ASME B18.3.2M-1979 (2003) — Metric Series Hexagon Keys and Bits for Socket Screw Products

🔧 ANSI/ASME B18.3.2M-1979 is the dedicated standard for metric-series hexagon keys and bits, specifying the complete dimensional, mechanical, and performance requirements for nominal sizes from 0.7 mm to 36 mm. First approved in 1979 and reaffirmed in 1990, 1998, 2002, and 2003, the Standard states that these tools are primarily intended for tightening and loosening metric-series hexagon socket screw products, and may also be suitable for other products with metric hexagon socket wrenching provisions. The 0.7, 0.9, and 1.3 mm sizes are soft conversions of the 0.028, 0.035, and 0.050 in keys required for wrenching the 1.6, 2, and 2.5 mm set screws already called for in ISO standards.

1. Scope and Interchangeability 📌

Clause 1 covers keys and bits from 0.7 mm to 36 mm nominal size. Clause 1.2 addresses interchangeability with ISO: keys of 2.5 mm and larger produced to this Standard conform dimensionally to ISO 2936-1973 (hexagon socket screw keys, metric series); ISO 2936 currently does not cover sizes below 2.5 mm. However, the Standard cautions that keys made to ISO 2936 lack the strength to drive high-strength screws such as property class 12.9 hexagon socket cap screws and property class 45H hexagon socket set screws — a critical consideration when selecting tools.

2. Large Keys and Bits 🔩

Clause 1.3 recommends that for nominal socket sizes above 24 mm, bits be used in conjunction with standard hexagon wrenches or power drives; where keys must be used, they shall conform to the dimensions specified herein. Bits are available in nominal sizes 24 mm and smaller, but their lengths are not standardized. The rationale: long key levers are awkward at large sizes and make controlled torque application difficult, whereas bits with torque tools are safer and more effective.

3. Dimensions, Materials, and Torsional Strength Testing 🔬

Table 1 of the Standard gives dimensions (width across flats, lengths, tolerances) for each key and bit size; Table 2 specifies the torsional moments for the torsional strength test; Table 3 gives sample sizes for mechanical testing. Keys are typically made of alloy steel and heat treated for a strength–toughness balance. Under test, keys must withstand the specified torsional moment without fracture or excessive permanent deformation. All dimensions are in millimeters (mm); plated keys are dimensioned before plating.

4. Engineering Practice Essentials 🛠️

• 🔍 Matching: select the key/bit by the screw’s hex socket across-flats; metric sizes pair one-to-one, so never substitute inch keys (0.035 in ≠ 0.9 mm, etc.).
• 🔧 Insertion and force: seat the key fully to the bottom of the socket and apply force perpendicular to the long arm; angled insertion wears the socket mouth.
• 📊 Torque control: for class 12.9 screws, use a bit with a torque wrench; when using a cheater bar at large sizes, watch operator safety and avoid over-torquing the screw.
• 📋 Inspection: in critical assembly, periodically check key across-flats wear against a gage; discard keys worn beyond limits.

5. Frequently Asked Questions ❓