CNC bending tolerances: what to spec on your drawing

October 12, 2026  ·  Indumor

Bending looks simple — folding sheet — but it is where most drawings fail: impossible radii, dimensions that do not close with the flat pattern, machining tolerances demanded from a press brake. At Indumor we bend every day; here is what your drawing should say.

Which tolerances are realistic

A modern CNC press brake repeats angles within ±0.5° and flange dimensions within ±0.2-0.5 mm depending on thickness and size. Demanding ±0.05 mm from a bent part is paying for machining you do not need: if one dimension is truly critical, we talk — sometimes you bend first and machine only that feature after.

The inside radius is not decorative

Every thickness has its minimum radius (quick rule: radius ≈ thickness). Specifying a sharp inside corner on thick plate cracks the material. If it is not critical, let the shop use its standard tooling: cheaper and faster.

The flat pattern: leave it to the shop

Sheet stretches when bent (K-factor). If you dimension the FINISHED flanges and leave the development to us, the software calculates it exactly for our tooling. Hand-calculated flat patterns on drawings are the #1 cause of out-of-spec parts.

Bending drawing checklist

• Dimensions on finished flanges, not on the flat.
• Thickness and material explicit (galvanized bends differently than stainless).
• Bend direction noted when it matters (cosmetic faces, grain).
• A realistic general tolerance with critical dimensions flagged separately.
• Holes near bends: closer than 2.5× thickness to the bend line they deform — we flag it and solve it (bend then cut, or compensate).

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