M3.5×0.6 Tap Drill Size
The number you came for, then the reasoning behind it.
M3.5×0.6 tap drill — 75% thread
2.9 mm
2.915 mm exact · nearest inch drill #33 (0.1130")
How it's calculated
Tap drill = major diameter − pitch → 3.5 − 0.6 = 2.9 mm
The exact 75% figure is major − (pitch × 75 / 76.98). The simple subtract-the-pitch rule lands within a hundredth of a millimetre of it, which is why every machinist uses it.
All engagement percentages
| % thread | Exact hole | Metric drill | Inch equivalent |
|---|---|---|---|
| 50% | 3.110 mm | 3.1 mm | 0.1200" (#31 drill) |
| 65% | 2.993 mm | 3 mm | 0.1200" (#31 drill) |
| 75% | 2.915 mm | 2.9 mm | 0.1130" (#33 drill) |
| 85% | 2.837 mm | 2.8 mm | 0.1100" (#35 drill) |
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.