What tap drill do I use for 1/2-13 UNC?
Answer
Drill 27/64 in to tap 1/2-13 UNC. That leaves 78% thread engagement, which is what workshop charts are built on.
How it is worked out
- The relation every chart is built on
drill = major diameter − (engagement% ÷ 100) × 1.299 × pitch - For this thread
12.7 − 0.75 × 1.299 × 1.95 = 0.4251 in - Rounded to a drill you own
0.4251 in → 27/64 in
1.299 = 2 × 0.6495, the truncation of the 60° thread profile
Worth knowing
- Clearance hole — close
- 13.59 mm, for accurately positioned holes only
- Clearance hole — normal
- 14.1 mm. Use this one unless you have a reason not to.
- Clearance hole — loose
- 15.24 mm
- Minor diameter
- 10.585 mm — the thread's own root, which is not the drill size and is the mistake worth avoiding
- Pitch diameter
- 11.431 mm
- How deep to tap
- One diameter of engagement in steel, one and a half in aluminium or cast iron, two in plastic. Deeper gains nothing measurable — the first few threads carry most of the load.
- Why not cut a full-depth thread
- The last quarter of engagement adds roughly 5% to the stripping strength and about doubles the torque on the tap. In anything harder than aluminium the tap breaks first, and a broken tap in a finished part is a much worse afternoon.
Tap drills for nearby imperial threads
Once it is tapped
Do it with your own numbers
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