ASME B18.3.6M-1986 (2002) — Metric Series Socket Set Screws

28 10 月, 2026
ASME Standards

🔧 ASME/ANSI B18.3.6M-1986 is the complete product standard in the fastener family covering metric series socket set screws in nominal sizes from 1.6 mm to 24 mm. Developed by Subcommittee 3 of the ASME B18 Committee on the Standardization of Bolts, Nuts, Rivets, Screws, Washers, and Similar Fasteners, it was recognized as an American National Standard in 1986 and reaffirmed in 1993 and 2002. In addition to complete dimensional and general requirements, the Standard provides appendices covering formulas for dimensions, a part-numbering system and preferred sizes for government use, and metric thread dimensions for socket screw products — making it the U.S. counterpart to ISO 4026 through ISO 4029.

💡 A set screw is a “headless” fastener that clamps directly on the shaft with its point to establish axial location or torque transmission. Socket set screws are the workhorse of shaft-hub fixing on pulleys, gears, couplings, and shafts.

1. Scope and Interchangeability 📌

The scope is precise: the Standard contains complete general and dimensional requirements for metric series socket set screws in nominal sizes from 1.6 mm to 24 mm recognized as American National Standard. Three appendices are included: Appendix I (formulas for dimensions), Appendix II (part-numbering system and preferred sizes for government use), and Appendix III (thread dimensions).

⚠️ The inclusion of dimensional data does not imply that every size is a stock production item. Consumers should consult manufacturers about lists of stock sizes before assuming availability of any particular size.

On interchangeability, the Standard states that socket set screws produced to it will interchange functionally with screws conforming to the international standards for hexagon socket set screws with flat point, cone point, dog point, and cup point: ISO 4026, ISO 4027, ISO 4028, and ISO 4029-1977, respectively. U.S. and ISO products can therefore be used interchangeably at the same nominal size.

2. Dimensioning and Thread Requirements 🔧

All dimensions are given in millimeters (mm) and apply before plating unless otherwise stated — a critical rule for controlling the dimensional chain through surface-treatment processes.

Element	Requirement	Remarks
Nominal size range	1.6 mm through 24 mm	Full metric series
Thread tolerance	Class 4g6g (U.S. practice)	ISO specifies 5g6g; the U.S. class is tighter
Drive type	Hexagon socket (default), spline socket	Hexagon socket supplied unless specified
Point styles	Flat, cone, dog, and cup points	Point dimensions per Table 2
Material	Alloy steel per ASTM F 912M; corrosion-resistant steel per applicable ASTM spec	Mechanical/chemical properties by reference
Designation	Spec no. – nominal size × pitch × length – drive – point/product – material & property class	Sequence per para. 1.7

The point style determines how the screw works: the flat point suits applications requiring repeated adjustment and a smooth contact surface; the cone point is for permanent location and can seat into softer shaft material; the dog point works with a drilled hole in the shaft for the most positive location; and the cup point is the most common general-purpose style, concentrating clamping stress inside a concave cup for better bite on the shaft.

🚨 Set screw retention depends entirely on the friction bite between the point and the shaft surface. Under vibration, cyclic, or high-speed service, supplementary locking (thread-locking adhesive, jam nuts, or end drilling) is essential — cup-point pressure alone is often insufficient to maintain preload long-term.

3. Spline Sockets and Gaging 📏

Besides the standard hexagon socket, the Standard also specifies spline socket dimensions. Spline drive distributes stress more evenly and concentrates it less than a hexagon, allowing higher torque in the same head envelope with less risk of slipping or broaching — which is why spline sockets are favored in aerospace and high-stress applications. Dimension tables for both hexagon and spline sockets are given, together with tables for the corresponding socket gages used for manufacturing and acceptance.

The dimensional formulas of Appendix I let designers interpolate sizes not tabulated directly, but there is a governing rule: when questions arise concerning acceptance of product, the tabulated dimensions prevail over recalculation by formula — a convention shared across the B18 series that keeps buyer and supplier on the same basis.

4. Materials and Mechanical Properties ⚙️

The Standard references ASTM specifications rather than restating strength data: alloy steel socket set screws must meet the mechanical and chemical requirements of ASTM F 912M (the metric-series alloy-steel socket set screw specification), and corrosion-resistant steel screws must meet the appropriate corrosion-resistant ASTM specification. This reference-based approach reflects the founding principle of the metric edition: the metric products must maintain the same quality and performance integrity as their inch counterparts.

💡 Material choice matters: carbon steel for general machinery, alloy steel where high strength is required, and austenitic stainless for humid or corrosive environments. Dimensions are controlled before plating, so account for coating thickness on thread fits when specifying finishes.

5. Engineering Practice Essentials 🛠️

🔍 Selection workflow: establish shaft size and clamping need → choose nominal size and pitch (ISO or U.S. series) → pick the point (cup for general use, dog for positive location, cone for permanent fixing) → choose hexagon or spline drive by torque → select material and finish.
🔧 Installation torque: tightening torque depends on size, material, and point style. Overtorque rounds the socket (“broaches” it) or snaps the screw; under-torque cannot produce enough axial clamp. Follow recommended torque tables for set screws.
📊 Thread fit: the 4g6g tolerance is tighter than ISO’s 5g6g, giving a closer fit and better location. When mating with ISO screws, verify the tapped-hole tolerance class is compatible.
📋 Identification: mark per para. 1.7 in the required sequence — spec number, nominal size, pitch (X), length (X), drive, point and product name, and material/property class — to preserve traceability.

6. Frequently Asked Questions ❓

❓ Q: How does B18.3.6M relate to B18.3, the inch socket screw standard?
A: B18.3 covers inch-series socket cap and set screws; B18.3.6M is the dedicated metric-series socket set screw standard. Units differ (inches vs. millimeters), so they cannot be mixed, but quality and performance levels are intended to match.

❓ Q: Can set screws made to this Standard replace ISO 4026–4029 screws?
A: Functional interchangeability is stated in the Standard. Note the U.S. thread tolerance is 4g6g versus ISO’s 5g6g, and point details differ slightly; verify diameter, length, and point contact geometry against the actual assembly before substituting.

❓ Q: Why are dimensions controlled before plating?
A: Because plating adds thickness that grows thread and point dimensions. Controlling the pre-plate size keeps plated product within tolerance; buyers should specify coating type/thickness and verify post-plate dimensions.

🔍 Q: Hexagon socket or spline socket?
A: Hexagon sockets use universal tools and lower cost, and are the default. Spline sockets distribute stress more evenly, handle higher torque with less slip, and suit high-reliability applications (aerospace, racing, precision drives), but require special tools.