How to Cut Square Threads on a Conventional Lathe I. Understanding Square Threads First
Square threads have a square-shaped thread profile, with the thread thickness equal to half the pitch. They offer high transmission efficiency but suffer from low alignment precision and weak root strength. Precise manufacturing of square threads is relatively difficult, and the clearance after wear of the spiral pair is hard to compensate for or repair. A common example is the threads on a vise. They require a lot of force when tightened and loosened frequently. Threads on valves and water pipes, regardless of size (small ones for faucets), are all square threads, with only slight differences in dimensions. Jacks and spiral presses also use such threads. Therefore, square threads are suitable for heavy loads, where the strength of the square thread is required to be robust, and there are no stringent demands on the precision of the threads.
II. Thread Cutting Process Technology
Cutting Principle The principle of thread turning requires a strict motion relationship between the lathe spindle and the tool: that is, for every complete rotation of the spindle (workpiece), the tool moves uniformly by a distance equal to the lead.
Thread Turning Process The process for turning an ordinary external thread includes:
Facing the end
Turning the thread major diameter
Turning the retreat groove
Chamfering
Turning the thread
Square threads do not have a fixed thread profile like other threads Generally, when the thread profile dimensions are not specified, the thread flank is made into a square. The turning tool feeds straight to process square threads, and the groove depth (thread height) dimensions are processed accordingly.
III. Turning Tools Thread turning tools, considering the shape of the thread, are suitable for short threads. The width of the tool's cutting edge should be half the pitch, and since there are many wide-pitch threads, high-speed steel flexible turning tools are often used. The biggest problem in processing square threads is the grinding method of the turning tool. The cutting edge width should be half the pitch, which is not easy to operate. If the pitch and lead increase, the thread rise angle (lead angle) will be larger, and the back angle of the turning tool may hit the side of the thread flank. Additionally, for threads with a large lead, the thread rise angles at the top and bottom of the thread flank will be different. Therefore, on the outer circumference (thread top), the width of the cutting edge should be the same as the groove width, and at the bottom of the groove, the thread flank will be recessed. To avoid these phenomena, a trapezoidal turning tool with a smaller front cutting edge width is used for the recessed part.
IV. Turning Square Threads Unlike triangular and trapezoidal threads, square threads do not have the trouble of single or double cutting, nor is there an issue with the minor diameter, because the precision requirements are not high. In fact, this is relatively simpler in thread processing.
