DFM checklist for CNC machined, sheet metal and welded parts

Drawings and tolerances (all processes)

• Apply tight tolerances only to functional features: fits, sealing faces, datums and interfaces.
• Give a general tolerance block (for example ISO 2768-m or your company standard) for everything else.
• Use GD&T where it expresses function better than plus/minus dimensions, not as decoration.
• Specify material grade, temper and finish explicitly, including coating thickness if it affects fits.
• Make sure the drawing revision matches the 3D model you send.

CNC machining

• Internal corners: add a radius slightly larger than the cutter radius; sharp internal corners need EDM or extra operations.
• Pocket depth: deep, narrow pockets need long tools that chatter. Keep depth to a few times the cutter diameter where possible.
• Holes: use standard drill sizes; specify reamed tolerances only where a fit requires them.
• Threads: thread engagement beyond about 1.5 × diameter adds little strength; call out thread depth, not full-depth threads.
• Thin walls: very thin walls deflect and vibrate during machining; thicken them or add ribs.
• Setups: features on many faces mean more setups. Group features so the part can be machined in as few orientations as possible.
• Stock: design around standard bar and plate sizes to reduce waste.

Sheet metal

• Bend radius: use the same inside radius on all bends — often close to material thickness — so one tool does every bend.
• Flange length: very short flanges cannot be held in the press brake; a common rule of thumb is at least about four times the thickness.
• Holes near bends: keep holes and slots away from bend lines (roughly two to three times thickness plus the bend radius) or they will distort.
• Bend relief: add relief cuts where a bend meets an edge to prevent tearing.
• Minimum hole size: holes smaller than the material thickness are hard to cut cleanly.
• Hardware: use self-clinching inserts instead of welded nuts where possible.
• Flat pattern: send DXF flat patterns only if they match your bend allowances — otherwise let the supplier unfold the model.

Welding and fabrication

• Access: leave room for the torch and the welder's line of sight on every weld.
• Weld symbols: use AWS A2.4 or ISO 2553 symbols; specify size and length, not just "weld all around".
• Distortion: balance welds on both sides and avoid unnecessary long continuous welds on thin plate.
• Machining after welding: if a frame needs flat or aligned faces, machine them after welding and stress relief.
• Standard sections: design with standard tube, channel and angle sizes available in your supplier's market.
• Finishing: allow drain and vent holes for galvanising, and note masking for powder coating.

Assemblies

• Check tolerance stack-ups on critical fits before release.
• Reduce the number of unique fasteners.
• Design parts so they can only be assembled one way where orientation matters.

Want a second pair of eyes? Our DFM review service checks drawings before you send them out for quotes. Then use the RFQ checklist to request accurate prices.

Frequently asked questions

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