Quote Originally Posted by hardenum View Post
The main DOC concern was that when I put different pieces with different height onto the vacuum table I'd hit the higher ones with the vertical beam. Like machining the bottom and top piece (http://www.jt-precision.com/uploads/...7293225867.jpg) one after another without actually having to go and change the piece. Sure I could put the higher one to the front, it would work, but knowing me, I'd switch the 2 unconsciously and mess some part of the machine.

Anyway, do you recommend a ballscrew on each column or one thicker in the middle?
I suppose it's possible gantry brace could hit if your having very different height materials but I don't think it will be a massive issue and how often are you likely to do that.? It certainly won't be an issue cutting the part you show.
Also there is nothing stopping you having both a lifting gantry for very high parts and fitting a conventional Z axis to this. This way you have the best of both worlds. Again this is commonly done on Large gantry mills, and when I say large I mean something that could mill a full size Car from a billet of steel.!!

Regards the ballscrews then it only works with one on each column. . . . How could you fit a central ballscrew and still have the material move thru the opening.