Drilling Case-Hardened Shafts: Tips for Long Tool Life
Case-hardened shafts have a hard outer layer — typically 55-62 HRC — with a softer core around 30-40 HRC. The hard case makes drilling difficult. The drill has to cut through the hard skin at entry and exit while drilling through the soft center. I have drilled thousands of these shafts for hydraulic cylinder rods, gearbox shafts, and machine-tool spindles. The approach is completely different from drilling through-hardened materials. The drilling is best done before case hardening if the part design allows. But sometimes the shaft needs to be drilled after hardening because of manufacturing sequence or design changes. I would estimate that about 30% of my case-hardened shaft jobs are post-heat-treat drilling, and those are the ones that require the most process care. ...