A letter q drill measures 0.3320 inch (8.433 mm). The closest fractional drill is 21/64" (0.3281") and the closest metric drill is 8.4 mm (0.3307"). It is the standard tap drill for 3/8-24.
What are the letter q drill equivalents?
| Drill | System | Decimal (in) | mm | Difference (in) |
|---|---|---|---|---|
| Q | letter | 0.3320 | 8.433 | 0 |
| 8.4 mm | metric | 0.3307 | 8.400 | -0.0013 |
| 8.5 mm | metric | 0.3346 | 8.500 | +0.0026 |
Tap drill for: 3/8-24.
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 q drill run?
| Material | RPM (HSS) | Feed (in/rev) |
|---|---|---|
| 6061-T6 aluminum | 2,301 | 0.0066 |
| 360 free-machining brass | 1,726 | 0.0060 |
| 1018 cold rolled steel | 920 | 0.0050 |
| 304 stainless steel | 460 | 0.0033 |
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 q drill in inches?
0.3320 inch, which is 8.433 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, 8.4 mm is within 0.0013 inch.
What tap uses a letter q drill?
It is the standard 75% thread tap drill for 3/8-24. Tap drills are chosen to leave about 75% of full thread depth, the balance of strength and tapping torque that published charts use.