Dividing Head

I have a bit of a conundrum, hoping someone might be able to shed some light on it. I have a commercial 4" centre height dividing head which I use on the mill. The spindle nose has been modified the take my Myford chuck. Runout on the nose register is less than .0002 TIR. If I place a piece of stock in the chuck ("Griptru" on the lathe, roughly true it then skim it to clean it up so that it reads essentially "0" TIR and transfer the chuck to the dividing head on the mill, that same surface shows a runout of between 4 1/2 and 5 thou TIR as I rotate the head and chuck through 360 degrees. It doesn't matter how many times I remove the chuck and replace it back on the dividing head the runout remains the same (within tenths) with the high reading at the same relative position of the head and chuck. If I then zero it with the Griptru adjusting screws to as close to "0" TIR as I can get and transfer it back to the lathe it is then out the same 4 1/2 to 5 thou but with the high point 180 degrees relative to the chuck body as it was on the dividing head, I am totally flumuxed. Anyone have any ideas as what could be causing this??? ( Runout in the lathe spindle nose is essentially zero.)

John

Have you tried clocking the dividing head spindle runout on the back face where the chuck backplate seats as well as the diameter, a very small deviation there will be much magnified on the workpiece at the front of the chuck.

It could be a tiny bit of swarf embedded or something similar.

Phil

John, you say the dividing head spindle has been modified to a Myford spindle nose, was this done by you or another or was it supplied with this nose?
I don't know the original tolerance on the Myford nose but the register is on ones I have measured typically size to minus 2 tenths usually with size to size -- .00015 max. I tried purchasing attachments with a myford nose and they are hopeless usually -- .003 to -- .005 thus a very poor location. Also have a Vertex dividing head this is a little better but not in the same league as the Myford lathe spindles. Dependant on what i am making and accuracy required I sometimes use a 4 jaw on the Vertex to zero the component. Not convenient but !!!
This could explain your problems.

Some thoughts

Measure the diameter of the cylindrical register on the div head and on the Lathe and compare.

Also check the inside diameter of the register on the griptrue. This will tell you how much "slop" you could get if the threads are not concentric or are mucky.

There does have to be some clearence otherwise the chuch will not screw on so it's the thread that does the centering with the register acting as a solid limit.

Have a really good clean up of the threads and as others have said check for any sign of bruising.

regards Martin