I've been using a lot of 1/2 thick aluminum bar stock for making parts the last few years. Most of it I buy online from one of a half dozen sources because I can buy it cut to length (+/-) for less than I can buy 20' bar uncut locally. Anyway, in the last month or so I have been buying more from a vendor that is nearer my side of the continent. The last couple orders of1/2 x 6 have been warped across the grain (not lengthwise) by as much as .02 inches. Most of it is bowed in the middle by atleast as much as .01. Now I know this is mill rolled aluminum stock and its not going to be perfect, but I've been buying this sort of bar stock for almost 5 years. I've been buying more in the last 2-3 years. In the past I've found .003 to .005 facing pass was enough to get a clean straight surface to work from. I just plan on skimming .005 off every piece and I don't worry about it.
Imagine my surprise when I had to take four passes to get a clean face on a piece of this stuff. I mentioned it to the vendor and he told me he was a retired machinist before he started selling metal online and its all like that, and if I got any that was finishing with only .003 to .005 skim pass it must have been plate because all bar stock is off by more than that.
Well, that might be true, but I have to say I've never gotten and 6061 bar stock that was warped that much before... and if the stuff I was getting from other vendors was plate it did a good job of mimicking bar stock with roll marks, and rounded corners like bar stock.
So, is it really all that bad and I've just been lucky or what?