Quote:
Originally Posted by bkahl
-Always do CAD estimates, but know that the estimate is lighter than what it will actually be. CAD is just mean like that.
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CAD isn't mean. It's a computer program. GIGO. Your calculation is only as good as the information you put in.
Just for general information, material nominal dimensions are sometimes given at what is known as "minimum material conditions" (unless specifically noted otherwise with a tolerance). This means that the 0.125" sheet you buy might actually be 0.127". The 1"x1"x0.0625" square tubing usually has pretty good dimensionality on the 1"x1", but the 0.0625" is very much a minimum requirement. Strength calculations for structural tubing are based on the nominal size, so the piece has to be at least that strong. Suppliers will target thicker materials to make sure that with manufacturing tolerances, they meet/exceed the minimum. They, of course, have an incentive to minimize the excess material to save on cost, but there is always the extra bit. Extrusions also don't have sharp intersections on internal corners. The fillet of material will add to the weight.
CAD models can account for this if you model all features and include the maximum material conditions.