#12-28 UNF Thread
Recommended tap drill #14
Tap drill Ø 0.1820″ / 4.623 mm
Major Ø 0.2160″ / 5.486 mm
Threads per inch 28
The chart recommendation gives approximately 73.3% thread engagement — the conventional target for average materials. It is not the only valid choice; see the options below.
Specification
| Designation | #12-28 UNF |
|---|---|
| Thread family | Unified National Fine (UNF) |
| Nominal size | #12 |
| Basic major diameter | 0.2160″ / 5.486 mm |
| Threads per inch / pitch | 28 TPI (0.907 mm pitch) |
| Recommended tap drill | #14 (0.1820″ / 4.623 mm) |
Tap drill options by thread engagement
Any of these standard drills produces a usable engagement for #12-28 UNF; smaller drills give stronger threads but higher tapping torque. Percentages use the conventional approximation (60° thread form).
| Drill | Decimal in | mm | Approx. engagement |
|---|---|---|---|
| #16 | 0.1770 | 4.496 | 84.1% |
| 4.5 mm | 0.1772 | 4.5 | 83.7% |
| #15 | 0.1800 | 4.572 | 77.6% |
| 4.6 mm | 0.1811 | 4.6 | 75.2% |
| #14 — chart recommendation | 0.1820 | 4.623 | 73.3% |
| #13 | 0.1850 | 4.699 | 66.8% |
| 4.7 mm | 0.1850 | 4.7 | 66.7% |
| 3/16″ | 0.1875 | 4.762 | 61.4% |
Nearest drills to the tap drill in other systems
| Drill | Size | vs #14 | Resulting engagement |
|---|---|---|---|
| 4.6 mm | 0.1811″ / 4.6 mm | −0.023 mm | 75.2% |
| 3/16″ | 0.1875″ / 4.762 mm | +0.14 mm | 61.4% |
Related threads
- #12-24 UNC — same nominal size, 24 TPI (Unified National Coarse (UNC)) · compare
- #10-32 UNF — next size down in UNF
- 1/4″-28 UNF — next size up in UNF