ACME Thread Dimensions Chart (Internal & External) | ACME Thread Profile, Formula, Classes & Sizes

ACME Thread Dimensions Chart (Internal & External) | ACME Thread Profile, Classes, Sizes and More
ACME threads are vital components in modern machinery and equipment. Their robust design, standardized specifications, and versatile application make them indispensable in mechanical engineering. This article’ll break down the ACME Thread designation, profile, angle, calculation formulas, series & classes, and dimensions with size charts for internal and external ACME threads.

What Is ACME Thread?

The ACME thread, developed by Acme Screw Machine Company in 1894 and standardized in 1921, represents a significant milestone in mechanical engineering. Despite its name often being mistaken for an abbreviation, “Acme” is simply derived from the company’s name. While metric threads have become dominant for many other thread types worldwide, the inch-based ACME thread maintains its position as the international leader among trapezoidal threads. The ACME thread is characterized by its distinctive trapezoidal profile, featuring a 29° flank angle and a thread height equal to half the pitch in general-purpose applications.

ACME Thread Angle

ACME Thread Designation

This thread follows a specific designation system where threads are identified by their nominal diameter in inches and pitch in threads per inch. An ACME screw thread is indicated by D – P – ACME SC.

D refers to the nominal diameter: Expressed in inches (fractional or decimal, 1/4″ to 5″)
P refers to pitch: Threads Per Inch (TPI) or pitch distance
S refers to Series: G (General) or C (Centralized)
C refers to class: 2 (lower precision) to 4 (higher precision)

For instance, a 1/2-10-ACME 2G designation indicates an ACME Thread with 1/2″ diameter, 10 threads per inch, General purpose series, Class 2 precision.

ACME Thread Profile and Calculation Formula

d = diameter from designation
n = TPI from designation
P = n/TPI
h = P/2
d2 = d-h
d1= d-P

ACME Thread Series and Classes

According to ANSI/ASME B1.5 standard, there are 3 typical series or ACME screw threads.

1. G Series (General Purpose)
The most commonly used ACME thread type with a 29° included angle. It is susceptible to wedging under loads and requires individual support for screws and nuts. This series is recommended for standard applications and features three precision classes: 2G, 3G, and 4G.
Classes:

2G: Least accurate, commonly used.
3G: Medium accuracy.
4G: Highest accuracy, recommended to reduce backlash.

2. C Series (Centralized)
This series maintains the same basic dimensions as the G series but with tighter tolerances and limited clearance at the major diameter of both internal and external threads. It prevents wedging on flanks and part sagging, making it better suited for high-load applications. The series features three primary precision classes: 2C, 3C, and 4C, with additional classes 5C and 6C (not recommended for new designs).
Classes:

2C: Similar to 2G but with tighter tolerances.
3C: Medium accuracy.
4C: Highest accuracy, minimizes backlash.

3. Stub Series
The Stub series features a 43% shorter thread height compared to standard ACME and has a truncated thread profile. It is ideal for applications with space constraints and only has one precision class (corresponds to 2G). Defined in ASTM B1.8, it can be applied to both General and Centralizing ACME threads and uses the same major/minor diameter allowances as G and C series.