# What are wire/number/letter drills used for?



## CarlosA (Dec 14, 2017)

Now, that is kind of a click bait title because we all know what _most of them_ are used for.

More specifically: What are the odd drills in the range used for? For example a number 35 (.110") drill does not show up on any chart I have. But a number 30 shows up for a metric tap m4 .7, or a number 33 shows up for a 6-40 tap thread. A number 33 shows up for the body size of a 4-36,40,48 machine screw. I would say maybe the #35 is for a reamer, but the closest reamer is 7/65" at .109" - Drilling for an oversized reamer for a 7/64 would still be too sloppy. A number 36 drill is .111" - is there ever a reason to ream a #36 hole?

So what are the rest of the drills used for? I have not really researched this yet, just curious what you all think.


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## Dave Paine (Dec 14, 2017)

Related to wire gauge sizes.   A wiki page.  Scroll down to the number and letter section.  I think since these are common in the US they are often used if they are the closest drill bit for a given use.

Number and letter gauge drill bit sizes


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## CarlosA (Dec 14, 2017)

Dave Paine said:


> Related to wire gauge sizes.   A wiki page.  Scroll down to the number and letter section.  I think since these are common in the US they are often used if they are the closest drill bit for a given use.
> 
> Number and letter gauge drill bit sizes



Nice chart, thanks!

Size 97 at 5 thousandths is impressive, would like to get ahold of some of those.


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## Bob Korves (Dec 14, 2017)

CarlosA said:


> Now, that is kind of a click bait title because we all know what _most of them_ are used for.
> 
> More specifically: What are the odd drills in the range used for? For example a number 35 (.110") drill does not show up on any chart I have. But a number 30 shows up for a metric tap m4 .7, or a number 33 shows up for a 6-40 tap thread. A number 33 shows up for the body size of a 4-36,40,48 machine screw. I would say maybe the #35 is for a reamer, but the closest reamer is 7/65" at .109" - Drilling for an oversized reamer for a 7/64 would still be too sloppy. A number 36 drill is .111" - is there ever a reason to ream a #36 hole?
> 
> So what are the rest of the drills used for? I have not really researched this yet, just curious what you all think.


They are used for whatever you need them for, whether it is an orifice to control air, oil, or water, for making starter holes for wood and sheet metal screws in various materials, if you want a bit looser hole for tapping in hard materials, etc.  They help you to make the size hole you think you need for a job.  If drill bits came in .001" (or .01mm) increments like gage pins, I would be tempted to buy a set...


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## kd4gij (Dec 14, 2017)

Drilling holes that size.


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## FOMOGO (Dec 14, 2017)

I have several sets of 61-80 for doing appliance and automotive jetting. Never really pursued the why  of it. He who is there first makes the rules, pertains to many things. Mike


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## woodchucker (Dec 14, 2017)

CarlosA said:


> Nice chart, thanks!
> 
> Size 97 at 5 thousandths is impressive, would like to get ahold of some of those.


99, 97 is 6 thou.


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## 4GSR (Dec 14, 2017)

woodchucker said:


> 99, 97 is 6 thou.


Drilling holes in hypodermic needles....


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## Wreck™Wreck (Dec 14, 2017)

kd4gij said:


> Drilling holes that size.


Entirely to obvious.
Number drills are for drilling holes in numbers.
Letter drills are for drilling holes in letters except for the letter Q, the letter Q can not be drilled at all, it's a nasty little bugger.
Likewise fractional drills will drill fractions in 1/64" increments, if you require 1/128" increments this requires a non sinusoidal high modulus zero size drill.

Hope this helps


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## Mitch Alsup (Dec 14, 2017)

I have used a D drill (0.246) to put bearing races (1/4-1/8-3/32 ball bearings:: 0.250)  in Al so that the bearings can be pushed in with thumb pressure but not fall out under vibration nor gravity. I simply use calipers or micrometers and measure the thing being fitted, then run some tests in spare material to check for fits.

Once you get smaller than about 1/3rd of an inch (8mm) you depend more on drilling and less on boring.


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## Ulma Doctor (Dec 14, 2017)

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


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## Aaron_W (Dec 14, 2017)

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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## 4GSR (Dec 14, 2017)

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...........................................................


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.


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## westsailpat (Dec 14, 2017)

Fill in the gaps with some MM drills .


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## Bob Korves (Dec 14, 2017)

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".
> 
> ...


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


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## Tony Wells (Dec 14, 2017)

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.


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## Reddinr (Dec 14, 2017)

For when the next smaller drill is broken.


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## Bob Korves (Dec 14, 2017)

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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## Aaron_W (Dec 15, 2017)

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".



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


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## EmilioG (Dec 15, 2017)

" 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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## Bi11Hudson (Dec 15, 2017)

Wire sized drills fill many different functions. A Nr 30 drill at 0.128" leaves .003" for a .125 rivet. Basicaly 1/8 inch plus .003". A common clearance hole in the aircraft industry. Another is .1875 + .003 (3/16"). The end result is 0.1905, a really off the wall number. I do have a drill(several) this size. Rather tight spiral for deep cuts in aluminium.

Machine screws line up for many of the small sizes. A size "0-80" screw is 0.060". Add (or subtract) 0.013" for each screw number. ie. #2-56 screws are calculated as (2x.013")+060"=0.086". Smaller sizes work out negative. ie. A #00-90 is 0.060 *minus *0.013" =.047*" *. A 000-120 (3/0) is 0.034" and a 0000-160 (4/0) is .021". There are smaller sizes but 4/0 is the smallest I use for my models. The thread pitches noted are the ones I use. There are others, just as there are for larger sizes. Of course, each tap size requires a different size drill.....

I don't know the reason for .060 plus or minus .013 for machine screws but it dates *way, way *back. At least into my grandfather's time. (He died in 1912....) A curiosity here; the screw that holds a wall plate on your room lights is a #6, 0.138" dia. (Tap size for #6-32 TPI is a #36) It also is 3.5mm, although I don't remember what the tap size is in metric.

Metric measurement goes back to 1820 or so. Thomas Jefferson was a big proponent of metrics. But England still was using the Imperial system and *Con*gress wanted to maintain a good trade relationship with them. *Pro*gress was set on the "back burner" that time, for sure. (Pun intended......)

I've babbled on far too much here but you must consider the fractional inch measurements as somewhat primative as a system that was held onto because of politics.

Bill Hudson


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## 7milesup (Dec 16, 2017)

One practical use for the extremely small (.25mm) or thereabouts size drills is cleaning out the nozzle on my 3D printer.  Don't do it very often but it is nice to have an actual drill bit to remove the plastic residue vs. just trying to use an accupuncture needle or similar.  I also use very small bits for adding details to my RC models.


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