Home › 9/16-12 UNC › 9/16-12 UNC tap drill size
9/16-12 UNC Tap Drill Size
The number you came for, then the reasoning behind it.
9/16-12 UNC tap drill — 75% thread
31/64
0.4844" · 12.2 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 0.5625 − (0.01299 × 75) / 12 = 0.4813"
The nearest standard drill above that is 31/64 at 0.4844".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 0.5084" | 33/64 (0.5156") | 12.9 mm |
| 65% | 0.4921" | 31/64 (0.4844") | 12.5 mm |
| 75% | 0.4813" | 31/64 (0.4844") | 12.2 mm |
| 85% | 0.4705" | 15/32 (0.4688") | 12 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.