Most quotation arguments are tolerance arguments in disguise. A drawing that applies a blanket ±0.05 mm to every dimension — including formed walls, radii and hole positions produced by different stations — cannot be quoted honestly, because the tolerance that governs the price is not stated.
Which tolerances can a stamping tool hold?
In metal stamping, tolerances are process-dependent. Pierced and blanked features in thin material can typically hold tighter tolerances than formed features, because forming adds springback, thinning and radius variation. Hole-to-hole position depends on piloting and strip layout; wall thickness on a drawn part varies with the draw ratio and cannot be held uniform; and bend angle tolerances are wider than flat-feature tolerances. Very tight tolerances in the 0.001 mm range are achievable only on selected critical machined features where geometry, material and inspection conditions allow, and must be confirmed from the drawing during DFM review rather than promised as a general capability.
Read the drawing the way the tool will
- Separate cut features from formed features. They come from different stations and carry different tolerance capability.
- Check the datum scheme. A drawing that dimensions everything from a formed edge is harder to inspect than one that uses a stable datum.
- Look for stacked tolerances. Three features each ±0.05 mm can consume the assembly allowance before the part is even made.
- Mark the critical characteristics. Two or three marked features produce a better price than twenty unmarked ones.
- State the inspection expectation. Full layout inspection on every lot changes the price and the lead time.
When to machine instead of forming to tolerance
If a feature genuinely needs a tolerance the forming process cannot hold — a bore diameter, a seal face flatness, a thread — the honest route is to form close and then machine the feature, with dedicated inspection at that characteristic. Attempting to hold it purely with the tool means chasing a moving target at every sharpening, and the cost appears later as rejects and sorting.
What a good stamping quotation tells you
- Which tolerances the supplier proposes to hold as standard for your material and thickness
- Which features are proposed for secondary machining, and why
- The inspection method and frequency for the critical characteristics
- The tool stage count and what it assumes about volume
- Any tolerance the supplier recommends relaxing, with the cost effect
Key point
A quotation without a tolerance discussion is a placeholder. The number only becomes real once both sides agree which features are critical and how they will be measured.
Frequently asked questions
Can stamping hold ±0.05 mm on a formed wall?
Usually not as a blanket tolerance. Formed walls thin and vary with the draw ratio; the practical tolerance is agreed per feature during DFM review.
How do drawings cause price differences between suppliers?
Because an ambiguous drawing lets one supplier assume generous tolerances and another assume tight ones. Marking critical characteristics removes most of the gap.
Does a tighter tolerance always cost more?
Not always — it depends which feature it applies to. A tight tolerance on a machined bore is routine; the same tolerance on a drawn wall is not.