Copying machining-style GD&T onto formed sheet-metal parts is one of the fastest ways to inflate quotes, trigger exceptions, or create inspection fights after parts arrive. Process capability differs—and drawings should reflect that.
Forming is not a precision mill
Brake forming, punching, and welding introduce springback, heat distortion, and stack-up that machining datums do not assume. A 0.005 in true position on a hole-to-bend relationship may be realistic after secondary machining and impossible as-formed. Decide early which features need machining after form versus which can live with forming capability.
Datums that match how parts are made
Datums should reflect functional mating and fixturing reality—not arbitrary CAD origin corners. On brackets, a formed face that sits against a weld fixture is often a better primary datum than a sheared edge that varies by nest orientation. Align inspection intent with how the shop will hold the part.
Flatness and profile with context
Large panels after welding rarely meet aggressive flatness without secondary ops. If flatness matters for sealing or appearance, specify where and how it will be measured—and accept process steps (stress relief, machining, shimming strategy) in the plan. Silent flatness callouts without process context become reverse-engineering exercises for the vendor.
Position tolerances tied to function
Hole patterns that locate fasteners to a machined casting may justify tighter position after form. Cosmetic vent patterns usually do not. Group features by function so fabricators know where to spend process control—and where to keep cost down.
Practical release checklist
- Are as-formed tolerances separated from post-machined features?
- Do datums match fixture and mating intent?
- Are profile/flatness callouts achievable without unnamed secondary ops?
- Would a fabricator pad the quote because inspection criteria are ambiguous?
Need help aligning GD&T with shop capability before you release? We provide DFM audits and production drawing support that survive real fabrication—not just CAD checks.