Engineering Drawing and CAD Conventions

Why a drawing must be exact

An engineering drawing is a precise, shared language. It must be exact and free of doubt, so that anyone, anywhere, can make the part exactly as intended. To make that possible, engineers agree conventions: rules for the lines they draw, the views they show, the way they add sizes, and the scale they work to. Because the conventions are shared, every engineer reads a drawing the same way. Computer aided design, known as CAD, produces these drawings quickly and lets you show a part as a 3D model, as flat views, or as an exploded picture of how the parts fit together. This module covers the main conventions and the role of CAD.

Drawing and CAD words

Learn these before you read any engineering drawing. They name the views, the rules and the tools that let a drawing be understood the same way by everyone.

Match each line to its job

First angle against third angle

Orthographic views can be arranged in two agreed systems. A drawing shows which one it uses with a standard symbol, so a reader anywhere knows how to lay the views out.

Which view is this?

A drawing shows a part from directly in front, from directly above and from one side, as three separate flat pictures. What are these three drawings called?

• The three orthographic views
• A single isometric view
• An exploded assembly view
• Three separate scale ratios

Read the drawing before you cut

A costly mistake in a workshop starts with a misread drawing. Before cutting anything, work through the drawing in order. First check the projection symbol so you know how the views are laid out. Then check the scale, so you read every size correctly. Then read the dimensions, each given once and clearly. Finally check the line types, so a hidden edge is not mistaken for a visible one. Symbol, scale, dimensions, lines: reading in that order stops wasted material and wasted time.

Pick the drawing conventions

Select every statement below that follows correct engineering drawing convention.

• A hidden edge is shown with a dashed line
• A centre line is a thin chain line
• Each dimension is given once and clearly
• A visible outline is drawn with a faint dotted line
• Scale means a drawing is always life size

Order making a CAD model

Put the steps of producing a part in CAD in order, from a blank screen to a drawing the workshop can use.

• Sketch the rough profile on screen
• Add exact dimensions and constraints
• Build the profile into a three-dimensional model
• Create the orthographic views from the model
• Add dimensions, the scale and the projection symbol
• Check against British Standards and export the drawing

Complete the drawing facts

Showing a part as separate flat views is _____ projection. A single three-dimensional view with edges at equal angles is an _____ drawing. The ratio between the size drawn and the real size is the _____. A hidden edge is shown with a _____ line.

One part, three views

Suppose you must draw a small metal bracket with a hole through it. In first-angle projection you place three views. The front view shows the height and width and the outline of the bracket as thick continuous lines. The top view, drawn below the front view, shows the width and depth. The side view shows the height and depth. The hole cannot be seen from the front, so its edges appear as dashed lines, and a thin chain line marks its centre. Each size is given once, the whole drawing is drawn to a stated scale, and a projection symbol tells the reader the system used. From these three flat views alone, a workshop anywhere can make the exact bracket.

Spot the drawing conventions

Tap the TWO statements that describe a correct engineering drawing convention.

• A hidden edge is drawn as a dashed line
• A projection symbol tells the reader which system is used
• Dimensions can be left off when the part looks obvious
• Line thickness never matters on a technical drawing

Work out the scale

A bracket is 80 mm long in real life. It is drawn at a scale of 1 to 2, which means half size. How long is the bracket on the drawing, in millimetres?

Draw it for the workshop

You are preparing a drawing for the workshop. Make the right choice at each step.

• The workshop needs to see a hole that runs hidden inside the part. How should you show its edges?
• The part is too large to fit the page at full size. What should you do?
• A reader in another country must know which projection system you used. How do you tell them?

Build a rule about drawings

Choose the word for each gap to complete one correct rule about engineering drawings.

Explain why drawings follow rules

A new apprentice asks why engineering drawings have so many strict rules instead of just being neat sketches. Explain why the conventions matter and how a drawing communicates a design.

• Explain what makes a drawing exact and free of doubt
• Explain the job of different line types
• Explain what a scale tells the reader
• Explain why a projection symbol is needed
• Explain how CAD helps produce accurate drawings