Problem with M3 head moving left

[ben]

I'm using an M3 with Fusion 360 to mill ABS, and finally have found the necessary setup to get it to work properly (Single flute 1/8" mill, high DC spindle speed, .010" stepdown, never plunge).  But I still have a problem when the head is moving to the left.

I noticed, while milling a rectangle, that the head is steady when moving in three directions, but the travel is jerky when the head is moving to the left.  Then I was milling another part and twice I heard the servos knock, and found the machine had lost it's X position, when the head was positioning to the left between cuts, when the mill was not even in the material.  This happened twice in the same spot in the code.

When I assembled the M3 it seemed that the X axis head was stiffer than the other rails, but assumed this was because it is carrying a large, unbalanced load, and I saw no way to adjust it.  

Could the belt to too tight or too loose?  Is there a way to reduce the speed of travel when the head is moving between operations (in Fusion 360 CAM).

If you have any suggestions about how to resolve this, I certainly would appreciate it....

[aforww]

Well there might be a few variables at play here. Climb cut or regular? Belt might be loose depending on how the cutter is traveling it can mess with one d direction and not the other. The other thing I suspect is that the spindle isn't perpendicular to the material on that axis. If it's leaning left, the cutter will be digging in on the leading edge which caused problems only obvious when it's traversing right to left.

I suggest checking belt tension all the way around, check the gear spindles and make sure the set screws are tight, check that the Vwheels will slightly spin by hand if you hold the Z assembly still. Then, once all that is checked, check plumb of the spindle.