I agree Bill. Also I imagine there is some variability/play in the two screws that hold each tube into the blocks at either end.
I have been working on my Onefinity build for a few weeks now - I am using a Masso G3 controller, as well as homing sensors.
To test for square, as well as accuracy and repeatability I used a CNC pen holder I had designed and 3D printed, then fitted with a sewing needle as a test tip. I taped small squares of cardboard to various test locations, and lowered the Z axis until the needle made a hole in the cardboard. Measurements between needle holes were made with a metal ruler and tape measure with 1 mm increments (I am saving up for a precision linear scale).
I set up a work offset, and repeatedly jogged between various start positions, and home and the work offset. Happily the needle lowered into the exact hole in the cardboard every time at each location (I felt confident after about 5-6 replications).
I tested my X and Y axes for square and found them to be out by 1 mm (using 600mmx450mmx750mm right angle triangle reference points). Before working to resolve this, I thought it would be best to first calibrate my X and Y axes. I went through the calibration process two times each for the X and Y axes until my distance readings were as accurate as possible using my measuring tools. (I do not know if the 1F controller has this option). After rehoming, and resetting my 600m and 450mm needle reference holes, I was now showing square. Keeping in mind I am only as good as my current measuring tools, and prescription glasses.
My set up is still in the testing phase, with my CNC temporarily mounted using 3D printed mounts I designed - see below. The idea is that when I mount it to its final resting place - probably a torsion box - I can make sure it is set up perfectly, then mark the exact position of the 16 screw holes using the alignment holes in the posts of each mount. So far, using screws in the four external mounting holes has been working extremely well for testing purposes. Keep in mind the only dynamic testing has been a first run of gcode to test the drawing ability of my marker holder.
