Before I embark on cutting any high tolerance parts with the mill I think it is worthwhile adjusting the column for perpendicularity. I don't have a cylindrical square which would be the reference of choice for this task. They are fairly cheap, but I can't see much use for one other than this job for moment. I do have a cnc 4.5" angle plate (cnc in this case being cheap n' cheerful) which I think will do. Given the quality, it is pretty unlikely to be terribly square, but it will at least be consistently not square. By measuring once, rotating the tool 180 degrees around the axis of measurement and then measuring a second time, the bias of the tool will be revealed. This is a similar technique to the calibration of water levels. Measure any nearly flat surface with the level, flip the tool around and measure again. The bubble should be in the same position no matter which way round the tool is.
So here is the setup on the table. I bolted the angle plate down and marked its position on the table and the section of the surface that was measured. Once the measurement is done, I flip the plate 180 degrees and realign it with the marks.
So here is the setup on the table. I bolted the angle plate down and marked its position on the table and the section of the surface that was measured. Once the measurement is done, I flip the plate 180 degrees and realign it with the marks.
Same thing for the X axis direction
The results show that the angle plate is around 2 thou off square over 4.5" and that the column is off by about ten thousandths with respect to the X axis and two or three in Y. I really should do a series of measurements and take an average, but until I'm certain the column doesn't have to come apart again and I get myself a 10 thou test dial indicator I'm only interested in getting within a couple of thou.