CNC Machining Design Guide: 12 DFM Rules That Keep Your Part Cheap & in Tolerance

Most "expensive" CNC parts are expensive because of the drawing, not the shop. A few design-for-manufacturing rules — applied early — cut cost and scrap and make the part come out right the first time. Here are the twelve that actually move the needle.

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The 12 Rules

None of these change what the part does. Together they are the difference between a quote you wince at and one you accept. Tighten only what the function demands — that single habit beats the other eleven.

#	Rule	Why it matters	The fix
1	Wall thickness ≥ 0.8 mm (metal), ≥ 1.5 mm (plastic)	Thin walls chatter and deflect	Thicken, or accept more setups
2	Put a radius in every internal corner	Sharp corners need EDM or tiny tools	Standard corner radius (e.g. R0.5+)
3	Hole depth ≤ 4× diameter	Deep holes wander and snapped drills	Open the back, or relax position tol
4	Use standard thread sizes	Off-size taps are custom and slow	M3/M4/M5/M6/M8 etc.
5	Tolerance only critical dims	Every tight dim costs time + checks	Standard commercial tol elsewhere
6	Avoid overly thin features & fins	They vibrate and break tools	Widen or support
7	Standardize hole diameters	Fewer tool changes, faster	Reuse a drill set
8	No draft needed (unlike molding)	Machining has no pull-out	Keep vertical walls if you want them
9	Mark only functional surfaces	Finish on every face wastes cost	Call out Ra only where it seals
10	Design for one setup where possible	Re-clamping shifts dimensions	5-axis for multi-face parts
11	Choose the grade from function	Over-spec material costs money	6061 before 7075 unless load needs it
12	Send 3D + 2D together	One file alone invites guesswork	STEP/IGES + PDF with GD&T

Two Rules Worth More Than the Rest

Tolerance only what is critical. A part toleranced at ±0.1 mm everywhere is cheap; the same part at ±0.01 mm everywhere costs more and risks scrap for no function gain. Our tolerance chart shows what is routine versus what needs a controlled setup, and the cost-reduction guide shows the curve.

Mark geometry with GD&T, not just sizes. When the relationship between features matters — a flat mating face, a hole pattern, a concentric bore — GD&T tells the shop exactly what is critical and stops it over-machining harmless variation. The six symbols buyers actually use are in our GD&T guide for buyers. Surface requirements belong in the same drawing: the surface-finish chart lists the common Ra values.

Materials and Processes Still Shape the Rules

These rules are general, but material changes the margins. Plastics move with heat and moisture, so clamp lightly and machine near room temperature. Titanium work-hardens, so keep the tool moving. Aluminum is the forgiving one. The full set is in our materials overview.

Frequently asked

What is DFM for CNC machining?

Design for Manufacturability is the habit of shaping a part so it machines accurately, quickly and cheaply — without changing what it does. On a CNC part that means sensible wall thickness, radii instead of sharp internal corners, hole depths within reason, standard thread sizes, and tolerance only where the function needs it. Good DFM is invisible: the part just comes out right the first time.

What is the minimum wall thickness for CNC machining?

As a practical rule, keep walls at least 0.8–1.0 mm on metal and 1.5 mm on most plastics. Thinner walls chatter, deflect under the tool and go out of tolerance. If the design truly needs a thin web, expect more setups and a higher scrap rate — and say so on the drawing so the shop plans for it.

When should I specify GD&T instead of plus/minus tolerances?

Use GD&T when the relationship between features matters more than each size alone — flatness of a mating face, position of holes in a pattern, concentricity of a bore. It tells the shop exactly what is critical and stops it from over-machining harmless variation. The six symbols buyers actually use are in our GD&T guide for buyers.

Does tighter tolerance always cost more?

Almost always, and not linearly. Holding ±0.1 mm is routine; ±0.01 mm needs a controlled setup, more measurement and slower passes; anything tighter may need a second operation or inspection. The cheapest part is the one toleranced only where the function demands — so mark critical dimensions and let the rest sit at the standard commercial tolerance.

Not sure your drawing is DFM-clean?

Send the STEP and PDF. We'll flag thin walls, sharp corners, over-tight dims and anything that adds cost without adding function — before you pay for the first part.

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Keep Reading

Plan your part against our printable DFM checklist, or look up any term in the CNC machining glossary.