DFM guidelines for CNC machining — the rules that actually save money.

1. Generous internal corner radii. Pockets and internal features should have corner radii at least 1/3 of the pocket depth. R3 is usually cheaper than R0.5.
2. Minimum wall thickness based on material. Aluminum ≥ 1 mm, stainless ≥ 1.5 mm, titanium ≥ 2 mm. Thinner walls deflect during machining and fail inspection.
3. Standard thread pitches only. M6×1.0 and M6×0.75 are both standard, but the shop has tools for coarse pitch. Specify fine pitch only when functionally needed.
4. General tolerances via ISO 2768-m. Let the block tolerance handle 90% of dimensions. Explicit ± tolerances only on critical features.
5. Chamfer over "break sharp edges". "Chamfer 0.3×45°" is measurable and specific. "Break all edges" is unclear — ends up as 0.5 mm chamfer by default, more if the shop is cautious.
6. Datums from machined surfaces. First operation creates a flat, square datum set. All subsequent dimensions reference these machined features, not raw stock surfaces.
7. Minimize setups. Consolidate features onto fewer part faces. Each additional setup costs $50-200 in labor and introduces tolerance stack-up error.
8. Standard drill sizes. Use drill sizes that match available taps (Ø5.0 for M6×1.0 tap). Non-standard holes require custom reamers.
9. Surface finish callouts at needed level. Ra 3.2 standard mill finish vs Ra 0.4 precision grind — 4× cost difference. Spec the highest Ra value that works.
10. Material standard grades. 6061-T6 and 304L stainless are always in stock. Less common grades (5083, 17-4 PH) require sourcing time and cost premium.

DFM rules are guidelines, not hard constraints. Break them when:

• Function actually requires it: a sealing surface really does need Ra 0.4 μm; a precision bearing really does need ±0.005 mm. Don't compromise function for cost.
• Regulatory requirements mandate specific tolerances: aerospace, medical, automotive specs often require tighter tolerances than DFM would suggest. Follow the spec.
• Volume justifies the cost: at 100,000 units/year, a 5% cost reduction per part pays for custom tooling that's uneconomic at 1,000 units.
• Performance testing validates the design: if a stress analysis specifically shows you need 7075, use 7075 despite cost.

What DFM review does: ensures each design decision has a reason tied to function or requirement. Decisions made by habit or "what we've always done" are the targets for DFM improvement.

When you ask for DFM review on a quote, provide:

1. STEP file: shows the full geometry in 3D
2. PDF drawing: shows your tolerance callouts, surface finish, material spec
3. Functional context: what is the part for? What does it mate with? What load or environment?
4. Volume and cost targets: 10 prototypes vs 1,000/year changes which DFM improvements matter

A shop that offers real DFM review will come back with:

• Specific feature-by-feature suggestions
• Cost delta for each suggestion
• Any concerns about current design vs manufacturability
• Material and process alternative recommendations

DFM review should be free (it's part of the quoting process). If a shop wants to charge for DFM review, they're probably not applying it routinely to other quotes either.