cnc machining design rules
CNC Machining Design Rules
CNC machining does not have a mould to fill or a bed to support it — it has a cutter, and the cutter is the constraint. Every rule below comes from the same fact: a rotating tool has a diameter, a length and a deflection, and geometry that ignores those three will be slow, inaccurate or impossible.
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- Min wall (metal)
- 0.8–1.0 mm
- Pocket depth
- ≤ 4 × cutter diameter
- Internal radius
- ≥ 1/3 of pocket depth
- Hole depth
- ≤ 4 × diameter (drilled)
Machining is a subtractive process with a round tool. Every design rule is a consequence of that round tool: its diameter sets corner radii, its length sets reach, and its deflection sets what can be held accurately.
Specifications
| Minimum wall | 0.8 mm aluminium · 1.0 mm steel |
|---|---|
| Pocket depth | ≤ 4 × cutter diameter |
| Internal radius | ≥ 1/3 pocket depth |
| Drilled hole depth | ≤ 4 × diameter before a second operation |
| Standard tolerance | ±0.01 mm locating · ISO 2768-m general |
| Standard finish | Ra 1.6 µm functional · Ra 3.2 µm general |
Frequently asked questions
What is the minimum wall thickness for CNC machining?
About 0.8 mm in aluminium and 1.0 mm in steel for a wall up to 4× its thickness tall. Taller or thinner walls deflect, so the machine takes lighter passes and the price rises.
Why can't a pocket have sharp internal corners?
Because the cutter is round. A 6 mm end mill leaves a 3 mm radius, and anything sharper needs a smaller cutter with less reach, or EDM. Sharp corners are quoted as an extra operation.
How deep can a drilled hole be?
About four times the diameter before the drill wanders and the exit face distorts. Deeper holes are drilled undersize and then reamed or bored, which is a second operation.
What is the most expensive feature on a machined part?
A feature that needs a second setup. Re-fixturing doubles the handling time, and handling is the least visible line in a quote — which is why moving a feature to the same face often cuts the price more than a discount request.
Should I tolerance every dimension?
No. Tolerance the dimensions that locate the part and add a general note (ISO 2768-m) for the rest. Dense tolerancing multiplies inspection time without improving function.
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