Tailstock Die Holder
From DT Online
The end of the Quill of a Drill Press or Centre Lathe, or a chuck held in it, can be used to ensure a Die is offered Square to the end of a chamfered bar.
A Tailstock Die Holder makes good use of the true alignment between a Centre Lathe Tailstock and Chuck. They are fitted with a Morse Taper such that they can be gripped accurately in the tailstock quill. With the power off the lathe chuck is held by hand (or the lathe placed in a low gear or its spindle locked), the handles of the Tailstock Die Holder can be rotated forwards and backwards to produce concentric threads on the bar stock.
Safety Point! Tailstock Die Holders are designed for thread cutting only by hand, using the lathe for thread alignment, but without using the lathe power. NEVER turn on the lathe power while holding the Die Holder handles. This can be extremely dangerous, even for short lengths of thread.
Industrially, a Die Heads are used in a Centre Lathe under power to cut or roll external threads: NOT Tailstock Die Holders.