Single-Point Thread Turning

Single-point thread turning is the way a screw thread is cut on the lathe, one form tool cutting the thread into the rotating work, pass after pass, until the thread is full depth. A screw thread is a helical groove of a precise shape, and on the lathe it is cut by a tool whose edge is ground to that shape: the single-point threading tool, a cutting tool whose point carries the form of the thread — the V of a common thread, its 60-degree included angle, its root and its crest. The machine drives the tool along the work, advancing one lead of the thread for every revolution of the spindle, so the tool cuts a helical groove of the thread’s pitch; then the tool is withdrawn, returned, and fed deeper, and another pass cuts the groove deeper still, until the thread reaches its full depth. It is the classic, exacting threading method — the one that made the tap and the thread mill alternatives to it — and it is how threads are cut when they must be right.

The tool and its form

The single-point threading tool is made for the thread it cuts, and its form is its identity. Its cutting edge is ground to the thread form — for the common V-thread, the 60-degree included angle of the thread’s sides, with the tool’s point shaped to reach the root of the thread and its sides to cut the flanks. The tool may be a high-speed-steel tool ground in the shop for the thread, or a carbide threading insert made in the standard forms, each ground to cut one thread form at one pitch. The tool is set in the lathe with its point at centre height and its sides square to the work’s axis — the threading tool must be set true, with the bisector of its angle perpendicular to the axis, or the thread it cuts will lean and its flanks will not be the angles they should. Set right and fed along the rotating work at one lead per revolution, the tool cuts a helical groove whose shape is its own.

The many passes

A thread is not cut in one pass, and the sequence of passes is the heart of thread turning. The full depth of a thread is far more than a single cut could take — the tool would be overloaded, its edge broken or the work deflected — so the thread is cut in a series of passes, each cutting the groove a little deeper. The tool runs along the thread, cuts its pass, withdraws, returns to the start, feeds in by the next increment of depth, and runs again; pass after pass, the groove deepens, each pass cutting a thin slice from the flanks and the root until the thread is full depth and the tool’s form is fully reproduced in the work. The number of passes grows with the size of the thread — a coarse, deep thread needs many passes, a fine thread fewer — and the passes are often made lighter as the cut deepens, because the tool’s engagement grows. The last pass or two are finishing passes, cutting only a little, to clean the thread’s surfaces and bring it to its final form.

Feeding in: radial and flank

The way the tool feeds deeper between passes decides how the thread is cut, and the two ways are the radial feed and the flank feed. In radial (or plunge) threading, the tool feeds straight in, perpendicular to the axis, each pass cutting both flanks of the thread and the root at once — the simple way, standard on the CNC lathe, cutting a clean full-form thread with the tool cutting on both sides. In flank (or compound-rest) threading, the tool feeds in at an angle, along one flank, so that each pass cuts mostly one side of the thread — the traditional way on the manual lathe, where the compound rest is set to the flank angle and the tool cuts on one edge, leaving the other flank to be finished by later passes. Radial threading cuts both flanks and is simpler to program; flank threading lets the tool cut on one edge at a time, which suits a ground tool and the manual machine. The choice shapes the cutting and the finish of the thread’s flanks.

Internal threads and the bore

Thread turning is not only for the outside of the work, and the internal thread is the harder of the two. An external thread — a screw or a stud — is cut on the outside of a rotating shaft, the tool reaching in from the side, easy to see and to set. An internal thread — a nut or a threaded bore — is cut inside a hole: the threading tool is a small boring-style bar that reaches into the bore, and the same series of passes cuts the thread up the wall of the hole, the tool withdrawn at the end of each pass to clear the chips. Internal threading is harder than external for the reasons that all internal work is harder: the tool is smaller and less rigid, the cut is out of sight, and the chips must clear a confined hole; but it is the method that cuts a thread where a tap cannot reach or a thread must be cut in a bore that is part of a larger turned part.

Thread turning in the family of threading

Single-point thread turning is the oldest of the ways this wiki’s entries describe, and it still has its place beside the newer methods. It is the method of the lathe, cutting a thread in the same setup as the rest of the turned part, so the thread runs true to the part’s axis and its diameters — the reason that threaded shafts, studs, spindles and the threads that must be concentric with their part are thread-turned rather than made another way. It cuts threads of any pitch the machine can feed, from the finest to the coarsest, external and internal, right-hand and left-hand, with one tool that is ground or chosen for the form. For the high-volume small thread, the tap cuts faster, and for the machining centre the thread mill interpolates a thread without a threading setup; but where the thread is part of a turned part and must be true to it, single-point thread turning is the method, cutting the helical groove pass by pass to the full depth of the thread.