Tolerance Stack-Up for Stamped Parts: What Your Engineer Needs You to Know

Why individual tolerances do not add up the way you think

In a stamping operation, each pierce, blank, and bend lands where the die puts it. The die is machined to a tolerance — typically ±0.02 mm on critical features. The press itself has a positional repeatability — usually ±0.05 mm for a well-maintained press under 300T.

When you stack two stamped features on the same part, you are adding the variation of each operation. The statistical rule: when two independent dimensions each at ±0.1 mm combine, the assembly tolerance is ±√(0.1² + 0.1²) = ±0.14 mm (RSS, root sum square) — not ±0.2 mm (arithmetic sum).

Most engineers use arithmetic sum by default. This is safe but expensive — it assumes the worst case on every part. For high-volume production (50,000+ pcs/yr), RSS stack-up is the correct approach because the probability of every feature hitting worst-case simultaneously is near zero.

Where stack-up bites you hardest

Hole-to-edge distance: a hole punched at ±0.1 mm and an edge stamped at ±0.1 mm gives a hole-edge dimension tolerance of ±0.14 mm by RSS. On a 6 mm nominal dimension, that means the hole edge could be as close as 5.86 mm to the edge — check your material ductility before approving.

Bend-to-holes: a bend downstream of a punch operation introduces springback, which is itself a tolerance. The hole position relative to the bent flange is not purely die location — it is die location plus springback variation. For ±0.05 mm hole positions near a bend, specify a draft angle and hold the bend before the punch.

Assembled sub-panels: two stamped brackets each at ±0.5 mm on mounting hole positions can produce a ±1 mm mismatch at assembly. If you are bolting two stamped parts together, specify positional tolerance (e.g. ±0.3 mm) on the hole pattern, not just on the individual features.

The GD&T basics stamping engineers use

Position tolerance (ASME Y14.5): a hole pattern specified as Ø0.3 mm positional tolerance at maximum material condition means the hole can shift 0.15 mm from datum before the part is rejected — far more production-friendly than two independent ±0.1 mm dimensions.

Flatness: for parts under 3 mm thick, specify flatness ≤0.15 mm (not just ±0.1 mm positional) because stamping introduces bow between the draw beads.

Perpendicularity: critical for brackets that mate against a flat surface. A 90° ±1° bend on a 50 mm leg gives a ±0.87 mm positional error at the tip — significant if you are bolting through it.

When in doubt, send the 2D PDF or STEP file with your RFQ and note which features are critical. We will flag any dimension where your tolerance and your process capability do not match.

What we do with your drawing

On every RFQ we review the tolerance column before quoting tooling. If your drawing specifies ±0.05 mm on a 3 mm thick stamping in 65Mn, we will tell you: this requires hard tooling (±0.02 mm achievable) and in-process CMM inspection — add $X to tooling.

If your drawing specifies ±0.15 mm and you have a 3 mm thick SPCC part, we will often suggest relaxing to ±0.2 mm — it costs nothing to make, halves the tooling wear rate, and your assembly probably does not need ±0.15 mm.

DFM feedback is free. Send the drawing with your RFQ and we will give you a tolerance compatibility report as part of the quote package.