I’ve tried everything with adjusting the gibs. It won’t get any better it seems. This is very peculiar as it’s only 3 years old and now milling something that’s 18” long is quite difficult when using the y with the x locked.
You did not say what was on the table. Also, in your description, it is implied that if the X- is NOT locked that the y-axis does not bind!?
1) Uneven weight on the table can cause excessive tilt. A large vise? Try counter balancing any uneven weight along the X-axis. I have a lead brick I sometimes place on the long x-table of my PM940M to counter balance a vice that I sometimes locate at one end. This tends to yield a symmetry for the y-axis saddle pressures, but really works best when I am working near the center of the table. My PM vise weight in at 90+ #s and the mill table is pretty long and heavy duty. The worst case situation might be that you are working on one end and have a vice on the extended end.
2) Even without this an over tighten gib will result in binding. Also, unfortunately some ways are not perfectly parallel, this will result in a stiff response at the narrow end when the gib has been optimized at the wide end. This stiffness can drastically increase backlash.
3) If Y-axis lead screw is fastened down (bearings) at both ends then results suggested by
@MrWhoopee can result. Even then the stiffness should differ at the center vs ends of the y-axis positions. My machine had bearings at both ends as there was a CNC stepper motor at the back and a crank at the front. However, the rear bearing and stepper motor was not centered properly for the y-axis way travel. I removed the front bearing and the lead screw was no longer bent as it position changed at the front. It may have helped a little with the stiffness at the end position. It was hard to tell.
3) Assuming there is no or little binding when the x-axis locks are not tighten it could be that tightening them actually twists the x-table resulting in the change in the y-saddle that causes binding. Again, analogous but different, to the comment made by
@MrWhoopee . It is common that the locks are simply screws pushing in on the back side of the gib at two points. This can actually bend the gib and distort it considerably. It is hard to fix this other than to lock the position via some other means. Perhaps there is something wrong with the x-axis gib or its fit and these locks are causing strange stresses/strains in the Y saddle via the fit to the X ways.
Just some thoughts.... Good luck.
