Some thoughts on cnc machine design principles. . .

[irving2008]

good stuff...

haven't checked through all the math :whistling: but looks good. I'm surprised how poor I-Beam is in torsion, but its clear box section is the way to go... I would expect C-section to perform equally poorly..

I did the calcs once before in a similar way for box section for side supports to gantry instead of plate, you might want to add that option. Also there, I- & C-section might make sense as there is little torsion...

[FatFreddie]

good stuff...

haven't checked through all the math :whistling: but looks good. I'm surprised how poor I-Beam is in torsion, but its clear box section is the way to go... I would expect C-section to perform equally poorly..

I did the calcs once before in a similar way for box section for side supports to gantry instead of plate, you might want to add that option. Also there, I- & C-section might make sense as there is little torsion...

As I'm sure you know, for the best stiffness to weight ratio in both bending and torsion you basically want to move the material as far away as possible from the neutral bending axis (and have a closed section for torsion), which means ideally a round tube but box section is pretty good and much easier to work with. If you look at bicycle frames where they care a lot about both stiffness and weight the tube forms are generally close to circular, elliptical or a rounded off box

I like the idea of an option for using box on the side supports - it always looks like the weak point on a lot of gantry designs and it would be nice to be able to quantify that.

I'm also interested in the idea of placing bearings close together - I have been designing to get them as far apart as possible working on the principle that it's not expensive to make the axes a little bit longer and it will reduce the effect of any flex in the bearings. It will also let me put the leadscrews in a convenient position rather than the best place to minimise the forces on the bearings - any comments?

Good thread.

Mark