The best way to mill the underside is to machine setup blocks with protrusions on which the flat (hopefully) machined surfaces will rest. You don't have to make a complete "negative" of your part, 4 -5 "pads" will do, depending on the size of the part. You can design the blocks along with your part in your CAD program.
No, if the underside is flat this is indeed a waste of time. You could use a sacrifisial base or put the part on parallels in the vise, secure the vise and remove the parallels. Keep two things in mind:
1. Tighten the vise as much as you would if you had the finished part in it, otherwise you might crash it when material has been removed from the center.
2. As mentioned before, no loose cutouts. You can leave thin connecting tabs and cut & file them off later. I've left aluminum tabs so thin I could cut them with a hobby knife and still worked fine.
HTH, Nikolas |