A letter t drill measures 0.3580 inch (9.093 mm). The closest fractional drill is 23/64" (0.3594") and the closest metric drill is 9.1 mm (0.3583").
What are the letter t drill equivalents?
| Drill | System | Decimal (in) | mm | Difference (in) |
|---|---|---|---|---|
| T | letter | 0.3580 | 9.093 | 0 |
| 9.1 mm | metric | 0.3583 | 9.100 | +0.0003 |
| 23/64" | fraction | 0.3594 | 9.128 | +0.0014 |
Drill sets that include this size
Number and letter sizes come in indexed sets; buy the set rather than single drills.
What speed should a letter t drill run?
| Material | RPM (HSS) | Feed (in/rev) |
|---|---|---|
| 6061-T6 aluminum | 2,134 | 0.0072 |
| 360 free-machining brass | 1,600 | 0.0064 |
| 1018 cold rolled steel | 854 | 0.0054 |
| 304 stainless steel | 427 | 0.0036 |
Starting points from conservative HSS drilling speeds. Small drills often call for more RPM than a benchtop drill press offers; run at the top speed and feed gently. Use cutting oil in steel and stainless, and clamp the work: never hold it by hand.
Cutting fluid for drilling
Frequently asked questions
What is a letter t drill in inches?
0.3580 inch, which is 9.093 mm. Letter drills run from A (0.234 inch) to Z (0.413 inch), continuing upward from the number drills. If you do not have this exact size, 9.1 mm is within 0.0003 inch.
What tap uses a letter t drill?
No common UNC or UNF thread uses it as its standard 75% tap drill, but it often serves as an alternative drill for a slightly different percentage of thread, or as a clearance or pilot size. The tap drill chart lists every standard pairing.