Widgetmaster, I was just arguing with myself to figure this thing out and you are completly correct.
The closer to the plane with the bearings both the top of the gantry and the ballscrew are the better and more ridged. And from basic statics principles it seems that it doesn't make much difference the location, as long as both lever arms(the distance from the bearings to either the gantry or the ballscrew) are minimized, so where ridgitity will be affected is if I can minimize one lever arm without changing the other, so the moment forces wouldn't be so great on the bearings supporting the gantry. I hope that made sense, and thanks for all of the replies you have helped me to figure out what system I am going to go with. As far as ridgitity is concerned I believe the two ballscrews would be more rigid, but not necessarily worth the cost, it seems that I could go with larger/higher quality bearings for cheaper and obtain similar rigitity. The one thing that would make two ballscrews worth it is if I had to span a large distance, and needed to have supports in the middle to eliminate table and bearing sag. For this current table I should be able to overcome those issues without difficulties. My final design will consist of bearings on the side of the table, so that the round thompson linear bearings will have the same strength for holding the gantry and for withstanding the gantry lifting do to cutting forces etc. It will also yeild the benifit of dust protection. I also plan on using a spinning nut idea to try to maximize feed speeds and maintain accuracy. Thanks for the help, I will try to post some drawings etc in the diy router section when I am closing in on my final design. Once again thanks, you all have helped me in my decisions very much.  |