Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
ASME B1.15-1995 — Unified Inch Screw Threads (UNJ Thread Form)
✈️ ASME B1.15-1995 covers Unified Inch screw threads of the UNJ thread form. The UNJ form is the UN thread form modified to 9/16 H (75% h) basic thread depth, which permits a controlled root radius between 0.15011P and 0.18042P in the external thread, while the minor diameter of the mating internal thread is increased to ensure the necessary clearance. The form was introduced for applications requiring high fatigue strength where working stress levels are high, to minimize size and weight in parts — as in aerospace applications and in commercial products where stresses are critical. The Standard is similar to Military Specification MIL-S-8879 and is equivalent to ISO-3161-1977 for thread Classes 3A and 3B; British Standard BS 4084:1978 (with Amendment 1) is technically identical to ISO-3161-1977 except for its Appendix A. The Standard was approved as an ASME Standard on June 22, 1993, and issued on August 15, 1995.
💡 The soul of UNJ is the external root radius: a continuous circular arc as large as the design form allows, minimizing stress concentration — a design element proven by tension-tension fatigue testing of aircraft fasteners in 1942.
1. Thread Form and Geometric Basis 📐
The Standard uses the Unified thread symbol system: H is the height of the sharp vee thread formed by extending the flanks until they meet, H = 0.866025P; the symbol h equals 0.75H and represents the theoretical height of the American National thread form, still used as the basis for “percent of thread.” The UNJ form is the UN form modified to 9/16 H (75% h) basic depth, which allows the 0.18042P maximum root radius in the external thread; the internal thread minor diameter is enlarged to clear the flank tangency point.
Tension-tension fatigue testing in 1942 isolated four elements of good external thread root design: the root should be radiused, not sharp; theoretically it should be a continuous circular arc blending smoothly with the flanks; the radius should be as large as possible within the allowable design form; and the root contour should be smooth throughout, free of imperfections, tool marks, or other minor notches.
| Geometric Element | UN Form (B1.1) | UNJ Form (B1.15) | Design Intent |
|---|---|---|---|
| Basic thread depth | 5/8 H class (about 62.5% h typical internal) | 9/16 H (75% h) | Leaves room for the root radius |
| External root radius | Up to about 0.14434P (radius type) | 0.15011P–0.18042P (controlled) | Lowers stress concentration, raises fatigue life |
| Internal minor diameter | Conventional chip-clearance design | Enlarged to clear the maximum root radius | Guarantees clearance at flank tangency |
| Typical application | General mechanical fastening | Aerospace, high-stress commercial parts | Minimum size and weight, fatigue resistance |
⚠️ The symbol h equals 0.75H and remains the basis for “percent of thread.” A 75% thread in engineering documents means this value; do not confuse the UNJ 75% h basic depth with actual tapped internal-thread depths, which typically run from 60% to 68% h.
2. Classes and Field of Application 🏷️
B1.15 covers Classes 2A/2B and 3A/3B. Specifically:
- 3A/3B: meet the requirements of Military Specification MIL-S-8879 and are the recognized classes for aerospace applications, with the tightest tolerances and precise form control.
- 2A/2B: appropriate for commercial applications commensurate with the fatigue and stress levels required; the Standard does not recommend 2A/2B for aerospace use.
The body of the Standard also covers: profile of thread (basic profile, design profiles, root radius), series of threads, thread characteristics (length of thread engagement LE, tolerance system, limits of size, thread form tolerances), dimensional accommodation of coatings and platings (coated external threads Class 3A, dry-film lubricant coatings, coated external Class 2A, coated internal Classes 2B and 3B), and designation rules (basic designation, standard series designation, coated threads, modified crests, special length, and special series).
🚨 Class red line: aerospace fastening systems may use only 3A/3B. Although 2A/2B appear in B1.15, the Standard explicitly does not recommend them for aerospace — neither fatigue life nor assembly control meets MIL-S-8879.
3. Historical Development: From “NR” to “Hi R” to UNJ 📖
The first known U.S. standard of similar form was SAE Aeronautical Standard AS 82, published in March 1942 — a modified American National form at 75% h basic depth with a 0.108P–0.180P root radius, symbolized NR (National Round), developed for aircraft-engine applications. In April 1952 the military published MIL-B-7838A, a bolt procurement specification for aircraft based on the Unified form at 83⅓% h for 160,000 psi tensile bolts. Because production experience showed tapped internal threads generally run between 60% and 68% h (rarely 70%), the aerospace fastener industry in 1955 raised the external root radius to 0.150P minimum and 0.180P maximum for bolts of 180,000 psi and higher tensile strength — the “Hi R” form. In September 1960, working with SAE Committee E25 and the AIA National Aerospace Standards Committee, the Air Force and Navy published MIL-S-8879 featuring the Hi R root radius and an internal thread modified to 9/16 H (75% h). With the advent of aircraft gas turbine engines, high-temperature threaded fasteners using the MIL-S-8879 UNJ root radius showed greatly improved stress-rupture life.
💡 Except for electrical hardware and thread sizes 0.138 in. and smaller (which may use the UN form), the UNJ form has been adopted by the aerospace industry as the all-purpose thread standard — note this exception clause in design and procurement.
4. Dimensional Accommodation of Coatings and Platings 🔧
The Standard devotes a full chapter to coated/plated threads. The core idea is that coatings change the effective size of the thread, so the design form and tolerances must reserve space. Typical provisions:
- Coated external threads, Class 3A: reserve tolerance position so the coated functional size remains within 3A limits.
- Dry-film lubricant coatings: the coating process affects effective size and must be included in tolerance allocation.
- Coated external threads, Class 2A: reserved for commercial practice.
- Coated internal threads, Classes 2B and 3B: reserve for internal coatings by the corresponding class.
The designation rules make coated threads identifiable, avoiding ambiguity between nominal size and post-coating size at acceptance.
⚠️ Plating trap: if zinc or cadmium plating is not reserved in the tolerance allocation, a plated external thread can exceed the maximum-material boundary and fail to assemble, or a plated internal thread can drop below minimum material and interfere. Chapter 5 of B1.15 exists precisely for this.
5. Engineering Practice Essentials 🛠️
- ✈️ Aerospace selection: always specify 3A/3B with conformance to MIL-S-8879; the external root radius must fall within 0.15011P–0.18042P.
- 📏 Fatigue review: the root must be free of tool marks, rolling flakes, or any minor notches; rolled threads are preferred over cut threads for the compressive-strengthening effect.
- 🔧 Engagement length LE: group engagement lengths per para. 4.1 and select tolerance widths accordingly; short engagements need tighter tolerances.
- 🔩 Mixing with UN: UNJ external threads and UN internal threads do not match by design (different root/depth systems); keep one thread form throughout a joint.
✅ One-line selection: high stress, fatigue resistance, and weight reduction first → UNJ 3A/3B; general commercial fastening → UNJ 2A/2B or conventional UN threads.
6. Frequently Asked Questions ❓
❓ Q: What is the fundamental difference between UNJ and UN threads?
A: UNJ changes the external basic depth to 9/16 H (75% h) and mandates a controlled root radius of 0.15011P–0.18042P, with the internal minor diameter enlarged accordingly. The UN form does not force this radius range, so stress concentration and fatigue life differ markedly.
A: UNJ changes the external basic depth to 9/16 H (75% h) and mandates a controlled root radius of 0.15011P–0.18042P, with the internal minor diameter enlarged accordingly. The UN form does not force this radius range, so stress concentration and fatigue life differ markedly.
❓ Q: Why can 2A/2B not be used in aerospace?
A: Only Classes 3A/3B satisfy MIL-S-8879’s requirements on form, root radius, and tolerances. The wider tolerance bands and looser form control of 2A/2B cannot deliver the fatigue and assembly precision aerospace fasteners need, and the Standard says so explicitly.
A: Only Classes 3A/3B satisfy MIL-S-8879’s requirements on form, root radius, and tolerances. The wider tolerance bands and looser form control of 2A/2B cannot deliver the fatigue and assembly precision aerospace fasteners need, and the Standard says so explicitly.
❓ Q: Why is the internal minor diameter enlarged?
A: The external maximum root radius of 0.18042P needs space; tapping a conventional minor diameter would make the radius interfere at the flank tangency point. Enlarging the internal minor diameter secures clearance there, while actual internal depths commonly remain at 60%–68% h.
A: The external maximum root radius of 0.18042P needs space; tapping a conventional minor diameter would make the radius interfere at the flank tangency point. Enlarging the internal minor diameter secures clearance there, while actual internal depths commonly remain at 60%–68% h.
🔍 Q: How are plated threads handled dimensionally?
A: Follow Chapter 5: reserve tolerance position for the coating — external threads by Class 3A (or 2A), dry-film lubricant cases, and internal threads by Classes 2B/3B — and mark coated threads per the designation rules; acceptance is based on the coated functional size.
A: Follow Chapter 5: reserve tolerance position for the coating — external threads by Class 3A (or 2A), dry-film lubricant cases, and internal threads by Classes 2B/3B — and mark coated threads per the designation rules; acceptance is based on the coated functional size.
