3/4" shank with a 1/2-20 thread — straight threaded shank.
A straight threaded shank is the arbor that holds a threaded-shank cutter. The plain 3/4" end goes into your holder or collet; the cutter screws onto the 1/2-20 thread at the other end and seats against the 3/4" shoulder, which is what squares it to the axis and takes the cutting load.
The shank is 1-1/2" long with 1/2" of thread. High-speed steel throughout.
What screws onto it. Any of our 1/2-20 outside chamfering mills: 3/4" at 60°, 3/4" at 90°. The number is the largest work diameter each one fits over.
