CNC Spindle Alignment, Shims, and Vibration

A CNC owner asks whether there are drawbacks to using too many shims behind his spindle. August 3, 2009

I operate a CNC machine that has multiple shims behind the main spindle. Has anyone else corrected spindle alignment using shims without runout and vibration? How many shims are acceptable before causing damage to the spindle bearings? Has anyone rebuilt the air cylinder product alignment pins?

Forum Responses
(CNC Forum)
From contributor M:
The number of shims shouldn't matter as long as the spindle connection is tight. The shims are just used to align the spindle perpendicular to the table. Itís pretty unlikely without at least one shim. If you are not sure how to tram the spindle, call the manufacturer. It really isn't that hard once you have done it once or twice.

From the original questioner:
My main concern is the evidence of vibration in terms of runout. We would benefit from less runout with savings in tooling. We have repeatedly corrected the spindle alignment to the table, but mainly due to operator error (hitting alignment pins) and time, the shims are working loose. One or two shims are acceptable; however we have three or four.

From contributor M:
My initial thought would be the spindle is the source of the runout, not the attachment to the slide.

From contributor R:
I would agree that too many shims are a bad thing but I'm not sure how thick your shims are. I've see manufacturers put three or four shims behind their spindles. You might measure the thickness of the four and put one thicker shim in. If your router has a tool changer you need to make certain that it is still properly aligned for tool changes. The shims are used to make certain you have a perpendicular cut on the side of the board to the top of the board. They should not be used to space the spindle in order to get the proper dimensions with your cut. If the location of your cut is off you would need to change the parameter that relates to the location of the spindle or the parameter that gives you your zero point on the table depending on which one is really the problem. I do agree with contributor M that runout will be the result of the spindle bearings not the number of shims behind the spindle.