#3-56 UNF tap drill size
Tap #3-56 UNF with a #45 drill (0.0820 in, 2.08 mm), about 73% thread. Taps this fine break easily, so hand-tap with cutting oil and back off often.
| Drill | inch | mm | Thread % | Typical use |
|---|---|---|---|---|
| 5/64" | 0.0781 | 1.98 | 90.0% | Tight: high tapping torque, tap breakage risk |
| #47 | 0.0785 | 1.99 | 88.4% | Tight: high tapping torque, tap breakage risk |
| #46 | 0.0810 | 2.06 | 77.6% | General purpose |
| #45 (chart drill) | 0.0820 | 2.08 | 73.3% | General purpose |
| 2.1 mm (metric drill) | 0.0827 | 2.1 | 70.4% | General purpose |
| #44 | 0.0860 | 2.18 | 56.0% | Low engagement: check strength |
| Thread | Tap drill | Tap drill (mm) | Clearance close / free |
|---|---|---|---|
| #1-72 UNF | #53 (0.0595 in) | 1.51 mm | #48 / #46 |
| #2-64 UNF | #50 (0.0700 in) | 1.78 mm | #43 / #41 |
| #3-56 UNF (this page) | #45 (0.0820 in) | 2.08 mm | #37 / #35 |
| #4-48 UNF | #42 (0.0935 in) | 2.37 mm | #32 / #30 |
| #5-44 UNF | #37 (0.1040 in) | 2.64 mm | #30 / #29 |
Practical notes
At 56 TPI the basic thread depth is only about 0.0097 in per side, so small errors in drill size change engagement a lot. Use a sharp, correctly sized drill, a tap guide to keep the tap square, and limit thread depth to about 1.5 × diameter.
Thread-forming taps are popular for #3 sizes in aluminium and brass because they make no chips; they need a larger hole, roughly 0.0901 in (2.29 mm, D − 0.5 × pitch). Confirm against the tap maker's chart.
If a #3 thread in plastic or a printed part must survive repeated assembly, a threaded insert is usually more durable than tapping the base material.
UNF (fine) threads have more threads per inch than UNC, a larger tensile stress area and better resistance to loosening, but they strip more easily in soft material and are more sensitive to drill size.
Clearance drills for a #3 screw: close fit #37 (0.1040 in, 2.64 mm), free fit #35 (0.1100 in, 2.79 mm).
FAQ
What size drill for a #3-56 UNF tap?
The standard tap drill for #3-56 UNF is a #45 drill, 0.0820 in (2.08 mm), which gives about 73% thread.
What clearance hole for a #3 UNF screw?
Use #37 (0.1040 in, 2.64 mm) for a close fit or #35 (0.1100 in, 2.79 mm) for a free fit. Clearance depends on diameter only, so it is the same for UNC and UNF #3 screws.
What metric drill can I use for #3-56 UNF?
The closest metric drill is 2.1 mm (0.0827 in), giving about 70% thread on #3-56 UNF.
What is the difference between #3-56 UNF and #3-48 UNC?
Both are #3 diameter. #3-56 UNF has 56 threads per inch and uses a #45 tap drill (0.0820 in); #3-48 UNC has 48 TPI and uses a #47 drill (0.0785 in). The threads are not interchangeable.
Sources: ASME B1.1 — Unified inch screw threads (UN and UNR form) · Machinery's Handbook — tap drill sizes and percentage-of-thread formula · ASME B94.11M — twist drills (number and letter drill sizes) · Common machinist clearance-drill chart (close / free fit)
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