The single most important cutting tool for a drill machine is the drill bit. They are found in any size you want. However, standard sizes are being maintained all over the world with defined sets of sizes. The sizes are maintained by different systems of measurements followed in different parts of the world.
In the U.S. the measurement system for drill bits is the fractional-inch and gauge bit sizes. In nearly all other countries, the metric system sizes are maintained. What is the difference between these sizes?
Well, the metric drill sizes are defined by the diameter of the bit in terms of metric lengths. Fractional drill bit sizes, on the other hand, make inch-sized products.
What is a lettered drill bit?
Other than the drill bit sizes mentioned earlier we also have numbered and lettered drill bit sizes. The numbered ones have gauge sizes ranging from 80 to 1; the number 80 is the smallest while 1 is the biggest.
Then starts the lettered drill bits. They are denoted by the letters from A to Z. Here A is the smallest and Z is the largest. These lettered drill bits fall in between the normal 1/64 or 1mm jumps between the sizes of the other two sets.
In other words, lettered bits are simply the larger versions of the numbered wire sizes.
What are letter drill bits used for?
As we’ve seen, most of the measurement systems followed for drill bits are metric and fractional measurements. However, they have their limitations. That is why numbered and lettered drill bits were developed to make up the gaps between these two sizes.
The most common uses for lettered and numbered drill bits are for using taps since you won’t probably find the exact match for fractional or metric sizes, but you can get close to them by using lettered or numbered bits. Since they fall in the sizes between those two types, they are used whenever a precision-sized hole is needed to be made.
Moreover, number and letter sizes are also used for twist drill bits, more than other forms, as their ranges encompass the sizes used more in twist drills. Besides, when you want to obtain maximum thread depth, lettered drill bits are used.
The chart for lettered drill sizes
The chart below will show you the drill bit sizes of all the lettered drill bits from A to Z. In the second column the letter drill sizes in inches are given.
| Drill Size | Inch | mm |
|---|---|---|
| A | 0.234 | 5.94 |
| B | 0.238 | 6.05 |
| C | 0.242 | 6.15 |
| D | 0.246 | 6.25 |
| E | 0.250 | 6.35 |
| F | 0.257 | 6.53 |
| G | 0.261 | 6.63 |
| H | 0.266 | 6.76 |
| I | 0.272 | 6.91 |
| J | 0.277 | 7.04 |
| K | 0.281 | 7.14 |
| L | 0.29 | 7.37 |
| M | 0.295 | 7.49 |
| N | 0.302 | 7.67 |
| O | 0.316 | 8.03 |
| P | 0.323 | 8.20 |
| Q | 0.332 | 8.43 |
| R | 0.339 | 8.61 |
| S | 0.348 | 8.84 |
| T | 0.358 | 9.09 |
| U | 0.368 | 9.35 |
| V | 0.377 | 9.58 |
| W | 0.386 | 9.80 |
| X | 0.397 | 10.08 |
| Y | 0.404 | 10.26 |
| Z | 0.413 | 10.49 |