G0704 Z Axis ball screw run out Help?

[awleaphart]

Hello,
Im new to the CNC world and Im needing help with runout on my Z-Axis. I have taken it apart 3 times and replaced the stepper motor mount-ball screw bearings. Today I think I have found the problem just don know how to fix it.
As I was trying to tram the head to the Colum with a 1/2 drill rod in the spindle I noticed as I raised the head the test indicator was moving back and forth about .050 . I also could see the ball screw through the opening in the column moving back and forth also. Is this normal or did I miss something in the insulation.
Thanks Anthony L

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[awerby]

Are you sure the machining on the end of the ball screw, where it goes into the bearing, is concentric with the rest of the screw?

[awleaphart]

Andrew I don't know. What the easiest way to check?
I have the ball screw out of the machine and rolled it on a surface plate. It has like a slight bend in it. I can slide a peace of paper under the screw about 8 inches from the motor connector end. Maybe .005 Dif from the plate to the screw.
Anthony L

[awerby]

A bent screw can also cause problems like the one you noticed. To check for a discrepancy between the machined end and the rest of the screw, roll it on your surface plate with a dial indicator on the end.

[gd.marsh]

Sounds like you might need to adjust the gib on the Z dovetails.
Movement of the head is a sign that the dovetails are too loose.
A screw that has an .005 bend should not cause any wobble in the head with the dovetails properly adjusted.
A screw with end machining that's out of concentric by .005 would likely bind up near the end that's out (if the dovetails are properly adjusted)

[awleaphart]

Ok,
I put the Z back together and run it up and down fast and I can see the saddle twisting left to right. MARSH I have not tried to tighten the gibs up. That would be great if its that all needs to be adjusted.
I don't have any V blocks but I have some 123 block. I rolled the screw on the blocks and the screws worse spot is in the middle .008 and on the bearing end where the screw has no threads I'm getting .016

[awleaphart]

Guys,
I feel like I'm missing something here. I wish I would have taken a video. This thing look bad when running up and down. Can the double ball nut mount and the stepper motor/bearing mount be out of aline?
Anthony L

[awleaphart]

ok,
I put the Z axis back together and tighten up on the gib. It got indicator sweeping down to .003 well now I have .020 sweep on the Y axis. It can be more or less. Depending which way the ball screw is turn.