Copper Plated Cams

I often see cams that are copper plated everywhere but on the face of the gear teeth and on the actual ground working surface of the cam
lobes. Why is this done? Is it used as a heat treat mask? I would suspect that the case depth of the gear teeth would be less than that desired on the lobes if it is a relativly fine pitch gear. Is it carburized twice? Once for the gear teeth and then a second time for the cam lobes with aditional mask applied in between? Why not use the paint on masks that are available? Thanks Mark
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For masking off.

No, for masking off Nitrogen for the heat treatment. You want hardness where the copper is missing and that's where the N has to diffuse in. See: "Nitroc" hardening.
Nick
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The copper plating is a stopper for case hardening, ie. the carbon will not diffuse through the copper plating.
It's used on low carbon steel components where only certain features require hardening.
Typically the component is rough machined followed by copper plating. Then the feature to be hardened is machined such as cam surfaces and gear teeth as you mentioned.
Next is Case hardening, a process which diffuses carbon into the surface of the steel, followed by quenching.
The depth of the case may be from .003' TO .030" deep depending on type of steel, carbonizing process, and length of time in the carbonizing atmosphere.
The real beauty of this process is that gears can be made from a steel which, when heat treated normally, would provide an exceedingly tough steel, but lacking the hardness required for wear resistance. By case hardening the gear teeth only one gets the best of two worlds, extremely hard wear surfaces combined with a tough and impact resistant core structure.
Automotive gears are typically made this way.
Cheap cutting tools and dies can be made from hot rolled steel followed by case hardening; but it won't have the tough core.
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And aircraft gears. --Glenn Lyford
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The copper plating is a stopper for case hardening, ie. the carbon will not diffuse through the copper plating.
It's used on low carbon steel components where only certain features require hardening.
Typically the component is rough machined followed by copper plating. Then the feature to be hardened is machined such as cam surfaces and gear teeth as you mentioned.
Next step is case hardening, a process which diffuses carbon into the surface of the steel, followed by quenching.
The depth of the case may be from .003' TO .030" deep depending on type of steel, carbonizing process, and length of time in the carbonizing atmosphere.
The real beauty of this process is that gears can be made from a steel which, when heat treated normally, would provide an exceedingly tough steel, but lacking the hardness required for wear resistance. By case hardening the gear teeth only one gets the best of two worlds, extremely hard wear surfaces combined with a tough and impact resistant core structure.
Automotive gears are typically made this way.
Cheap cutting tools and dies can be made from hot rolled steel followed by case hardening; but it won't have the tough core.
Wolfgang
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