Hi all - I am having trouble with the Y axis of my router shuddering when doing short moves. It seems like something is binding, the question is how to determine what.

The machine is a an old Maxym router - an odd cantilevered design but heavily built. The Y axis runs on a pair of THK linear rails, 16" on center. There are two trucks on each rail, about 8" on center. The Y axis is driven by a chinese 2010 ballscrew with a plain ballnut (not backlash limiting). The steppers are driven by a Gecko g540.

The shudder started after I tore down the Y axis, disassembled and cleaned the bearing trucks, and replaced a 1/2" acme screw with the 2010 ballscrew. Now I am wondering if cleaning the bearing trucks allowed them to have too much side to side play, allowing the set of them to rack, and therefore bind. Or is it an issue with the ballscrew, and particularly the lack of backlash limiting.

The shudder only happens during a series of short moves. I can move rapidly (I have only gone 300 ipm) and it appears very smooth during acceleration and at speed. It is also smooth during arcs (G2, G3). I use spiral tool paths a lot, for aesthetic reasons, and my CAM software tends to make them into a lot of G1's with deltas of .0005" or so. These used to be fine prior to my "fixing" things, and they are still fine on the X axis (which has similar linear guides, and a 1/2" acme screw). The shudder happens anywhere on the Y axis.

I was worried there was some interplay between the axis motion, so I stripped out all the Y and Z movement in a test file. The machine shudders. I ran the same program, but swapped to the X axis (and then to Z), no shudder. So it seems like it is isolated to the Y axis.

I have played with acceleration and max speed. Bringing the acceleration way down (less than 2 ips in Mach3) does not eliminate the problem. Using my phone as a seismograph, I found that in Mach3 - 5ips acceleration produced max vibrations of >300 gal. 2ips produced vibrations of >175gal. I tried LinuxCNC - it is much smoother, it turns out. Acceleration of 10ips in LinuxCNC produced max vibrations of <125gal. The steppers do not stall until over 15ips. This is with a max velocity of 180ipm in both Mach and LinuxCNC. I am using LinuxCNC now, but you can still hear the machine banging around so I would like to find a way to tighten it up.

I have also tried to measure backlash in the screw. I did this by moving my Y axis from 0 to 1 and indicating that spot with a dial indicator. I then moved from 1 to 2, and back to 1. My indicator says I am within .001" of where I started, which is within the error of my indicator. So I think the backlash is within at most .001", and I am not sure how to measure it more precisely. Is that enough to cause shudder?

I should also note that I have lubricated all these parts with grease - Mobilux EP2. I started using way lube on the ball nut, but I could hear the balls rattling around and finally switched to grease. It is quieter now, but does not seem to have made much difference.

Another note - the end seals on the bearing trucks were trashed - a gooey waxy substance rather than rubbery. I have new seals, but they have not yet been installed. Could it be these trucks are worn out?

I am not sure how to break the problem down further. How can I rule out either the ballscrew or the linear guides? If it is the guides, could I load them laterally so they were less able to rack as a unit? That is to say, push them outward from the center between the two rails, then bolt them to their mounting plate. It seems like it would cause more wear on the inside of the rails, but it seems like it could work, maybe. How much to load them? Maybe I do not care about guide wear in a low-production machine. I know I would rather not replace all the trucks to find that it was the screw all along.

Thanks for your time!

Bart

I have attached some images

1) The machine itself, sorry about the clutter. It worked very nicely to machine the brass seen on the table, no shudder, but just nice straight moves.

2) The axis in question

3) A closer view of the linear guides and the ball nut mount.