Trapezoidal thread

Trapezoidal threads are screw thread profiles with trapezoidal outlines. They are the most common forms used for power screws. They offer high strength and ease of manufacture. They are typically found where large loads or high accuracy are required.

The original trapezoidal thread and still probably the one most commonly encountered worldwide is the Acme thread. The Acme thread was developed sometime in the second half of the nineteenth century 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 Acme thread form name appears as early as 1895.

Trapezoidal metric thread

The trapezoidal metric thread is similar to the Acme thread, except the thread angle is 30°.  Although metric screw threads are generally more prevalent worldwide than nonmetric threads, the Acme thread is very common worldwide, and may be more widely used than the trapezoidal metric thread.

Manufacturers today are usually capable of making whichever threads metric or nonmetric are best for any given application based on customer expectations. 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.

What are the similarities?

Both thread forms serve the purpose of producing linear motion when rotated, usually under heavy load. Some common uses include lead screws for linear actuation on machinery, table lifts, clamps and vises, valve stems, medical diagnostic device drives, trailer jacks and jack-stands.

The uses for these threads are essentially the same. Actually, taking manufacturing tolerances allowed into consideration, they may be interchangeable when threads per inch is the same.

The demands placed on the tools that produce these threads are higher due to the amount of material being removed per tooth in the process. Controlling chip-load is critical. To design a tap for these threads requires specific detail on the application. Information on the material being tapped, tapping depth, condition of the hole through or blind and Class of Fit is essential to engineer the proper tap.

Frank Xu is the author of this article on Trapezoidal thread taps. Find more information, about Trapezoidal thread gauge.