I'm with Ger on this one.
Draw slides take loads in one plane only. Sit at your desk and half open a draw, lift it up and down and it shouldn't move, move it side to side and it will move a long way, wrack it diagonally and it will move about the same.
A gantry cnc will load its bearings in two dimensions and try to wrack the gantry in the same way as the draw test above. Add the leverage and distance of the cutter from the bearing and you've got problems.
I don't know what brand the slides are that were proposed for a diagonal mount but I've got some Home depot full extension high capacity slides here and trying it (before posting) they bind quite badly on a 45%.
Reading this post and the other recent posts about the new designs it strikes me there's little talk of the loads the machines have to be built for and how the different materials and bearings will need different design points in the construction of the machine.
For example given the draw slide option and the loads a cnc machine imposes the slides has to suport two planes of load and so in the ideal solution you'd need two sets one 90% to the other as Ger suggested.
Similarly the black iron pipe skate bearing arrangement has limits and challenges on the span a pipe of a given size can support, the need for support and the need for adjustment. It's not practical to significantly increase the dimensions of the JGRO router and expect it not to deflect big time.
In the same vein a lot of the gantry designs have simple uprights and back plates with no triangulation or support while the tube/rod is supported at the ends only by the thickness of MDF or the ends of bolt heads. They need way more support than that.
Some thought needs to go into the manufacture of the parts for alignment too. Since most of us ( except Ger apparently ! ) can't get our parts cut on a cnc machine first, then the design should avoid excessive number of joins or hole alignments, use techniques like cutting parallel dado's in one go on one board and then cutting the parts from that, bolting together enda and bdrilling all as one part to approximately line up holes, that sort of stuff.
It strikes me that there's a lot of enthusiasm ( which is great) to develop the machines but the engineering of each option has yet to catch up.
Andrew  |