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Lesson 09·Surface finish

Polishing and surface finish for forge service

Working Ra targets by die type, why mirror finish on a hot forge cavity is wasted effort and sometimes harmful, and how to pick stoning, diamond paste, or abrasive flow machining for the job.

6 min readLesson 9 of 13

Tying it together

What this means on the shop floor

Forging cavities get Ra 0.4-0.8 µm with a directional texture aligned to flow when the application is aluminum or brass. The print calls out the texture, not just the Ra.

Flash lands get Ra 0.8-1.6 µm by stone or paste, with no polish call beyond the working number.

Gates and runners get Ra 0.2-0.4 µm by diamond paste or AFM, depending on accessibility.

Extrusion bearings get Ra 0.1-0.2 µm by diamond paste in tight grit progression, because the bearing finish transfers to the extruded section.

Post-nitride polish allowance shows on the traveler as a removal limit in microns, with the compound layer specced 2-3 µm above the post-polish working target on the nitride print.

Pushback questions for the polisher

  1. What method are you using on each feature (stoning, diamond paste, or AFM), and how does that match the access and the finish target?
  2. How much material will you remove on each pass, and is that within the post-nitride white layer budget on the traveler?
  3. Will you leave a directional texture on the cavity walls, and if so, oriented to which flow direction?
  4. What Ra measurement instrument and probe location will you use to verify the finish at delivery?

A polisher who answers in SPI grades alone has not engaged with the forge application. A polisher who asks about lubricant, flow direction, and post-nitride case structure is engineering the finish.

Common confusions

A mirror polish is not better than a moderate polish for forge service. It is a different specification for a different application. The right finish is the one that matches the failure mode of the die in service.

SPI A-1 across a forging cavity is rarely the right call outside extrusion bearings and die-casting runners. When the print specifies A-1 on a forging cavity, the print is probably wrong.

Polish after nitriding removes case. Every polishing pass eats into the compound layer or the diffusion zone, and the removal has to live on the traveler instead of in the polisher's head.

Texture at a given Ra performs differently from uniform polish at the same Ra in service. The print has to call out the texture if it is wanted, because the default polishing practice is uniform.

AFM is a method for reaching surfaces that hand methods cannot. On accessible features hand stoning and diamond paste are faster. AFM earns its place where geometry forces it.

Up next: vendor handoff.

Sources