For an existing ACME screw or nut, choose a matching ACME thread form; for a metric Tr component, choose the specified metric trapezoidal form. In an ACME vs trapezoidal threads comparison, the distinction matters because ACME itself has a trapezoidal profile, but its standard included angle differs from that of metric Tr.
ACME vs Metric Trapezoidal Threads: Key Differences
Standard ACME threads have a 29° included angle, with each flank at 14.5°; metric Tr threads have a 30° included angle, with each flank at 15°. ACME commonly uses inch dimensions, while metric Tr uses millimetres. Converted dimensions alone don’t identify the thread form.
| Feature | Standard ACME | Metric Trapezoidal Tr |
|---|---|---|
| Profile | Trapezoidal | Trapezoidal |
| Included angle | 29° | 30° |
| Angle of each flank | 14.5° | 15° |
| Common dimensions | Inch-based | Millimetre-based |
| Tool and mating-part selection | Match the specified ACME form and dimensions | Match the specified metric Tr form and dimensions |
The included angle is the angle between the two flanks. A small difference in appearance isn’t a reliable basis for treating two threads as interchangeable.
ACME Threads: Uses and Limits
ACME is a trapezoidal thread form suited to a component specified with an ACME thread. Its practical advantage in a replacement or machining job is that you can select the tool, gauge and mating part against the same specified form.
The 29° included angle also sets a limit: a tool for a 30° metric Tr profile won’t produce the specified ACME flank angles. Check the drawing for the applicable standard and revision, pitch, diameter, hand, number of starts and required clearance before selecting an insert or mating nut.
For a job calling for an ACME insert, our 16 ER 8 ACME WLD listing is available to review. Confirm its specifications and compatibility against the job requirements before selecting it.
Metric Trapezoidal Threads: Uses and Limits
Metric Tr is a trapezoidal thread form specified in metric dimensions. It’s the appropriate choice when the drawing or mating component calls for that form, rather than ACME.
For a single-start thread, Tr 14 × 3 specifies a 14 mm major diameter and a 3 mm pitch. Don’t apply that reading automatically to a multistart designation; check the notation and the specified standard. Metric Tr’s 30° included angle also means that an ACME profile tool doesn’t produce the specified flank angles.
Choosing an ACME or Metric Tr Thread
Choose ACME if:
- The drawing specifies an ACME thread and its applicable standard.
- The replacement must match an existing ACME screw or nut.
Choose metric Tr if:
- The drawing specifies a metric trapezoidal Tr thread and its applicable standard.
- The replacement must match an existing metric Tr screw or nut.
Before ordering, check the standard and revision, major diameter, pitch, thread hand, number of starts, lead and required clearance. A pitch gauge can help check pitch, but pitch alone doesn’t establish the complete profile or fit.
Pitch is the axial distance between adjacent thread grooves. Lead is the distance the thread advances in one revolution: lead = pitch × number of starts. Pitch and lead are equal on a single-start thread; on a multistart thread, lead is greater.
FAQ
Are ACME and Tr Threads Interchangeable?
Don’t assume they are. Standard ACME and metric Tr have different included angles, and a mating part must also match the specified dimensions, hand, starts and clearance.
Is ACME a Trapezoidal Thread?
Yes. ACME has a trapezoidal profile, but standard ACME has a 29° included angle, while metric Tr has a 30° included angle.
Does Pitch Always Equal Lead?
No. Pitch equals lead for a single-start thread. For a multistart thread, multiply pitch by the number of starts to find lead.