MachiningShops, People and Places

Hard and Fast Rules – Lessons Learned with Hard Turning

Advanced cutting tool materials such as polycrystalline cubic boron nitride and ceramic have made the turning of hardened steel a cost-effective alternative to grinding. Indeed, many machine shops have retired their cylindrical grinders in favor of less expensive and more versatile CNC lathes.

Today’s sophisticated CNC lathes offer accuracy and surface finishes comparable to what grinders provide. Many metalcutting experts, particularly those who sell machine tools, assert that an extremely rigid, high-performance lathe is a prerequisite for hard turning. That’s not always the case.

Often, all you need for hard turning is a 2-axis CNC lathe and the correct cutting tools. (See accompanying article, page 45.)

You also need to run tests before hard turning a customer’s parts. You’ll learn a lot, and, if you’re like me, you’ll discover that the testing can be a lot of fun.

A number of years ago, I worked as an applications engineer for a machine tool distributor. One of our customers, a shop that manufactured tooling, was seeking a quicker way to push custom punch and die sets out the door.

The shop was already finish turning hardened tool steel on a regular basis. The process entailed three steps: rough turning soft tool-steel blanks, heat-treating them and finish turning them. The shop wanted to take hard turning to the next level: roughing hardened steel. This would save it from having to rough-turn tooling blanks before heat treatment and permit it to always have hardened blanks on hand. The company estimated that it would save one to two days on every order it processed.

Hard roughing also meant that the shop would need to remove up to 2″ of material per side from hardened blanks, then finish-turn them to the required dimensions. That, obviously, would prove to be a daunting task.

The shop asked me for help. Until then, my experience with hard turning had been limited. And removing such massive quantities of hardened tool steel was, frankly, beyond my imagination.

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