Home › 1-1/2-12 UNF › 1-1/2-12 UNF tap drill size
1-1/2-12 UNF Tap Drill Size
The number you came for, then the reasoning behind it.
1-1/2-12 UNF tap drill — 75% thread
91/64
1.4219" · 36 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 1.5000 − (0.01299 × 75) / 12 = 1.4188"
The nearest standard drill above that is 91/64 at 1.4219".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 1.4459" | 93/64 (1.4531") | 36.5 mm |
| 65% | 1.4296" | 91/64 (1.4219") | 36.5 mm |
| 75% | 1.4188" | 91/64 (1.4219") | 36 mm |
| 85% | 1.4080" | 45/32 (1.4062") | 36 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.