South Bend 9 spindle 'jump' in reading radial play.

[dansawyer]

The lathe is a 1945 South Bend 9a. The lathe is in reasonably good condition.
Currently there is a 5/8 collet in the spindle with a 5/8 rod being used as a mandrel. There is an aluminum cylinder with a 5/8 bore affixed to rod with a set screw.
I machined the cylinder to be concentric with the rod. A dial indicator reads both the rod and cylinder are circular in the horizontal to plus / minus half a thou.
However when I turn the spindle clockwise and then counter clockwise the indicator jumps about 5 thou. After the initial 'jump' the reading remains within the plus / minus half a thou. This is valid in both rotations.
I have not checked the vertical run out. Should I? Will it provide useful information?
What is causing the jump?

 

[tazzat]

the dial indicator itself? or the setup of the dial indicator?

 

[SLK001]

I have not checked the vertical run out. Should I? Will it provide useful information?
What is causing the jump?

It could be your indicator or setup taking up slack when you reverse directions. It could also be the spindle oil film re-positioning itself when you reverse (it rides on a film of oil). This could indicate that your spindle may have too much play in it, necessitating an adjustment of same. Are you sure that it is jumping 0.005"? That is an awful lot of jump.

 

[benmychree]

I have seen erratic indicator readings when the screws that hold the back of the indicator are loose.