Polishing and surface finish for forge service

A controlled cross-hatch or directional texture can outperform a uniform polish at the same Ra value because the asperity distribution is engineered rather than random. Shop-floor practice is to leave the final stoning or paste pass with a deliberate directional pattern oriented to the material flow, so the lubricant grooves carry along the flow rather than against it. Cross-hatch patterns at 30-45 degrees to the primary flow direction hold lubricant longer than parallel grooves, at the cost of more polishing time. Trade-press writeups call this functional finishing.

The print should specify the pattern when it is wanted: a textured Ra 0.6 µm with directional pattern aligned to flow, not just "Ra 0.6 µm." Otherwise the polisher polishes uniformly and the texture is lost.

Three methods cover most forge die work.

Stoning. Manual abrasive stones in aluminum oxide or silicon carbide, in grit progression from roughly 220 to 1200. Fast, and the right call for accessible flat surfaces, large radii, and flash land faces. A skilled die polisher with a stone hits Ra 0.4-0.8 µm in tens of minutes. The limitation is access. Stones do not reach inside deep cavities or around tight internal radii.

Diamond paste. Diamond abrasive in an oil or water carrier, applied by hand with felt bobs, dental tools, or hardwood sticks. Grit progression typically runs 15 µm down to 1 µm for moderate finish, 0.5 µm or finer for mirror. Slower than stoning, but reaches deeper into cavities and around radii. Hits Ra 0.1-0.4 µm on accessible surfaces. The output depends heavily on the operator.

Abrasive flow machining (AFM, also called extrude honing). A viscous polymer carrier loaded with abrasive grit is forced through the die cavity under pressure, often in alternating directions. Used post-EDM to remove recast and to polish gates, runners, and internal passages that cannot be reached by hand. Hits Ra 0.2-0.8 µm depending on grit and cycle time. Repeatable across parts in a lot. AFM is the right call for complex internal geometries and for production runs where consistency matters more than absolute finish.