Fig. 15. Method of setting and using Inside Thread Tool
Internal Threading.—Internal threading, or cutting threads in holes, is an operation performed on work held in the chuck or on a faceplate, as for boring. The tool used is similar to a boring tool except that the working end is shaped to conform to the thread to be cut. The method of procedure, when cutting an internal thread, is similar to that for outside work, as far as handling the lathe is concerned. The hole to be threaded is first bored to the root diameter D, [Fig. 15], of the screw that is to fit into it. The tool-point (of a tool for a U. S. standard or V-thread) is then set square by holding a gage G against the true side of the work and adjusting the point to fit the notch in the gage as shown. The view to the right shows the tool taking the first cut.
Very often the size of a threaded hole can be tested by using as a gage the threaded part that is to fit into it. When making such a test, the tool is, of course, moved back out of the way. It is rather difficult to cut an accurate thread in a small hole, especially when the hole is quite deep, owing to the flexibility of the tool; for this reason threads are sometimes cut slightly under size with the tool, after which a tap with its shank end held straight by the tailstock center is run through the hole. In such a case, the tap should be calipered and the thread made just small enough with the tool to give the tap a light cut. Small square-threaded holes are often finished in this way, and if a number of pieces are to be threaded, the use of a tap makes the holes uniform in size.
Fig. 16. Cross-slide equipped with Stop for Regulating Depth of Cut when Threading