OldTools Archive

Recent Bios FAQ

274458 Don Schwartz <dks@t...> 2021‑08‑30 Re: Set of Mathieson plough iron (in metric sizes?)
This reminds me of the distinction between absolute and relative 
measure. What matters most in woodworking is not the absolute measure, 
but the fit, arguably also in metalwork.

And ... if the shoe fits, wear it ( no matter the size )!  Hurrah for 
common sense!

Don



On 2021-08-29 7:20 p.m., Thomas Conroy via groups.io wrote:
> Chris wrote:
>
>
> I recently bought a set of Mathieson plough irons.
> They are all in very good condition, marked from 1 to 8, all with the
> same stamp at the same location "ALEX. MATHIESON GLASGOW".
> This looks like an original set to me and by the look of it, they have
> been barely used....
>
> The sizes puzzle me, it looks almost like a metric set, as I can't
> make any sense of the imperial sizes.
>
> Here are the figures:
> Number mm inch mm Error (%)
> 8 15.44 0.607 15 +6.6
> 7 13.05 0.509 13 +0.7
> 6 11.83 0.465 12 -2.0
> 5 9.78 0.385 10 -2.2
> 4 8.21 0.33 8 +1.7
> 3 7.04 0.277 7 +0.3
> 2 4.97 0.198 5 -0.2
> 1 4.00 0.158 4 +0.0
>
> I can't make any sense of the imperial sizes (eg, numbers are not in
> 8th or a 16th of an inch), but the metric sizes almost match, except
> for #8 maybe....The #8 is off by 3/16th of
> an inch
> And then if it's metric, why some random numbers instead of the size in mm.
>
> Hum... Anyone seen something like that?
>
>
>
>    Hi, Chris,
> You are inhabiting the wrong mental universe. Get rid of all those little
numbers; they are useless and meaningless in the world of the traditional
woodworker. The first owner of those blades probably didn't have a measuring
tool graduated finer than sixteenths, and may not have had one finer than
eights. Wouldn't have known how to use one, or what to use it for. Why would you
need one? You weren't making things from measured drawings that had to be
precise to a thousandth of an inch or they wouldn't work. It wasn't until Henry
Maudsley started scattering larval machinists like ragweed pollen (1820s or
1830s, call it) that anyone cared about tolerances of less than a sixteenth of
an inch or so; and it wasn't until Laroy Starrett invented a machine for
precision marking rules mechanically (1870s, IIRC*) that fine rules became
affordable for the ordinary workman. They would work by divider and
straightedge, not by numerical measurement and sight comparison with rulers.
> For decades I wanted a set of classic chisels graded by sixteenths up to two
inches. Well, they don't exist. Oh, the old catalogues listed them, but they
don't actually exist. You can get sixteenths up to about 7/16", with a precision
of plus or minus1/32" if you are lucky. From 1/2" up to 1" or maybe 1-1/4" you
can find them by eights with a tolerance of a sixteenth. Eventually I got a
series of 2-1/2" butt chisels up to 1-1/2" by sixteenths, but I did it by
regrinding about a third of them; and it was only possible because many of the
great makers tapered their chisels back from the edge, sometimes by almost 1/8"
over 6". By starting with firmer chisels worn down to short butt length, I
sometimes got close to the dimension I wanted before I had to regrind.
> OK, your plough irons are a little irregular in change from one to the next
(that is the proper number to look at if you insist on numbers, not the actual
inches or milimeters). But not enough to be a real problem in use; if you just
choose the one that is closest to what you want, you will never be as much as
1/16" away from the "ideal" measurement; apart from the big 7-to-8 gap, you
won't be further off than about 1/32", roughly, if my numbers are right. Who
would need better than that anyway? If you need precision, use dividers and you
can go as close as you want, provided you don't try to cram an irrational
universe into a rational straightjacket (rational and irrational in the correct
mathematical sense, not in the vague half-assed common speech sense.)
>
> Tom Conroy
> Yes, I know that scattering machinist larvae like ragweed pollen is a mixed
metaphor. It's vivid, though. Betcha you'll remember it.*I checked. Graduationg
machines for steel machinists rules started with Darling, Brown and Sharpe
around 1850, not with Starrett around 1870. Doesn't invalidate the basic point.
Precision rules are machinists' tools, not woodworkers' tools. A woodworker
never needs a measurement as small as 1/32"; hell, a six-inch wide board will
shrink and swell more that that over the course of a year back where you have
summers and winters. If you need a tighter fit, it is faster, easier, and
actually more accurate to use divideres or direct comparison.
>
>
> 
>
>

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