#3-48 UNC Tap Drill Size
The number you came for, then the reasoning behind it.
#3-48 UNC tap drill — 75% thread
#47
0.0785" · 2 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 0.0990 − (0.01299 × 75) / 48 = 0.0787"
The nearest standard drill above that is #47 at 0.0785".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 0.0855" | #44 (0.0860") | 2.2 mm |
| 65% | 0.0814" | #46 (0.0810") | 2.1 mm |
| 75% | 0.0787" | #47 (0.0785") | 2 mm |
| 85% | 0.0760" | #48 (0.0760") | 1.9 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.