CNC DFM Checklist + Aerospace/Medical Requirements (2026 Edition)

Why DFM matters

Design for Manufacturing (DFM) is the practice of designing parts so they can be made faster, cheaper and more reliably without sacrificing function. The earlier in the design process you apply DFM, the more it pays back.

• 10–30% cost savings on the production quote, no design changes needed beyond DFM optimisations.
• 2–5× shorter lead times when the part doesn’t need special tooling, fixtures or hand operations.
• Better first-pass yield — fewer scrapped parts, fewer revisions, fewer surprises at FAI.
• Easier scaling — what works at 10 units works at 10,000.

The 20-point DFM checklist

Run through every item before sending a part to quote. Each one fixes a real cost or quality issue we see weekly:

1. Internal corner radii ≥ 0.5 mm. Sharp internal corners need expensive small tools. Default to 1 mm where possible.
2. Avoid deep narrow pockets. Pocket depth more than 4× tool diameter causes chatter and poor finish. Split into multiple operations or shallower pockets.
3. Add tool-clearance to bottoms of pockets. Leave at least 0.5 mm flat clearance for the cutter to land on.
4. Standard hole sizes for tapping. Use M3, M4, M5, M6, M8, M10 metric or #4-40, 1/4-20 imperial. Custom thread sizes need custom taps.
5. Hole-to-edge distance ≥ 2× hole diameter. Closer distances cause edge break-out, especially on aluminium and plastics.
6. No blind holes deeper than 4× diameter. Drill cycle becomes complex and chip evacuation fails. Add a relief or split the hole.
7. Threaded hole depth = 1.5× thread diameter. Anything more is wasted; the engagement length doesn’t add strength.
8. Wall thickness ≥ 0.8 mm in metals, 1.5 mm in plastics. Thinner walls deflect during cutting and warp on cool-down.
9. Wall thickness consistency. Sudden changes in wall thickness cause warping. Taper transitions over a length of 3× thickness.
10. Avoid undercuts where possible. Undercut tools are expensive and slow. If unavoidable, design for a standard T-slot cutter (e.g., 6 mm slot).
11. Datum scheme: A-B-C. Pick the largest stable face as A. B and C perpendicular. Reference all GD&T from this scheme.
12. One-sided machining when possible. Reduce setups by designing all critical features on one side or two opposing sides.
13. Avoid sharp external corners. Add 0.2–0.5 mm chamfers to all external edges — improves handling, prevents burrs, looks more professional.
14. Standard fastener clearance holes. M4 clearance = 4.5 mm (close), 5.0 mm (free). Don’t use 4.3 or 4.7 mm — they’re non-standard drills.
15. Avoid mirror-finish surface specs unless required. Ra 0.8 µm is standard. Ra 0.4 µm requires extra polishing time.
16. Specify a single overall material per part. Multi-material parts are an assembly, not a single CNC operation.
17. Tolerance only what matters. 80% of dimensions can stay at ISO 2768-m default. Tighten only critical features.
18. Provide a 3D STEP file. 2D drawings as the only source of truth lead to interpretation errors. Send STEP + PDF drawing.
19. Mark up critical features clearly. Star or note the 3–5 features that absolutely must be in spec. Helps the shop prioritise inspection.
20. Communicate the part’s purpose. One sentence — “this bracket holds a sensor in a vibration test rig” — helps the engineer suggest better DFM that you might never have considered.

Aerospace requirements (AS9100 and friends)

When a part is destined for aerospace use, several layers of additional requirements kick in. They are not optional:

JLYPT operates an ISO 9001 certified quality system with AS9100 capability for designated aerospace work. See our certifications page for current scopes and our aerospace manufacturing overview.

Medical device requirements (ISO 13485 and FDA)

Medical CNC work — surgical instruments, orthopaedic implants, drug-delivery components — operates under similar but distinctly different rules:

• ISO 13485 — the medical device quality management standard. Equivalent role to AS9100 in aerospace.
• FDA 21 CFR Part 820 — the US Quality System Regulation. Required for parts entering the US medical device supply chain.
• EU MDR (Medical Device Regulation) — replaced MDD in 2021. Stricter clinical evidence and unique device identification (UDI) requirements.
• Material biocompatibility — ISO 10993 testing for parts that contact patients. Common implant materials: Ti-6Al-4V ELI, CP-Ti Grade 4, 316L stainless, PEEK.
• Lot traceability — every part traceable to material lot, machine, operator, inspection record.
• Cleaning validation — for parts shipped clean or sterile, validated cleaning procedure with residue testing.
• Environmental controls — controlled humidity and temperature in the machining cell prevent thermal expansion errors and contamination.

Documentation you must provide

For regulated work, the documentation package is part of the deliverable. Plan for this from day one:

Common pitfalls

• Specifying a tolerance you can’t inspect. “±0.005 mm true position over 200 mm” on a $10 part is impossible to verify economically.
• Assuming the supplier knows your standards. If you need MIL-DTL-13924 black oxide rather than commercial black oxide, say so explicitly.
• Forgetting the supply chain. If your customer requires DFARS-compliant materials, your supplier must source them — typically 20–40% premium and longer lead time.
• Mixing up similar materials. Ti Grade 2 ≠ Ti Grade 5. 316L ≠ 316. CP Titanium ≠ Ti-6Al-4V. Specify the exact grade.
• Treating documentation as optional. For aerospace and medical, the paperwork IS the deliverable. Without certs and traceability, the parts are unusable.
• Late spec changes. Changing the alloy after machining starts, or the surface finish after kit-out, cascades into rework and lost time.

Cost implications of regulated work