A Tailstock Die-Holder for the 7x Lathe:
There are two main methods of cutting screw threads on a "screw cutting" lathe. For large and precise threads or long threaded shafts the thread is cut by a shaped bit that is driven by a threaded shaft matched to the revolutions of the lathe chuck that moves the lathe carriage in a precise speed ratio compared to the lathe spindle and chuck. Changable gearing in the back of the lathe allows selection of different gear ratios that will move the carriage (and the cutter) to produce different pitch threads. This method is called "single-point thread cutting". It's a very accurate method but it requires a time consuming resetting of the lathe gears, and a good deal of skill.
The second way is to simply hold a "button die" or normal hand threading die in a fixed position and allowing the lathe to rotate the work and thread the workpiece onto the button die. In this case the die is usually held by a "die holder" fitted to the lathe tailstock. This method is useful for small threads that might be difficult to cut by the single point method, and for shorter threads. The device used to hold the die is called a Tailstock Die-Holder.
In both cases the lathe can be used under power (at slow speeds) or driven by a hand crank in the lathe spindle for very precise control.
In the above picture you can see my version of the hand-crank inserted into the lathe spindle, and you can also see the the gears that can be changed to produce various thread pitches. While using a hand-crank the lathe is carefully isolated from electrical power so it can't accidentally be switched on with the hand-crank in place which would cause it to spin dangerously.
My Version of a Tail-Stock Die Holder:
About 10 years ago I bought an early version of the 7x10 Mini Lathe to replace my tiny Unimat db200. Most of my projects are small, and with the short bed length I viewed the 7x10 as a sturdy and powerful replacement having roughly the same "footprint" but with much more capability.
The short bed-length, for me, had a single disadvantage. Fitting a commercial tail-stock die-holder on such a short bed tended to limit the threading projects to short items only, because by fitting a morse taper drill chuck, along with a threading die spindle and a die holder designed for two sizes of dies, there's not much room left for the workpiece.
I ended up designing my own version of a dieholder with the attributes of being very short, having a long "pass-through" for cutting longer threads, and a clamp lever rather than tommy-bars. While most of my threading is done using a spindle crank, sometimes threading under power is convenient and in this case tommy-bars can be dangerous if there's a hang up in the process.
My design uses a die holder that's designed to rotate and slide on the outside of the extended tail-stock ram, and can turn freely. The newly formed thread can then pass through the die, through the die-holder body and well into the tailstock ram's morse socket giving room for a longer continous thread. To turn the die-holder, a clamp and turning handle is clamped on the die-holder body allowing the handle to clamp and release as the die is turned. This is much more convenient than "tommy bars" and in my opinion, much safer too. In the case of threading under power, the clamp can be adjusted to slip on the tailstock ram, to drag slightly, or to clamp and stop rotation completely to form the thread. It's very controllable, and if there is any catching, or you accidently thread to a shoulder, the clamping of the handle can be immediately reduced allowing the holder to rotate harmlessly. Even if your reaction time is slow, being conservative with the clamping pressure will allow the holder to spin in case of a problem, much different than the damage a tommy bar can produce in a powerfull lathe.
The sliding body is made of brass to alleviate any wear problems on the tailstock-ram although at normal slow threading speeds I doubt there would be much wear, and the factory finish in the ram is nothing special anyway. A drop or two of oil would preclude any tendancy to grab.
The steel rim around the actual dieholder was turned from a piece of 2" mild steel bar, as was the clamp body. The rim was carefully bored to be a snug fit on the brass body then forced over the brass tube and soft soldered using a butane torch. The brass tube was then clamped in the 3 jaw chuck and the die holder recess machined.
The clamp body was bored off center by about 10mm in the 4-jaw chuck, drilled and tapped for the handle, then the slot was hacksawed to allow it to clamp on the holder body.
In use under power the holder is brought into contact with the rotating workpiece with the handle and then the handle is slowly turned to tighten the clamp and start the thread. Varying the drag on the clamp handle to regulate the cutting force seems intuitive, but I made the handle long enough that in case of a jam, my fingers won't be in the way.
I usually cut threads with the lathe off, and use my hand powered spindle crank. This is the best way, although I've found that normal short threading jobs are easiest with the lathe off, holding the lathe chuck with my left hand, and simply turning the die-holder with the handle, backing off to get another purchase.
My normal dies are not adjustable but I've included an adjusting grub screw position used to fine tune the die centering. The die in my holder is currently clamped by two allen grub screws 90 degrees apart. The grub screw tips were shaped to fit the dimples on my dies by turning them in a drill press while grinding with small hand held grinder.
This is a simple project that could be built using available materials around the shop, and no real precision is required.