ISO 13485 instrument housing: 0.8 mm walls, zero rejections in eight lots

The problem

A medical device manufacturer needed an instrument housing in 316L stainless: 0.8 mm walls, a 4:1 deep internal cavity, bead-blasted and passivated, with a dimensional report and material certificate for every single unit. The previous supplier passed the first article and then drifted — by lot four the wall thickness was outside the band on two of ten parts.

Two things made it hard. The thin wall deflects under normal clamping and cutting forces, and the deep cavity traps coolant and swarf that later show up as contamination on a wipe test.

What we changed

• Single set-up on a five-axis centre. Every feature that interacts — bore, face, mounting holes — is cut without re-clamping. Re-fixturing a 0.8 mm wall is where the drift came from in the first place.
• Low-pressure hydraulic fixture. Clamping pressure is set by a regulator to 6 bar instead of the shop default. The wall is supported over its full length rather than at three points.
• Dedicated tooling for the cavity. A reduced-shank carbide end mill with through-tool coolant reaches the 4:1 depth without chatter, at a step-over that keeps the radial load below the wall's deflection limit.
• Cleanliness as a process step. Ultrasonic wash, DI rinse, then a filtered-air dry before the part enters the clean packing area. Wipe-test samples are taken per lot and kept with the batch record.

The result

Eight lots, zero rejections, every unit CMM-inspected with a per-unit report and material certificate traceable to heat number. Wall thickness Cpk is 1.7 across the eight lots, and the wipe test has stayed inside the customer's limit on every batch.

The process card for this part is now fixed: same fixture, same tooling, same clamping pressure. If the part number comes back in two years, it runs the same way — which is what ISO 13485 is actually asking for.