1/2" shank with a 5/16-24 thread — straight threaded shank.
A straight threaded shank is the arbor that holds a threaded-shank cutter. The plain 1/2" end goes into your holder or collet; the cutter screws onto the 5/16-24 thread at the other end and seats against the 1/2" shoulder, which is what squares it to the axis and takes the cutting load.
The shank is 1-1/4" long with 5/16" of thread. High-speed steel throughout.
What screws onto it. Any of our 5/16-24 outside chamfering mills: 1/4" at 60°, 1/4" at 90°. The number is the largest work diameter each one fits over.
