Working hard to avoid workhardening

When a component must resist corrosion while withstanding a high level of heat, such as one found in the combustion chamber of a jet engine, part designers often decide to make the part from a nickel-base superalloy. Whether Inconel 600, Haynes 230, Hastelloy B or another metal from that group, they all are challenging to thread or otherwise machine.

That is because those alloys have a high level of ductility, so they tend to easily workharden when cut, according to Marlon Blandon, product manager for thread milling at Emuge Corp., West Boylston, Massachusetts. As a result, prior to threading, drilling can workharden the surface of a hole to be threaded. A continuous threading application like tapping, in turn, can generate such a high level of heat that it workhardens the bore to the point that it can’t be threaded.

Emuge reports that its Threads-all ZGF S-Cut thread mills offer aggressive cycle time reductions and impart a fine finish when threading nickel-base alloys. Image courtesy of Emuge

Blandon suggests thread milling with the appropriate tool instead. The thread mill should be made of solid carbide and have a positive rake angle and sharp cutting edges. “We use a proprietary micrograin carbide that allows us to grind very sharp edges,” he said. “The finer the carbide grade, the easier it is to create a sharp edge.”

Those cutting edges should then be coated to protect them against heat. A TiCN coating can be deposited to resist heat without adding too much expense to a tool. However, a multilayer TiAlN coating is even more effective because once the coating is heated, it hardens more to enhance the protection, Blandon noted.

Hold On

Once an end user has the correct thread mill, a suitable toolholder is needed. According to Blandon, avoid ER collets at all costs because they are designed for linear cutting, either in and out or side to side but not for 3-axis circular interpolation, which thread milling requires.

In addition, shrink-fit toolholders might not be adequate because they often don’t have enough thickness to surround the tool shank and enable vibration-free thread milling, he added. Vibration produces chatter marks on threads, which require re-cutting. “You don’t want to make too many passes,” Blandon said, “because every time you make a pass, you are generating heat and the material is workhardening.”

A better choice is a holder with a side-locking screw that pushes the entire wall of the shank into the holder and improves rigidity, he said.

Blandon added that Emuge offers the FPC toolholding system. “It’s a gear-driven system that pulls the entire shank of the tool into a clamping sleeve. Then this clamping sleeve gets tightened with a specific torque into the holder itself. That is more than enough for thread milling.”