Is there any way to rotate the coord plane in XY around Z so as to compensate for fixture inaccuracies?

This would be faster than measuring and adjusting my code or trying to get my fixture dead on.

The short answer is NO.

In actual fact Mach 3 does not do anything but drive the axis, which can then be at any angle to each other that you like. It does, however in the Maths, assume that axis are at 90 degrees to each other where computing a derived path - i.e. x and y moving together, or even x and z or y and z.

I am trying to envisgae what you mean - do you mean your cutter (z) does not move parrallel to your table in either direction, or what.
If the error was constant, you might be able to make some adjustment using a Vis Basic macro.

No, I am fixturing a part for a second op. Since I am not doing the first op I have little control on how its aligned (not always square with the edge of the
material). So I can indicate the off a feature of the part and trig out the angle the part is out but now I need to adjust my path to also be angled.
I can do this in my code, but life would be better if I could just rotate the coordinates.

For example for simplicity lets say Im cutting a straight line
G01 X0 F20
X10

Well maybe the part is tilted a little, rather than monkey with it to get it straight because the slot im cutting isnt parallel to the edge of the part
I really need
G01 X0 F20
X10 Y0.010

So if I could just rotate the XY plane around Z I could make this adjustment on the fly and not edit my code.

Hope this makes sense, my application is more complex than this but I think this gets the point across. My machine runs true the part I'm cutting is not.

I checked out a post on the forum that was controlling a CNC embroidery machine and they had a rotate function.
http://www.hqcomputerizedquilting.com/hub.php?a=VD&id=3
So either they are generating that code on the fly via script, rotating the code via script using these new flash screen setup, or something along those lines or I am missing something.

Mach does support coordinate rotation. See the doc for G68. I think it is for exactly what you want to do.

One note- jog will still occur along the unrotated axis, so you will get some strange jog moves, but for a small rotation it may not be a problem.

My apologies if that is correct - it was a feature I was unware of.