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Unified inch · UNF

1/2-20 UNF tap drill size

For general work, OSG suggests 65–70% of full thread. Harder materials and deep holes want less; the calculator sets it from your material.

Cut tap

29/64

0.4531" · 72% thread

Or 11.6 mm for 67%

Form tap

31/64

0.4844" · 46% thread

Or 12.1 mm for 69%

Open 1/2-20 UNF in the calculator

Pick your material for its % of thread, tapping RPM and the rigid-tap code.

Thread dimensions

Major Ø
0.5000" (12.700 mm)
Threads per inch
20 (pitch 0.0500")
Pitch Ø (basic)
0.4675" (11.875 mm)
Minor Ø (internal, basic)
0.4459" (11.325 mm)
Thread depth (internal)
0.0271" (0.687 mm)

Basic sizes of the 60° form. Tolerance classes (2B/3B) set the limits around them.

Cut tap drills

For a cutting (fluted or spiral) tap. Every standard drill from 50% to 85% thread; suggested rows are in the 65–70% band.

Cut tap drills for 1/2-20 UNF
DrillØ inØ mmThread
11.3 mm0.444911.3085%
11.4 mm0.448811.4079%
11.5 mm0.452811.5073%
29/640.453111.5172%
11.6 mmsuggested0.456711.6067%
11.7 mm0.460611.7061%
11.8 mm0.464611.8055%

Form tap drills

A form (roll) tap displaces metal instead of cutting it, so it needs a bigger hole. Every standard drill from 50% to 85% thread; suggested rows are in the 65–70% band.

Form tap drills for 1/2-20 UNF
DrillØ inØ mmThread
12 mm0.472412.0081%
12.1 mmsuggested0.476412.1069%
12.2 mm0.480312.2058%

3-wire measurement

Best wire size
0.0289" (0.733 mm)
Over wires, at basic pitch Ø
0.5108" (12.975 mm)

For the external thread (the screw). The best wire touches the flanks at the pitch diameter.

Sources