Sometimes "gauge" is just used to mean "measured thickness", and a
gauge of ".006" probably refers to a thickness of 0.006 inches. An
equivalent figure in "ML" is probably intended to refer to a thickness
measured in millimeters (for which the correct abbreviation is "mm").
There is also a measurement for steel based on "Gauge numbers". If you
see a "gauge" with an integer (such as "24 gauge") it's likely to be
referring to a gauge number.
A conversion chart between gauge numbers, thickness in inches, and
thickness in millimeters can be found here:
Maryland Metrics - Steel Thickness Conversion Table
On this table, "B.W.G." means Birmingham Wire Gauge, used for steel
wire. "U.S.G." means US Standard Gauge, used for stainless steel.
Other gauge scales also exist, so whenever a gauge is quoted it is
important to add the appropriate initials (for example "24 B.W.G.").
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