Acme threads appeared sometime in the late 1800’s, as an improvement on the square thread form. Square threads were the first choice for motion transfer and heavy loads. But, square threads were difficult to produce with available cutter technology. Although the square form was relatively efficient for the purposes required, it was inherently weak at the base of the thread due to the sharp 90-degree angle of the flank. Modifying the included-angle to 29-degrees widened the base of the thread making it stronger. Over time, standards in Acme diameters and pitches were established, all with Imperial Inches in diameter and Threads-per-Inch units of description.
In Europe, similar standards followed using metric units of measure, and the slightly different included flank angle of 30-degrees. Metric standard trapezoidal threads are covered under DIN103. Diameter and pitch descriptions are in metric units of measure.
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 CNC machinery, table lifts, clamps and vises, valve stems, medical diagnostic device drives, trailer jacks and jack-stands. The American Acme form has an included flank angle of 29-degrees. The metric Trapezoidal thread is at 30-degrees. The uses for these thread forms are essentially the same. In fact, taking manufacturing tolerances allowed into consideration, they may be interchangeable when TPI (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. In order to design a tap for these threads, North American Tool 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. General purpose taps are available, but are definitely not suitable for every application.
So what’s the difference between Acme and Trapezoidal? Not much physically. Although the American Acme thread-form is used and accepted throughout the world, the choice of which to use is usually dictated by the origin and user destination of the finished part. To that manufacturer, the difference is everything!