A letter r drill measures 0.3390 inch (8.611 mm). The closest fractional drill is 11/32" (0.3438") and the closest metric drill is 8.6 mm (0.3386").
What are the letter r drill equivalents?
| Drill | System | Decimal (in) | mm | Difference (in) |
|---|---|---|---|---|
| R | letter | 0.3390 | 8.611 | 0 |
| 8.6 mm | metric | 0.3386 | 8.600 | -0.0004 |
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 r drill run?
| Material | RPM (HSS) | Feed (in/rev) |
|---|---|---|
| 6061-T6 aluminum | 2,254 | 0.0068 |
| 360 free-machining brass | 1,690 | 0.0061 |
| 1018 cold rolled steel | 901 | 0.0051 |
| 304 stainless steel | 451 | 0.0034 |
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 r drill in inches?
0.3390 inch, which is 8.611 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, 8.6 mm is within 0.0004 inch.
What tap uses a letter r 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.