Power tapping small size fastners

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Well-Known Member
Jan 8, 2018
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Just beginning to machine the excellent Ransom & May mill engine casting set provided by Hemmingway in the UK.
A #2-56 is about the smallest fastener these eyes and fingers are capable of manipulating. Have always been a little leery of blind hole tapping under power with any mill. My later model Grizzly mill with the variable speed control will get down to a slow speed crawl where you can easily watch and count the number of rotations for a given thread depth. To avoid tap breakage of this small size I use a tap hole for a 50-55% thread depth. I usually try to drill the tap hole about 2x the depth of my finished thread. Securely anchoring the workpiece is a must, unless you have gorilla strength like my brother in law. A lot of tap breakage is caused by a workpiece not being secured properly.
After lubing the hole to be tapped I power tap on the slow RPM range of my mill, usually about 20 RPM. and count the revolutions till I get about half way of the desired tap depth, reverse and let the tap wind out. Next I clean the fresh tapped hole with a blast of comp air to clean the hole of chips and lube. At this point I change to a bottom tap that threads in the partially tapped hole and proceed with the tiny tap handles with the square hole to fit the tap( ava from LMS in 1.5 and 2.5" lengths) and finish hand tapping to the desired hole bottom.
The advantage of starting with a powered tap situation is you get to start the tap straight and then get to finish to the bottom desired without having to worry about tap breakage.
Probably nothing new but just wanted to pass along a technique that has worked great for me. I may get real foxy and try a 1-72 tap next!

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