If it is a thread mill then quite unusual not to state the thread form and I've not seem them with the square end for a tap wrench and as there is not helix to a thread mill the square would not be of any use if being held in ER tapping collets.

I think it is just one of those wood drill and router bits where they set the machine wrong and forgot the drill part and the square on the shank is just because all their blanks are pre-formed like that whatever the use, or to help register it during manufacture.

Given that it seems a bit pricey. A whole pack are that price from Lidl.

Edited By Bazyle on 15/01/2023 14:31:23

Bit of a mystery, the "thread" angle is clearly 90 degrees rather than the common 60 or 55. The square shank is also strange. I would very much like to get hold of something similar with a 90 degree shape and 1/16", 0.0625" pitch, about 3/4" diameter and with about 3" of usable length.

All I can think of is that it was made to special order by a tap making company and they used a tap blank which already had the square on the end. We have some strange taps at the museum, usually foundin boxes of tools and cutters donated to us. I keep them hidden out of harms way in case somebody uses one by mistake.

The site also sell a 9/16 x 22 Acme tap.

https://www.jurassictools.com/product/bates-9-16-22-acme-tap-acme-9-16-tap/ Acme tap

I'm surprised at such a fine acme thread, since they were used to provide accurate motion with lower friction than vee threads, and easier machining than square threads. 22 tpi doesn't seem a handy pitch for precise movement purposes. The image's resolution is low, however the thread looks to me more like a vee shaped thread.

I wonder if Michael's example was a thread milling cutter ground onto a tap blank? A tap maker would be able to produce such a cutter on a relieving lathe with grinding spindle or a similar grinding machine.

Bill