Acme thread form

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, is the Acme thread form (/ˈækmiː/ ACK-mee). 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 via 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 square (because the wear can be compensated for); it is stronger than a comparably sized square thread; and it makes for smoother engagement of the half nuts on a lathe leadscrew than square does.

The trapezoidal metric thread form is similar to the Acme thread form, except the thread angle is 30°. It is codified by DIN 103. Although metric screw threads are generally more prevalent worldwide than imperial threads, the Acme thread is very common worldwide, and may be more widely used than the trapezoidal metric thread. This is not surprising, as manufacturers today are usually capable of making whichever threads (metric or imperial) are best for any given application (based on customer expectations or tooling availability). It may be that the tooling for Acme threads has been so dominant (compared to trapezoidal metric) that customers tend to want Acme threads for power screws regardless of metric standards used elsewhere in the product.

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