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Concept
Trapezoidal thread form
Summary
Trapezoidal thread forms are screw thread profiles with trapezoidal outlines. They are the most common forms used for leadscrews (power screws). They offer high strength and ease of manufacture. They are typically found where large loads are required, as in a vise or the leadscrew of a lathe. Standardized variations include multiple-start threads, left-hand threads, and self-centering threads (which are less likely to bind under lateral forces).
The original trapezoidal thread form, and still probably the one most commonly encountered worldwide, with a 29° thread angle, is the Acme thread form (ˈækmiː ). The Acme thread was developed in 1894 as a profile well suited to power screws that has various advantages over the square thread, which had been the form of choice until then. It is easier to cut with either single-point threading or die than the square thread is (because the latter's shape requires tool bit or die tooth geometry that is poorly suited to cutting). It wears better than a square thread (because the wear can be compensated for) and is stronger than a comparably sized square thread. It allows smoother engagement of the half nuts on a lathe leadscrew than a square thread. It is one of the strongest symmetric thread profiles; however, for loads in only one direction, such as vises, the asymmetric buttress thread profile can bear greater loads.
The trapezoidal metric thread form is similar to the Acme thread form, except the thread angle is 30°. It is codified by DIN 103. While metric screw threads are more prevalent worldwide than imperial threads for triangular thread forms, the imperially sized Acme threads predominate in the trapezoidal thread form.
The Acme thread form has a 29° thread angle with a thread height half of the pitch; the apex (or crest) and valley (or root) are flat. This shape is easier to machine (faster cutting, longer tool life) than a square thread. The tooth shape also has a wider base which means it is stronger (thus, the screw can carry a greater load) than a similarly sized square thread.
Official source
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