#6-40 UNF Tap Drill Size
The number you came for, then the reasoning behind it.
#6-40 UNF tap drill — 75% thread
#33
0.1130" · 2.9 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 0.1380 − (0.01299 × 75) / 40 = 0.1136"
The nearest standard drill above that is #33 at 0.1130".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 0.1218" | #31 (0.1200") | 3.1 mm |
| 65% | 0.1169" | #32 (0.1160") | 3 mm |
| 75% | 0.1136" | #33 (0.1130") | 2.9 mm |
| 85% | 0.1104" | #35 (0.1100") | 2.8 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.