ASME B5.38-1958 — Driving and Spindle Ends for Portable Air and Electric Tools

28 10 月, 2026
ASME Standards

🔩 ASME B5.38-1958 (originally ASA B5.38-1958) is the American Standard that coordinates the driving and spindle ends of portable air and electric tools, approved by the American Standards Association (ASA) on October 29, 1958. It applies to portable power tools used for drilling, grinding, polishing, sawing, and driving threaded fasteners, and standardizes the dimensions and tolerances of both the power-output end and the driven elements so that chucks, sockets, abrasives, and saw blades made by different manufacturers are interchangeable. It is a foundational interface standard for the hand-tool and power-tool industry.

💡 B5.38 solves the “wrong arbor” problem: before the Standard, every builder used its own spindle thread, taper, and square drive, and accessories could not be exchanged between tools. Once the interface is standardized, the same chuck or socket fits any compliant tool.

1. Purpose, Scope, and Definitions 📌

The scope of the Standard is defined as follows:

Coverage: portable power tools for drilling, grinding, polishing, sawing, and driving threaded fasteners. Other tools not classed as percussion tools belong in this category and may be added by revision through the usual procedure.
Definition: “tool” means a portable device for performing a mechanical operation with power from compressed air or electricity.
Exclusions: percussion tools — hammers, chisels, scalers, tampers, clay diggers, and rock drills — are excluded.
References: B1.1-1949 (Unified and American Screw Threads) and B7.1-1956 (Safety Code for the Use, Care and Protection of Abrasive Wheels).

The Standard states explicit dimensions and tolerances for driving and driven elements where coordination is important and not already covered by other American Standards; all dimensions are in inches.

⚠️ Note: B5.38 governs only interface dimensions, not tool performance (power, speed, torque). Check two things before use: whether the tool belongs to a covered category, and whether its interface type appears in the Standard’s tables.

2. Chuck Spindles: Threaded and Tapered 🔧

2.1 Threaded Spindles (Geared Chucks)

The Standard defines threaded spindle series for geared chucks (drill chucks). The main series are 3/8-24, 1/2-20, 5/8-16, and 3/4-16, all right-hand, with pitch diameter tolerances, end radius R, and spindle lengths:

Nominal Size and Thread	Pitch Dia. Max (in.)	Pitch Dia. Min (in.)	Radius R	Length L (in.)
3/8-24	0.3479	0.3455	1/16 ± 1/64	9/16 (+0.000 / -0.030)
1/2-20	0.4675	0.4649	1/16 ± 1/64	9/16 (+0.000 / -0.030)
5/8-16	0.5844	0.5812	3/32 ± 1/64	11/16 (+0.000 / -0.030)
3/4-16	0.7094	0.7062	3/32 ± 1/64	11/16 (+0.000 / -0.030)

2.2 Tapered Spindles (Jacobs Tapers)

The Standard also covers tapered spindles, commonly known as Jacobs tapers, in sizes No. 1, No. 2 Short, No. 2, No. 33, No. 6, and No. 3, with large-end diameter and taper per foot:

Taper Number	Large-End Dia. DM (in.)	Length LM (in.)	Taper per Foot (in./ft)
No. 1	0.335–0.333	0.656	0.92508
No. 2 Short	0.490–0.488	0.750	0.97861
No. 2	0.490–0.488	0.875	0.97861
No. 33	0.563–0.561	1.000	0.76194
No. 6	0.626–0.624	1.000	0.62292
No. 3	0.748–0.746	1.219	0.63898

Master plug gage dimensions are provided to verify taper fit, and Appendix A gives recommended spindle-to-chuck correlation.

3. Threaded-Fastener Drives: Hexagonal Chucks and Square Drives 🔩

For tools that drive threaded fasteners (bolts, nuts), the Standard defines three interface types:

Hexagonal chucks: nominal hex sizes 1/4, 5/16, 7/16, 5/8, and 3/4 in., with across-flats limits (e.g., the 1/4-in. chuck accepts hex from 0.253 to 0.255 in.).
Hexagonal shanks: the mating tool shanks, with the same across-flats tolerances.
Square drives: male and female ends in sizes 1/4, 3/8, 1/2, 5/8, 3/4, 1, and 1-1/2 in., with two design variants (Design A with retaining cross-hole, Design B solid). Tables 5 and 6 give full dimensions, fillet radii, and fit clearances (minimum across-flats clearance 0.001 in. through 3/4-in. size; 0.002 in. in 1-in. and 1-1/2-in. sizes).

⚠️ Watch the male-fillet / female cross-hole fit on square drives: if the male tang has a fillet radius R_M at its shoulder and the female end is not chamfered, the female cross-hole diameter must be enlarged or the drive cannot be assembled. For impact wrenches keep the male across-flats dimension as close to the maximum as practical.

4. Abrasive Tool Spindles and Circular Saw Arbors 🛠️

Sections IV and V of the Standard cover the spindles of grinding/polishing tools and the arbors of circular saws:

Sanders and polishers: spindle thread and flange seating dimensions are defined.
Vertical and angle grinders: typical spindle 5/8-11 UNC-2A (spindle length measured from the wheel bearing surface of a revolving cup guard), plus 3/8-24 UNF, 1/2-13 UNC, etc. Appendix C recommends a safety sleeve nut when using unthreaded wheels with a 7/8-in. hole.
Straight wheel grinders: spindle threads 3/8-24 UNF, 1/2-13 UNC, 5/8-11 UNC, and 3/4-10 UNC, coordinated with B7.1-1956; Appendix B recommends organic-bond wheels rated 9500 ft/min for pneumatic and induction-motor grinders and vitrified wheels rated 6500 ft/min for light-duty electric grinders, with maximum wheel diameter/thickness tables.
Circular saw arbors: 5/8-in. arbor for blade diameters 6 through 8-1/2 in.; 3/4-in. arbor for blades 9 through 12 in.

🚨 Safety red line: the maximum wheel speed must match the motor power and spindle design. Organic-bond and vitrified wheels have different allowable speeds; fitting the wrong wheel type can cause wheel bursting and serious injury.

5. Engineering Practice Essentials 🛠️

🔍 Accessory selection: identify the tool spindle type (threaded, tapered, or square drive) and size, then choose the chuck, socket, or adapter from the Standard’s correlation tables; never force-fit a “nearly fits” accessory.
🔧 Thread care: clean burrs and nicks on threaded spindle noses before mounting chucks; the length tolerance is a (0, -0.030) in. negative tolerance, and grinding the spindle too short degrades axial chuck location.
📊 Taper verification: check Jacobs taper contact surfaces with a master plug gage periodically; insufficient bearing area causes chuck runout and early wear.
📋 Impact tools: keep the male square-drive across-flats dimension near the maximum for impact wrenches to minimize play and extend drive life.

6. Frequently Asked Questions ❓

❓ Q: Does B5.38 apply to rotary hammers and jackhammers?
A: No. The Standard explicitly excludes percussion tools — hammers, chisels, scalers, tampers, clay diggers, and rock drills — which require their own interface specifications.

❓ Q: Is a 1/2-in. square drive the same “1/2” as a 1/2-in. socket?
A: Yes. Square drives are named by the across-flats size of the drive head (1/4, 3/8, 1/2, 5/8, 3/4, 1, 1-1/2 in.), and sockets have matching square holes. When buying, the critical size is the square drive, not the fastener size.

❓ Q: What are the tradeoffs between Jacobs tapers and threaded spindles?
A: Tapered spindles self-center on their conical seat, give good alignment, and allow fast chuck changes — the mainstream chuck interface. Threaded spindles provide a more positive connection but must be screwed fully home. The two suits different chuck sizes; Appendix A gives a recommended correlation.

❓ Q: Why does the Standard mention 9500 and 6500 ft/min wheel speeds?
A: They match the wheel bond: organic-bond wheels are stronger and are rated up to 9500 ft/min, while vitrified (glass-bond) wheels are rated at 6500 ft/min. Choose the wheel for the spindle speed class and comply with B7.1 safety rules.

ASME B5.38-1958.pdf