Hi Stuart,

Well you seem to have covered all the bases there with your tooling.

It would appear that the combination of chrome plating + case hardened lever (another strange item ? ) was too much for your HSS tap. I know from past experience that chrome plating is very hard, having had to grind off a built-up "lip" around the edges of some BMW exhaust pipes I had rechromed in the UK to get them to fit into the heads. Went though Dremel grinding bits like they were going out of fashion !

A quick search suggests that the hardened case layer of case hardened steel is very similar in hardness to that of HSS (around 62 HRC for case hardness, 62-65 HRC for HSS), with chrome being much harder than either at 68-72 HRC. I would expect the chrome layer to "machine" the HSS tap ! Probably explains also why you couldn't drill the chromed layer either.

On the odd bit of hard (as opposed to decorative) chroming I had done for work (usually building up worn tailstock barrels or spindles) the platers masked off areas such as threads with wax before palting to prevent the plating getting into those areas - presumably due to the difficulties that could be had trying to clean out the plating with cutting tools.

I have found references online to tap manufactures recommending carbide taps for materials up to 70 HRC - unlikely to be inexpensive & probably more brittle than HSS. And even more difficult to remove should you be unfortunate enough to break on in the part !

Nigel B.