Rpk
Is Rpk measured directly on the machine?
No. Rpk is normally obtained from a surface profile measurement and reported by metrology software as part of the Rk family; it cannot be inferred from spindle load, feed, or Ra alone.
What does a lower Rpk mean?
A lower Rpk means fewer high peaks projecting above the core profile, which generally improves initial conformity and reduces wear-in severity on mating surfaces.
What should a programmer change if Rpk is too high?
Reduce remaining peak structure by adjusting finishing allowance, honing or abrasive strategy, feed, or dwell, then remeasure; toolpath changes alone are not enough unless they change the functional profile.
Rpk is the reduced peak height parameter in the ISO 13565 surface-roughness family, used with Rk and Rvk to quantify functional surface texture. It represents the height of top peaks that wear away during initial run-in; lower Rpk generally means less break-in wear and more stable contact on honed bearing, seal, or cylinder surfaces.
On the shop floor, Rpk is a functional texture callout commonly found on plateau-honed bores, bearing journals, seal surfaces, and cylinder walls. The CNC operator or finisher controls toolpath, feed, abrasive size, and honing cycles to produce a surface whose reduced peak height fits the drawing's Rpk/Rk/Rvk window. Because Rpk captures the small peaks that will be worn off during run-in, a low value indicates that the surface is ready for stable sliding, sealing, or reciprocating contact without excessive early wear. Verification requires surface profilometry, not a simple Ra gauge; the metrology report must use the same cutoff, filter, and direction as the original spec. In millwork or cabinetry, Rpk is generally irrelevant unless the drawing explicitly references ISO 13565, so the term should be confirmed before treating it as a finish requirement.
Rpk mistaken for Ra: Treating Rpk as a general roughness average misses the functional peaks that Ra ignores, so a part can meet Ra yet still have excessive high peaks, causing early wear or seal damage during run-in.
Smooth-looking surface, high Rpk: Honing or reaming can leave a visually smooth bore with elevated peak height, because the functional peak zone hasn't been removed, so break-in becomes abrasive and lubrication film stability suffers.
RPK read as a machine code: The acronym also names a brand or model prefix, so the drawing can be misread as a product identifier, leading to wrong setup sheets, incorrect gauges, and rejected parts for the wrong reason.