I do not think you will get the 0.013" error across the part as you will be cutting right under the head which will be a constant distance, sure the table sags at the end with weight but facing/milling the material under the head you will get just a little variance. nothing like 0.013", I would take a piece of aluminum 1' long maybe 3/4" thick for the test and mount it down centered in the bed, and with the weight you tested at the end of the table place it there at the end of the table to simulate the deflection go ahead and mill/face the 1' strip of aluminum and then measure the thickness, it may be well within your tolerances. Its pretty hard to deflect a 4" slab of aluminum with the table/slide but we do not know hoe much of the material is machined away. This would be an interesting experiment to see how much deviation in the thickness you have.
Thanks,
Joe
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