Number series drills - history and origins

Needing to once again drill a few holes shown on the drawings as number sizes but using my metric drill set, as usual I turned to Google to find the equivalents. This happens frequently enough that I really could save myself tons of time if I took the trouble to print out a table to stick on the workshop wall once and for all.

One of the first result returned by Google, was a link to the very useful but unfortunately not very reliable Wikipedia.
A case in point being the Wikipedia - Drill bit sizes page which has a subsection dealing with the "US number and letter gauge drill bit sizes" where one can find the largely irrelevant statement that they are "analogous to, but different from, American wire gauge".

Now, to the best of my knowledge the number drill series originated in the UK and not in the US, I also believe that the numbers size are related to but not identical to the Stubs steel wire gauge.

As so many people seem to take what they find on Wikipedia as gospel, when I come across any glaring errors, I do try to set them right. To this end I would appreciate it if anyone could shed some light on the history and origins of the number drill series and preferably provide some links to reference material.

Also, are the Stubs iron wire and Stubs steel wire gauges two different standards and are either or both the same as the Birmingham wire gauge? Wikipedia is about as clear as mud on the subject.

Thanks,
Clive

The words " can of worms" springs to mind on this question as there are so many different systems depending on the intended use of the wire or sheet.

If you can get a look at a Machinery's Handbook you will see what I mean.

Can't give you a history but can show scan from 'Newnes mechanical Engineers Pocket book.. 1990...

George.

"Also, are the Stubs iron wire and Stubs steel wire gauges two different standards and are either or both the same as the Birmingham wire gauge?"

Stubs Iron Wire and Stubs Steel Wire are different from each other, and both are different from Birmingham Gauge, - vide "Machinery's Handbook"

On drills I take Zeus tables as gospel. Works for me but when buying number drills now there may be a chance that they aren't to UK standards if there are any differences.

Looking at an american burner design it calls for a #71 drill and then mentions 0.026" which is the same size as a brit #71 according to pre metric equiv Zeuss tables. More recent Zeuss give 0.65mm = 0.0256"

The number drills do not relate to wire sizes etc. #16 for instance is 0.177" dia.

John

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Birmingham Wire Gauge is just another name for Stubs Iron Wire Gauge. It was modified slightly and became British Standard Wire Gauge in 1883.

It is however totally different from number drill sizes - no idea where they came from.

In both cases they are in a more or less exponential series which makes a lot of sense.

Russell.

From Machinery's Handbook (10th Edition - 1941)

"The sizes of small twist drills are designated (1) by gage numbers; (2) by letters; (3) by fractional dimensions. Many of the smaller sizes conform to the Twist Drill and Steel Wire Gage. Stub's Steel Wire Gage has also been used to some extent, especially for carbon steel drill rod, but the Twist Drill and Steel Wire Gage is now employed almost universally for twist drills made to wire-gage sizes"

So I think that we can assume that twist drill "gage numbers" were a standard in their own right as defined by the "Twist Drill and Steel Wire Gage" and therefore not directly connected to Stub's wire sizes.

Regards,

IanT

Most, if not all, gauges relate back to how much you get from a given weight of product.eg- how thick is the wire if 1lb of iron is drawen out ? Hence the differences, lead wire and brass wire will give different dias. Shotgun gauges are the size of that no. of balls from 1lb of lead. Glass is similar, or was 'till metrication. I know this is over simplified, and has been modified over the many years.

I have been looking for years and it seems the origin of drill letter and number gauges is lost in time.

The series shows no obvious logical progression, this graph shows the gap between successive drills. As you might expect it gets bigger towards the larger drill sizes, though there are a few big leaps at apparent random.

Someone patient might want to see if the actual sizes have individual purposes (e.g. different fits on various sizes of stock or wire).

More likely, someone, somewhere was making drills drawing the wire though dies made to no particular standard, some more worn than others. The irregular succession of sizes became the de-facto standard, perhaps?

Neil

Although we like to think of everythig being invented by either the Brits or Yanks it is worth considering the useage of this product at the time of conception. Primarily the watch and clock making industries which had been predominately German and Swiss with the latter probably getting supplies from the former.

If you plot size in mm agaionst number you see distinct discontinuities at or near round figures of mm. this indicates it the larger sequence was the drawing together of several pre-existing sequences with a bit of filling in the gaps. This would tie in with the multiple thread size sequences in use in watchmaking in th e19th century.

When the UK industry created a demand someone pulled together a number of sequences to make an overall list probably only to make their catalogue easier.to order from. Railways and communications in mid C19 was making centralised production and supply of standardised parts and materials more common.

It is quite likely that some sizes were only included to fill in the table and were never commercially made except to make drill sets!

It's interesting to note comments about watches and clocks. Some feel there is evidence that europe basically made cheap copies of UK stuff in all sorts of areas. Pass. Also that German quality is largely a result of buy German policies to help get their economy up and running post the wars - in other words part fiction rather than fact.

The step in sizes of the drills could well be similar to wire gauges but a little more exact. Wire gauges and their tolerances were formulated on the basis of what ever was made could be sold. In the case of drills what ever size they actually turned out to be.

John

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A "Can of worms" as described by Brian in a posting above really does seem most appropriate when it comes to wire and other gauges.

However, a US Department of Commerce and Labor publication - Circular of the Bureau of Standards No.31. Copper Wire Tables (1st Edition 1912) contains a "Short history of wire gages" which does offer some insight into the subject. Unfortunately it makes no specific mention of number drills, it does however confirm that the Birmingham Gauge (Stubs Iron Wire Gauge) numbers refer to the number of times the wire was drawn, each subsequent drawing causing a reduction in size. So for example the first drawing is gauge 1, the second gauge 2, etc.

The above mentioned publication also includes a reference to an article published in The Electrical World Vol 10, 1887 by S.S. Wheeler that describes "over 30 gages, 19 of which were wire gages" and has elsewhere been described as "the most complete exposition of the subject yet made". Unfortunately this article must have been of some value to the vandal who removed the relevant pages, 253 to 259, from the copy scanned by Google sometime in the intervening 128 odd years since publication.

In regard to the number drills. Many references state that the Stubs Steel Wire Gauge is the basis of, though not identical to, the numbered twist drills sizes. From a cursory comparison of a few of the number drill sizes in relation to the same number Stubs Steel size, it seems that the number drills are sized between 1 and 3 thou larger than the equivalent numbered Stubs size. So it could be possible that the number drill series were originally intended to provide clearance holes for Stubs Steel Wire Gauge sized material. Naturally the letter/number series sizes now cover a much wider range than the Stubs sizes which would explain why the Stubs is now regarded as only a basis.

Lacking any further information, and not being sufficiently interested in the subject to devote much more time to it, for now I will just amend the the current US-centric Wikipedia entry to more correctly reflect the international nature of the numbered drill series.

Clive

Edited By clivel on 04/08/2015 08:58:54

Are number drills still necessary. Noticing that there is only a few thou difference on consecutive number drills, is that level of accuracy needed on a drilled hole and can twist drills create that level of accuracy. A set of imperial fraction drills and metric drills increasing in steps of 0.1 are all that are needed. Number and probably letter drills seem a novelty and a curiosity of past times.

Handy comparator of different wire gauges here.

Russell

If You make parts commercially they are necessary and more common than You would think.

From Machinery's Handbook 1914 First Edition Replica:

When I worked at K shoes making punch dies we used number drills almost exclusively, the tiny difference between sizes helped us get a good fit on pressed in punch tubes.

No idea of origin or if they matched anything else though.

Steve

In aviation engineering I remember that we used a #30 drill for 1/8" rivets.

Ian S C

Ian has summed up the sort of things that they are used for perfectly and why the differences are small increasing with size. Another is gas jets and items like that. They and letter drills probably largely came about to fill in the holes in 1/64" drill sizes. Metric 0.1mm steps in size can replace a lot of them but taking 1mm for instance that's 10% and a fair difference in area. The other point though at some size 0.1mm steps are smaller than the really need to be.

Actually there are a sort of metric equivalent

**LINK**

My source of unusual drills but in this case I suspect I will stick to # drills. Much cheaper.

John

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I find the 0.1 mm increment size is too big a step for small tapping drills. Colin