We offered a smoother finish. The customer said no — and they were right.

The brief

A driveline manufacturer sent us a prototype release drawing for an ISO-series clutch pressure plate: compacted graphite iron — EN-GJV-300 to EN 16079 on the drawing, supplied as RuT300 to GB/T 26655 — 190–240 HB, roughly 2.8 kg finished, machined from solid rather than cast. Twelve pieces — eleven to the drawing, and one with the total height reduced by just under a millimetre, the material taken off the fulcrum side, for a comparison test.

It was our first job in compacted graphite iron, which is worth saying plainly, because it is the reason for everything that follows about inspection.

What we questioned

Two callouts decide whether this part works, and neither is obvious from the outline. The friction face carries a conical taper of 0.14–0.23 mm — a constant 15 arcmin, falling linearly from the outer zero plane to the inner diameter of the friction ring. And that same face has a roughness window of Ra 2.5–6.5 µm with concentric machining lines, nominally around Ra 4.5 measured radially.

We proposed Ra 1.6–3.2 µm — a finer finish, and the one most shops would offer as an upgrade. The customer turned it down: too smooth would reduce initial friction torque transmission. They were right, and we would far rather hear it at quotation than after a bench test.

What we did

The customer supplied a 3D model of the friction face in its nominal taper condition, and we drove the toolpath straight off that model instead of reconstructing the cone from the 2D drawing. Turning was not mandatory — only the roughness range and the concentric pattern were binding — which left us free to pick the process that held both.

What shipped with the parts

A ten-item inspection report on the customer’s own template, ballooned against their drawing. The taper (15′ ±4′, on the CMM) and the friction-face roughness (measured by an independent test house, minimum three points) were measured on 100% of the parts, not a sample. The other eight items — the three controlling diameters, the overall height and two step heights, all to ±0.1 mm, plus parallelism to datum A and flatness of the friction face — were measured on our CMM, by our own programmer, in house.

Where it ended up

All twelve pieces were delivered 15 days after the drawing was approved. Every one passed the customer’s own testing. Twelve parts is not a quantity most shops will run a 100% CMM check on. We did, for two reasons: on this part the taper and the friction face are the whole job, and a sample of three tells you nothing about the other nine; and it was a material we had not cut before, so we wanted the numbers on every piece, not an assumption.