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ASME B1.8-1988 (2016) — Stub Acme Screw Threads
⚙️ ASME B1.8-1988 (2016) covers Stub Acme screw threads, including the thread form, basic dimensions of the standard series, tolerances and allowances on major and minor diameters, pitch-diameter allowances, and gage requirements. Stub Acme belongs to the 29° thread-form family along with the General Purpose Acme thread, but its basic depth is reduced from 0.5P to 0.3P, with correspondingly wider flats at crest and root. The shallower depth gives a larger minor diameter and a stronger core, while shortening the cutting length of the thread tooth. It is widely used in actuators, jacks, slides, and machine-tool feed screws where axial space is limited or high load capacity is required. This edition is a revision of ANSI B1.8-1977, approved as an American National Standard on January 11, 1988, and reaffirmed in 2016.
💡 One-line understanding of Stub Acme: where space must be saved, use a shallower trapezoidal tooth to gain a larger core diameter and a shorter machining stroke — load capacity holds while the axial envelope shrinks.
1. Background and Historical Development 📌
National standardization of Acme screw threads in the United States began in 1932, when Subcommittee No. 4 on Acme Threads of Sectional Committee B1 held its first meeting in New York. A survey report by C. W. Bettcher and F. L. Woodcock on the types, sizes, and pitches in use evolved into the draft standard approved as ASA B1.3-1941, which already contained introductory notes and tables for general-purpose screws and nuts, basic dimensions of general-purpose Acme threads with special and standard pitches, basic dimensions of 29° stub threads, three-wire measurements, and basic proportions for modified square threads. In December 1942, to meet the war emergency, the National Aircraft Standards Committee requested that ASA consider a war standard for special Acme threads for aircraft construction. At the London Conference of 1944 it was proposed that a Stub Acme war standard also be drawn up; the March 1946 draft resulted from discussions with British and Canadian experts at the Ottawa Conference of October 1945. Although a war standard was never issued, the January 1948 draft was circulated for comment, the final draft was completed in June 1951, and approval as an American Standard was granted on May 7, 1952. Subsequent minor revisions were approved on May 14, 1973, and May 11, 1977, leading to this 1988 edition.
⚠️ Historical note: Stub Acme was born to serve space-constrained, high-load power transmission such as aircraft structures. If space permits, prefer General Purpose Acme (B1.5) for easier machining and common gages; switch to Stub Acme only when the envelope demands it.
2. Thread Form: Stub Acme vs. General Purpose Acme 🔧
Stub Acme and General Purpose Acme share the same 29° included angle (14.5° half angle); they differ in depth and flat widths:
| Form Element | General Purpose Acme (B1.5 family) | Stub Acme (B1.8) | Effect in Service |
|---|---|---|---|
| Included angle | 29° | 29° | Same radial-to-axial displacement ratio |
| Basic depth | 0.5P | 0.3P | Shallower tooth, larger core diameter |
| Basic height (from sharp vee) | 0.5P | 0.3P | Less cutting, more core strength |
| Crest flat width (basic) | ≈ 0.3707P | ≈ 0.4224P | Wider crest, more even bearing pressure |
| Root flat width (basic) | ≈ 0.3707P | ≈ 0.4224P | Wider space, easier chip relief and fit |
| Typical application | Screws, jacks, general drives | Actuators, space-limited high-load parts | Trade-off of space vs load |
The main body (Section 1, “Specifications for Stub Acme Threads”) provides the design dimensions of the form (Table 2), standard series basic dimensions (Table 3), tolerances and allowances on major and minor diameters (Tables 1 and 4), and pitch-diameter allowances (Table 5), to be used with Figure 1 (Stub Acme form) and Figure 2 (disposition of allowances, tolerances, and crest clearances).
✅ Three engineering benefits of Stub Acme: ① for the same major diameter, a larger minor diameter means higher torsional load capacity; ② the shallow tooth shortens turning/milling strokes and extends tool life; ③ the wide flats reduce contact pressure against the nut and even out wear.
3. Tolerances and Allowances 📏
Stub Acme tolerance allocation likewise centers on the pitch diameter:
- Major- and minor-diameter tolerances (Tables 1 and 4): separate tolerances for each diameter of internal and external threads, keeping crest clearances and contact conditions per design.
- Pitch-diameter allowances and tolerances (Table 5): allowances are tabulated by pitch to accommodate fit and manufacturing scatter; the disposition of allowance, tolerance, and crest clearance is shown in Figure 2.
- Standard series (Table 3): basic dimensions from small to large diameters, covering the common actuator and feed-screw sizes.
⚠️ Unlike V-form threads, Acme-family pitch-diameter allowances and crest/root clearances must be checked together: with only 0.3P depth, any over-generous major-diameter tolerance eats the crest clearance and causes abnormal fit or interference. Always verify the three-layer allocation of allowance + tolerance + clearance.
4. Gage Requirements 🔩
Section 2 of the Standard is devoted to gages for Stub Acme screw threads:
- Functional gages: manufactured to the design form; the GO end checks the maximum-material boundary and the NOT GO end the minimum-material boundary.
- Three-wire measurement: pitch diameter is measured with wires whose best size is derived from the 29° angle and the flat crest/root form — not from a 60° thread formula.
- Crest-clearance checks: dedicated gages or projection verify the clearance allocation between the external major diameter and the internal minor diameter.
🚨 Measurement red line: the 29° angle and the flat-topped, flat-bottomed Stub Acme profile make the three-wire best-size formula entirely different from the 60° thread. Reusing 60° wire tables produces pitch-diameter misjudgments — always use wires derived from the Stub Acme form.
5. Engineering Practice Essentials 🛠️
- ⚙️ Selection decision: choose General Purpose Acme when axial space is ample; choose Stub Acme when space is limited and high load and torque transfer are required.
- 🔩 Material and heat treatment: actuator screws often pair a hardened screw with a bronze nut; verify contact stress and wear life.
- 📏 Lead accuracy control: for positioning duty, check cumulative lead error; consider ground flanks or lapped fits.
- 🔧 Assembly and lubrication: trapezoidal drives need adequate, replenished lubrication to prevent stick-slip and wear; use protective boots in dusty or abrasive environments.
💡 Placed within the B1 family, Stub Acme is the shallow-tooth option that the Acme family (29° included angle) offers for space-versus-load trade-offs. It is not interchangeable with V-form systems such as M/UNJ, so mind the specification conversions when selecting.
6. Frequently Asked Questions ❓
❓ Q: What is the difference between Stub Acme and General Purpose Acme?
A: Both use the 29° included angle, but Stub Acme reduces the basic depth from 0.5P to 0.3P and widens the crest/root flats from about 0.3707P to about 0.4224P. The result is a larger core, higher strength, shorter machining, and more even crest contact — for space-constrained high-load drives.
A: Both use the 29° included angle, but Stub Acme reduces the basic depth from 0.5P to 0.3P and widens the crest/root flats from about 0.3707P to about 0.4224P. The result is a larger core, higher strength, shorter machining, and more even crest contact — for space-constrained high-load drives.
❓ Q: When should Stub Acme be used instead of General Purpose Acme?
A: When axial space is limited yet the load capacity of a large-diameter screw must be kept (actuators, slides, machine-tool feed screws), or when you want to shorten the thread-cutting stroke and reduce tool consumption. For ordinary ample-space drives, General Purpose Acme remains the first choice.
A: When axial space is limited yet the load capacity of a large-diameter screw must be kept (actuators, slides, machine-tool feed screws), or when you want to shorten the thread-cutting stroke and reduce tool consumption. For ordinary ample-space drives, General Purpose Acme remains the first choice.
❓ Q: How are Stub Acme pitch-diameter allowances assigned?
A: Table 5 tabulates basic pitch-diameter allowances by pitch, Tables 1 and 4 give major/minor-diameter tolerances, and Figure 2 shows the disposition of allowance, tolerance, and crest clearance. Verify all three together rather than relying on the pitch-diameter tolerance alone.
A: Table 5 tabulates basic pitch-diameter allowances by pitch, Tables 1 and 4 give major/minor-diameter tolerances, and Figure 2 shows the disposition of allowance, tolerance, and crest clearance. Verify all three together rather than relying on the pitch-diameter tolerance alone.
🔍 Q: What should be watched when measuring Stub Acme by the three-wire method?
A: Use the best-size wires computed for the 29° angle and the flat crest/root profile, and account for the crest flat’s effect on the contact point. The 60°-thread wire tables cannot be reused, or the pitch-diameter readings will be noticeably off.
A: Use the best-size wires computed for the 29° angle and the flat crest/root profile, and account for the crest flat’s effect on the contact point. The 60°-thread wire tables cannot be reused, or the pitch-diameter readings will be noticeably off.
