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Screw Cutting

Screw threads can be produced in various ways: by hand with the aid of such devices as a screw tap, a die plate or a screw die, or by mechanical methods which comprise turning (on a lathe), milling, rolling, pressing or casting. Which of these methods is most suitable in any particular case will depend on the number of screw-threaded components to be produced, the desired precision of the thread, and the quality of the surface finish to be attained.

Screw cutting for a limited number of components can be done cheaply and simply by hand with a screw tap (Fig.1) or a screw die (Fig.2). The tool is rotated by hand, the workpiece being gripped in a vise. Same way, the workpiece may be gripped in the chuck of a lathe and rotated, while the tap or die is guided by the tailstock. The screw tap cuts an internal thread in a hole drilled beforehand. The screw die cuts an external thread on a rod or bolt and is held in a device called a stock, which has handles for manipulation. A simpler device is the die plate, which is merely a steel plate provided with threaded holes of various sizes for cutting screw threads.

A screw-cutting lathe is a machine capable of cutting very accurate screw threads via single-point screw cutting (which is the process of precisely guiding the linear motion of the tool bit in relation to the rotating motion of the workpiece). Until the early 19th century, the notion of a screw-cutting lathe stood in contrast to the notion of a regular lathe, which lacked the parts needed to guide the cutting tool in the precise path needed to produce an accurate thread.

Since the early 19th century, it has been common practice to build these parts into any general-purpose metalworking lathe; thus, the dichotomy of regular lathe and screw-cutting lathe does not apply to the classification of modern lathes. Instead, there are other categories, some of which bundle single-point screw-cutting capability among other capabilities (for example, regular lathes, tool room lathes, and CNC lathes), and some of which omit single-point screw-cutting capability as irrelevant to the machines' intended purposes (for example, speed lathes and turret lathes).

A higher degree of precision can be obtained with machine-cut screw threads, especially those produced on a lathe. For the majority of screwed work a tap is used for internal threading (Fig.3, showing the thread being cut in a nut) and a die head is used for external threading. The die head, a device that is clamped to the lathe, comprises a cylindrical body containing chasers for cutting the thread.

Greater precision can be attained with a single-point cutting tool (Figs.4 and 5). On the lathe the required pitch of the screw thread is obtained by gearing the lead screw up to the main spindle of the lathe by means of a train of gears. The lead screw is a long threaded rod extending along the lathe and serving as a master screw for cutting screw threads. The gearing enables screws of varying pitch and diameter to be cut, by varying the speed of rotation of the lead screw. When the lathe is started, the lead screw rotates and, by means of a nut that engages with it, moves the saddle (which carries the cutting tool) along the lathe at a definite rate, so that the tool cuts a thread of the requisite pitch.

When the tool reaches the end of the workpiece, the lead screw is disengaged, the tool withdrawn, and the saddle returned to its starting position ready to take another cut. Instead of a single-point tool a so-called chaser is sometimes used; this tool has a serrated cutting edge to produce the screw-thread profile.