z car becomes loose on y axis about halfway across

Let me elaborate a little.

The machine is a 2007 PRSalpha 4x8 with a spindle.

If the z car is adjusted so that it is tight in the home position (all the way right on the y axis), the z car becomes loose (meaning I can shake it forward and backward a little) on the y axis about halfway, then it becomes tight again at the extreme left position.

I've checked the parallel of the v rails and they are dead on all the way across the beam. I originally thought that there might be a dip in one of v rails, but measuring across the upper and lower with calipers doesn't really show an issue. Now this is measuring tip to tip of the rails (not necessarily where the v rollers ride) and there could possibly be some wear to the rails I guess, but visually they don't really appear to be worn.

There isn't enough eccentricity in the lower v rollers to set them to be tight in the center of the machine. If there were more eccentricity I could probably just tighten it in the center and call it a day.

Any help is much appreciated

Thanks in advance,
Mark

Mark Now I do not have a PRS , But Are the rails tight ? Did you measure between the rails with the car out in the middle ? If the rails are bolted on you may be able to loosen the middle and spread then a bit and retighten .

My initial thought was to shim out the bottom rail. This was before I was able to measure across the two rails which verifies that they are equidistant along the beam.

How likely is it that maybe the upper and lower rails need to be replaced? The machine doesn't see a lot of hard, day in and day out use.

Mark, my machine is the same model as yours and my Y axis rails were off in parallel by quite a bit. The hardened rails shouldn't wear out to that degree with the kind of use you mention. What I did was to first make sure that the top rail was flat, no dips etc. with a straight edge, then checked it for parallel to the table surface as best I could. If your table has been surfaced with it out of alignment, you can't use the table surface, but can check with a straight edge laying across the X rails. Once satisfied with the top rail, check the lower rail for flat with the straight edge and adjust if needed by loosening the small screws holding the rail to the aluminum bar first. If you can't get flat with those, then you can tweak more by loosening the cap screws on the aluminum bar and gain a bit more. Mine were flat, so I just loosed the lower rail cap screws that hold the aluminum bar with v-rail on and using some spacers of the same/proper length, clamped them in place between the inside edges of the v-rail, not the aluminum bar, and then clamped the lower rail in place from top to bottom and then tightened the cap screws a little at a time working from the middle out. You can gain a little spread by loosening the cap screws so you should be able to get the eccentrics to snug up afterwards. Be sure to loosen them before spreading the rails apart. A little blue loctite may be a good idea on the small rail screws if you loosen them.

Some oiler/wipers for the rail is a good idea too as the older PRS tools had none.