From: shoppa@alph02.triumf.ca (Tim Shoppa)
Newsgroups: rec.crafts.metalworking
Subject: Re: Gauge thickness history?
Date: 22 Feb 1998 01:47:32 GMT

In article <34EDB8B9.2C91@lanl.gov>, MEMoore  <memoore@lanl.gov> wrote:
>What is the reason or the rationale or the history
>for the system of gauge thickness for sheet metal?
>
>Why as the gauge number decreases does the thickness of the metal
>increase?

The earliest standard for sheet metal gauges actually ran the
other way.  This is the American Institute of Mining Engineers Standard
Decimal Gauge, from 1877.  The gauge number was the thickness
of the sheet in thousandths of an inch.  The series ran from
2 to 22 by 2's, 25, 28 to 40 by 4's, 40 to 100 by 5's, 110 to
180 by 15's, and 200, 220, 240, 250.

The gauges that you're more familiar with began with the "US Standard
Gauge" for tax-levying purposes in 1893.  They defined the mass
of a cubic foot of wrought iron to be 480 lb.  A 1 foot by
1 foot by 1/2" sheet would weigh 20 pounds, or 320 ounces, and
the gauge for sheet steel weight 320 ounces per square foot was
defined at 7/0.  From #7/0 to #0, the sequence went down by
20 ounces, 320, 300, ..., 180.  Then from #0 to #14 it went down
by ten ounces; from #14 to #16 by five ounces; from #16 to #20
by four ounces; from #20 to #26 by two ounces, from #26 to #31,
ounc ounce, from #31 to #36, by half an ounce, and from
#36 to #38, a quarter of an ounce.  In this scheme, assuming
that 480 lb is the weight of a cubic foot of sheet metal, #16
is 1/16" an inch - a nice round number.  But no, life couldn't
be that simple!

The US standard gauge wasn't very useful, because most sheet metal
was rolled steel, not wrought iron, and rolled steel weighed closer
to 502 pounds per square foot.  It was never clear whether steel was
being sold/taxed by thickness or by weight per square foot.

After two years of confusion, the American Railway Master Mechanics
Association called for a return to the decimal gauge.  Since then,
the decimal gauge has often been called the "Master Mechanics Gage".

Despite this plea, most of the manufacturs continued to use a variant
of the US standard gauge, interpreted by weight and not by thickness.
This is called the "Manufacturer's Standard Gauge", and is the one
in common use in the US for sheet steel.  This is why #16, which
was originally 1/16"=0.0625" thick in the US Standard gauge, is actually
480/502 this thickness, or 0.0598".

Aluminum, copper, magnesium, and zinc are measured in  completely
different gauge(s) and have stories of their own!

Tim. (shoppa@triumf.ca)