What Tolerances Can You Hold on a Deep Drawn Housing?

Which dimensions behave how

Not every dimension on a drawn housing is equally easy to hold, and treating them as if they were is the most common source of unnecessary cost:

• Outside diameter is set by the final draw or sizing stage and is one of the more controllable dimensions, especially if the die is sized for the material and thickness.
• Inside diameter follows the punch and the material thickness; it is tighter when the wall is ironed or the bore is finish-machined.
• Wall thickness varies around the circumference and along the height because of material flow. A single nominal value on the drawing is a starting point, not a guarantee at every point.
• Concentricity and runout depend on how many stages the part passes through and on the tooling alignment rather than on the press alone.
• Height and flatness are affected by springback and by whether the flange or bottom is coined or trimmed after forming.

What sets the achievable tolerance

Four variables decide how tight a drawn housing can actually be made: the number of forming stages and their sequence, the annealing schedule between stages, the material grade and temper, and whether functional surfaces are machined after drawing. A sealing face that must hold a fine tolerance is usually best turned or ground after drawing rather than asked of the forming process — that is a deliberate process choice, not a compromise.

A practical tolerance strategy

Define the functional datums first, then mark the dimensions that genuinely matter — typically bore diameter, concentricity, height and any sealing surface. Indicating which dimensions are critical-to-function and which are nominal lets us optimise the forming route instead of over-constraining it. Over-tolerancing a drawn surface that is not functional raises tooling cost and inspection time without improving the part.

Capability is not asserted, it is demonstrated: first article inspection reports every dimension, and capability studies are run on the characteristics that the control plan marks as significant.

How we measure it

Tolerances are only meaningful if they can be verified, so the gauging lives in-house: a 2.5D optical measuring projector, an instant vision measuring system, a digital height gauge with 0.001 mm resolution, a portable surface roughness tester, a Rockwell hardness tester and a model 20 salt spray chamber, supported by outside micrometers, three-point bore micrometers, inside caliper gauges, plug and go-no-go gauges and a concentricity gauge.

The full list is published on the equipment list, and the documentation behind it on quality & certifications and PPAP.

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