Via my web page, I get frequent requests for a formula that relates numbered drill size to the number designation. This typically from the poor benighted souls who think that there should be some sort of logical relationship in American metalworking so-called 'standards'. I respond to these people by saying that it is possible to fit a practical (i.e., first order) least squares equation but the maximum error for a particular drill size runs as high as 10% which is just too much for precision metalworking. (If you're curious, the fit is included below.) Higher order fits can decrease this error slightly but the resulting fifth or sixth order polynominal is just too complex to be practical in a shop environment. That's why everyone uses a table for these drill sizes.
The last fellow who inquired about this also wanted to know the origin of the numbered drills. That set me off to do a bit of research and, since I'll soon be out of the business of writing articles for the club, I decided to pass my findings along to all of you.
You may hear oldtimers referring to the numbered drills as 'wire gage' drills. This gives us a hint of where to start looking. Perhaps the numbers are related to a particular wire gage 'standard'. Now the nice thing about standards is the fact that there are so many of them. There are literally dozens of so-called 'standards' for wire sizes.
Wire is made by taking an ingot and running it through a rolling mill. This is done again and again to produce the finer wire sizes. Now, the idiots who run rolling mills decided to start labeling their wire by the number of times it's been rolled. It's hard to understand how anyone would be dumb enough to use such a stupid system. I can only suggest that this was due to the presence of too many ex-pat Brits in the early rolling mills. After all, what can one expect of people who use a unit of weight called a hundredweight (cwt) which actually weighs 112 pounds? If ever there was an argument for adopting the Metric System approach of labeling things by their size, wire (and sheet) gage numbers are it.
Labeling by the number of times rolled explains why the higher numbers correspond to smaller sizes. Not only is the system stupid, it's backwards as well! However, in keeping with the anything-to-maximize-confusion nature of the Imperial system, I must point out that there *are* wire gages where the size *increases* with the gage number. Music wire makers especially seem to favor this approach. I'm frankly surprised that these cretins didn't label their wire with the notes of the musical scale so one could run around measuring wire by 'twanging' it.
At any rate, some judicious Googling led me to the Stub's *steel* wire gage. In the table below, this 'standard' is compared side-by-side with the numbered drill sizes. They're close (but, of course, not quite equal) across the full range from 1 to 80 gage numbers. Since an agreement this close doesn't exist for any of the other 'gage' standards, I can only conclude that the numbered drill series was derived from the Stub's *steel* wire gage, with a bit of finagling thrown in to prevent any accidental total agreement, and thus preserve the sacred illogic and confusion of the Imperial system of measurement.
Should you decide to research this further, note that there is also a Stub's *iron* wire gage. By this time, it will probably not come as a surprise to learn that it's not the same as the Stub's *steel* wire gage.
Finally, before you ask, I don't have a clue about the origin of the letter drills. I can, however, picture some British boffin in a post-colonial New England factory muttering to himself, "Blimey, nobody will ever need more than 26 drill sizes. Let's just label the blighters with letters."