I have tapped 10s of thousands of blind 4-40 holes with my Procunier 1E tapping head with excellent results on both manual and CNC mills. A nice thing about using the Procunier on my Tormach CNC mill is that the code is so simple 2 lines:100% downfeed rate, no dwell, and retract at 200% downfeed rate. Also I tap aluminum 4-40 with a Balax form tap at 1200 rpm so fastand and no chips. With the cushioned double-cone clutch small taps i.e. 0-80 work great. In addition I installed the Pro-Quik quick change tap holders so changing tap sizes is just a snap away. http://rockford-ettc...Pro-QuikTap.pdf I just enter the z height for each tap size in the tool table and quickly swap out tap size during tool change. I made a bracket with anti-rotation rod and TTS quick change tooling so changing the entier Procunier tap head on my Tormach is quick and easy especially with the power drawbar.
Don
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