Acme Power Threads: Geometry, Tolerances and Selection: Stub Acme Fit and Tolerances

Stub Acme Fit and Tolerances

The fit of Stub Acme threads corresponds to the Class 2G General Purpose Acme thread. This means:

• Pitch diameter allowances = same as Class 2G General Purpose (Table 4)
• Pitch diameter tolerances = same as Class 2G General Purpose (Table 5)
• Major and minor diameter allowances = same as General Purpose

For a fit having less backlash, the tolerances and allowances for Classes 3G or 4G General Purpose Acme threads may be used.

Formulas for Determining Stub Acme Diameters

External Threads (Screws):

Internal Threads (Nuts):

Stub Acme Screw Thread Data — Complete Preferred Series

Stub Acme Thread Designations

0.500-20 Stub Acme

• ½-inch major diameter
• 20 threads per inch
• Right hand
• Single thread
• Standard Stub Acme

0.500-20 Stub Acme-LH

• Same as above, but left hand

Alternative Stub Acme Threads: Modified Forms 1 and 2

Since one Stub Acme thread form cannot meet every application, the standard includes two additional modified forms in its appendix. Both use the same tolerances, allowances, and major diameter as the Standard Stub Acme, and both have the same basic thread thickness at the pitch line (0.5P).

Comparison of All Three Stub Acme Forms

What This Means in Practice

• Form 1 (h = 0.375P) threads are deeper than Standard Stub Acme but still shallower than full Acme. The pitch and minor diameters will be smaller than Standard Stub values for the same major diameter.
• Form 2 (h = 0.250P) threads are the shallowest of all Acme variants. The pitch and minor diameters will be larger than Standard Stub values.

Calculating Modified Form Dimensions

To calculate Form 1 or Form 2 dimensions, use the same 12 formulas from Table 13a (Stub Acme), but substitute:

Alternative Stub Acme Designations

0.500-20 Stub Acme M1 — Modified Form 1 thread

0.500-20 Stub Acme M2 — Modified Form 2 thread

Degree Stub Thread: The Legacy Profile

The 60-Degree Stub thread comes from former American Standard B1.3-1941. It uses a 60-degree included angle (like a Unified thread) but with a stub depth, making it a hybrid between the V-thread world and the power-thread world.

When to Consider a 60-Degree Stub

Use this thread when design or operating conditions could be better satisfied by a 60-degree profile than a 29-degree Acme profile. It retains the familiar tooling compatibility of standard V-threads while offering a shallower, more robust form.

Degree Stub Thread Formulas

Important: A clearance of at least 0.02 × pitch is added to the basic depth h to produce extra depth, thus avoiding interference with threads of mating parts at the minor or major diameters.

General Purpose vs. Centralizing vs. Stub: The Decision Matrix

This is the chart the practitioner should have had on his wall. Use it every time you specify an Acme thread:

Measuring Acme Threads: The Three-Wire Method

Accurate pitch diameter measurement on Acme threads requires extra care because of the 29-degree thread angle.

Why the Small Thread Angle Matters

In measuring Acme threads (or any thread with a comparatively small thread angle), it is particularly important to use a formula that compensates for the effect of the lead angle, especially in gage and precision work.

The effect of the lead angle on wire position and the resulting measurement M is much greater in a 29-degree thread than in a 60-degree thread. This occurs because the cotangent of the thread angle increases as the angle becomes smaller. The reduction in thread groove width in the normal plane (caused by the lead angle) pushes a wire of given size higher in the groove of a 29-degree thread than in a 60-degree thread.

For Single-Start Threads (Lead Angle < 5°)

Use the approximate three-wire formula with the best wire size from standard tables.

For Multiple-Start Threads (Lead Angle > 5°)

Use the direct pitch diameter determination formula:

$$E = M - (C + c)$$

Where:

• $E$ = actual pitch diameter
• $M$ = measurement over wires
• $(C + c)$ = constant from the three-wire measurement table

To enter the table, calculate the lead angle:

$$\tan B = \frac{L}{\pi \cdot E_1}$$

Where $L$ = lead and $E_1$ = nominal pitch diameter.

The best wire size is found by taking the value of $w_1$ from the table for lead angle $B$ and dividing by the number of threads per inch.

the practitioner's Resolution: How Two Letters Changed Everything

Six months after his costly mistake, the practitioner had this guide laminated and bolted to a clipboard that hung from every CNC lathe in his shop. His quality checklist now had three mandatory verification points before any Acme thread was cut:

1. Thread type confirmation: General Purpose (G), Centralizing (C), or Stub?

2. Class verification: What class is specified? Is it appropriate for the application?

3. Dimensional cross-check: Do the limiting dimensions from the standard match the programmed toolpath?

The hydraulic press manufacturer not only forgave the mistake—they gave the practitioner a follow-up order. Because now they knew he'd never make that mistake again.

And neither will you.

Quick-Reference: Master Formula Card

Clip this out. Keep it in your toolbox.

General Purpose Acme

Stub Acme

Degree Stub

Tolerance Ratios (All Acme Types)

Allowance Formulas (Pitch Diameter)

Your Next Step

Pull up the last Acme thread you specified or machined. Ask yourself three questions:

1. Did you select the right thread type — General Purpose, Centralizing, or Stub — for the actual application?

2. Did you verify the class against the drawing, or did you assume 2G because it's the default?

3. Could your thread pass a gage check against the limiting dimensions in this guide?

If the answer to any of those is "I'm not sure," you've just found the gap that this guide was built to fill.

Print the Quick-Reference Formula Card. Verify your next thread against the tables. Make this the last time you guess about Acme specifications.

Reference Standards: ASME/ANSI B1.5-1988 (General Purpose and Centralizing Acme Screw Threads) · ASME/ANSI B1.8-1988 R1994 (Stub Acme Screw Threads) · Former ANSI B1.3-1941 (60-Degree Stub Thread)

The Screw That Shut Down a Production Line

the practitioner stared at the CNC lathe's output bin and felt his stomach drop. Forty-seven leadscrew assemblies — every single one from the night shift — failed incoming inspection. The Acme threads on the external screws wouldn't engage smoothly with their mating bronze nuts. Some bound halfway through. Others wobbled with so much backlash the linear stage drifted 0.008 inches under load.

His supervisor wanted answers by noon.

the practitioner knew the threads looked right. The 29-degree angle was correct. The pitch was correct. The major diameter measured within spec. But when the quality engineer pulled the pitch diameter readings, the truth became painfully clear: the pitch diameter allowances had been calculated for a Class 4G fit, but the mating nuts were Class 2G.

That mismatch — a few thousandths of an inch in the wrong direction — scrapped an entire production run worth tens of thousands in materials, labor, and lost delivery time.

This is the guide the practitioner wished he had on his desk that morning. Every allowance, every tolerance, every formula, every table you need to specify, manufacture, and inspect Acme screw threads — pulled directly from ASME/ANSI B1.5-1988 and organized so you can find what you need in seconds.

What You Will Learn in This Guide

• Why tolerances and allowances exist on Acme threads (and the costly consequences of ignoring them)
• The three General Purpose classes (2G, 3G, 4G) and how their tolerance ratios differ
• The three Centralizing classes (2C, 3C, 4C) and their unique major-diameter clearance requirements
• Complete pitch diameter allowance tables for both General Purpose and Centralizing types
• Complete pitch diameter tolerance tables with worked examples showing the two-increment calculation method
• Major and minor diameter tolerances and the formulas behind them
• Stub Acme thread tolerances and how they borrow from General Purpose data
• Multiple-start thread adjustments — the additional allowances most engineers forget
• Step-by-step worked examples calculating every limiting dimension from raw formulas
• A master quick-reference card you can print and tape to the shop wall

The Difference Between Allowance and Tolerance

Before diving into the numbers, you need to lock in two definitions that the entire ASME/ANSI B1.5-1988 standard is built upon:

• Allowance — An intentional, prescribed difference between the basic (nominal) dimension and the closest limit of size for a mating part. It creates the minimum clearance (or in some systems, maximum interference) between mating threads. Think of it as the designed gap that ensures the screw can enter the nut.

• Tolerance — The total permissible variation in a dimension. It defines the range within which the actual manufactured part must fall. Think of it as the manufacturing window — how much the machinist is allowed to deviate from the target.

Allowance sets the starting line. Tolerance defines how far you can drift from it.

On Acme threads, allowances are applied to the pitch diameter of the external thread (reducing it below basic) and to the major and minor diameters (creating crest clearances). Tolerances are then applied away from the mating part — minus on external threads, plus on internal threads — to ensure that every thread within tolerance will assemble with every mating thread within tolerance.

General Purpose Acme Threads: The Complete Tolerance System

The American National Standard ASME/ANSI B1.5-1988 provides three classes of General Purpose Acme threads:

Key principle: External and internal threads of the same class should be used together. Class 2G is the preferred default for general assemblies.

Basic Diameter Relationships

Every Acme thread dimension traces back to three basic diameters:

$$D = \text{Basic Major Diameter (Nominal Size)}$$

$$D_2 = D - h = D - \frac{P}{2} \quad \text{(Basic Pitch Diameter)}$$

$$D_1 = D - 2h = D - P \quad \text{(Basic Minor Diameter)}$$

Where:

• $P = \frac{1}{n}$ (pitch, with $n$ = threads per inch)
• $h = \frac{P}{2}$ (basic thread height for General Purpose)

Thread Form Essentials

Basic Dimensions by Pitch (Table 1)

Note: The "allowance" used in total height ($h_s$) and root flat calculations is 0.020 inch for 10 threads per inch and coarser, and 0.010 inch for finer pitches. This is the major/minor diameter allowance, not the pitch diameter allowance.

Pitch Diameter Allowances: The Critical Clearance Zone

This is where the practitioner's production run failed — and where most Acme thread problems begin.

The pitch diameter allowance is the intentional gap between the maximum pitch diameter of the external thread and the minimum pitch diameter of the internal thread. It creates the minimum backlash or end play in the assembly. The tighter the class, the smaller the allowance, and the less backlash in the finished mechanism.

General Purpose Acme — Pitch Diameter Allowances (Table 4)

ASME/ANSI B1.5-1988. All dimensions in inches. Values apply to any nominal size within the stated range, calculated from the mean of the range.

General formulas for any diameter:

$$\text{Class 2G Allowance} = 0.008\sqrt{D}$$

$$\text{Class 3G Allowance} = 0.006\sqrt{D}$$

$$\text{Class 4G Allowance} = 0.004\sqrt{D}$$

Critical Rule: An increase of 10 percent in the allowance is recommended for each inch, or fraction thereof, that the length of engagement exceeds two diameters.

What the Allowance Ratios Tell You

The relationship between classes follows a consistent 3:2:1 pattern based on the multiplier under the square root:

• 2G → 3G: Multiply the 2G allowance by 0.75 (or 3G = 0.006/0.008 of 2G)
• 2G → 4G: Multiply the 2G allowance by 0.50 (or 4G = 0.004/0.008 of 2G)
• 3G → 4G: Multiply the 3G allowance by approximately 0.667

This means a Class 4G thread has half the designed backlash of a Class 2G thread of the same size.

Pitch Diameter Tolerances: The Two-Increment Method

This is one of the most frequently misunderstood aspects of Acme thread specification. The pitch diameter tolerance is not a single lookup value. It is the sum of two increments — a diameter increment and a pitch increment — found in separate halves of the same table.

How to Calculate Pitch Diameter Tolerance

Step 1: Find the diameter increment from the upper half of Table 5 using the nominal diameter.

Step 2: Find the pitch increment from the lower half of Table 5 using the threads per inch.

Step 3: Add them together.

Worked Example: 1/4-16 ACME-2G

• Diameter increment for D = 1/4 inch, Class 2G: 0.00300
• Pitch increment for n = 16 TPI, Class 2G: 0.00750
• Total pitch diameter tolerance = 0.00300 + 0.00750 = 0.0105 inch

The equivalent thread thickness tolerance = 0.259 × 0.0105 = 0.00272 inch

General Purpose Acme — Pitch Diameter Tolerances (Table 5)

ASME/ANSI B1.5-1988. All dimensions in inches. For a nominal diameter between two tabulated values, use the increment for the larger tabulated diameter.

Diameter Increments:

Pitch Increments:

General formulas:

$$\text{Diameter Increment} = \begin{cases} 0.006\sqrt{D} & \text{Class 2G} \\ 0.0028\sqrt{D} & \text{Class 3G} \\ 0.002\sqrt{D} & \text{Class 4G} \end{cases}$$

$$\text{Pitch Increment} = \begin{cases} 0.030\sqrt{1/n} & \text{Class 2G} \\ 0.014\sqrt{1/n} & \text{Class 3G} \\ 0.010\sqrt{1/n} & \text{Class 4G} \end{cases}$$

Tolerance Class Ratios

The ratios of pitch diameter tolerances for Classes 2G, 3G, and 4G are:

This means a Class 2G tolerance is three times the Class 4G tolerance, and Class 3G is 1.4 times the Class 4G tolerance.

The thread thickness tolerance is always 0.259 × the pitch diameter tolerance for any class.

Major and Minor Diameter Tolerances

While the pitch diameter gets most of the engineering attention, the major and minor diameters have their own tolerance rules that ensure proper crest clearance and thread engagement.

General Purpose Acme — Major & Minor Diameter Rules

Major and Minor Diameter Allowances

The standard creates minimum diametral clearances at both the crest and root of the thread:

• Minor diameter clearance (external thread): Maximum minor diameter of the screw is set 0.020 inch below the basic minor diameter of the nut for 10 TPI and coarser, and 0.010 inch for finer pitches.

• Major diameter clearance (internal thread): Minimum major diameter of the nut is set 0.020 inch above the basic major diameter of the screw for 10 TPI and coarser, and 0.010 inch for finer pitches.

Application of Tolerances — Direction Rules

This is non-negotiable and often the source of errors:

• Internal thread tolerances are PLUS — applied from the minimum size upward (the part can only get bigger than minimum)
• External thread tolerances are MINUS — applied from the maximum size downward (the part can only get smaller than maximum)

This unidirectional system guarantees that every in-tolerance internal thread will assemble with every in-tolerance external thread of the same class.

Given Data

• D = 2.0000 inches (nominal major diameter)
• n = 4 TPI
• P = 1/4 = 0.25000 inches
• h = P/2 = 0.12500 inches
• Class = 2G

Step 1: Look Up Allowances

Pitch diameter allowance (Table 4, size range 1-7/8 to 2-1/8): 0.0113 inch

Major/minor diameter allowance: 0.020 inch (since 4 TPI is coarser than 10 TPI)

Step 2: Calculate Pitch Diameter Tolerance

From Table 5:

• Diameter increment for D = 2 inch, Class 2G: 0.00849
• Pitch increment for n = 4 TPI, Class 2G: 0.01500
• Total = 0.00849 + 0.01500 = 0.02349 inch

Step 3: Calculate All Limiting Dimensions

External Thread (Screw):

Internal Thread (Nut):