Step 3 of 4
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Vendor handoff: what goes on the print to the heat treater, nitride house, and polisher
Three annotated print callouts for the three outside vendors in a forge die build, and the line items to keep off the print so the spec stays right-sized.
Step 3 of 4The polish callout
A complete polish callout for a forge die is shorter than the heat-treat and nitride callouts but unforgiving where it is wrong.
Polish per zone, by drawing callout:
Cavity surfaces (zones C1-C4): Ra 0.4-0.8 µm. Diamond paste progression 6 µm to 3 µm to 1 µm. No silicone-containing compounds. No abrasive flow machining on gates without sacrificial allowance specified.
Flash land (zone F1): Ra 0.8-1.6 µm. Stone or diamond paste only.
Gates and runners (zones G1-G3): Ra 0.2-0.4 µm. Diamond paste only.
Material removal limit: ≤ 5 µm per side on any nitrided surface. Do not breach the diffusion zone. If damage exceeds 5 µm removal, stop and notify engineering before continuing.
Pre- and post-polish Ra measurements on the highest-risk surfaces (zones C1, F1, G1) recorded and shipped with the part. Witness sample of each polished zone on the witness coupon if a coupon is in the routing.
Handling: Cotton or nitrile gloves only. Cavity capped with foam insert for shipment. Parts shipped in VCI bag, no oil or grease applied.
Polish targets vary by surface because the surfaces do different jobs. A mirror finish on a hot forge cavity is wasted effort and sometimes harmful. It galls aluminum, holds scale, and offers no service advantage over a properly stoned 0.4-0.8 µm Ra. Gates and runners want the lowest Ra because material flow through them sets cavity fill and the surface state controls erosion. The flash land tolerates the coarsest finish because its job is back-pressure, not flow quality. The material removal limit is the line that protects the nitrided case from the polisher. A polish step that removes 15 µm where the spec said 5 µm punches through the compound layer into the diffusion zone, and the next vendor in the chain inherits a part with inconsistent surface state. Abrasive flow machining is fast and consistent on complex geometries, but it removes material everywhere the flow touches, and on a gate with tight tolerance it eats the geometry unless the print left sacrificial stock to absorb the loss.
Quick check
A polisher returns a part with cavity Ra at 0.5 µm (in spec), no Ra measurement on the flash land, and a removal of 9 µm on the cavity by their internal log. The nitride spec called out 6 µm compound layer at delivery. What is the as-shipped case structure and what should the routing have stopped?