Surface Finish Inspection in CNC Bores: Avoid Rework

Surface finish inspection on internal CNC features fails more often from access than from machining capability. A bore or pocket that meets Ra 0.8 µm on the bench may be impossible to verify with a skid-type profilometer once the feature is deep, narrow, or partly enclosed.

Check stylus and skid geometry before quoting the requirement. A standard skid may not enter a 20 mm bore or a pocket with 3 mm corner radii; a small-diameter skid or a portable unit with a slim probe is often needed. Traverse length must still cover the cutoff: at a 0.8 mm cutoff, plan roughly 4 mm of trace, and confirm the drawing states cutoff and evaluation length rather than Ra alone.

When contact measurement cannot reach the surface, agree on an alternative surface finish verification method in writing. Replica tape or compound can support a qualitative check on deep cavities, while a comparator plate is useful for visual screening of non-critical faces. For sealing surfaces on 6061-T6, 7075, or 316 stainless, specify the inspection location and traverse direction—axial or circumferential—because readings differ.

Put the method on the drawing: Ra value, cutoff, inspection zone, and acceptable alternative. That single note prevents a finished lot from being rejected over a surface no instrument can reach—and keeps rework and lead time under control.

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For more guidance on related work, see: Preventing CNC Machining Scratches During Handling.