Acme Power Threads: Geometry, Tolerances and Selection: Standard Abbreviations

Standard Abbreviations

These abbreviations are mandated for use on drawings, specifications, tools, and gages:

Note: Right-hand threads carry no special suffix—they are assumed right-hand unless explicitly designated otherwise.

Designation Examples

Single-start, right-hand:

1.750-4 ACME-2G

This designates a General Purpose Class 2G Acme thread with:

• 1.750-inch major diameter
• 4 threads per inch
• Single start
• Right hand

Single-start, left-hand:

1.750-4 ACME-2G-LH

Same thread as above, but left hand.

Multiple-start, right-hand:

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

This designates a General Purpose Class 3G Acme thread with:

• 2.875-inch major diameter
• 0.4-inch pitch
• 0.8-inch lead
• Double thread (lead ÷ pitch = 2 starts)
• Right hand

Formulas for Determining Diameters (Table 2a)

This is the formula set that makes everything else in this guide possible. If you're calculating dimensions for a non-standard diameter-pitch combination, this is where you start.

Definitions

• D = Basic Major Diameter and Nominal Size (inches)
• P = Pitch = 1 ÷ n
• E = Basic Pitch Diameter = D − 0.5P
• K = Basic Minor Diameter = D − P

External Threads (Screws)

*If P is between two recommended pitches in Table 3, use the coarser of the two in this formula.

Internal Threads (Nuts)

Key Rules for Diameters

Basic Diameters:

• The maximum major diameter of the external thread is basic (= nominal major diameter) for all classes
• The minimum pitch diameter of the internal thread is basic (= D − h)
• The basic minor diameter is the minimum minor diameter of the internal thread (= D − 2h)

Length of Engagement: Tolerances in this standard apply to engagement lengths not exceeding twice the nominal major diameter. For longer engagements, increase the allowance by 10% for each inch (or fraction) that the engagement exceeds 2D.

Major and Minor Diameter Tolerances and Allowances

Diameter Tolerances

Diameter Allowances (Clearances)

At the minor diameter of external threads: The maximum minor diameter is set 0.020 inch below the basic minor diameter of the nut for ≤10 TPI, and 0.010 inch below for finer pitches.

At the major diameter of internal threads: The minimum major diameter is set 0.020 inch above the basic major diameter of the screw for ≤10 TPI, and 0.010 inch above for finer pitches.

Application of Tolerances

This is where many machinists get tripped up:

• Internal thread tolerances are PLUS — applied from minimum sizes upward
• External thread tolerances are MINUS — applied from maximum sizes downward
• Pitch diameter tolerance = thread thickness tolerance ÷ 0.259 (or equivalently, thread thickness tolerance = 0.259 × pitch diameter tolerance)

Pitch Diameter Tolerances (Table 5)

The pitch diameter tolerance for any specific thread is found by adding two values from this table:

1. The diameter increment (based on the nominal diameter)
2. The pitch increment (based on the threads per inch)

Diameter Increment

General Formula for Diameter Increment: 0.006√D (Class 2G), 0.0028√D (Class 3G), 0.002√D (Class 4G)

Pitch Increment

General Formula for Pitch Increment: 0.030√(1/n) (Class 2G), 0.014√(1/n) (Class 3G), 0.010√(1/n) (Class 4G)

Worked Example

Problem: Find the pitch diameter tolerance for a 1/4-16 ACME-2G thread.

Step 1: Diameter increment for D = 1/4 inch, Class 2G = 0.00300

Step 2: Pitch increment for n = 16 TPI, Class 2G = 0.00750

Step 3: Total pitch diameter tolerance = 0.00300 + 0.00750 = 0.0105 inch

Step 4: Equivalent thread thickness tolerance = 0.259 × 0.0105 = 0.00272 inch

Rule: For a nominal diameter between two tabulated values, use the diameter increment for the larger of the two.

Recommended Thread Series (Table 3)

The standard recommends these specific diameter-pitch combinations to minimize the inventory of tools and gages. Use these whenever possible.

*Shear Area is per inch of engagement length at the minor diameter crests of the internal thread (minimum values based on max D₁ and min d₂). Stress Area is the minimum based on the mean of minimum minor and pitch diameters of the external thread. Both are for Class 3G.

Limiting Dimensions (Table 2b) — The Numbers That Matter on the Shop Floor

This is the table you'll have taped to the side of your lathe. These are the actual maximum and minimum diameters for every size in the recommended series, broken out by class.

Sizes 1/4 through 1-3/8

Sizes 1-1/2 through 5

Stress Area and Shear Area Calculations

When you're designing a lead screw assembly, you need to know whether the thread section can handle the load. These formulas give you the tools.

Stress Area of General Purpose Acme Threads

For computing the tensile strength of the thread section, use the minimum stress area based on the mean of the minimum pitch diameter and the minimum minor diameter of the external thread:

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

Where:

• d₂ min = minimum pitch diameter of the external thread (from Formula 4 or Table 2b)
• d₁ max = maximum minor diameter of the external thread (from Formula 5 or Table 2b—note this is labeled "max" because it's the least removed from basic)

Shear Area of General Purpose Acme Threads

For computing the shear area per inch of engagement length of the external thread:

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

Where:

• D₁ max = maximum minor diameter of the internal thread (from Formula 12 or Table 2b)
• D₂ min = minimum pitch diameter of the external thread (from Formula 4 or Table 2b)
• n = threads per inch
• 14.5° = half the 29-degree thread angle

Multiple-Start Acme Threads

The tabulated data in Tables 2b through 5 are for single-start threads. However, multiple-start threads are frequently used when fast traversing motion is required.

Key Rules for Multi-Start Threads

Two-start, Class 2G: The tabulated data may often be used as-is, but generally requires reduction of working tolerances to provide greater clearance for satisfactory assembly.

Three or more starts, or classes tighter than 2G: Additional allowances and/or increased tolerances are needed.

Recommended Internal Thread Allowances for Multi-Start

Use the allowances from Table 4 for all external threads. For internal threads, apply the following percentages of the Table 4 allowances (columns 3, 4, and 5):

Example: Additional Clearances

For a 0.25-16 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

For multi-start threads with more than four starts, the 100 percent allowance is generally adequate since index spacing variables would typically be no greater than on a four-start thread.

the practitioner's Acme Thread Decision Matrix

Step 1: Read the print designation.

Step 2: Look up the correct tolerance class.

Step 3: Pull limiting dimensions from Table 2b for the specific size and class.

Step 4: Verify pitch diameter tolerance by calculating from Table 5 (diameter increment + pitch increment).

Step 5: Set up and cut. Measure pitch diameter first—it's the critical dimension.

Since implementing this system, the practitioner's shop has had zero Acme thread rejections across three shifts. The matrix cost nothing to make. The knowledge behind it was already in the standard—it just needed to be organized for the people actually holding the wrenches and pushing the buttons.

Quick-Reference: Master Formula Card

Cut this section out and laminate it. Tape it to the side of your lathe. Save it to your phone. Whatever you do, don't lose it.

Basic Geometry

Allowance Rules

Tolerance Rules

Class Tolerance Ratios

Tolerance Direction

Engineering takeaway

the practitioner didn't become a better machinist overnight. He became a better systems builder. The difference between a shop that produces perfect Acme threads and one that doesn't isn't talent—it's access to the right information at the right time.

The data in this guide isn't secret. It's been sitting in the ASME/ANSI B1.5-1988 standard since 1988. But how many machinists have actually read it cover to cover? How many engineers have walked the formulas backward to understand why a Class 4G thread has one-third the pitch diameter tolerance of a Class 2G?

Now you have.

Every table. Every formula. Every class. Every dimension from 1/4-inch to 5-inch diameter. Organized, cross-referenced, and ready to use.

Your Next Step

Print the Master Formula Card from the section above and post it where you work. Then pick one Acme thread from the Recommended Series table and manually calculate every limiting dimension using the formulas from Table 2a and the allowances from Tables 4 and 5. Check your answers against Table 2b.

If your numbers match—you own this standard. If they don't—you just found your knowledge gap before it became a rejection slip.

What's the most common Acme thread size in your shop? Have you ever verified that the dimensions on the floor match the standard? That's worth investigating before your next production run.

All dimensional data in this guide is extracted from ASME/ANSI B1.5-1988, American National Standard for Acme Screw Threads. This guide is intended as a technical reference and does not replace the official standard for contractual or legal purposes. All dimensions are in inches unless otherwise noted.