I went back to confirm that the D1-4 back plate was properly mounted to the spindle and took a cut on the back plate to make sure it runs true.With the chuck remounted on the back plate there is less than .0005’’ TIR on the outside of the chuck body. A test bar that I know is true was indicated it to less than.0005’’ at the chuck. However, 6.5’’ out from the jaws the TIR of the test bar is.015’’. This set up started to shake over 500 rpm. I have run my 6’’ scroll chuck with the same test bar (but with much less runout) to the maximum of 1200 rpm and there was no sign of vibration at any speed.
I had not spun the 8’’ chuck without the jaws. But now that you asked I tried it. Without the jaws it does not vibrate at all up to the maximum of 1200 rpm. At 700 rpm there was only a hint of vibration, but it was nothing like when the jaws are in the chuck. Interesting, thanks Alphawolf.
While the jaws were out I weighed them. One jaw weighs a half once less than the other three.
The jaws seem to be very poorly machined. The holding pads of the jaws do not contact the work evenly. When the outer pad of the jaw contacts the work there is .015’’ of daylight showing under the inner pads.
So now it’s about how to fix the jaws. Unless there is a problem correcting these jaws it does not seem like downsizing to a 6’’ chuck would be worth the expense.