I came across this article recently:

http://www.homepages.mcb.net/howe/NewsMillSpline.htm

The author had developed a clamping gadget to lock down the spindle on it's splines in order to reduce chatter when face milling. It would seem to me something like this would be helpful, particularly if you have done a CNC conversion that no longer requires the quill function. It looks extremely easy to make and it is a reversible change.

Thought you guys might like to take a look.

Best,

BW