Rack and pinion maintenance

[phil_o]

I watched the Shopbot Training video on machine maintenance. The part about checking for wear on the rack and pinions got my attention. I have a 12 year old benchtop PRT 4G. With the machine turned on I tried to move the X Y and Z axis as recommended. Only the X axis showed movement. Using a dial indicator I saw that there was approx. 0.020" of play. With a visual check of the pinions I could not see any wear or misalignment.
Is 0.020" of play an amount to be concerned about?
Also I am aware that most of the use of my machine is within the center of the X axis, middle to center of the Y axis and a fairly short range on the Z axis. It makes sense to me that there would be wear to the racks in the range most used. Would it be a good idea to flip all three racks end for end to access parts of the racks with far less use?

[Brady Watson]

Phil,
Check the rest of the mechanicals on that machine, like the horizontal V-roller bearings and make sure that they are running true in conjunction with the vertical ones on the Y car. Many were never shimmed properly from day 1 and this causes things to either wear poorly (rails) or not track properly creating a gap over time that results in backlash.

Furthermore, the gearboxes on those machines can get sloppy over time. If you have the control box on, and you physically push on each side of the axis, you can often sense backlash in the gearbox. It is *possible* to adjust the gearbox, but you kinda have to know what you are dooing because if you make it too tight, you'll kill it...Keep in mind that on a good day these machines are going to show .008-.015" of backlash, so your .020" figure isn't that far out of range.

-B

[cowboy1296]

Dumb question I know, but when aligning the pinion with the rack, do you do it with the power on or off?