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#12-28 UNF Tap Drill Size
The number you came for, then the reasoning behind it.
#12-28 UNF tap drill — 75% thread
#14
0.1820" · 4.6 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 0.2160 − (0.01299 × 75) / 28 = 0.1812"
The nearest standard drill above that is #14 at 0.1820".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 0.1928" | #10 (0.1935") | 4.9 mm |
| 65% | 0.1858" | #13 (0.1850") | 4.7 mm |
| 75% | 0.1812" | #14 (0.1820") | 4.6 mm |
| 85% | 0.1766" | #16 (0.1770") | 4.5 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.