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Given the choice between 3 and 4 jaw, I'd go with the 4-jaw, have found that it can be tightened down more, and holds more securely than the 3-jaw. Also, you can put a hold-down clamp into the slot around the rim (the screw adapter or the rotary table came with one, I think) that will keep it from un-screwing itself from the table, very important when milling!Another option is to turn the mill vise on its side to grip the casting, though it may be too rough to work well. However it is secured, use light cuts - that is a lot of leverage up to the base of the casting.You mention getting the base perfectly centered to mill the bottom flat. Why? The base just has to be flat and perpendicular to the rest, why does it have to be centered?
Hey JimI have retrofitted both of my shop chairs to polyurethane wheels, they are about the size of roller blade wheels and not too terribly expensive; and they work great!https://www.amazon.com/s?k=office+chair+wheels+for+hardwood+floors&crid=2XC6QY8WTV083&sprefix=office+chair+wheel%2Caps%2C218&ref=nb_sb_ss_i_2_18Dave
Another option would be to mount it to the mill table bottom side down with some shims to bring it into plumb. This could be verified using a precision square and the parting line of the casting. Then a clean up cut can be made on the upper surface. Then flip it over and skim the bottom. You could do it in stages incase some adjustments need to be made. I would need to go back and look but I think this is how I did the bottle frame on my Pacific engine.Dave
Surus mentioned before you start milling anything you might want to do some Casting fondling and find the centre line down the casting so that when you mount it you will know that it is going end up with the ends matching the main bottle shape. There is one centre to find down the open side and one down the solid side with the casting line (which could be slightly off the centre line). You might also consider a little fettling first to make it easier to hold/mark the casting Edit: I like Dave's idea of milling the top and bottom flat by clamping the bottle on its side to the milling machine base How about bolting an angle plate to the milling table and clamping the casting vertically against that rather than trusting on just holding the bottom of the casting ... The casting is rather tall and if it moved while milling Jo
Picture!!! Picture!!!