Drill press into milling machine

[astroracer]

Agree with above. A cheap, desktop drill press uses the downward force while drilling to retain the collet/chuck in the quill. This "upward force" goes away when you try to side mill with an endmill cutter. Any chatter (and there will be chatter) will walk the standard taper drill chuck right out of the quill... A true mill uses a drawbar to hold the collet in the quill. It will not fall out at the first hint of chatter.
And, as ncollar also alluded too, the bearings in the drill press quill are not designed to operate under a sideways force.
Just so you know.
Mark