The tap drill for a #10-32 UNF thread is a #21 drill (4.04 mm), which leaves about 76% of full thread with a cutting tap. The ideal hole for 75% thread is 0.1596 in, and #21 is the nearest standard inch drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 0.1697 in | #18 | 0.1695 in / 4.31 mm | 50% |
| 60% | 0.1656 in | #19 | 0.1660 in / 4.22 mm | 59% |
| 65% | 0.1636 in | #19 | 0.1660 in / 4.22 mm | 59% |
| 70% | 0.1616 in | #20 | 0.1610 in / 4.09 mm | 71% |
| 75% | 0.1596 in | #21 | 0.1590 in / 4.04 mm | 76% |
The drills either side of the target are 5 points of thread apart, so either is a reasonable choice. #21 (0.1590 in) gives 76% and a stronger but harder-to-tap thread; #20 (0.1610 in) gives 71% and less tapping torque. On #10-32 UNF the choice is mostly about tap life in hard materials.
#10-32 is the standard fine thread for instruments, optics mounts, small pneumatic fittings and precision fixtures. Its basic pitch diameter is 0.1697 in and its tensile stress area is 0.0200 in². A bolt of this size passes through a 0.2210 in normal-fit clearance hole (ASME B18.2.8).
Example: #10-32 UNF at 75% thread
Result: #21 (0.1590 in), 76% thread
| Major (nominal) diameter | 0.1900 in / 4.83 mm |
|---|---|
| Pitch | 32 TPI (0.0313 in) |
| Basic pitch diameter (d − 0.6495·P) | 0.1697 in |
| Basic minor diameter, internal (d − 1.0825·P) | 0.1562 in |
| Tensile stress area | 0.0200 in² |
| Fit | Hole diameter |
|---|---|
| Close | 0.2060 in / 5.23 mm |
| Normal | 0.2210 in / 5.61 mm |
| Loose | 0.2380 in / 6.05 mm |
All UNF sizes:
Use #21 (0.1590 in). It leaves 76% of full thread with a cutting tap; the ideal 75% hole is 0.1596 in. For an easier-to-tap 50% thread, #18 (0.1695 in) gives 50%.
#21 is 4.04 mm. The nearest metric drill to the 75% target is 4.1 mm (0.1614 in), which gives 70% thread.
A forming tap needs a bigger hole: 0.1762 in for 65% thread, so drill #16 (0.1770 in). Forming taps work in ductile metals such as aluminum and mild steel.
For #10-32 UNF, ASME B18.2.8 gives close fit 0.2060 in, normal fit 0.2210 in and loose fit 0.2380 in. The hole depends only on the diameter, so #10-24 UNC uses the same sizes.
0.0200 in², from As = 0.7854 × (D − 0.9743 ÷ n)². Multiply it by the bolt material's proof or tensile strength to get the load the thread can carry.
No. The drill depends on the pitch, not only the diameter. #10-32 UNF takes #21 (0.1590 in); #10-24 UNC takes #25 (0.1495 in). Check the pitch on the drawing before you drill.
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