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Plate Thickness Tolerances

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Because of the discussion of plate thickness tolerances as applied to HSS 
shapes and the assertion that mills are able to roll to closer and closer 
tolerances, I pulled out ASTM A6 to see what the under tolerances are.

Table 1, for plates ordered to thickness, Note 2 states, "Permissible 
variation under *specified* thickness, 0.01 in." [emphasis added].

Table 2, for plates ordered to weight, the under tolerance depends on the 
thickness of the plate, running from 3.0 psf for the thinner plates (plates 
<= 3/8"), to 1.0 psf for the thicker plates (plates >= 4").  For the assumed 
weight of steel at 490 pcf, the 3.0 psf underweight tolerance represents an 
under thickness of 0.07 inches (1/16 inch), 7 times the tolerance for plates 
ordered to thickness, and the 1.0 psf underweight tolerance represents an 
under thickness of 0.02 inches, still twice the tolerance for plates ordered 
to thickness.

Charlie Carter:  Can you provide a rational explanation for the disparity in 
tolerances for plates ordered by the two methods?

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

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