Trapezoidal, Acme and how to identify a thread

November 3, 2026  ·  Indumor

The scissor jack in the trunk of a car and the vise on the bench run on the same thread as the hundred-ton press in a die shop, just at a different scale. A bolt clamps and stays put; these screws push, lift or drag all day long, and so the thread is a different animal: nearly straight flanks, a wide root and a coarse pitch. That is the trapezoidal thread, the Acme in inch sizes, and in presses that always push the same way, the buttress. At the end comes the page most consulted in any shop handbook: how to tell what thread you have when a part arrives without a drawing.

ISO trapezoidal: the 30° form

The metric trapezoidal thread lives in four ISO standards (2901 the profile, 2902 the series, 2903 the tolerances, 2904 the dimensions), which match DIN 103. The form has 30° between flanks and the contact height H₁ is half the pitch. At crest and root there is a clearance a_c that grows with the pitch, from 0.15 mm at 1.5 mm pitch to 0.5 mm for pitches 6 to 12. Every diameter follows from that: the pitch diameter d₂ is d − 0.5·P, the screw minor d₃ is d − P − 2·a_c, and the nut has its major D₄ = d + 2·a_c and its minor D₁ = d − P.

It is designated by diameter and pitch: Tr 40×7. The usual tolerance classes are 7e for the screw and 7H for the nut. The table lists first and second choice diameters with their preferred pitch.

d = D: nominal diameter. d₂: pitch diameter. d₃: screw minor diameter. D₄: nut major diameter. D₁: nut minor diameter. h₃: screw thread height (0.5·P + a_c). Choice 1: preferred diameters of ISO 2902.

Acme and Stub Acme: the inch version

Acme is the American trapezoidal thread: 29° instead of 30°, a basic height of half the pitch and the pitch in threads per inch. It comes in three general purpose classes (2G, 3G and 4G, from loosest to tightest) and in a centralizing series that locates on the major diameter, for long screws under side load. Stub Acme keeps the angle with a height of 0.3·P, for thin walls or coarse pitches on small diameters.

A Tr and an Acme do not interchange even when the diameters look close: 1-5 Acme is 25.4 mm with a 5.08 mm pitch, and the nearest Tr is 24×5 or 26×5.

Basic minor Ø = D − P. The actual screw carries a root clearance (0.010″ per side for 10 tpi and coarser, 0.005″ for finer) and the nut the same at the crest. Stub Acme: height 0.3·P, for thin walls.

Buttress and round threads

When the load always pushes the same way, the working flank should be nearly square: DIN 513 sets it at 3° and the other at 30°, with a rounded root that resists fatigue. The DIN 405 round thread goes the other way: 30° flanks with large crest and root radii, so dirt cannot jam it and knocks cannot dent it. You find it on fire hydrant valves, railway and hose couplings, crane hook nuts (DIN 15403) and, under its own standard (DIN 40400), on the base of a light bulb.

DIN 513: load flank at 3° and trailing flank at 30°, screw thread height 0.868·P. DIN 405: 30° flanks with large crest and root radii, height 0.5·P, pitch given in fractions of an inch. Uses are indicative.

Screw, nut and efficiency

A power screw is an inclined plane wrapped around a cylinder. The helix angle λ comes from the lead and the pitch diameter, and the efficiency depends on that angle and on friction. With a single start and ordinary friction a trapezoidal screw returns 25 to 40 %: most of the torque goes into rubbing the nut, and that same loss is what makes it self-locking. Multiple starts raise the efficiency and can lose that property; a ball screw runs above 90 % and needs a brake.

The nut is made of bronze or cast iron so the wear lands on the cheap part. On the lathe and in many presses it is split: two halves that close on the thread with a lever and release the carriage when they open. How that is used for threading is in chapter 8.

How to identify an unknown thread

Six steps, with a caliper, a rule, a pitch gauge and a thread angle gauge (reading the caliper to its last division is in chapter 2).

1. Outside diameter over the crests. On an internal thread, measure the hole and compare it with the minor diameters in the tables.
2. Pitch: with the pitch gauge, or with a rule over 10 threads divided by 10. On fine threads, count 20.
3. Angle with the gauge: 60° is metric or UN, 55° Whitworth or BSP, 30° trapezoidal, 29° Acme. Without a gauge, rounded crests give the Whitworth away.
4. Parallel or tapered: measure at both ends of the thread. If it grows 1 mm every 16 mm it is a pipe thread, and belongs in chapter 6.
5. Threads per inch or metric pitch: count the threads in 25.4 mm. A whole number means an inch thread; 16.9 or 14.5 means metric (1.5 and 1.75 mm). The pairs that fool people: M6 and 1/4-20 (1 vs 1.27), M8 and 5/16-18 (1.25 vs 1.41), M10 and 3/8-16 (1.5 vs 1.59), M12 and 1/2-13 (1.75 vs 1.95), M16 and 5/8-11 (2 vs 2.31), M20 and 3/4-10 (2.5 vs 2.54).
6. Hand: with the screw standing upright, threads rising to the right mean right-hand.

With the diameter and pitch measured, the table says what it may be; the full metric and inch charts are in chapters 4 and 5.

Search

One fact no table carries is worth more than the caliper: where the part came from. German or Japanese machine, metric and Tr. American, UNC and Acme. British before the 1970s, Whitworth and BSP. Ask before you measure.

Frequently asked questions

What is the difference between trapezoidal and Acme threads?

The ISO trapezoidal thread (Tr) has 30° between flanks and a pitch in millimeters; Acme has 29° and a pitch in threads per inch. Both have a contact height of half the pitch. They do not interchange: neither the diameters nor the pitches match.

How do I measure thread pitch without a gauge?

With a rule over 10 threads: measure crest to crest and divide by 10. If 10 threads span 70 mm, the pitch is 7. If a whole number of threads fits exactly in 25.4 mm, it is an inch thread and that number is its designation.

How efficient is a trapezoidal lead screw?

Between 25 and 40 % with a single start and a greased bronze nut; the rest is lost to friction. With two or three starts it climbs to 50 or 60 %, but it may stop being self-locking, and then the load can back-drive it.

Sources: ISO 2901:2016, ISO 2902:2016, ISO 2903:2016 and ISO 2904:2020 (metric trapezoidal thread); DIN 103 (parts 1 to 9); ASME B1.5-1997 (R2014) (Acme); ASME B1.8 (Stub Acme); DIN 513 (buttress); DIN 405 (round); DIN 15403 (crane hooks).