What are wire/number/letter drills used for?

i use # drills for many machine screw threads as follows
#8-32 tpi takes a #29 tap drill
#10-24 tpi takes a #25 tap drill
#10- 32 take a #21 tap drill
1/4"-20 tpi takes a #7 tap drill
 
I use them primarily because they are not Metric. I don't want to encourage practical systems of measurement that offer no challenge.

It is an interesting question, the different sizes must have come from specific purposes, because they don't change uniformly and the size differences don't seem to be entirely in relation to bit size. The smallest common sizes #79 (0.0145) to #80 (0.0135) is a difference of 0.001, #57 to #68 also changes in 0.001" increments, but you find numbers in the 69 to 78 range that change by 0.015 to 0.02".

You also find oddities like #55 which has a 0.0055" increase over #56, but is only 0.003" larger than #54. The change from #56 to #57 is 0.0035 so why such a big gap between #55 and #56? That is the biggest gap you find until you get into the low single digits and lettered bits.

From the 40s down to the teens they settle back into fairly standard increases of 0.0025 to 0.035, with some increasing upwards of 0.004" in the low teens and single digits. Then you come to #5 and #6 which are back to a change of only 0.0015, and then a big increase starts between #2 to #3 (0.008"), #2 to #1 (0.007") and #1 to A (0.006"). The letter drills are even more erratic ranging from lows of 0.004" to highs of 0.011" without any apparent pattern.
 
Aaron_W said:
I use them primarily because they are not Metric. I don't want to encourage practical systems of measurement that offer no challenge...........................................................
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I have metric drills too, we can add to the conversation. Have a bunch of them that literally match up to letter drill sizes as well as fractional sizes, too. Why they do that I don't know. I'm sure they are left over from the automotive industry. Definitely not from the oilfield industry! Could be military, too.
 
Aaron_W said:
I use them primarily because they are not Metric. I don't want to encourage practical systems of measurement that offer no challenge.

It is an interesting question, the different sizes must have come from specific purposes, because they don't change uniformly and the size differences don't seem to be entirely in relation to bit size. The smallest common sizes #79 (0.0145) to #80 (0.0135) is a difference of 0.001, #57 to #68 also changes in 0.001" increments, but you find numbers in the 69 to 78 range that change by 0.015 to 0.02".

You also find oddities like #55 which has a 0.0055" increase over #56, but is only 0.003" larger than #54. The change from #56 to #57 is 0.0035 so why such a big gap between #55 and #56? That is the biggest gap you find until you get into the low single digits and lettered bits.

From the 40s down to the teens they settle back into fairly standard increases of 0.0025 to 0.035, with some increasing upwards of 0.004" in the low teens and single digits. Then you come to #5 and #6 which are back to a change of only 0.0015, and then a big increase starts between #2 to #3 (0.008"), #2 to #1 (0.007") and #1 to A (0.006"). The letter drills are even more erratic ranging from lows of 0.004" to highs of 0.011" without any apparent pattern.
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It is based on, but not identical to, the Stubs Steel Wire Gauge, from 19th century England:
https://en.wikipedia.org/wiki/Drill_bit_sizes
 
One application I can think of is a little more modern that the origins of the nomenclature systems, so I doubt this is the rationale behind them, but they are used for rivets. Perhaps this would apply to tubular and solid rivets as well, giving the thought a little more age. Rivet sizes tend to be close to nominal, so you would probably want the holes you had to insert them into to be a little larger, but not a lot. Usually you can find the right size drill that will make insertion of the rivet (either a hot heading rivet or a modern "Pop Rivet") much easier and allow mislocation error less a problem. Of course there are all sizes rivets made, and in the past I suspect many more. Rivets date back quite a while. You can use size selection to control fit as well. So that gives more flexibility for different materials, yet all achieving an effective zero clearance joint when the rivet is upset. Nobody wants a leaky steam boiler.
 
Right on, Tony!. Trying to do rivets with just the nominal size drill is a PITA, and the next fractional size is too big. Number drills rule, at least until I find that set of .010 to 1.000" by .001". :)
 
Bob Korves said:
Right on, Tony!. Trying to do rivets with just the nominal size drill is a PITA, and the next fractional size is too big. Number drills rule, at least until I find that set of .010 to 1.000" by .001". :)
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I use them with models for the same reason. It;s like the 3 bears, this bit is too small, this bit is too big but this bit is just right.

I found this while looking for an explanation of why the wire gauge bits don't change uniformly. It is actually a paper from an Anesthesiology journal, specifically looking at needle gauges, but is related. It also offers an explanation of why gauge systems have survived even in places and industries that have otherwise solidly adopted the metric system. There are some practical reasons beyond resistance to change.

It does not however explain why our wire gauge drill bits have the weird exceptions to a more or less uniform size change.

http://www.freelanceanesthesieassistent.nl/upload/docs/the story of the gauge.x.pdf
 
" One of the posts that I did follow suggests that the numbers follow
the "Stubbs" wire gage very closely and that, since the bits were often
made from drawn wire, that's where the numbers came from."
This is what I found on Google. There is a reference to Sellers system.?
 
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