Hmm, curious. Does it have stops at each end of the axis? Presumably for that to work, the machine would need to have a fair amount of "slop" in the linear guides so that it can twist? Or is some other mechanism employed?
I have 8 endstops, min and max for each rail. Being a meter in length, and a mixture of stainless steel and 3d printed parts, and gt2 belts, you can get up to 2cm off square if you move just one of the motors whilst keeping the other stationary. Doesnt seem to be a problem in use though, as both motors move together to keep it square. I have successfully milled aluminium on it, though usually I just mill wood or polcarbonate.