Acme thread form

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Threads formed around a shaft are used to translate rotational motion into linear motion. In this way fasteners such as screws and bolts convert the rotational motion or torque of a screwdriver or wrench into a compressional holding force. Threads around a shaft are also used to move or position mechanical components such as a tool post on a metal-working lathe, or to raise or lower a garage door.

The most common profile for threads is a V-form; the threads, if sliced parallel to the shaft would describe a series of V's, representing the apex of the threads and the valleys between them. In some applications in which the screw is used to exert a great deal of force, the size of the threads is inadequate to prevent them from being sheared off of either the shaft or the bore into which they are threaded. Jack screws for moving large structures, bench vises and C-clamps are common examples of this type of screw application.

Square threads were developed to address this issue. Threads are cut with a square profile, with a zero degree pitch which provides a stronger, more robust bearing shoulder for the rotating shaft. However, square threads are more difficult to machine and are difficult to thread. The Acme thread was developed to offer better characteristics and to overcome these problems with the square thread form.

The Acme thread form uses a 29 degree pitch with flat apex and valley. The Acme screw thread is stronger than V-profile 60 degree threads. Wear can be compensated for with a split nut. Typically found where large loads or accuracy is required as in vises or the lead screw of a lathe.