Acme Power Threads: Geometry, Tolerances and Selection: What Makes an Acme Thread an "Acme" Thread?

Context and scope

Standard Reference: ASME/ANSI B1.5-1988 (General Purpose & Centralizing) | ASME/ANSI B1.8-1988 R1994 (Stub Acme)

Thread Angle: 29°

Thread Types Covered: General Purpose (2G, 3G, 4G) · Centralizing (2C, 3C, 4C) · Stub Acme · Alternative Stub Acme (Modified Forms 1 & 2) · 60-Degree Stub

What Makes an Acme Thread an "Acme" Thread?

Before diving into the subspecies of the Acme family, you need to understand what separates these threads from the 60-degree Unified threads you see on bolts and nuts.

The defining feature is the 29-degree included angle.

Where a standard UNC/UNF bolt uses a 60-degree V-thread optimized for clamping force, the Acme thread uses a wider, trapezoidal profile optimized for power transmission and linear motion. That 29-degree angle creates a thread that:

• Transmits axial force efficiently — less radial "bursting" force on the nut
• Is easier to machine — wider flats at crest and root accept standard tooling
• Tolerates wear better — the trapezoidal form maintains function as surfaces erode
• Allows split-nut engagement — critical for lathe leadscrews and quick-disengage mechanisms

The line bisecting the 29-degree angle is perpendicular to the axis of the screw thread. This symmetry means Acme threads transmit force equally in both directions—unlike buttress threads, which are asymmetric by design.

The Acme Thread Family Tree: Five Variations, One Standard

Here's your first decision matrix. Every Acme thread you'll ever encounter falls into one of these five categories:

Let's break each one down completely.

General Purpose Acme Threads: The Workhorse

This is the thread you'll encounter most often. If a drawing says "ACME" without any centralizing callout, it's a General Purpose thread.

Three Classes, Three Levels of Precision

Class 5G exists in the standard but is not recommended for new designs.

The key principle: use external and internal threads of the same class together. Class 2G is the preferred starting point for any general purpose assembly. Only step up to 3G or 4G when backlash control is a documented requirement.

The Basic Formulas — Your Foundation

Every dimension in the General Purpose Acme system derives from these relationships:

Allowances: The Invisible Clearance That Makes Everything Work

Allowances are the intentional gaps between mating parts that ensure free assembly. Get these wrong, and threads either bind or wobble.

Minor Diameter Allowance:

• Maximum minor diameter of external thread is set 0.020 inch below the basic minor diameter for 10 TPI and coarser
• For finer than 10 TPI, the allowance is 0.010 inch

Major Diameter Allowance:

• Minimum major diameter of internal thread is set 0.020 inch above basic for 10 TPI and coarser
• For finer than 10 TPI, the allowance is 0.010 inch

Diameter Tolerances: Where Precision Lives

Major Diameter Tolerance (External Thread): $0.05P$ (minimum 0.005 inch)

Major Diameter Tolerance (Internal Thread): 0.020 inch for 10 TPI and coarser; 0.010 inch for finer pitches

Minor Diameter Tolerance (External Thread): $1.5 \times \text{pitch diameter tolerance}$

Minor Diameter Tolerance (Internal Thread): $0.05P$ (minimum 0.005 inch)

Thread Thickness Tolerance: $0.259 \times \text{pitch diameter tolerance}$

How Tolerances Are Applied

This is where mistakes happen. Pay attention:

• Internal thread tolerances are PLUS — applied from minimum sizes upward
• External thread tolerances are MINUS — applied from maximum sizes downward
• Pitch diameter tolerances for external and internal threads of a given class are the same

Length of Engagement Rule

All tolerances in the standard are valid for engagement lengths not exceeding twice the nominal major diameter. If your engagement length exceeds $2D$, increase the pitch diameter allowance by 10 percent for each inch (or fraction thereof) beyond that limit.

General Purpose Acme Thread Form — Basic Dimensions

The following table gives the complete basic dimensions for every standard pitch per ASME/ANSI B1.5-1988.

Note: Allowance is 0.020 inch for 10 TPI and coarser, 0.010 inch for finer pitches. All dimensions in inches.

Single-Start General Purpose Acme Screw Thread Data

This is the master reference table. It gives you basic diameters, thread data, shear areas, and stress areas for every standard size in the preferred series.

Sizes ¼" Through 1⅜"

Sizes 1½" Through 5"

Shear Area = per inch length of engagement of the external thread in line with the minor diameter crests of the internal thread (minimum, based on max D₁ and min d₂, Class 3G).

Stress Area = minimum stress area based on mean of minimum minor and pitch diameters of external thread (Class 3G).

Stress and Shear Area Formulas

When you're designing a power screw, you need to know whether the thread section will survive the load. These are the formulas from ASME/ANSI B1.5-1988:

Stress Area (Tensile Strength)

$$\text{Stress Area} = \frac{\pi}{4} \left( \frac{d_{2\,\text{min}} + d_{1\,\text{max}}}{2} \right)^2$$

Where:

• $d_{2\,\text{min}}$ = minimum pitch diameter of external thread
• $d_{1\,\text{max}}$ = maximum minor diameter of external thread

Shear Area (Per Inch of Engagement)

$$\text{Shear Area} = \pi \cdot D_{1\,\text{max}} \left[ 0.5n + \tan 14.5° \left( D_{2\,\text{min}} - D_{1\,\text{max}} \right) \right]$$

Where:

• $D_{1\,\text{max}}$ = maximum minor diameter of internal thread
• $D_{2\,\text{min}}$ = minimum pitch diameter of external thread
• $n$ = threads per inch

Pitch Diameter Allowances — General Purpose Acme

These allowances are applied to the pitch diameter of external threads. They represent the gap between the maximum external pitch diameter and the basic (minimum internal) pitch diameter.

Critical: Increase the allowance by 10% for each inch (or fraction thereof) that the engagement length exceeds 2× the nominal major diameter.

Note: Class 2G allowances also apply to American National Standard Stub Acme threads (ASME/ANSI B1.8-1988).

Pitch Diameter Tolerances — General Purpose Acme

Pitch diameter tolerance is calculated by adding the diameter increment to the pitch increment from the table below.

Example: A ¼-16 ACME-2G thread has a pitch diameter tolerance of 0.00300 (diameter increment) + 0.00750 (pitch increment) = 0.0105 inch.

The equivalent tolerance on thread thickness is 0.259 × pitch diameter tolerance.

Diameter Increments

Pitch Increments

General Formulas for Tolerance Calculation

Tolerance ratio between classes: 2G : 3G : 4G = 3.0 : 1.4 : 1.0

Acme Thread Designations: Reading the Code

Every Acme thread designation tells a complete story. Here's how to decode it:

General Purpose Designation Examples

1.750-4 ACME-2G

• 1.750-inch major diameter
• 4 threads per inch
• Single thread (single-start)
• Right hand
• Class 2G General Purpose

1.750-4 ACME-2G-LH

• Same as above, but left hand

2.875-0.4P-0.8L-ACME-3G

• 2.875-inch major diameter
• 0.4-inch pitch
• 0.8-inch lead
• Double thread (two-start: lead ÷ pitch = 2)
• Right hand
• Class 3G General Purpose

Standard Abbreviations

Multiple-Start Acme Threads: When Speed Matters

When you need fast linear traversal—think a quick-acting clamp or a multi-start leadscrew—multiple-start threads are the answer. But they come with tolerance complications that catch many engineers off guard.

The Core Problem with Multi-Start Threads

The standard tabulated diameter-pitch data with allowances and tolerances relate to single-start threads only. These values can be (and often are) used for two-start Class 2G threads, but this generally requires reduction of the full working tolerances to provide a greater allowance or clearance zone.

When the class requires smaller working tolerances than 2G, or when 3, 4, or more starts are needed, additional allowances or increased tolerances (or both) may be needed.

Additional Allowances for Multi-Start Internal Threads

The standard recommends applying the allowances from the pitch diameter allowance table (Table 4) to all external threads, and then applying allowances to internal threads in these ratios:

Practical Example: What This Means in Real Numbers

For a 0.250-16 ACME-2G thread:

• 2-start: 0.002 inch additional clearance
• 3-start: 0.003 inch additional clearance
• 4-start: 0.004 inch additional clearance

For a 5-2 ACME-3G thread:

• 2-start: 0.0091 inch additional clearance
• 3-start: 0.0136 inch additional clearance
• 4-start: 0.0181 inch additional clearance

GO thread plug gages and taps would be increased by these same values. To maintain the same working tolerances on multi-start threads, the pitch diameter of the NOT GO thread plug gage would also be increased by these same values.

Gage Designation for Multi-Start Threads

When specifying gages or tools, the designation covers allowance requirements like this:

GO and NOT GO thread plug gages for: 2.875-0.4P-0.8L-ACME-2G with 50 per cent of the 4G internal thread allowance

These percentages can also be applied to Class 4G allowances used on Class 3G internally threaded product. The standard does not recommend any changes to externally threaded products.

The Lesson the practitioner Learned: Centralizing Acme Threads

This is where the practitioner's story gets its resolution—and where your understanding of the Acme family takes a critical leap forward.

What Centralizing Means (and Why It Matters)

Centralizing Acme threads have limited clearance at the major diameters of internal and external threads. This creates a bearing surface at the major diameters that:

• Maintains approximate alignment of the thread axis
• Prevents wedging on the flanks of the thread
• Keeps the screw centered in the nut under load

Think of valve stems, precision positioning screws, and any application where the screw must track straight without relying on external bearings to prevent wobble.

Three Classes of Centralizing Threads

Classes 5C and 6C exist but are not recommended for new designs.

The Centralizing Requirement Formula

For a centralizing fit to work, this condition must be satisfied:

$$(\text{Major Dia. Tolerance}_{\text{internal}}) + (\text{Major Dia. Allowance}_{\text{internal}}) + (\text{Major Dia. Tolerance}_{\text{external}}) \leq \text{Pitch Dia. Allowance}_{\text{external}}$$

This is why a Class 2C external thread (which has a larger pitch diameter allowance) can be used interchangeably with a Class 2C, 3C, or 4C internal thread and still fulfill the centralizing requirement.

Basic Formulas for Centralizing Acme Threads

Key difference from General Purpose: The internal thread crest flat formula adds the minor diameter allowance term (using a plus sign) rather than subtracting a major diameter allowance. This is what creates the tighter fit at the major diameter.

Basic Diameters

The basic diameters for Centralizing Acme threads are identical to General Purpose:

• $D_2 = D - 0.5P$
• $D_1 = D - P$

But the minimum minor diameter of the internal thread is set 0.1P above basic (not the same as General Purpose).

Centralizing Acme Thread Data — Complete Preferred Series

Centralizing Acme Pitch Diameter Tolerances

Calculated exactly the same way as General Purpose — add the diameter increment to the pitch increment. The tolerance ratios for Classes 2C, 3C, and 4C are identical: 3.0 : 1.4 : 1.0.

Designation of Centralizing Acme Threads

1.750-6-ACME-4C

• Centralizing Class 4C
• 1.750-inch major diameter
• 0.1667-inch pitch
• Single thread, right-hand

1.750-6-ACME-4C-LH

• Same as above, but left-hand

2.875-0.4P-0.8L-ACME-3C (Two Start)

• Centralizing Class 3C
• 2.875-inch major diameter
• 0.4-inch pitch
• 0.8-inch lead
• Double thread (two-start), right-hand

2.500-0.3333P-0.6667L-ACME-4C (Two Start)

• Centralizing Class 4C
• 2.500-inch nominal major diameter
• 0.3333-inch pitch
• 0.6667-inch lead
• Double thread, right-hand

Acme Centralizing Thread — Alternative Series (Minor Diameter Control)

When Acme centralizing threads are produced in single units or very small quantities, particularly in sizes larger than the range of commercial taps and dies, and when the manufacturing process uses cutting tools such as lathe cutting, there's an economically superior alternative.

Why Use Minor Diameter Centralizing?

The standard centralizing method controls alignment at the major diameters. The alternative series controls alignment at the minor diameters. The two advantages:

1. Easier and faster inspection. It is much easier to measure the minor diameter (root) of the external thread and the mating minor diameter (crest or bore) of the internal thread than it is to determine the major diameter (root) of the internal thread.

2. Better manufacturing control. Greater ease of checking translates directly to tighter control of machined size.

How to Implement Minor Diameter Centralizing

If minor diameter centralizing is necessary, recalculate all thread dimensions, reversing major and minor diameter allowances, tolerances, radii, and chamfer. The basic formulas remain the same; you're simply swapping where the centralizing control occurs.

Stub Acme Threads: When Full Depth Is Too Deep

Now meet the thread that exists because sometimes metallurgy and mechanics fight each other.

Why Stub Acme Exists

Stub Acme threads per ASME/ANSI B1.8-1988 (R1994) are designed for unusual applications where, due to mechanical or metallurgical considerations, a coarse-pitch thread of shallow depth is required.

Think about this: you need a large-pitch thread for fast travel, but the material is hardened steel that chips at full thread depth. Or the wall thickness of a tube limits how deep you can cut. Stub Acme solves this by cutting the thread height to 60% of a standard Acme thread.

The Key Difference: Thread Height

The thread thickness at the pitch line is the same (0.5P), but the shallower depth means wider flats at crest and root.

Stub Acme Basic Dimensions

Allowance: 0.020 inch for 10 TPI and coarser; 0.010 inch for finer pitches.