Thinking about taking the leap into milling hardened steels? Why would you? After all, if your shop has been successfully grinding, jig boring and EDMing hardened materials, why change?
For starters, hard milling might be more profitable. Michael Minton, national application engineering manager for Methods Machine Tool Inc., Sudbury, Mass., explained that hard milling not only eliminates costly finishing operations, but produces a higher quality part than does traditional machining methods.
“By machining after heat treatment, you not only avoid secondary operations, but also eliminate problems with workpieces being twisted, bent or otherwise out of shape due to heat treating,” Minton said. “This allows you to make a part with higher accuracy and better dimensional characteristics compared to parts heat-treated after machining.”
In general, hard milling involves cutting primarily tool steel or precipitation hardening stainless steel, such as 15-5 or 17-4, that has been hardened to at least 50 HRC. After a workpiece is roughed in the soft state, it is sent to the furnace for hardening and then finish machined with coated carbide, ceramic or PCBN tools. The amount of metal removal in the hardened state is minimal—perhaps just 0.010 ” to 0.020 ” per surface—making this process feasible for most hardened parts.
Depending on the workpiece configuration, production volume and amount of stock removal, however, it may be feasible to machine the workpiece entirely from a hardened state. Modern machine tools, advanced cutting tool materials and sophisticated CAM programs make what was once a highly improbable machining operation into one within the reach of many shops. According to Minton, the advantage of machining from a hardened state is having to cut it only once.
However, you’re not going to accomplish this on an old knee mill. “As always, machine tool construction plays a large part in the level of success you’ll achieve with hard milling,” Minton said. “You can be successful with linear way machines, provided they are made well.”
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