This section is from the book "Amateur Work Magazine Vol6". Also available from Amazon: Amateur Work.
Mr. A. B. French, in the "American Machinist," gives the following information regarding a spring threading tool:
The drawings show a special spring threading tool that gives satisfaction and produces nice smooth threads. The body of the tool is in two pieces A and B, both made of tool steel. The bottom B is of 1/4-inch square stock and has a l/4xl/8-inch slot in it as shown. The top A is made of l/2x3/4-inch stock and has a l/4xl/8-inch tongue, a nice fit in the slot in B. A and B are held together by the screws C.
The front end of the body at G is hardened. The tongue and slot are ground and lapped and should be a nice sliding fit. The neck of the tool H is about 3/16-inch thick, the space for the spring is 3/4x1 5/8-inches. The spring is made of spring steel, tempered. On the bottom of the spring there is a tongue K to fit into the slot in the body B. The adjusting screw E is 1/4-inch, 20 threads, it is to adjust the tension of the spring. The thread tool F is made of 1/4-inch Stubs steel flatted on the top. It is held by the set screw 1, the hole in the body not being drilled quite through so as to back the tool up. The small pin J is 1/8-inch diameter driven into the lower half of the body B; in the upper half A there is an elongated slot 1/8x3/16-inch for it to work in.
This tool is not designed for heavy work; the largest I have ever used it on was a 1-inch 8-thread tap.
By adjusting the screw E you can get any tension on the spring you want to suit the size of thread being cut. As the nose of the tool is supported the tool cannot spring downward or to the side; when it-strikes a hard spot it springs back.