Drill Bit Size R: Letter Sizes A to Z Explained
Drill bit size r is a letter-size bit in the A to Z series, found in 26-piece letter sets and 115-piece combined sets beside fractional and wire-gauge sizes.
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A tap drill chart calls for a letter bit, and the drawer holds a row of bits marked with letters instead of fractions. Drill bit size r is a letter-size bit in the A to Z letter series, the series that sits beside fractional and wire-gauge sizes in a full drill bit set. Size R turns up in letter sets and in combined sets, so knowing where it sits in the series helps when a chart, a tap size, or a reamer calls for it by letter.
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Below: where size R sits in the letter series, which sets include it, how listings describe letter bits, and how to pick the right bit for the job. For the full range of sizes, see the guide to standard drill bit sizes and types of drill bit.
Drill Bit Size R: Where It Sits in the Letter Series
Letter drill bits run from A to Z, with R in the upper part of the series, after Q and before S. The letter is the size name, the same way a fractional bit is named by its fraction and a wire bit is named by its number. A listing for a single letter bit states the decimal diameter and the length for that letter, so the letter on the shank is a shorthand for a stated size. Century Drill and Tool lists its size F bit at 0.257 inches in diameter and 4-1/8 inches long, which shows how a letter size is published: letter first, decimal diameter and length beside it.
Letter sizes share a case with fractional and wire-gauge sizes in a combined set. Fractional sizes run from 1/16 inch to 1/2 inch in 1/64 inch steps, with 29 sizes in the set. Wire-gauge sizes run from #1 to #60, with 60 sizes covering 0.228 inch to 0.040 inch, or 5.79 mm to 1.02 mm. Letter sizes run from A to Z, with 26 sizes. A 115-piece combined set holds the full range: 29 fractional, 26 letter, and 60 wire-gauge bits. That is the set to look for when a job calls for a letter bit and the bench holds only fractional bits.
| Size system | Range | Sizes in the set |
|---|---|---|
| Fractional | 1/16 inch to 1/2 inch in 1/64 inch steps | 29 |
| Letter | A to Z | 26 |
| Wire gauge | #1 to #60, 0.228 inch to 0.040 inch | 60 |
Which Sets Include Drill Bit Size R
A 26-piece letter set runs from A to Z, so size R is in the case. HIDOTOL lists a 26-piece cobalt letter set, A to Z, in an indexed metal case, and Drill America lists a 26-piece high-speed steel letter set, A to Z, in a metal case. A 115-piece combined set also includes the letter range: HIDOTOL, COMOWARE, GuardPro, and Pro Edge Technology list a 115-piece cobalt set with 29 fractional, 26 letter, and 60 wire-gauge bits in an indexed metal case.
The cobalt letter bits in these sets are M35 high-speed steel with 5% cobalt, with a 135-degree split point and a round straight shank. The cobalt listings describe the split point as self-centering, so the bit starts on the mark instead of walking across the surface, and the fully ground twist clears chips while drilling. Those listings state the bits suit hardened metal, stainless steel, cast iron, wood, and plastic.
How to Pick the Right Letter Bit
- Match the letter on the chart. When a tap chart or a plan calls for size R, reach for the bit marked R, not the nearest fractional bit.
- Check the listing for the decimal diameter. A letter bit listing states the diameter in inches beside the letter, as the size F listing shows with 0.257 inches. Compare that figure with the hole the job needs.
- Pick the material grade for the work. M35 cobalt bits with 5% cobalt are listed for hardened metal and stainless steel, while standard high-speed steel suits general purpose work, as the Drill America listing states.
- Look for a 135-degree split point. Listings with a split point describe a self-centering start, which helps on smooth metal where a bit can walk.
- Keep the set in its indexed case. Letter bits look alike at a glance, and the indexed case keeps a letter in its marked slot.
What Letter Bits Cut
Listings for the cobalt letter sets name the same materials: hardened metal, stainless steel, cast iron, wood, and plastic. HIDOTOL and COMOWARE list titanium alloy and hard plastics among the materials their M35 cobalt sets cut, and the Drill America letter set is high-speed steel for general purpose work. The bit material matters more than the letter: the letter sets the hole size, and the steel grade sets what the bit can cut.
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FAQ: garage tools
What is drill bit size r?
Drill bit size r is a letter-size drill bit in the A to Z letter series. It is sold in 26-piece letter sets and in 115-piece combined sets that also hold fractional sizes from 1/16 inch to 1/2 inch and wire-gauge sizes from #1 to #60. The listing for a letter bit states its decimal diameter beside the letter.
What is a Z size drill bit?
A Z size drill bit is the last letter in the A to Z letter series, sold in the same 26-piece letter sets and 115-piece combined sets as size R. Letter sizes run in alphabetical order, so Z sits at the end of the series. Check the listing for the decimal diameter before drilling.
Is size R in a 115-piece drill bit set?
Yes. A 115-piece cobalt set holds 29 fractional, 26 letter, and 60 wire-gauge bits, and the letter range runs from A to Z, so size R is in the case. HIDOTOL, COMOWARE, GuardPro, and Pro Edge Technology list a 115-piece set with that mix.
What materials can a letter drill bit cut?
Listings state that M35 cobalt letter bits cut hardened metal, stainless steel, cast iron, wood, and plastic, with some listings adding titanium alloy and hard plastics. The steel grade sets what the bit can cut; the letter sets the hole size.
What does a 135-degree split point do?
A 135-degree split point is a self-centering tip. Listings describe it as preventing the bit from walking at the start of a hole and clearing chips while drilling, which keeps the hole on its mark and the bit cooler in use.
Do letter drill bits come in cobalt?
Yes. HIDOTOL, COMOWARE, and Pro Edge Technology list M35 cobalt letter bits with 5% cobalt in 115-piece sets, and GuardPro lists a 115-piece cobalt set with the same 29, 26 and 60 mix. MACXCOIP lists a 60-piece M35 cobalt number set covering #1 to #60. Drill America also lists a high-speed steel letter set for general purpose work.
Related: standard drill bit sizes, types of drill bit, the best drill bit set picks, and the best drill bit for steel.
