Thanks Bill. I did see the stock moving , just didn't redo the video. The mill tram is not perfect but actually close.
I'd have to say, from watching the video, that it wasn't all that close. Your cutter was taking a slice out of one half or less of the path of the tool. It should be cutting all the way around. Think about the forces involved: You are pushing the quill of the milling machine down onto (and into) the stock you're cutting. If the mill is trammed properly, it will help push the stock down against the parallels you have holding the stock up where you want it. That will reduce your vibration and chatter right there. Now, at the same time, it will be trying to shove the piece to the side, where the jaws of the vise are supposed to hold it from moving. If it's loose in there, it will move as the cutter moves, and that will add to your chatter. You need to fix both problems to get the best results. If you're just punching a hole through the metal maybe it doesn't need to be all that precise. If you need a precise hole, for a bulkhead fitting, a fan, or to hold a lens, or many other purposes, you need that mill trammed so the cutter isn't lifting off the surface and then slamming back down on to and in to it, causing it to move.