This is a tap for cutting acme thread. It is of “tandem” configuration; the lead section cuts a triangular thread and the second section forms the trapezoidal acme profile.
I bought it because I have the scheme to make a CNC engraver for decoration of reels.
The thread size is 3/8-12. This is a “standard” acme thread for which taps are readily available. I have not searched for an acme die because I plan to use threaded rod for the screws.
Here I am tapping a trial block of Delrin, from which I will make anti-backlash nuts for the engraver. An anti-backlash nut is two plain nuts preloaded against each other by a spring. The 3/8 diameter is critical for me because it is the largest diameter that passes through the bore of my lathe spindle.
I also plan to make brass threaded-bore clamp-on shaft collars for the thrust bearings of the engraver leadscrews. The brass will be more difficult to tap, but an oversize pilot bore is suitable for a collar.