Press-Fit CNC Machining Tolerances: How to Specify Holes That Assemble Right

Press-fit CNC machining tolerances determine whether an assembly holds under load or fails during insertion. For a truly interchangeable press fit, avoid relying on ISO 2768-m default values; instead, define critical bore and shaft diameters with an explicit tolerance based on planned interference, geometric position, and edge condition.

A practical starting point is an H7/p6 or H7/s6 fit for steel shafts, but the correct callout depends on the materials and application. For 6061-T6 or 7075 aluminum, thermal expansion and lower elastic modulus reduce effective interference compared with the measured fit. For POM/Delrin inserts, generous chamfers and low interference prevent cracking because creep can relax the joint.

Surface finish affects insertion force and holding strength. A Ra 0.8 bore on a machined part will behave more consistently than a 1.6 surface; rougher finishes can smear on assembly and cause galling in stainless grades such as 303, 304, or 316. Specify Ra only on the bearing surface and apply burr and chamfer controls at hole edges.

Finally, keep dimensional and geometric callouts as loose as possible for non-critical features. Tighter tolerances, custom inspection, and CMM verification add lead time and cost. Stating a tolerance class such as ISO 286 H7 and limiting it to functional bores lets a machine shop optimize process and quote realistically.