Subject: letter & number drills

From: [email-masked] (Kurt Rieder)
Date: 17 Sep 1996 16:05:19 GMT

All this posting about metric vs. non-metric (whatever you may want
to call the non-metric) has caused me to wonder about the many systems
used for drills (not to mention wire).  Why do we indeed have both
A-Z letter and 1-60 + 61-80 number drills along with inch fraction
and metric ?  Are any considered obsolete yet ?  Why did they limit 
the letter drills to just 26 (instead of AA, AB, AC, etc.) ?  I think
that there are actually duplications, with the same size showing up
in more than one system... don't make much sense to me ?  What's the
history behind all this ?

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From: [email-masked] (Jonathan M. Elson)
Date: 17 Sep 1996 18:08:34 GMT

Kurt Rieder ([email-masked]) wrote:
: All this posting about metric vs. non-metric (whatever you may want
: to call the non-metric) has caused me to wonder about the many systems
: used for drills (not to mention wire).  Why do we indeed have both
: A-Z letter and 1-60 + 61-80 number drills along with inch fraction
: and metric ?  Are any considered obsolete yet ?  Why did they limit 
: the letter drills to just 26 (instead of AA, AB, AC, etc.) ?  I think
: that there are actually duplications, with the same size showing up
: in more than one system... don't make much sense to me ?  What's the
: history behind all this ?  

The fractional sizes are obvious.  The number sizes are mostly in-between
the fractions, but for the larger sizes, what do you do after you get to
#0 (zero)?  It would get rediculous to have #00, #000, #0000 ad infinitum,
so the letter sizes extend the 'mostly in-between' sizes on the larger
end of the scale.  There ARE duplications, the 'E' is exactly .250"?
Extending the letter drills beyond Z gets to some pretty large sizes, as
the Z is .413".  fractional drills available in 64ths of an inch give
plenty of resolution when you get up near a half inch.  If you need an
EXACT size, you will drill undersize and ream or bore to final size,
anyway.

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From: [email-masked] (Bruce Holder)
Date: 20 Sep 1996 01:51:18 GMT

Kurt Rieder ([email-masked]) wrote:
> and metric ?

  It's fascinating to speculate about some of these traditional measures,
practices, nomenclatures, etc. I think it makes sense that in the smaller
sizes, the increments between each size are small and as the holes get
bigger, the differences between adjacent sizes can be bigger. So the
smallest fractional increment, 1/64 or .015625, is five times the 3 or 4
thousandths difference between the smaller numbered drills. By contrast,
while it is possible to get drills by 64ths from 1/2" to 1", you can't
get nearly as many different types and I've never seen sets of them in a
catalog.

  Now this part is purely speculation. I can think of two reasons for
the nomeclature: One, it's a easier to remember a one or two character
designation for a drill size than a three digit size in thousandths.
And two, I'm convinced there's an element of intentional jargonization.
(Is that a word?) We machinists, mechanical engineers, etc. are clearly
superior to the non machinists, etc. because we know that a #1 drill
is larger than a #10 drill, but an A drill is smaller than an M drill.

> Are any considered obsolete yet ?

  The letter, number and fractional sizes are all considered equally
obsolete by those who prefer metric. Otherwise no.

> Why did they limit 
> the letter drills to just 26 (instead of AA, AB, AC, etc.) ?

  As I noted above, I think it has to do with the increments between
adjacent sizes. By the time you get up to the Z drill, .413", the
fractional sizes are quite adequate to express the desired increments.

> I think
> in more than one system... don't make much sense to me ?

  Not as many duplications as you might think. There are obviously
quite a number that are close, but only one duplicate that I remember,
E = .250.

>  What's the history behind all this ?  

  Sorry, no history for sale here. But I'll speculate all day long.

Bruce

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From: Charles Snell <[email-masked]>
Date: 23 Sep 1996 00:12:49 GMT

If you look at a drill and tap chart you will find that there are several 
letter drills used with standard size taps, hence the reason for their 
existance. Yes, the letter and number drills fill in the gaps between the 
fractional drills, and are where you have to go if you want to drill for 
a close clearance, etc.
--

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From: Jeffrey Prothero <[email-masked]>
Date: Mon, 23 Sep 1996 19:22:42 -0700

On 23 Sep 1996, Kurt Rieder wrote:

>
> Well... that might explain a use for just a very few of the letter and
> number drills.  What about the rest of them ?  Filling the gaps doesn't
> nearly begin to explain it.... 1/128 inch fractional drill stemps might
> have done it better.  .................
>
> It would seem that the creators of letter and number drill series
> were marching to a different drummer.  What's the rational behind the
> step size in each series ?  It's obvious in both inch and metric drills.
>
>
  I think the number and letter drills are a geometric progression, where
each drill is the same percentage larger than the previous.

Useful if you were making drawing plates for wire, where you would want
the same amount of reduction of the wire.

Wire gauges are such that the diameter doubles every 6 numbers ie 6 gauge
wire is twice the diameter of 12 gauge.
					Cheers,

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From: [email-masked] (Kurt Rieder)
Date: 23 Sep 1996 20:39:29 GMT

Charles Snell ([email-masked]) wrote:

: If you look at a drill and tap chart you will find that there are several 
: letter drills used with standard size taps, hence the reason for their 
: existance. Yes, the letter and number drills fill in the gaps between the 
: fractional drills, and are where you have to go if you want to drill for 
: a close clearance, etc.

Well... that might explain a use for just a very few of the letter and
number drills.  What about the rest of them ?  Filling the gaps doesn't
nearly begin to explain it.... 1/128 inch fractional drill stemps might
have done it better.  And... look at the conflicts...
    1/4 inch and letter E are both 0.2500
    1/32 and #68.... 3/32 and #42.... 9/64 and #28.... 9/32 and K....
    etc, and many more, are 0.0002 or less apart in size... not enough 
    to be of any significance.... not to me, anyway.
It would seem that the creators of letter and number drill series
were marching to a different drummer.  What's the rational behind the
step size in each series ?  It's obvious in both inch and metric drills.

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From: [email-masked] (PnP Nolan)
Date: 25 Sep 1996 02:48:01 -0400

In article <[email-masked]>, [email-masked] (Kurt

>step size in each series ?  It's obvious in both inch and metric drills.

Errr, scuse me but...Aren't number and letter drills British Standard Wire
Gauge? If so, then where is the problem with E being 0.2500? Or perhaps we
should have the wire sizes range as... A, B, C, D, 1/4" F, G, H... <grin>

The rationale behind the sizes, I would suspect, has more to do with
cross-sectional area of the wire (and it's strength/moment of inertia etc)
rather than a uniform increment or step between the sizes. I doubt it was
a blatant attemp at a disordered range of drills.

Number drills are invaluable for replacing worn pins in all manner of
things. I often need to replace pins when accurizing competition hand guns
where the hole has worn a few thou. I use the next size up number or
letter drill and providing the straight shank portion of the drill is long
enough, I cut it off and use it as the pin. Perfect fit every time. At
$0.30 a piece I can make lotsa pins for not much money.

A competiton Hammer can cost $30+ to replace just because the hole is
worn. Not in my shop, since I have to ream and fit tighter pins anyway,
the number and letter drills hold a place of great use with me.

Peter

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From: [email-masked] (Carl)
Date: Wed, 25 Sep 1996 04:17:47 GMT

>step size in each series ?  It's obvious in both inch and metric drills.

   The different drill sizing systems were indeed created by people marching to
different drummers.

   Fractional drills are obviously sized by dividing intervals in half: eg. 1/32
is one half of 1/16, etc..

   Number drill sizes derive from numbered wire sizes.  Back before the era of
precision measurements each manufacturer had his own system of measurement, (if
he had a known good part he could make one like it). This applied to wire as
well as other products.  The wire drawer would know how he had made the
drawplates for a given batch of wire and could do a good job of duplicating them
when they wore out.  Anyone else would have a harder time duplicating the wire
sizes.  As a result each manufacturer offered a different set of wire sizes that
he felt would cover all needs.  Wire sizes were often called after the number of
the draw plate used for the final draw, ( a number 10 wire was last drawn on the
number 10 plate in a given manufacturers series).  The wire was drawn through
one plate after another to reduce it.  As the wire got smaller the amount of
reduction between sizes became less and less because the wire could stand less
and less strain before it would break. Thus the wire number was often an
indication of how many reduction steps it took to make it.

   Some common wire series are the B&S series,( which I think stand for either
Brown & Stubbs or Briggs & Stubbs)., AWG (American Wire Gage), and the Stubbs.
At one time there were probably many other series, just as there were once over
100 automobile manufacturers in the US.

   Ultimately drills made from the wire drawn to one particular set of standards
won the day and thus we have number drills.

   The Letter Drills  were an attempt by one, fairly sucessful manufacturer, to
differentiate his product by using letters instead of numbers and reversing the
order of progression, going from small to big.  I say "fairly sucessful" because
the system is still with us.

                                                               Carl Grinnell

[email-masked]