Hey John,
a gantry-type CNC machine can be twisted. This is usually the case after setting it up. In this case all your workpieces, including the flattened wasteboard will be twisted. Therefore such a machine needs to be adjusted for coplanarity. The QCW frame offers the Any Surface Leveling Feet. But you can also use heigt-adjusted feet or casters. In this case if you move the table around, it is necessary to repeat the check and adjustment of coplanarity each time you moved it. The fishing line test is useful for checking coplanarity.
Also a CNC machine needs to be adjusted for rectangularity (“squaring”). In the case rectangularity is not adjusted, the machine is no rectangle but a parallelogram. As the result all your workpieces intended to come out as rectangles will be parallelograms, i.e. will have no right angles. Also tiling (working on a larger workpiece part by part) or double side milling (reversing the workpiece to mill the underside) will not allow to get matching results.
Furthermore, on a machine with no accurately adjusted coplanarity and rectangularity, the Y movement can block, or have “hiccups” or loose its position reference coordinates.
I would consider a machine ready to use only if you first checked and ensured that it is
Only then I would proceed with
This is usually done after a first surfacing of the wasteboard. The Mitz Pellicciotta method, which is a method that works the better the larger the bit diameter is, because it looks at the direction of the patterns such a bit makes when it is not perpendicular to wasteboard surface, is to see as third step and not in all cases mandatory.