Guide
ACME or Metric Trapezoidal Thread for a Custom Lead Screw?
Metric trapezoidal thread is the ISO power-screw form with a symmetrical 30 degree thread, sized in millimeters of pitch, and ACME is its US counterpart with a 29 degree thread sized in threads per inch. They look almost identical, but they are separate systems, and the safe rule is to build the screw and its nut to the same one.
By Saola Manufacturing engineering team · Published · Updated
This guide is for engineers designing a linear-motion or adjustment screw who work in inches while the supplier or the mating parts work in metric, or the reverse. It compares the two forms, shows how each is written on a drawing, and ends with the questions to put to a machine shop. It does not claim any particular shop can cut either form; that has to be confirmed per supplier.
A note on sources. The standards themselves, ASME B1.5 and the ISO 2901 to 2904 series, are paywalled, so this page uses their public catalog entries and engineering explainers, and it does not reproduce dimension tables. Where the explainers disagree on how much the one degree matters, both views are given.
On this page
Are ACME and metric trapezoidal threads interchangeable?
In most cases no. Screw suppliers advise that the screw and nut follow the same thread standard, because angle, pitch series and tolerances all differ.
Helix Linear, a lead screw maker, puts it plainly: in most cases ACME and trapezoidal threads are not directly interchangeable, because of differences in thread angle, pitch standards and dimensional tolerances, and a mismatched screw and nut typically will not mate properly (Helix Linear, accessed 2026). It adds that the main distinction is the measurement standard rather than performance.
igus, a polymer-nut manufacturer, takes a more relaxed view of the angle itself. It says the one degree difference has a minimal effect in most applications and that load distribution is nearly identical, but also that swapping types means the whole system has to be reconfigured, not just the screw (igus, June 2026). The two views agree on the practical point: do not mix a screw from one system with a nut from the other.
| Item | ACME | Metric trapezoidal |
|---|---|---|
| Thread angle | 29 degrees | 30 degrees |
| Governing standard | ASME B1.5, general purpose and centralizing forms | ISO 2904:2020 basic dimensions, with ISO 2901 to 2903 (2016); DIN 103 in Germany |
| Size system | Inches, threads per inch | Millimeters, pitch and lead |
| Typical designation | 1/2-10, then the class (for example 2G) | Tr16x4 |
| Lead of a single-start thread | 1/10 in (2.54 mm) for 1/2-10 | 4 mm for Tr16x4 |
| Classes in the standard | General purpose 2G, 3G, 4G; three centralizing classes | Tolerance classes from the ISO tolerance part, 2903 |
| Multi-start example | Covered in an ASME B1.5 appendix | Tr 60x18(P9): two starts, lead 18, pitch 9 |
Why do lead and pitch matter more than the one degree?
Because the two systems give different travel per revolution, and the motor, controller and mating parts are all sized to that number. A swap changes the lead, which changes positioning.
Pitch is the distance between neighboring threads, and lead is the distance the nut travels in one turn. On a single-start screw they are equal; on a multi-start screw the lead is the pitch multiplied by the number of starts. igus warns that a different lead means the controller needs reprogramming or recalibration, and that ignoring step resolution can cause significant positioning errors (igus, June 2026).
An inch screw of 10 threads per inch has a pitch of one-tenth of an inch, which is 2.54 millimeters. That is not a value in the whole-millimeter pitches of the examples in the sources, so even a nominally similar size will not travel the same distance per turn. Always compare the lead, not the diameter.
The designations make the difference visible. A metric screw such as Tr16x4 states diameter and pitch in millimeters, and a multi-start screw lists both lead and pitch, as in Tr 60x18(P9) (Wikipedia, Acme thread form, accessed 2026). An ACME screw such as 1/2-10 states diameter in inches and threads per inch.
How do you write each thread on a drawing?
Write the full designation, the class, the hand and the number of starts, and name the standard in the note. Do not rely on a nominal diameter and a pitch alone.
For ACME, the ASME B1.5 catalog entry states that the standard covers general purpose and centralizing threads, with three general purpose classes 2G, 3G and 4G and three centralizing classes, and that its limits and tolerances apply to single-start threads (ASME, B1.5-1997 (R2024)). A typical callout therefore has the size, threads per inch, the form, and the class, then the hand if it is left-hand. The same standard has appendices on gaging and multiple-start threads, so a multi-start ACME screw should name the lead and number of starts explicitly.
For metric trapezoidal, ISO 2904:2020 gives the basic dimensions and refers to the design profiles in ISO 2901, and BSI lists it as current (BSI, BS ISO 2904:2020). A callout has the Tr designation with diameter and pitch, plus lead and pitch in brackets for a multi-start thread, the hand, and the tolerance class that matches the nut. Use the same wording on every drawing and the purchase note so the shop and the inspector read the same thing.
Whichever form you choose, add three notes beyond the thread: the thread length, any runout or relief at the ends, and how the thread will be checked. The general rules for writing a thread on a print are in the thread callout guide, and the checking tools for ordinary threads are in the go/no-go gauge guide.
When should a custom lead screw be ACME and when trapezoidal?
Match the nut and the system around the screw: choose ACME when the nut, the motor settings and the maintenance spares are inch-based, and trapezoidal when they are metric.
The sources agree there is no performance reason to prefer one form. Helix Linear calls the main distinction the measurement standard rather than performance, and igus says materials, lubrication, lead and self-locking behavior matter far more than the one degree. That leaves the supply chain as the decider.
| Situation | Lean toward | Why |
|---|---|---|
| Mating nut is a US catalog part | ACME | Nut and screw must share a standard |
| Mating nut is a European or Asian catalog part | Metric trapezoidal | Same standard, metric lead |
| Motor and controller already set in millimeters | Metric trapezoidal | Lead values match the existing motion settings |
| Machine ships to US customers who stock spares | ACME | Spares and drawings in the customer's system |
| Machine ships worldwide with one design | Pick one and state it on every document | Mixed systems create mismatched spares |
| Radial load on the nut is expected | Check the centralizing form of ACME | Roots and crests contact before the flanks |
What other ACME variants should you know about?
The general purpose form carries axial load only, the centralizing form is meant to also locate the nut radially, and the stub form is a shallower thread. Name the variant on the drawing.
The ASME B1.5 catalog entry separates general purpose threads, which have clearance on all diameters, from centralizing threads, which have limited clearance at the major diameter. Wikipedia’s summary adds that the general purpose form is not designed to carry external radial loads, so nut and screw should be supported independently. The centralizing form is designed so the roots and crests of opposing threads contact before the flanks do under radial load, and the stub form is a shallower thread (Wikipedia, Acme thread form, accessed 2026).
The same page compares ACME with a square thread: ACME is easier to cut and wears better because wear can be compensated, at the price of lower efficiency from higher friction. For a design that has to be made by a general machine shop, that ease of cutting is a practical reason to choose an ACME or trapezoidal form over a square one.
What should an RFQ ask a shop about ACME or trapezoidal threads?
Ask the shop to confirm the exact thread form and class in writing, name the gauges and method it will use to check lead and fit, and offer a trial fit with the mating nut before a production run.
- Confirmation of the full designation and class as understood, so a misread callout is caught at quote stage.
- How the thread is checked: a thread gauge set, measuring over wires, a lead measurement, or a trial fit with the real nut.
- Straightness of the finished screw over its length, which affects how the nut runs, and how the part is supported while it is cut.
- The material and heat condition of the bar, and whether the screw is stress-relieved before the final cut. See stress relieving before machining.
- The production site and country of origin, stated on the quote.
Lead screws are turned parts, and the CNC turning page describes the turning work that runs at the lead plant. Trapezoidal and ACME forms are not listed there as specific items, so send the drawing with the designation and the mating nut, and the quote will say whether the thread form is quoted and how it will be gauged.
Frequently asked
Can I run an ACME screw in a metric trapezoidal nut?
It is not advised. Suppliers recommend that screw and nut follow the same standard, and the lead will differ, so the nut will bind, wear fast or move a different distance per turn.
Is a trapezoidal thread stronger than ACME?
The sources do not say so. They describe the two as nearly identical in function, with the main difference being the measurement standard, so choose on the mating parts.
What does Tr16x4 mean?
Tr marks a metric trapezoidal thread, 16 is the nominal diameter in millimeters and 4 is the pitch. On a single-start screw the lead equals the pitch.
What does a multi-start designation such as Tr 60x18(P9) mean?
The lead is 18 and the pitch is 9, so the thread has two starts. The nut moves 18 millimeters per turn, twice the pitch.
Do I need to specify the class on the drawing?
Yes. ASME B1.5 defines several classes, and the ISO series has its own tolerance classes. Without a class the shop has to guess how tight the fit between screw and nut should be.
Sources
- ACME threads vs trapezoidal threads: key differences engineers should know, Helix Linear Technologies, accessed October 2026
- ACME vs trapezoidal lead screws, igus, June 2, 2026
- Acme thread form, Wikipedia, citing ASME/ANSI B1.5-1997 and DIN 103, accessed October 2026
- B1.5 Acme Screw Threads, ASME, 1997 edition, reaffirmed 2024
- BS ISO 2904:2020, ISO metric trapezoidal screw threads, basic dimensions, BSI, published February 2020