DFM Checklist for CNC Machining: Practical Rules

Tolerances: tighten only where function needs it

Over-tolerancing is a top cost driver. Apply tight tolerances only to CTQs.

• Define fits and mating interfaces clearly
• Use practical tolerances on non-functional features
• Specify GD&T intent where needed

Tool access and deep cavities

Deep, narrow cavities require long tools, which increase vibration risk and reduce surface quality.

• Avoid extremely deep pockets when possible
• Increase corner radii to allow stronger tools
• Consider splitting a part or changing geometry to reduce reach

Corner radii and internal geometry

Internal sharp corners are expensive. Use radii that match standard tooling.

• Use practical internal corner radii for pockets
• Avoid tiny fillets unless function requires them
• Plan chamfers/edge breaks to reduce burrs

Datums and inspection planning

Datums define how a part is built and inspected. Poor datum definition causes cost and quality issues.

• Define stable datums that reflect functional assembly
• Mark CTQs clearly on the drawing
• Align inspection/reporting expectations during RFQ

Inspection planning

Finishing and masking requirements

Finishing failures are common and expensive. Specify finish details clearly:

• Finish standard/type, color, thickness
• Masking requirements (threads, sealing surfaces)
• Cosmetic vs functional surfaces

Finishing guide

Packaging and surface protection

Surface damage often happens after machining. Specify packaging requirements early if surfaces are critical.

• Scratch protection for cosmetic/functional faces
• Corrosion prevention when required
• Labeling and kitting requirements