The challenge with hardened steel keyways isn’t just tool wear — it’s a cascade of problems that compounds as hardness increases:

Broaching above 35–40 HRC: Standard HSS broaches are rated for softer materials. As hardness climbs, chip formation changes — instead of shearing cleanly, hard material resists deformation, which drives cutting forces up and causes premature edge chipping. Above 40–42 HRC, even carbide CNC broach inserts have limited practical life, and the risk of crashes increases significantly. Machining forums and industry sources consistently report jammed and broken broaches, damaged tool bodies, and machines knocked out of alignment from hard material broaching attempts gone wrong.

The work hardening problem with stainless: Austenitic and precipitation-hardening stainless steels (304, 316, 17-4) work-harden rapidly during cutting. Each broach tooth passes through the work-hardened layer left by the previous tooth, accelerating wear and increasing the force required. A fresh, sharp broach in 316 stainless may work once — a slightly worn one can weld itself into the bore.

EDM as the only fallback: When broaching fails on hard materials, wire EDM is typically what engineers specify next. It cuts regardless of hardness since it doesn’t rely on mechanical cutting force. But EDM is slow — roughly 12 square inches per hour — and expensive per part. For anything beyond a true one-off, the economics are difficult to justify.

NMT keyseat millers in hard materials: The cutter geometry and material are specified for your application. In harder alloys, we design the cutter with appropriate geometry for the material’s characteristics — chip formation behavior, hardness, and any work hardening tendency — so the tool cuts rather than rubs or chips. The one-pass operation minimizes cutting time in hard materials, which directly reduces heat generation and tool wear compared to multi-pass approaches.