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

George.

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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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

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

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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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.

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

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

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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Interesting that in John's link above some like 0.35mm and 0.13mm are so much cheaper. Must be used a lot for something.

.

much cheaper indeed !!

There are quite a few bargains in the list "available while stocks last" and [I think] all these have shorter flute lengths. I suspect that they may have been sizes used for PCB drilling, and pretty-much obsolete now that most products have gone 'surface mount'.

MichaelG.