Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
ASME B18.3.4M-1986 (2002) — Hexagon Socket Button Head Cap Screws (Metric Series)
🔧 ASME/ANSI B18.3.4M-1986 is the dedicated standard for metric-series hexagon socket button head cap screws, covering complete general and dimensional requirements for nominal sizes from 3 mm to 16 mm. First approved in 1979 and revised in 1986 (incorporating by reference the companion ASTM mechanical and chemical specifications), it was reaffirmed in 2002. Button head screws feature a low rounded top with a large flat bearing surface, projecting much less above the work than a standard socket head cap screw, making them ideal where head-height clearance is limited. The Standard explicitly cautions that, because of the smaller socket and shallower key engagement, wrenchability is significantly reduced and the property class 12.9 tensile loads will not be met.
💡 Button heads exist for low-profile applications: when a standard cap screw head would strike an adjacent part, cover, or guard, the button head trades some drive capability for reduced head height.
1. Scope and Design Limitations 📌
Clause 1 covers metric-series hexagon socket button head cap screws from 3 mm to 16 mm; all dimensions are in millimeters (mm) and apply before plating unless stated otherwise; options are at the manufacturer’s discretion unless agreed otherwise. The Standard states two design limitations explicitly: (1) wrenchability is significantly reduced because the socket is smaller and key engagement shallower, and (2) property class 12.9 tensile loads will not be met because of the head configuration. Where full clamping capability must be developed, it may be necessary to assemble the product with a nut.
⚠️ Do not substitute button head screws for cap screws at the same load: button heads do not meet class 12.9 tensile performance. If full clamp force is required, the Standard recommends assembly with a nut so the nut carries the clamping load.
2. Size Series and Typical Applications 🔩
Table 1 of the Standard gives head diameter, head height, body diameter, socket size, and related dimensions; Table 2 gives hexagon socket dimensions; Table 3 gives hexagon socket gage dimensions. Common sizes and typical applications are summarized below.
| Nominal Size (dia × pitch) | Head Characteristics | Typical Applications |
|---|---|---|
| M3 × 0.5 | Low rounded head, small bearing face | Electronics covers, sheet-metal assembly |
| M4 × 0.7 | Low profile head | Appliances, automotive interior |
| M5 × 0.8 | Low profile head | General machinery, guards |
| M6 × 1.0 | Large flat bearing surface | Machine guards, electrical enclosures |
| M8 × 1.25 | Large flat bearing surface | Engine accessories, equipment housings |
| M10 × 1.5 | Large flat bearing surface | Heavy-equipment covers |
| M12 × 1.75 | Large flat bearing surface | Large-equipment low-profile joints |
| M16 × 2.0 | Large flat bearing surface | Heavy-structure low-profile service |
💡 The large flat bearing face spreads load on thin sheet and plastics, reducing sink-in and surface damage; when using washers, keep the washer OD within the head bearing face.
3. Interchangeability with ISO 7380 🔬
Clause 1.2 states that screws made to this Standard are considered completely interchangeable with those described in ISO/TC2 7380-1983 (metric button head screws). The primary differences are: (1) thread fit — 4g6g here versus 5g6g in ISO, and (2) the magnitude of the length tolerance. Dimensional differences are minor and screws are functionally interchangeable. Appendices give formulas for dimensions (Appendix I), the government part-numbering system and preferred sizes (Appendix II), and metric thread dimensions (Appendix III).
🚨 Interchangeability with ISO 7380 presupposes identical nominal sizes. When substituting, verify the thread fit (4g6g vs 5g6g) and length tolerance to avoid assembly deviations in tight-fit applications.
4. Materials and Mechanical Properties 🔩
Clause 3 sets material, processing, and mechanical requirements, incorporating the companion ASTM specifications (e.g., ASTM A 574M-type) for mechanical, chemical, and testing requirements. The strength class of button head screws must be understood in light of the head design: the low-profile head limits the bearing and drive structure, so even alloy steel screws are not assessed at class 12.9 tensile performance. Corrosion-resistant steel products are furnished to the applicable ASTM specification. Surface finishes (zinc, nickel, black oxide, etc.) follow the purchase agreement; dimensions apply before plating.
⚠️ Judge button head load capability from the manufacturer’s load data and tests — do not apply class 12.9 cap screw values. In highly vibrating service, also evaluate locking provisions.
5. Engineering Practice Essentials 🛠️
- 🔍 Selection: prefer button heads where head height is constrained; for load-sensitive applications switch to cap screws or a nut-tightened scheme.
- 🔧 Tightening: button heads have limited wrenchability — use a proper bit/key and control torque to avoid rounding the socket.
- 📐 Locking: in vibration service, add adhesive, nylon patch, or spring washers.
- 📋 Acceptance: verify dimensions per Table 1 and sockets per the gage table (Table 3); check the 4g6g thread fit and length tolerance.
6. Frequently Asked Questions ❓
❓ Q: Why don’t button heads meet class 12.9 loads?
A: The low-profile head limits socket size, key engagement, and head load-bearing structure; the Standard states class 12.9 tensile loads are not met. For full clamp force, assemble with a nut.
A: The low-profile head limits socket size, key engagement, and head load-bearing structure; the Standard states class 12.9 tensile loads are not met. For full clamp force, assemble with a nut.
🔍 Q: Are button head screws interchangeable with ISO 7380 screws?
A: Yes — completely interchangeable per Clause 1.2. Main differences are thread fit (4g6g vs 5g6g) and length tolerance; direct assembly works in most applications, verify tolerances for strict fits.
A: Yes — completely interchangeable per Clause 1.2. Main differences are thread fit (4g6g vs 5g6g) and length tolerance; direct assembly works in most applications, verify tolerances for strict fits.
❓ Q: What does “reduced wrenchability” mean?
A: It means reduced drive capability: the smaller socket and shallower key engagement limit the maximum tightening torque versus a standard cap screw, so button heads suit applications that do not need large preload.
A: It means reduced drive capability: the smaller socket and shallower key engagement limit the maximum tightening torque versus a standard cap screw, so button heads suit applications that do not need large preload.
❓ Q: When should I choose button head screws?
A: When the protruding head of a standard cap screw would interfere with an adjacent part, or when a large bearing face is needed on thin sheet or plastic.
A: When the protruding head of a standard cap screw would interfere with an adjacent part, or when a large bearing face is needed on thin sheet or plastic.
