E-Bait ER20 Chucks and ATC Grooves

Has anyone had any luck cutting the ATC grooves into the E-Bait ER20 chucks? I bought 20 of them before I built an ATC, and now need to put the grooves in, but my first attempt was a miserable failure. Tapering the top of the shank was easy, but the groove turned our to be much more difficult than expected. HSS does not cut well - seems too hard. So, I ground a custom cemented carbide lathe tool, but it broke half-way through. Any ideas how I can get this done? I'm tempted to try doing it on the mill, using a carbide slitting saw and circular interpolation, then just do the bevel as a second-op on the lathe, but I don't know if that would work well or not.

Any ideas?

Regards,
Ray L.