Inch, metric, and Acme

Thread and tap-drill reference

The thread dimensions we cut and tap to, across the forms the configurator supports: Unified inch, metric, and Acme, with threads per inch or pitch, major and minor diameters, and the tap drill for each size.

A thread is called out by its form, diameter, and pitch. Unified inch threads use a diameter and a thread count like 1/2-13, where 13 is threads per inch; metric threads use a diameter and a pitch in millimeters like M10x1.5. In both systems the coarse series is more tolerant of damage and faster to assemble, while the fine series holds better under vibration and gives finer adjustment. Below a quarter inch, Unified sizes are numbered (#4, #6, #10) rather than fractional.

The major diameter is the outside of an external thread and the nominal size of the callout. The minor diameter is the root, the smaller cylinder the thread is cut into; for a tapped hole the tap drill produces close to the minor diameter so there is enough material for the thread to form. Cut an external thread on a shaft end and the stock is turned to the major diameter first.

All of these forms are configurable, so pick the one your part needs. The Unified inch and metric tables cover fastening threads; the Acme table covers the power-thread form for lead screws.

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Unified National Coarse (UNC)

Diameters and tap-drill sizes in inches. TPI is threads per inch.

Unified National Fine (UNF)

Diameters and tap-drill sizes in inches. TPI is threads per inch.

Metric Coarse

Diameters and pitch in millimeters. Tap drill is the metric drill size.

Metric Fine

Diameters and pitch in millimeters. Tap drill is the metric drill size.

Acme (general purpose)

Trapezoidal power-thread form. Diameters in inches, TPI is threads per inch.

External threads and tapped holes

On a shaft, an external thread is single-point cut or die-cut on a turned diameter, usually at the major diameter shown here. A tapped hole is drilled to the tap-drill size and then tapped; the tap-drill column is what gives roughly 75 percent thread engagement, the usual target for strength without excessive tapping torque.

Class of fit (1A/1B loose, 2A/2B standard, 3A/3B close) sets how tightly the mating threads engage. Most work is 2A/2B. Tell us the class if it matters; otherwise we cut the standard class.

Acme and power threads

Acme threads have a 29 degree trapezoidal form built to move a load rather than clamp, so they are what you see on lead screws, jacks, and vises. The general-purpose Acme sizes in the table above are configurable the same way as a fastening thread; a shallower Stub Acme form is also available. Pick the size and set the length for a lead screw.

Related: Cylinder parts · Motor shafts · Shaft fits and tolerances.

Frequently Asked Questions

These references are for specifying a part; every value here is one the configurator applies when you build one. Numbers are representative of the standard and confirmed against your part before machining. See all guides on the guides page.