Decimal to Fraction Conversion - and Letter- and Number drill bits

I do not know about calculating. I have this chart stuck to the wall next to the drill press :-)

There probably is a simple mathematical rule used to generate the drill diameter tables. I don't know the rule, but I would guess that it is documented in the standard ANSI/ASME B94.11M-1993 (or earlier issue, which may be free), which is cited in Machinery's Handbook as the source of the tables.

Joe Gwinn

The wobbly metric conversion graph here suggests that the sizes were chosen arbitrarily to fill in the gaps in a fractional set:

didn't mention who decided on the number sizes. Stephen A. Morse invented the twist drill and Morse tapers.

jsw

I see a five-segment piecewise-linear function, slightly smeared by roundoff errors.

The sizes will not be arbitrarily chosen, as there is money at stake. The problem being solved is how to choose diameters such that the fewest number of sizes will cover the needed range. Drill gauges were modeled on wire gauges, and wire gauges most definitely follow a rule.

From the look of the wiki plot, I would guess that if one plots log of diameter against drill size (number or letter), one will get a roughly straight line, although the segments will remain visible.

Anyway, I imagine that ANSI/ASME B94.11M has the story, or at least cites the story.

Joe Gwinn

I think it's optimistic to assume that number drills follow a rational sequence. The drill gauge is more or less equivalent to the Stubs' Steel Wire Gauge which, according to Wikipedia, was derived by averaging the diameters of samples from a large collection of wires belonging to a Mr Stubs in the 18th century.

This reminds me of a case of "standardization" I ran across when doing some design work for GE's steam turbine division about 20 years ago. One of the things I worked on was adapting punch press tooling that was made in Lynn MA to fit a different machine in Bangor ME. The punch mounting plate had a hole pattern for the fasteners that was almost symmetrical, but not quite. The difference was small enough that it was not apparent when looking at the part or a drawing, and I was constantly checking to make sure that the mounting holes in the parts I was designing were offset in the proper direction. I finally had to go to Lynn to get some information from the old toolmaker who maintained the die sets and asked what the reason was for the odd hole spacing. I expected to hear something about avoiding interferences with their press or the like. What he told me was that the guy he was apprenticed to made the first prototype die set back in the 20s or so and, since it was only supposed to be a prototype, simply eyballed the holes. All I could think of was how many man-hours had been wasted over 60 years keeping track of those odd locations and reworking the resulting screw-ups.

Standards? Of course we have standards. We have LOTS of standards.

I sometimes flat-braid prototype wiring harnesses instead of twisting them, for fun or to fit slots better, and a few have made it into mass production. Somewhere in China an assembly girl must hate me.

jsw

The averaging could well be to overcome run-of-manufacturing variations in what wire diameters they could easily make. Mr Stubbs had to be important if they rummaged around in his shop to set the standard. Perhaps he was a leading maker of hard or hardenable steel wire in the day.

Whatever the article in question, all the manufacturers had their own gauge or number series, usually generated by some rule, no two exactly the same. The standard was written after one or a few of these manufacturers had won in the marketplace.

Sounds like the story of rail gauges in the age of steam.

We all get burned by that only-a-prototype scam, once.

In the programming world, the standard defense was to do the hack work in a language forbidden for the delivered system, so some desperate manager couldn't declare the hackwork to be "good enough - we need only a chevy, not a caddilac, ...". We all know this song.

Joe Gwinn

I like it. Now we need an analogous dodge for hardware. I delivered a "prototype" machine a few weeks ago that I fear is slipping into permanent status. Twenty years from now the new guy will be wondering what the hell I was thinking when some of the design decisions were made. What I was thinking was how to deliver in six weeks a prototype of a machine where I originally estimated a 4 month design and build.

You could build all the framework out of randomly spray-painted 80/20 that you cut with a hacksaw.

jsw

Or strapping, OSB, and drywall screws.

But neither aluminum nor wood would have lasted long enough to qualify even as a prototype in the case I was whining about -- lifting moly ingots out of a furnace at 2600F.

So go in one saturday, take a MIG Welder, and in big letters on the main structural member stitch "Prototype #1 - 2009, Ned S." Someplace where it's impossible to miss, especially when the CEO comes out for his annual inspection tour.

Perhaps in 20 years when they get ready to grumble, they will see that label and realize it wasn't built to last - and the "permanent" machine is about 19 years overdue.

Give it a year. And once it has been broken and patched and rethought and improved a few times (and it stops breaking and/or slipping and dropping the stock) the PHB's will realize they have just completed most of the design work for the permanent one.

-->--

...

Can't help you with that one.

Stunt people sometimes wear large funny hats when testing so the footage can't be used in the final cut.

jsw

Correct.

Maybe not; the original of the drill gage is a wire gage, and the wire-drawing process is more likely the driving technology, not the drill requirement.

Drawing wire is done by pulling through a succession of dies, each a little smaller than the one before. If you can narrow the diameter by 20% on each pull, without unacceptable breakage or die wear, that fraction will remain the same regardless of what the die diameter is.

So, a wire-drawing plant starts with a few pounds of hot-rolled rod, and after the first sizing die, calls the product 'zero gage'. Two pulls later, it's #2 gage, etc. Straight line on a semilog plot is exactly what one expects.

Maybe yes. It may be that the rule for number drills was driven by the rule for drawing wire, but there nonetheless is a rule.

Yep. My point exactly. Thanks for the details.

Joe Gwinn

Hi all -

We updated the decimal to fraction converter to include Letter/Number and Metric drill sizes. Just did a brute-force lookup table with appropriate sizes, after reviewing the information in this thread.

Not yet a graphical user interface, but maybe later.

The executable is compressed in "Fractions.zip" and downloadable from

Still Windows only... comments appreciated.

Help yourselves!

Carla

There are two kinds of statistics: the kind you look up and the kind you make up ~ Rex Stout

Carla F> Hi all -

Cool!

Thanks Carla!

--Winston

93.7% of all statistics are made up out of thin air. ;-)

Cheers! Rich

Sorry, it's only 43%...

David

But the number sizes were tweaked a bit to avoid duplicating fractional sizes, so there are more fine steps available for a complete set of drill bits. (The letter sizes have an exception -- isn't it 'F' which is precisely 1/4"?) For even more fine steps, get a set of metric bits in 0.1mm steps to keep with your fractional/number/letter set.

BTW -- How many of you have one of the 115 bit HUOT indexes which hold fractional, letter, and number sizes? How many of you have wondered what the little bent-up hook is for in the letter size section?

Well ... it is just right to hold a HUOT index for #61-#80 wire sized bits, so I have just exactly that in mine. Presumably somewhere it is documented that this is what it is for, but if so, I haven't seen it yet. :-)

Enjoy, DoN.