The big idea: A subtractive technique starts with more material than the part and removes the difference. It works on a 3D mass and on flat sheet alike.
Tell the processes apart by **what moves**
- Turning: the WORK rotates.
- Milling: the TOOL rotates.
- Cutting: a blade or beam travels along a line.
- Abrading: many small hard particles each take a little.
Cutting, milling, turning and abrading drawn by what moves in each.
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| Process | What moves | What it makes |
|---|---|---|
| Cutting | A blade or beam separates material along a line, with the sheet clamped still | Flat shapes from sheet — by saw, shear, laser, water jet or knife; the laser cutter is the one a school workshop owns |
| Milling | The TOOL rotates; the work is clamped and moved past it | Flat faces, slots, pockets, holes and complicated 3D surfaces — mould tools, jigs, machine parts, accurate metal prototypes |
| Turning | The WORK rotates; the tool is moved along and into it | Round shapes only — shafts, spindles, knobs, threads and bores, all about one axis |
| Abrading | Many small hard particles remove a little material at a time | Fine control of size and surface — filing, sanding, grinding, honing, polishing; it holds tolerances a cutter cannot |
The pair students swap: Turning and milling are confused constantly, and the difference is one fact.
Turning: the work spins. That is why it can only make round things. Milling: the tool spins. That is why it can make almost any shape a cutter can reach.
CNC, and cutting flat sheet
Accurate
The machine follows the CAD data exactly, to tight tolerances.
Repeatable
Same program, identical parts: they fit together and swap.
Runs unattended
Little supervision, high output, lower labour cost.
2D: cutting sheet
Laser cutting (acrylic, card), CNC routing (plywood, MDF) and die cutting (card nets) remove material from flat sheet.
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| What subtractive buys | What it costs |
|---|---|
| Accuracy and surface finish no other category reaches | Everything removed is waste — bought, transported and often processed before being cut away |
| It works on any material, hard or soft | Cutting tools wear and must be replaced, and hard materials wear them fast |
| No tooling to make first, so one part is viable | Cycle times are minutes to hours per part, so it does not scale to volume |
| It is how mould tools and dies are made in the first place | Internal shapes are limited to whatever a cutter can physically reach |
Why it is called wasting: A machined aerospace bracket can start as a block ten times its finished mass, and titanium swarf is expensive waste from an energy-intensive material.
That waste is the argument for additive manufacturing on complicated low-volume parts — and it is worth a mark whenever the two are compared.
How this is tested — explaining how components are produced by wasting techniques. It comes up two ways:
Paper 1 — multiple choice
- Identify a wasting process from a description of what moves.
- Choose the process suited to a stated shape.
Paper 2 — analysing a product
- Explain how a named component is machined.
- Justify a subtractive process against a forming or additive one.
The trap: Swapping milling and turning. Say which part rotates and the rest of the answer stays right.
Explain how a steel shaft with a threaded end and a flat machined along its length is produced.
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