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Engineering Drawing & CAD Page 1 of 53
School of Engineering Faculty of Technology University of Plymouth Drake Circus Plymouth PL4 8AA
Course notes - Engineering Drawing and CAD.
Contents:
References.
1 The design process and the role of the design model.
1.1 The design process 1.2 The design model 1.3 Types of design model
2 Representing the design model - Engineering Drawing.
2.1 Projections 2.2 Creating Orthographic Projection drawings 2.3 Drawing conventions 2.4 Sections 2.5 Dimensions 2.6 Tolerances, limits and fits 2.7 Assemblies
3 Representing the design model - 3D CAD & Solid Modelling.
3.1 Introduction to 3D Parametric Solid Modelling 3.2 Features, parts and assemblies 3.3 Using 3D CAD and Solid Modelling
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References.
For general guidance on Engineering drawing:
Engineering Drawing with CAD Applications. O Ostrouwsky Edward Arnold ISBN 0-340-50411-
Basic Engineering Drawing. Rhodes & Cook Pitman ISBN0-273-31887-X
Manual of British Standards in Engineering Drawing and Design. Editied by Maurice Parker British Standards Institute in association with Hutchinson ISBN 0-09-172938-
Manual of Engineering Drawing. Colin Simmons & Dennis Maguire Edward Arnold ISBN 0-340-58484-X
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1.2 The design model.
The concept of the designer working with a model of a design is fundamental to the design process.
The design model is a representation of the design. This model could be anything from a few ideas in the designers head, through to rough sketches and notes, calculations, sets of detailed formal engineering drawings, computer generated 3D representations, physical prototypes, etc.
The design model would be used by the designer to record and develop ideas and to provide a basis to evaluate the design.
Larger design projects are undertaken by more than one engineer. Design models are used to communicate and demonstrate ideas between all those concerned with the product design, development, manufacture and use.
A designer needs to have the skills to generate and work with this model in order to communicate ideas and develop a design.
1.3 Types of design model.
Designers use a variety of different models, depending on what property of the design is to be considered and for whom the information is destined.
Typically a designer may model:
- Function
- Structure
- Form
- Material properties, surface conditions
All of these areas probably encompass a large portion of the degree syllabus. Within this module we will concern ourselves primarily with form, i.e. the shape of parts or components and how they fit together.
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Figure 2.1a, two right angle planes of projection.
2.1 Projections.
2.1.1 Orthographic projection.
We have discussed both the role of the design model in the design process and the importance of the representation of the form or shape in this role.
Now we will consider in detail the methods designers use to represent the form of their designs.
Back in the 18th^ century a French mathematician and engineer, Gaspard Monge (1746-1818), was involved with the design of military armory. He developed a system, using two planes of projection at right angles to each other, for graphical description of solid objects.
This system, which was, and still is, called Descriptive Geometry, provided a method of graphically describing objects accurately and unambiguously. It relied on the perpendicular projection of geometry from perpendicular planes.
Monge's Descriptive Geometry forms the basis of what is now called Orthographic Projection.
The word orthographic means to draw at right angles and is derived from the Greek words:
ORTHOS - straight, rectangular, upright GRAPHOS - written, drawn
Orthographic projection is the graphical method used in modern engineering drawing. In order to interpret and communicate with engineering drawings a designer must have a sound understanding of it's use and a clear vision of how the various projections are created.
There are two predominant orthographic projections used today. They are based on Monge's original right angle planes and are shown fully in Figure 2.1b. They define four separate spaces, or quadrants. Each of these quadrants could contain the object to be represented. Traditionally however, only two are commonly used, the first and the third.
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For this example, you could say that the right hand side image is the plan or top elevation and the image to the left is the side elevation.
Whether you view the objects from the left or the right, the order in which the drawing views are arranged puts the image that you see after the object, object first then the image. This is always true for First Angle projection.
Put another way:
- Viewing from the left: The drawn image on the right is your view of the drawn object on the left.
- Viewing from the right: The drawn image on the left is your view of the drawn object on the right.
This can get confusing, particularly when also considering other drawings created using other projections. You may develop your own way of recognising First Angle projection. The author uses:
The OBJECT is FIRST for FIRST Angle projection. or... EYE > OBJECT > IMAGE or... You look through the object and place the image
An example of a component represented in a multiview drawing, in First Angle projection.
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2.1.3 Third angle projection.
Consider the third quadrant in Figure 2.1b. The resultant drawing of the cone would be obtained by flattening the two perpendicular projections planes, as shown in Figure 2.1d.
For this example of the cone, you would say that the left hand image is the plan or top elevation and the image to the right is the side elevation.
Whether you view the objects from the left or the right, the order in which the drawing views are arranged puts the image that you see before the object, image first then the object. This is always true for Third Angle projection.
Put another way:
- Viewing from the left: The drawn image on the left is your view of the drawn object on the right.
- Viewing from the right: The drawn image on the right is your view of the drawn object on the left.
Again, you may develop your own way of recognising Third Angle projection. Perhaps: EYE > IMAGE> OBJECT
Figure 2.1d, Third Angle.
Left. (^) Right.
The same component shown using Third Angle projection.
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2.1.5 Pictorial Drawing.
Orthographic projection is used as an unambiguous and accurate way of providing information, primarily for manufacturing and detail design. This form of representation can however make it difficult to visualise objects. Pictorial views can be created to give a more three dimensional impression of the object. There are three types of pictorial projections commonly used, as shown below.
Perspective, isometric and oblique pictorial projections.
Perspective: Used more with freehand sketching. Parallel lines appear to converge and meet at what is referred to as the vanishing point. You can have one, two or three vanishing points (VP).
Isometric: Receding lines drawn at 30º and are usually kept at true measured lengths.
Oblique: Front face sketched as a true shape. Starts with two axes, one horizontal, one vertical. The third axis is usually drawn at 45º and lengths are reduced by 50% of true lengths. Sometimes called 'cabinet' projection.
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2.2 Creating orthographic projection drawings.
This is an introduction into how to create and interpret multi-view orthographic projection drawings.
2.2.1 First angle projection.
The component:
Your drawing will, for this example consist of four views:
- Front F
- Left L
- Right R
- Plan (Top) P
Usual practice is to orient the component in a position that it is most likely to be found in.
Your aim is to create, from the front view, an orthographic projection drawing as shown below in Figure 2.2a. Note how the views are constructed in line with each other, allowing the features to be 'projected' between the views.
Figure 2.2a. A completed First angle projection drawing.
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2.2.2 Third angle projection.
The construction method used is the same. The difference between first and third angle projection when creating or reading really lies with the positions of the views. For the same component, an orthographic projection drawing with the same front, side and plan views would look like Figure 2.2b below.
Observe how, in third angle, the views give the image then the object. In other words, what you see then what you are looking at.
In first angle you are given the object then the image, or what you are looking at, then what you see.
Figure 2.2b. Third angle projection.
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2.3 Drawing conventions.
2.3.1 Introduction.
In order for anyone to be able to understand exactly what a drawing represents, sets of precise rules and conventions have to be followed, much like a language. These rules are usually referred to as Standards.
When a designer works with an engineerign drawing they must be familiar with the precise meaning of the various line styles, abbreviations, drawing simplifications and terminology as specifiied in the relevant standards. This section introduces you to some of the conventions defined in BS 8888.
Standards are developed both privately by companies and by internationally
recognised institutions.
Two such international standards are:
British Standard Institution: BS 8888 (Superceded BS 308)
American National Standards Institute: Y14 series
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No particular style is required, but characters should all be consisitent on the same drawing. Capital letters are preferred to lower case ones.
Size of lettering is given as a minimum height, relating to drawing size, as shown below:
Application Drawing sheet size Min. character height (in mm)
Drawing numbers A0, A1, A2 & A3 7 Titles, etc. A4 5
Dimensions & A0 3. Notes. A1, A2, A3 & A4 2.
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2.3.3 Terminology & representations of standard components.
Here are some examples of commonly used engineering components and features of components.
General: Housing: A component into which a 'male' mating part fits, sits or is 'housed'.
Bush/bearing: A removable sleeve or liner. Known also as a simple or plane bearing.
Boss: A cylindrical projection on surface of component.
Curved slot: Elongated hole, whose centerline lies on an arc. Used usually on components requiring adjustment.
Rib: A reinforcement, positioned to stiffen surfaces.
Fillet: A radius or rounded portion suppressing a sharp internal corner.
Key: A small block or wedge inserted between a shaft and a mating part (a hub). Used to prevent relative rotation of the two parts. Key way: A parallel sided slot or groove cut into a bore or a shaft, to 'house' a mating key.
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Fasteners: Bolts, screws & studs: Threaded fasteners. Bolts have a shank partially threaded, whereas screws are threaded along the entire length.
For guidance on dimensioning, see next page.
The last three examples here are called set screws and are used to position or lock components.
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This diagram gives approximate dimensioning methods for drawing hexagon headed metric bolts, nuts and plane washers.
(Manufacturers data sheets may give more accurate measurements.)
Circlip: Internal & external.
