Computer-Aided Design and Drafting CAD

Computer Aided Design
& Drafting

007.
Explain and demonstrate basic CAD commands and techniques

007.01
Explain basic CADD terms and concepts

007.02
Explain basic 2D CAD commands

007.03
Explain basic 3D CAD modeling commands and concepts

007.04
Construct a 2D CAD drawing

007.05
Construct a 3D CAD model

 
UNIT VII: Computer-Aided Design and Drafting (CAD)

Competency: 007.00
Explain and demonstrate basic CAD commands and techniques.

Objective: 007.01
Explain basic CAD terms and concepts.

Introduction: The purpose of this unit is to introduce students to basic 2D and 3D CAD concepts and commands. Over the last 20 years, CAD technology has advanced from only being able to construct simple 2D drawings to the functionality to create sophisticated, rendered, 3D solid models. The curriculum team feels strongly that 3D CAD should be introduced to students in Drafting I. This gives them a good foundation for the upper level courses. This unit will cover terms related to CAD, reasons for using CAD, set-up, draw, and modify commands, point-entry methods, and basic 3D modeling terms and commands. The 2D CAD material can be sufficiently covered with AutoCAD LT, AutoCAD, CADKEY, DATACAD, or the equivalent.

• Identify the following acronyms: R1(46):R2(43)
• CAD – This term has come to represent many different things. The most common are computer-aided design, computer-aided drafting, and computer aided design/drafting. The usage of the term really depends on the context. If one is producing mainly 2D documents, computer-aided drafting is probably appropriate. Computer-aided design generally reflects design utilizing a 3D modeling database.
• CAM – Computer-aided manufacturing. The use of computers to control the production process. The 3D CAD database can be used to run numerically controlled machine tools.

• Identify reasons for using CAD in place of manual drawing: R1(34-46):R2(43-45)
• Can reduce drawing time and improves productivity
• Prevents having to make repeated drawings of often-used symbols
• Improves overall appearance and readability of drawing
• Allows for easy revision of drawings
• Can be transmitted electronically
• 3D models, as a 3D database, can be used to:
• generate multiview drawings
• construct prototypes
• generate code for CAM
• increase visualization
• analyze mass properties of objects (volume, center of gravity, moments of inertia, etc.)

 

• Printers and plotters are used to produce hardcopies of CAD files. R1(45-46):R2(777-778)

UNIT VII: Computer-Aided Design and Drafting (CAD)

Competency: 007.00
Explain and demonstrate basic CAD commands and techniques.

Objective: 007.02
Explain basic 2D CAD commands.

R1(176-189):R4(AutoCAD Release 14 - 2002 Users Guide)

• Identify and explain the following setup commands:
• Units
• Limits
• Object Snap
• Snap
• Grid
• Ortho
• Polar
• Layer Controls
• Color
• Linetype
• Line weight
• On/Off
• Freeze/Thaw
• Lock/Unlock
• Save/Save As

 

• Identify and explain DRAW commands used to create:
• a straight line at a stated length, using the keyboard or mouse
• circles
• arcs
• regular polygons
• ellipses
• text of a stated size and font
• lines parallel, perpendicular, or tangent to other lines

• Display commands
• Zoom
• Extents
• All
• Window
• Dynamic
• Previous
• Limits
• Pan
• Change layers
• Regenerate (REGEN)

 

• Dimensions commands
• Linear
• Angles
• Circles (diameter)
• Arcs (radius)
• Center mark/line
• Leaders
• Modify properties (Lines & Arrows, Text, Fit, Primary Units)

• Other commands used to create geometry
• Hatch
• Blocks
• Polylines
• Divide
• Mirror
• Scale
• Stretch
• Array

 

• Modify commands          
• Selection options and techniques
• Crossing window
• Window
• Crossing polygon
• Fence
• All
• Last
• Previous
• Remove
• Copy lines and/or entities to a new location
• Erase
• Fillet
• Chamfer
• Move
• Rotate
• Trim
• Extend
• Offset
• Grips

• Edit commands
• Inquire/List (area/length)
• LTSCALE
• Break
• Change properties
• Undo
• Explode
• Polyline edit

 

• 2D Point Entry Methods 
• Cartesian or Rectangular Coordinates
• Polar Coordinates (Distance and angle constraints)
• Absolute Coordinates (global)
• Relative Coordinates (local)
• Direct distance entry

UNIT VII: Computer-Aided Design and Drafting (CAD)

Competency: 007.00
Explain and demonstrate basic CAD commands and techniques.

Objective: 007.03
Explain basic 3D modeling commands and concepts.

NOTE: When covering the 3D CAD material, it is recommended that one of the following programs be used: AutoCAD® (not AutoCAD® LT), Inventor®, Rhinoceros®, ProDesktop®, SolidWorks®, or SolidEdge®.

References: Users Guides, Tutorials, and/or Help menus provided with each software.

A.  Explain 3D modeling concepts: Types of modeling - Wireframe, Surface, and Solid.
1.   Wireframe models
a.   Object has no surfaces, but instead is composed of wire-like edges.

• Can see through the object (transparent).
• Visualization of object may be difficult because it can be tricky to tell which “wires” are on the front and which are on the back.
• Software used for creating wireframe models may include “hidden line” features that make model visualization more understandable by hiding “wires” on the back.
• Is very easy and fast for computer to calculate wireframe shapes.

2.   Surface models
a.   Surfaces define the shape of a hollow model.
b.   Surfaces are defined by using light, color and shadow to identify surface shape.

• Solid models

a.   Objects are defined as a solid mass.
b.   May contain information about the density, mass, moment of inertia, volume and center of gravity of the object.
B.  Set up a 3D scene and view 3D space

• Basic software interface

a.  Command line
b.  Menu bar
c.  Toolbars
d. Graphics area
e.  Status bar

• Viewports
• Creating and changing viewports
• Zoom and Pan
• Undo/Redo

C.  Use basic Boolean commands and capabilities for 3D solid modeling.

• Solid Primitives.
• Box or rectangular prism
• Wedge or triangular prism
• Cone
• Cylinder
• Sphere
• Torus
• Ortho mode
• Shade

2.   Boolean Commands.

• Union (+ or È) – adds parts together
• Subtract or Difference ( - ) – removes parts or features
• Intersection ( * or Ç ) – Intersects overlapping volumes into a single feature
• Extruding 2D profiles or surfaces
• Revolving 2D profiles or surfaces

E.   Display presentations of the model using shade and basic rendering techniques.
1.   Hide – hides edges that are not visible.
2.   Shade – a flat coloring system that takes minimum lighting into account.
3.   Render allows for more realistic and complex surfaces.

• includes sophisticated lighting and shadows.
• includes texture and bump maps.
• includes sophisticated backgrounds.
• raytracing generates reflections, refraction and more precise shadows.
• Plotting and/or exporting drawings and files.
• Basic 3D operations that the student should be able to perform:
• Generate a model by creating a profile and extruding or sweeping.
• Generate a model by creating a profile and revolving it around an axis along a circular path.
• Generate a model by using primitives and Boolean operations.
• Rotate parts by understanding which axis to use and what angle is required.
• Shade a model (no shadows or textures required).
• Plot the drawing.
• Export the drawing.

Using Rhinoceros® 3D NURBS Modeling Software

• Introduction
•  Rhino combines the accuracy of traditional CAD with spline-based modeling to create objects with NURBS curves and surfaces rather than line segments or polygon meshes. 
• Non-Uniform Rational B-Splines (NURBS) are mathematical representations that can accurately define any shape. 
• Rhino has a menu structure and operating procedure that is similar to AutoCAD®, making a quick and comfortable transition to 3D modeling for the 2D AutoCAD® user.
• The program is very powerful and is suitable for constructing varied shapes and objects.  Rhino would be used by a wide range of designers who work in 3D, especially where the forms are free flowing.  It is suggested that you visit their web site to view various applications. (www.rhino3d.com)  Rhino is not a parametric feature-based solid modeler like Inventor®, ProDesktop®, SolidWorks®, or SolidEdge®. It may not be as suitable for the type of modeling required in Engineering II or Engineering III as a constraint-based CAD program.
• Training materials and tutorial problems are available with the software, at bookstores, on-line, or through the software vendor.
• A sample schedule for learning software basics (taken and modified from Rhinoceros Teacher Workshop Guide, R30-TWSGUIDE 31-Dec-02)
• As you introduce the software, have the students learn only those commands that will help them finish the project.  Other commands can be learned as needed and as they are applicable to more advanced projects.
• Using a 90 minute class, students should reach a reasonable level of proficiency within two to three weeks of instruction. 
• After a reasonable amount of teacher guided/demonstrated projects, students should be allowed some time to explore and create on their own. Consider demonstrating the power of Rhino by exposing them to shapes that would not be found in the typical Drafting I textbook (using commands such as the loft and rail revolve or control point modification.
• Sequence of learning
• Basic interface: command line, changing viewports, zoom, pan, undo/redo
• Solid primitives: cone, sphere, box cylinder, torus, ortho, shade
• Basic editing: move, copy, rotate, delete, mirror, scale, object snap
• Boolean operatives: union, difference, intersection
• Rendering: properties, spotlight, render
• Creating curves and surfaces: line, curve, arc, trim, join
• Extruding surfaces: extrude
• Revolving surfaces: revolve
• Sweeping surfaces: sweep 1 rail, sweep 2 rail, rail revolve
• Lofted surfaces: loft

 

Creating 3D solid models in SolidWorks®, SolidEdge®, Inventor®, or ProDesktop®

SolidWorks®, SolidEdge®, Inventor®, and ProDesktop® are constraint-based CAD programs. They all function in a similar fashion. The biggest difference between these programs and software such as AutoCAD® is the way they take advantage of the 3D database. Within the constraint-based modeler environment the 3D solid model is typically the first type of file that is created. Once the 3D model files are created, assemblies and drawings of parts can be generated. Most of these programs take advantage of bi-directional associativity between the files. In other words, if a dimension is changed in the model, the drawing file and assembly files automatically update. If a change is made in the drawing file, the change is reflected in the part and/or assembly files.

Constraint-based programs also organize solid models into a series of modifiable features. These features are organized in some type of feature tree (see Figure 1). The feature tree includes the default planes of projection, the origin, and the individual features that make up each object.

Figure 1. Solid Model of the BASE PLATE with its Feature Tree.

Another important concept for students to understand when working within constraint-based programs is how constraints work. Within a single part a constraint may be a dimension or it may define the relationship between geometric elements. In Figure 2, three dimensional constraints are shown, but there are many geometric constraints that were applied to make sure the geometry changes correctly if the dimensions are modified. For example, tangent constraints were applied between the four arcs and their corresponding lines. An equal constraint was applied between the four arcs. The two vertical lines are symmetric about the vertical center line. The two horizontal lines are symmetric about the horizontal center line.

By adding the dimensions and constraints in this fashion the part designer is establishing some type of design intent. In other words, if a dimension is modified, the geometry should change only in a way defined by the designer. These changes should reflect how the part works within the assembly. For the part in Figure 2 the intent is to always keep the part centered about the origin. The symmetric constraint will maintain this intent when either the 3.000 or 5.750 dimensions are modified.

Figure 2. Sketch of the BASE.

Creating Parts within a Constraint-Based Modeler
The following procedure is recommended when modeling using a constraint-based program:
A.  Think about design intent. How might things be changed later?
B.  Define/select a sketch plane. Use one of the default planes (Frontal, Horizontal, Profile), select a planar surface, or construct a new plane.
C.  Sketch the new profile.
D.  Constrain the profile by adding relations and dimensions.
E.   Define the sweep parameters.
1.   Extrude/Revolve/Path/Blend
2.   One Side/Two Side
3.   Distance
4.   Outside/Inside
5.   Boolean
F.   Execute/Revise.
G.  Repeat the procedure for the next feature.

How to Create Student Success on VoCATS Section 007.03

• Students should know that shapes are typically formed by extrusion, Boolean operations, creating a profile and revolving it, or by sweeping operations and should have experience with each of these operations.
• Students will be expected to know the most efficient method for 3D shape construction.  For example, “Should I draw the shape and extrude or would it be easier to create a profile and revolve?”
• Students should be able identify basic solids and should be able to visualize the results of Boolean operations.
• Students should be able to explain the advantages/disadvantages of wireframe, surface and solid models.
• The teacher test item bank provides examples of questions that are typical of the VoCATS exam.

UNIT VII: Computer-Aided Design and Drafting (CAD)

Competency: 007.00
Explain and demonstrate basic CAD commands and techniques.

Objective: 007.04
Construct a 2D CAD drawing.

Requirements: Each student is required to create a 2D CAD drawing.
1.   Using the equipment provided, make a CAD drawing of the object shown on the next page.
2.   When plotted the drawing should fit on a size A (8.5” X 11”) sheet of paper at a scale of 1:1 (Full Scale). Ask your test administrator for plotting limits.
3.   Center the drawing on the sheet.
4.   Letter your name, problem number (007.04.001), scale, and date in the title block.
5.   Save your work on the diskette provided or as directed by the test administrator.
6.   Time Limit = 60 minutes.
7.   Your drawing will be evaluated on its accuracy and completeness.
8.   Dimension the drawing as shown.
9.   Use the following specification:
a.   Units – decimal                                               b.   Grid – 1 inch
c.   Limits – 10 X 7 inches                                    d.   Snap - .1 inch
e.   Layers

Layer Title	Linetype	Color
VISIBLE	CONTINUOUS	BLACK (WHITE)
HIDDEN	HIDDEN	DARK BLUE
CENTER	CENTER	RED
DIMENSIONS	CONTINUOUS	RED

Assessment: The 2D CAD drawing should be evaluated based on the following criteria:
CAD set-up                                                                                                     30 points
Accuracy                                                                                                         30 points
Lines                                                                                                               20 points
Dimensioning text and notes                                                                          15 points
Layout and balance                                                                                         5 points

Rubric for BASIC CAD – Construct a 2D CAD drawing - 007.04

CAD Setup

Numerous errors in setting up layers, limits, units, grid, snap, colors, and linetype.	Some errors in setting up layers, limits, units, grid, snap, colors, and linetype.	Layers, limits, units, grid, snap, colors, and linetype are constructed according to the specifications.	Total Points
0-21 points	22-27 points	28-30 points

Accuracy

Numerous errors in measurements.	Some errors in measurement.	When measured, the sizes of features and their locations agree with the given problem.	Total Points
0-21 points	22-27 points	28-30 points

Lines

Numerous stray lines or pieces of lines. Intersections are not formed. ANSI standards for coding not followed.	Some intersections are not formed. Some lines do not meet ANSI standards.	Line quality meets ANSI standards for coding. Drawing demonstrates an obvious use of object snaps. No stray lines.	Total Points
0-14 points	15-18 points	19-20 points

Dimensioning Text and Notes

Dimensions and text styles and sizes do not meet accepted standards. More than one spelling error. Misplaced or missing dimensions and/or text.	Some errors in settings for dimensions and text. No more than one spelling error.	Styles and sizes for dimensions and text meet standards as determined by the teacher. All dimensions and text are given as shown. Spelling is correct.	Total Points
0-10 points	11-13 points	14-15 points

Layout & balance

The drawing is not centered vertically or horizontally. Major errors in constructing the border and title block.	The drawing is centered vertically but not horizontally (or horizontally but not vertically). Some errors in constructing the border and title block.	The drawing is centered within the working space. Border and title block are drawn as specified.	Total Points
0-1 points	2-3 points	4-5 points

                                                                                                   Total Score                          

UNIT VII: Computer-Aided Design and Drafting (CAD)

Competency: 007.00
Explain and demonstrate basic CAD commands and techniques.

Objective: 007.05
Construct a 3D CAD model.

Requirements: Each student is required to create a 3D model.
1.   Using the equipment provided, create a 3D model of the object below and use an appropriate method to shade/color the model as specified by your test administrator.
2.   When plotted the drawing should fit on a size A (8.5” X 11”) sheet of paper. Ask your test administrator for printing or plotting limits.
3.   Include your name, problem number (007.05.001), and date in the file.
4.   Save your work on the diskette provided or as directed by the test administrator.
5.   Time Limit = 90 minutes.


Assessment: The 3D solid model should be evaluated based on the following criteria:
CAD setup and shading                                                                                  10 points
Accuracy of features                                                                                       30 points
Orientation of the model in 3D space                                                             15 points
Features modeled correctly                                                                             40 points
Text and notes                                                                                                5 points

Rubric for BASIC CAD – Construct a 3D CAD model - 007.05

CAD Setup and shading

Numerous or flagrant errors in file setup.	Some errors in file setup.	CAD file setup (such as units, shading, etc.) is constructed according to the specifications.	Total Points
0-6 points	7-8  points	9-10  points

Accuracy of features

Numerous errors in measurements.	Some errors in measurement.	When measured, the sizes of features and their locations agree with the given problem.	Total Points
0-21 points	22-27 points	28-30 points

Orientation of the model in 3D space

Important features are hidden from view. Object is not modeled in natural or specified position (upside-down, etc.).	Object is modeled in its natural or specified position. Some features are not clearly presented.		Object is modeled in its natural or specified position. Object features are clearly presented.		Total Points
0-10 points		11-13 points		14-15 points

Features modeled correctly

Numerous errors in feature size and/or location (slots, holes, fillets, etc.).	Some errors in feature size and/or location (slots, holes, fillets, etc.).	All features (slots, holes, etc.) are sized and located correctly. Fillets and rounds properly formed.	Total Points
0-29 points	30-37 points	38-40 points

Text and Notes

Text style and size does not meet accepted standards. More than one spelling error.	No more than one spelling error.	Text style and size meets accepted standards. Spelling is correct.	Total Points
0 points	3 points	5 points

                                                                                                   Total Score                          

AUTHENTIC ASSESSMENT: Product Development

Computer-Aided Design and Drafting (CAD) Project Assessment

• Each team member will construct a 2D CAD drawing of individual design concept (if previous drawings are drawn using board techniques).
• Evaluate drawings based on 2D CAD Performance Objective 007.04

 

• Each team member will construct a 3D CAD model of individual design concept (if previous drawings are drawn using board techniques).
• Evaluate drawings based on 3D CAD Performance Objective 007.05