. Straight-fluted drills are occasionally used.

Figure 1: Straight shank twist drill

Figure 2: Tapered shank twist drill

Twist drill terms

The drilling process

Drilling is a continuous cutting operation that produces a circular hole in solid material. The cutting edges are in continuous contact with the work.
When drilling do not let long curls of steel develop as the drill cuts, and do not try to break them or brush them away with the hands. The curl may be broken by releasing the feed for a second. Another method is to take an open end spanner and run it down beside the drill and strike the curl — this will break it.

Drilling different materials

It is a mistake to use the same drill at the same speed for all materials. Different drills are designed to cut different materials. Always check the drill pack before use. You will avoid wasted time, wasted drills and possible accidents.
See the next page for a comprehensive guide on drill types for different materials. The angles are either increased or decreased depending upon the hardness or softness of the material to be drilled.
For drilling hard metals such as tool steel a point angle of about 136° can be used, and the lip clearance angle decreased to about 9°. For heat treated steel an point angle of 125° and lip clearance of 8° to 10° is recommended, and for soft grade cast iron an included angle of 90° and a lip clearance of 15° to 18° is recommended. For softer materials, the point angle may be further decreased.
In addition to the recommended drill points of Figure 3, the following notes will also be useful.

Figure 3: Recommended drilling angles for various materials.

Carbide tipped drills

Drills with cemented carbide cutting tips are used at very high speeds for drilling non-ferrous metals, but are not so practical for cast iron and steel because the carbide tips are not as well reinforced as are lathe, shaper, and other types of carbide-tipped tool bits.
Remember that cutting ability and hardness are different things. A high speed drill so hard that it cannot be filed may, by exercising great care, be made to drill extremely hard material, but such a drill will be so brittle as to be worthless if used for softer materials.

Drill speeds

Table 1 is intended as a guide and in many cases it will be found that improved results will be obtained by increasing the speeds.
A good method of checking if the speeds and feeds are correct is to inspect the point after drilling a few components. If the outer corners of the cutting edges are showing signs of wear before the point, this is an indication of the speed being too high. If the point wears before the corners, this indicates the feed is too great.
Table 1: Drill speeds and lubricants

1     List and describe the main parts of a double-fluted twist drill and the functions of each.



2     What are the lip clearance angle and chisel angle for a correctly sharpened twist drill?



3     What is the purpose of flutes on a twist drill?



4     Name at least three types of shank design used on twist drills.



5     How are the sizes of drills up to 13 mm checked?



Check your answers with those given at the end of this section.

Cutting fluids

To maintain the speeds and feeds recommended without damage to the drill or the work, it is often necessary to use lubricant or cutting fluid.
The three functions of a cutting fluid are to:

• lubricate the tool
• cool the tool and the work
• assist in the removal of chips.

Care must be taken that the cutting fluid reaches the cutting edge, otherwise its value is lost and the drill may be damaged by the generated heat drawing the temper of the drill.
Table 2: Recommended cutting fluids

Special techniques

Deep holes

When drilling deep vertical holes, release the feed occasionally and withdraw the drill. Remove the chips from the bottom of the hole with an old round file that has been magnetised.

Drilling marble

The drilling of marble and similar materials, especially when the drill is fed by hand, can be facilitated by grinding a narrow slot in the point of the drill (See Figure 17). This slot should be about 3 mm to 6 mm deep according to the size of drill, and at an angle of a little less than 90° with the cutting edges. If the slotting is carefully done the drill will give good results.

Figure 4: Drilling of marble

Drilling slate

Slate, on account of its peculiarly stratified formation, may present some difficulty when drilling. It is necessary to have something under the slate slightly larger than the diameter of the drill, to support the pressure of the drill, otherwise the slate will fracture. Drill No. 9 of Figure 16 is suitable for this purpose, the flutes being wide and usually chromium plated.
The flat drill ground thin at the point is also suitable for drilling slate.
When drilling marble or slate, excess pressure may damage the back of the panel as the drill emerges.

Starting a drill on a curved surface

When starting the drill on a curved surface, the latter should be cut away with a chisel to allow the drill to have a flat seat to commence cutting, otherwise the hole may be out of position and the drill may snap.

Making slots

Drilling can be used to advantage in the making of slots, especially where hand processes with chisels and hack saws are to be later carried out.

Figure 5: Setting out
A series of holes are drilled in the waste portion and along the desired contour of the slot. Position the holes with centre spacing 1 mm greater than the drill diameter, and with the edges 1 mm from the finished contour: this allowance provides for finishing the job. The marking off of the holes will be facilitated by the use of a centre punch. 
The shearing punch or drift, (Figure 6), in small jobs may drive the web out in one piece, but in other cases the web may have to be sheared from both sides. Do not use an ordinary chisel as this has a wedging action - in some jobs a chisel will distort the work. Note the square shearing profile of a drift, as against the wedge shape of a chisel.

Figure 6: Drift for shearing thin web.
Figure 7 shows a five-hole slot with the web sheared between four of the holes. If the job is small it may be held in a vice with the jaws just below the line of the contour. Large pieces should be placed on a metal bench block or anvil having a recess to line up with the holes.

Figure 7: How to drill for a slot

Templates

Where the same drilling operation is to be repeated, it is advisable to adopt some system whereby the complete marking off of each piece can be avoided. The process adopted must permit reasonable speed according to the degree of accuracy and number of pieces required.
A marking template or template consists of a piece of material having the same shape as the finished article.
Templates are used where the parts have to be marked out, a record of sizes or the shape of layout of some part kept for future reference, or holes transferred from one part to another.
The template is usually laid over the surface and located on centre lines or by some other method, and if necessary clamped in position, after which the outline, etc., is marked on the surface with a scriber. Where required the various lines are pop marked.
Templates are made from different materials depending on the accuracy required and materials at hand, for example building paper, sheet zinc, mild steel plate.
Paper templates can be made by placing paper over the object and tapping lightly with a mallet round the piece thereby cutting the paper.
Usually, it is preferable to have only centre holes in the template rather than full sized holes. There is less likelihood of distorting the template, less time is required for marking off, and the template may be used for marking tapping size holes in one member and clearance holes in another. Dividers can be set to mark out clearance or tapping holes as desired.

Drilling problems

Table 3 summarises common drilling problems.
Table 3: Drilling problems

Sharpening twist drills

The twist drill is a precision tool and must be ground accurately. Sharpening should be carried out in a special drill sharpening machine. Small, inexpensive drills are usually considered as disposable when blunted.
When it is necessary to sharpen by hand on an ordinary abrasive wheel, great care must be taken to ensure that the angles and lengths of cutting edges are equal.
The sharpening should be done on the peripheral face which should be running true. Hold the shank of the drill with the right hand, the left hand resting on the tool rest with fingers holding the body of the drill.
Stand with the axis of the drill at approximately 60° with the axis of the wheel spindle. Commence grinding with the drill practically horizontal and the cutting edge just touching the face of the wheel. Exert a light pressure on the cutting edge and pivot the drill by lowering the shank, thus grinding from the cutting edge to the heel, and increasing the pressure on approaching the heel, thus providing for lip clearance. Do not attempt to grind heavily, but instead repeat the above process a number of times, being careful to prevent overheating. After repeating the grinding process several times for one cutting edge, revolve the drill and repeat for the other cutting edge.
When grinding a drill, use a gauge to check the:

• length of lips
• point angle
• lip clearance
• correct location of dead centre.

Precautions when sharpening

Care must be taken to avoid overheating. With carbon steel drills great care is necessary to avoid discolouring due to heat. The drill should be cooled off in water from time to time to avoid any possibility of overheating.
High speed steel drills should not be plunged into water when hot, as this may cause fracture.

Student exercise 2

1     State the probable cause and remedy for:
(a)   breaking of a drill




(b)   a change in the colour, length and character of the waste curls (or chips).




2     Name three functions of a cutting fluid used in a drilling operation.



3     Name the recommended cutting fluids for the following:
(a) soft (mild) steel

(b) brass

(c) cast iron

Check your answers with those given at the end of this section.

Summary

• Drilling, which is the making of holes in materials, should not be confused with boring, which is widening an existing hole.
• The twist drill is commonly used because the flutes which help to form the cutting edges, remove the chips and permit lubrication of the point.
• The point supports and forms the cutting edges or lips. For general use an included angle of 118 degrees is usual.
• The land or margin is the raised portion along the edge of the flutes.
• Body clearance is the portion that gives backing to the land or margin and is ground back to give clearance to the land.
• The shank is the end of the drill that engages in the chuck or socket.
• Unequal cutting lips and angles produce oversize holes.
• When drilling plastics a long sharp point with an included angle of 60 degrees and plenty of lip clearance is most suitable.
• A special tungsten alloy drill should be used for brick, concrete and glass.
• For grinding large drills a special machine should be used. For small drills satisfactory results can be obtained with hand grinding.

1     Name the types of shanks used with double-fluted twist drills.



2     List the conditions that determine whether a drill cuts true and to the correct size.



3     List the three most common spindles used to accommodate the tool holder of a bench drilling machine.

4     What safety precautions should be taken prior to using a bench drilling machine?



5     What is a suitable point angle for a twist drill if used to cut mild steel?

6     Across which parts of a twist drill is the true diameter of the drill measured?

7     When securing a tapered shank drill in a spindle socket, what important factor must be observed, if the drill is to receive its full driving torque?

Answers to Check your progress are at the end of the section.

Answers

Student exercise 1

1     The main parts and functions are:

• shank – portion that is gripped in the drill drive or chuck
• body—spiral helix from shank to cutting face giving the drill its depth of cut
• point – cutting face of the drill.

2     Lip clearance angle and chisel angle are:

• lip angle – 12 degrees to 15 degrees
• chisel angle – 120 degrees to 130 degrees.

3     The purpose of the flutes is to:

• provide suitable rake angles for the lips
• form cutting lips at the point
• curl the drill cuttings into tight helixes
• provide an exit for swarf
• allow cutting fluid to lubricate the cutting lips.

4     Types of shank design are:

• taper
• straight
• parallel
• round without flat
• round with flat.

5   Drill plate gauges are used.

Student exercise 2

1     (a) Broken drill - Probable Causes: Spring or back lash in press or work; too little lip clearance; too low speed in proportion to the feed; dull drill. Remedies: Test press and work for rigidity and alignment; regrind properly. Increase speed or decrease feed. Sharpen drill
(b) Overheating - Probable Causes: Change in condition of the drill such as chipping of cutting edge, dulling, etc. Remedy: Regrind drill
2          (a)        Lubricate the tool
(b)       Cool the tool and the work
(c)        Assist in the removal of chips
3          (a)        Soluble oil
(b)       Nil (dry) or kerosene
(c)        Nil (dry) or compressed air jet for cooling only

Check your progress

1     The types are taper, straight, parallel, round without flat, round with flat.
2     The conditions are: the two cutting lips must be of equal length; the two cutting lips must be at equal angles to the axis of the drill; both lips must be free of cracks and chips; the chisel angle must be central to the drill diameter; the drill must be straight.
3     The spindles are:

• threaded
• external taper
• internal taper.

4     The operator must wear eye protection, be suitably dressed (including cap if needed) and be sure the chuck key is removed and that all guards are in place.
5     118 degrees
6     across the lands
7     Locate the tang of the drill in the matching slot in the spindle, or sleeve.