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Unified inch · UNEF

1/2-28 UNEF 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

15/32

0.4688" · 67% thread

Form tap

31/64

0.4844" · 64% thread

Or 12.3 mm for 65%

Open 1/2-28 UNEF 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
28 (pitch 0.0357")
Pitch Ø (basic)
0.4768" (12.111 mm)
Minor Ø (internal, basic)
0.4613" (11.718 mm)
Thread depth (internal)
0.0193" (0.491 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 1/2-28 UNEF
DrillØ inØ mmThread
11.7 mm0.460611.7085%
11.8 mm0.464611.8076%
11.9 mmsuggested0.468511.9068%
15/32suggested0.468811.9167%
12 mm0.472412.0059%
12.1 mm0.476412.1051%

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 1/2-28 UNEF
DrillØ inØ mmThread
12.2 mm0.480312.2081%
12.3 mmsuggested0.484312.3065%
31/640.484412.3064%

3-wire measurement

Best wire size
0.0206" (0.524 mm)
Over wires, at basic pitch Ø
0.5077" (12.896 mm)

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

Sources