Computer Aided Drawing — Role, Advantages and Practice

Exam note, from the official syllabus: "CAD, only internal evaluation". No end-semester question can be set on this topic. It is assessed in the laboratory.

That does not make it unimportant — it is the only part of this course you will still be using in ten years. But when revising for the written paper, spend your time on Modules 1 to 3.

What CAD is

Computer Aided Drawing or Design — creating, modifying and documenting a design using software rather than instruments.

The distinction between drawing and design is worth noting. Early CAD merely replaced the drawing board. Modern CAD holds a model from which drawings are generated, so the drawings are consequences of the model rather than the primary artefact.

Role in product design and development

CAD sits at the centre of the cycle:

1. Conceptual design — quick sketches and layout studies.
2. Detailed design — full geometry, dimensions, tolerances.
3. Analysis — the model feeds directly into simulation of stress, flow, thermal behaviour and motion, without redrawing it.
4. Prototyping — the same model drives 3D printing.
5. Manufacture — CAM generates toolpaths from the model, so the machine cuts what was designed rather than what a machinist interpreted.
6. Documentation — drawings, exploded views, bills of materials.
7. Revision — change the model and every derived drawing updates.

Point 7 is the one that matters most in practice. On a drawing board, a design change meant redrawing every affected view by hand, and any view that was missed became a latent error. In CAD the change is made once and propagates. Most of the real productivity gain is here, not in the speed of drawing lines.

Advantages over manual drafting

• Accuracy. Geometry is exact to many decimal places; pencil work is limited by the thickness of the line.
• Speed of revision, as above.
• Reuse. Copy, mirror, array and pattern operations; standard parts are inserted from libraries rather than redrawn.
• Storage and retrieval — no plan chest, no degradation, instant search.
• Sharing — the same file goes to analysis, manufacture and the customer.
• Visualisation — 3D views, sections and renderings on demand.
• Interference checking — the software detects parts that collide during assembly, which a set of 2D drawings cannot show.
• Integration with CAM and CNC, removing a manual transcription step and the errors that come with it.

The honest limitations: cost of software and hardware; a real learning curve; dependence on power and data integrity, so backups matter; and the danger of misplaced confidence — CAD draws exactly what it is told, including exactly the wrong thing, and its precision can make a nonsense look authoritative. It does not check that a design is sensible.

Core concepts for two-dimensional work

Coordinate entry

• Absolute — a point measured from the origin, entered as x,y.
• Relative — measured from the previous point, @dx,dy.
• Polar — a distance and an angle from the previous point, @distance<angle.

Most work uses relative and polar entry, because a drawing is naturally described as a sequence of moves rather than as a list of absolute positions.

Drawing aids

• Snap and grid — constrain the cursor to regular positions.
• Ortho — restrict to horizontal and vertical.
• Object snap (osnap) — attach exactly to endpoint, midpoint, centre, intersection, tangent, perpendicular. This is the single most important habit to acquire. Lines that merely look joined but are not will fail to hatch, fail to trim, and produce wrong areas.

Layers. Separate lines by purpose — outlines, hidden, centre, dimensions, hatching, text — each with its own colour, linetype and thickness. Layers are how the BIS line conventions of Module 1 are implemented in CAD, and they let whole categories be hidden or printed selectively.

Common commands, in the vocabulary shared by most packages:

Draw: line, circle, arc, polyline, rectangle, polygon, ellipse, spline Modify: erase, copy, move, rotate, mirror, offset, array, trim, extend, fillet, chamfer, scale Annotate: dimension (linear, aligned, radial, diameter, angular), text, leader, hatch View: zoom, pan, regenerate

Model space and paper space. Draw at full size in model space — a 5 metre beam is drawn 5000 mm long — and apply scale only when laying out the sheet for printing. Drawing to a scale in model space is a habit from the drawing board and causes trouble later.

Practical advice for the laboratory

1. Draw full size, always. Scale at plotting time.
2. Use object snaps for every connection. Never place a point by eye.
3. Set up layers first, before drawing anything.
4. Use construction geometry and delete it afterwards, exactly as you would erase pencil construction lines.
5. Dimension on a dedicated layer, following the same BIS rules as manual drawing — the conventions do not change because the tool did.
6. Save often, and keep versions.