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ASME B18.3-2012 — Socket Cap, Shoulder, Set Screws, and Hex Keys (Inch Series)
🔧 ASME B18.3-2012 is one of the foundational fastener standards in the U.S. National Standard system, tracing its ancestry to ASA B18.3 approved in 1936. It covers complete general and dimensional data for hexagon socket head cap screws, shoulder screws, set screws, and hexagon keys recognized as an American National Standard, in the inch series. Appendices provide specifications for hexagon socket gages and gaging (Mandatory Appendix I), protrusion gaging of flat countersunk heads (Mandatory Appendix II), Type VI recess dimensions (Mandatory Appendix III), and, in the nonmandatory appendices, drill and counterbore sizes (Appendix A), formulas for dimensions (Appendix B), and the 1936-series cap screws (Appendix C). Where questions arise concerning product acceptance, the dimensions in the tables govern over recalculation by formula.
💡 Socket screws are driven through a hexagon recess in the head by a hex key, allowing high tightening torque in tight spaces with the head fully recessed in a counterbore — the standard interface fastener of machine tools, dies, aerospace, and racing.
1. Product Family and Scope 📌
The Standard covers the following product family, with dimensions given in Tables 1 through 18:
- Hexagon socket head cap screws: flat chamfered top, smooth or knurled cylindrical sides, flat bearing surface (Tables 1–7); shank straightness per Table 6.
- Drilled hexagon socket head cap screws: 2, 4, or 6 holes drilled in the head for safety-wire applications (Table 7), common in aerospace.
- Socket flat countersunk head cap screws: conical bearing surface of approximately 82 deg for flush installation (Tables 8–10).
- Socket button head cap screws: low rounded top with large flat bearing surface (Table 11).
- Socket low head cap screws: reduced head height and smaller socket for height-clearance-limited applications (Table 12).
- Socket head shoulder screws: precision ground shoulder serving as a locating journal (Table 13) — die bolts / stripper bolts.
- Hexagon socket set screws: cone, cup, flat, half-dog, and full-dog points (Tables 14–16).
- Hexagon keys and bits (Table 17) and hexagon sockets (Table 18).
⚠️ The Standard provides general and dimensional data; material, hardness, and heat-treatment requirements are established by referenced material specifications. Table dimensions govern acceptance — do not substitute formula-recalculated values.
2. Socket Head Cap Screws and Key Sizes 🔩
Table 1 gives head diameter, head height, body diameter, socket size, and related dimensions for each size of socket head cap screw. The table below lists common sizes with the corresponding hex socket (key) size across the flats and typical applications.
| Nominal Size | Thread | Hex Key (across flats) | Typical Applications |
|---|---|---|---|
| #4 | 4-40 UNF | 1/16 in | Precision instruments, small mechanisms |
| #6 | 6-32 UNC | 5/64 in | Electronics, general mechanisms |
| #8 | 8-32 UNC | 3/32 in | General machinery, tooling and fixtures |
| #10 | 10-24 UNC | 1/8 in | General machinery, hydraulic components |
| 1/4 | 1/4-20 UNC | 3/16 in | Machine tools, dies, engines |
| 5/16 | 5/16-18 UNC | 1/4 in | Machine ways, heavy fixtures |
| 3/8 | 3/8-16 UNC | 5/16 in | Heavy machinery, pressure equipment |
| 1/2 | 1/2-13 UNC | 3/8 in | Heavy structural connections |
💡 Use accurately sized, well-finished hex keys: an undersized key slips and rounds the socket. Periodically verify keys and finished sockets with the hexagon socket gage system of Mandatory Appendix I.
3. Set Screw Point Styles 🎯
Set screws fix hubs, gears, handles, and similar parts onto shafts. Table 14 gives dimensions, Figure 1 shows cup point variations, and Table 16 gives optional cup point dimensions. The point style controls locating accuracy and holding power:
| Point Style | Characteristics | Typical Use |
|---|---|---|
| Cone point | Pointed end for precise location | Precise hub-to-shaft location |
| Cup point | Cup-shaped end bites into the surface | General locking of gears and sprockets |
| Flat point | Flat end does not mar the surface | Light-duty, frequent adjustment |
| Half-dog point | Short cylindrical end fits a shaft hole | Torque transmission and location |
| Full-dog point | Long cylindrical end, deep engagement | Positive location under heavy load |
🚨 Under vibration and shock, set screws require positive locking — cup point bite-in, dog point engagement into a shaft hole, or thread-locking media. A plain flat point pressing on a bare shaft will loosen readily under strong vibration.
4. Type VI Recess and Gaging 📏
Mandatory Appendix III specifies the dimensions of the Type VI (hexalobular-style) recess — a six-lobe socket that reduces cam-out at high torque, improves tool life, and suits automated assembly. Mandatory Appendix I specifies hexagon socket gages and gaging (covering forged and broached sockets), and Mandatory Appendix II specifies protrusion gaging of flat countersunk heads. Nonmandatory Appendix A lists drill and counterbore sizes for socket head cap screws, Appendix B gives the formulas used for dimensional data, and Appendix C tabulates the 1936-series cap screws for maintenance of legacy products.
⚠️ The Type VI recess defined here is not identical in geometry to the ISO hexalobular (Torx®-style) drive used on many metric products. Always state the governing standard and appendix when ordering to avoid tool mismatch and stripped sockets.
5. Engineering Practice Essentials 🛠️
- 🔍 Selection: choose the head style by load and space (standard / low head / button / flat countersunk), then size by diameter, pitch, and length, and finally match the hex key or bit size.
- 🔧 Tightening: observe wrench orientation and insertion depth, maintaining the minimum key engagement of Table 15 for short set screws; use a bit with a torque wrench for high-torque applications.
- 📊 Locking: in vibration service, use drilled-head screws (Table 7) with safety wire, or add adhesive/nylon-patch locking aids.
- 📋 Acceptance: gage sockets per Mandatory Appendix I; check flat countersunk head protrusion per Mandatory Appendix II.
6. Frequently Asked Questions ❓
❓ Q: Socket screws or regular hex bolts?
A: Socket screws sink into counterbores, occupy less space, and allow high torque in confined areas — ideal for precision and compact assemblies; hex bolts suit large sizes, rapid assembly, and universally available tooling.
A: Socket screws sink into counterbores, occupy less space, and allow high torque in confined areas — ideal for precision and compact assemblies; hex bolts suit large sizes, rapid assembly, and universally available tooling.
🔍 Q: Where are shoulder screws (die bolts) used?
A: The precision ground shoulder acts as a locating journal for rotating or reciprocating parts (e.g., die stripper plates, cam followers); the thread only secures, while the shoulder provides radial location and axial support.
A: The precision ground shoulder acts as a locating journal for rotating or reciprocating parts (e.g., die stripper plates, cam followers); the thread only secures, while the shoulder provides radial location and axial support.
❓ Q: How do I pick cone, cup, or dog points?
A: Choose cone for precise alignment, cup for strong bite-in locking where surface marring is acceptable, half/full-dog for torque transmission into a shaft hole, and flat where the shaft must not be damaged and adjustment is frequent.
A: Choose cone for precise alignment, cup for strong bite-in locking where surface marring is acceptable, half/full-dog for torque transmission into a shaft hole, and flat where the shaft must not be damaged and adjustment is frequent.
❓ Q: What is the Type VI recess?
A: It is the six-lobe hexalobular-style drive defined in Mandatory Appendix III — it reduces slip at high torque, extends tool life, and suits automated assembly.
A: It is the six-lobe hexalobular-style drive defined in Mandatory Appendix III — it reduces slip at high torque, extends tool life, and suits automated assembly.
