9/16-18 UNF tap drill size
The standard tap drill for 9/16-18 UNF is a 33/64" drill (0.5156 in, 13.1 mm), giving about 65% thread. At this size pilot-drill first and power-tap, thread-mill or single-point the thread.
| Drill | inch | mm | Thread % | Typical use |
|---|---|---|---|---|
| 1/2" | 0.5000 | 12.7 | 86.6% | Tight: high tapping torque, tap breakage risk |
| 13.0 mm (metric drill) | 0.5118 | 13.0 | 70.2% | General purpose |
| 33/64" (chart drill) | 0.5156 | 13.1 | 65.0% | Hard or tough material, deep or blind holes |
| Thread | Tap drill | Tap drill (mm) | Clearance close / free |
|---|---|---|---|
| 7/16-20 UNF | 25/64" (0.3906 in) | 9.92 mm | 29/64" / 15/32" |
| 1/2-20 UNF | 29/64" (0.4531 in) | 11.51 mm | 33/64" / 17/32" |
| 9/16-18 UNF (this page) | 33/64" (0.5156 in) | 13.1 mm | 37/64" / 19/32" |
| 5/8-18 UNF | 37/64" (0.5781 in) | 14.68 mm | 41/64" / 21/32" |
| 3/4-16 UNF | 11/16" (0.6875 in) | 17.46 mm | 49/64" / 25/32" |
Practical notes
Drilling 33/64" (0.5156 in) is a heavy cut: pilot-drill, then open up, and check the hole size before tapping. An undersize hole is the usual cause of a seized tap at this size.
Use a tapping head, rigid tapping or thread milling on a machine. For one-off parts, single-point threading on a lathe or thread milling avoids the cost and risk of a large broken tap.
Thread-forming taps need about 0.5347 in (13.58 mm, D − 0.5 × pitch); that is approximate, so check the tap maker's chart.
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 9/16 screw: close fit 37/64" (0.5781 in, 14.68 mm), free fit 19/32" (0.5938 in, 15.08 mm).
FAQ
What size drill for a 9/16-18 UNF tap?
The standard tap drill for 9/16-18 UNF is a 33/64" drill, 0.5156 in (13.1 mm), which gives about 65% thread.
What clearance hole for a 9/16 UNF screw?
Use 37/64" (0.5781 in, 14.68 mm) for a close fit or 19/32" (0.5938 in, 15.08 mm) for a free fit. Clearance depends on diameter only, so it is the same for UNC and UNF 9/16 screws.
What metric drill can I use for 9/16-18 UNF?
The closest metric drill is 13.0 mm (0.5118 in), giving about 70% thread on 9/16-18 UNF.
What is the difference between 9/16-18 UNF and 9/16-12 UNC?
Both are 9/16 diameter. 9/16-18 UNF has 18 threads per inch and uses a 33/64" tap drill (0.5156 in); 9/16-12 UNC has 12 TPI and uses a 31/64" drill (0.4844 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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