A Practical Guide to Surface Finish Callouts on Machining Drawings
Ra values get specified more often than they're understood. A short, practical guide to what surface finish callouts actually control.
A surface finish callout is one of the cheapest lines on a drawing to get wrong, and one of the most expensive to over-specify. Every step tighter than necessary adds a secondary operation, and secondary operations add cost and lead time.
Match the callout to the function
A sealing face, a bearing bore, and a non-functional external surface have nothing in common functionally, and shouldn't share a finish callout out of habit. We ask what the surface actually does before quoting a finish tighter than the print requires.
What changes the cost
- Ra 3.2 μm and coarser: typically achievable directly off a standard finishing pass
- Ra 1.6 μm: usually needs a dedicated finishing toolpath, minor cost add
- Ra 0.8 μm and finer: often needs grinding or lapping as a secondary operation
Getting this right at the drawing stage, before quoting, is one of the simplest ways to keep a program's per-part cost predictable across a production run.
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