#3-56 tap drill size
#45
- Inches
- 0.0820
- Millimetres
- 2.08
- Thread
- 73%
Drill a #45 hole before cutting a #3-56 thread. That is the standard tap drill for about 75% thread.
#3-56 thread and hole sizes
| Thread | #3 diameter, 56 threads per inch, fine (UNF) |
|---|---|
| Major diameter | 0.0990 in 2.51 mm |
| Pitch | 0.0179 in 0.454 mm |
| Tap drill, about 75% thread | #45 0.0820 in |
| Tap drill, about 50% thread | #43 0.0890 in, for stainless and other hard materials |
| Clearance hole, close fit | #37 0.1040 in |
| Clearance hole, free fit | #35 0.1100 in |
| Nearest metric bit | 2.1 mm |
Why #45
A tap cuts threads into the wall of a hole, so the hole has to be smaller than the screw. For a #3-56 thread the hole that leaves exactly 75% thread is 0.0816 in. The closest standard bit is #45 at 0.0820 in, which leaves 73%.
More thread than that adds almost no strength and makes the tap much easier to break. In hard or tough material, go one step larger.
Tools for this job
- #3-56 tap
- #45 drill bit
- Tap and drill set
- Drill bit size gauge
- Thread pitch gauge
- Cutting oil for tapping
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