1-12 UNF Tap Drill Size
The number you came for, then the reasoning behind it.
1-12 UNF tap drill — 75% thread
59/64
0.9219" · 23.3 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 1.0000 − (0.01299 × 75) / 12 = 0.9188"
The nearest standard drill above that is 59/64 at 0.9219".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 0.9459" | 61/64 (0.9531") | 24 mm |
| 65% | 0.9296" | 59/64 (0.9219") | 23.6 mm |
| 75% | 0.9188" | 59/64 (0.9219") | 23.3 mm |
| 85% | 0.9080" | 29/32 (0.9062") | 23.1 mm |
Which percentage should you actually use?
75% for through-holes in mild steel and aluminium. 65% for blind holes, stainless, titanium and anything where the tap is at risk. 50% for hand-tapping deep holes. The strength you give up going from 75% to 60% is around 5%; the tapping torque you give up is around 30%. In practice the threads almost never fail — the tap does.
75% for through-holes in mild steel and aluminium. 65% for blind holes, stainless, titanium and anything where the tap is at risk. 50% for hand-tapping deep holes. The strength you give up going from 75% to 60% is around 5%; the tapping torque you give up is around 30%. In practice the threads almost never fail — the tap does.