Some thoughts on cnc machine design principles. . .

[Colin Barron]

Maybe this extra stiffness of the box section is why the likes of axyz and chinese makers use this design for the y axis?
Do they use aluminium or mild steel box section. I bet its steel, stronger and easy to weld plates onto it as well as better stiffness?
what is the main reason diy builders use aluminium, softer so easier to machine, lightness or does not rust?
Any ideas?
I am not American!

[routercnc]

Hi williamturner1

You might be reading off the wrong rows.

I've compared the 2 shape:
'I beam' width 100mm, depth 200mm, flange 10mm, web 7mm
RHS width 120mm, depth 80mm, wall 3mm

Assuming length 2000mm, and your weight 60kg, and vertical Z deflections are:
I beam 23 microns
RHS 393 microns

Which tallies with your jumping experiment, showing the vertical stiffness difference, where the I beam is much stiffer.

Unless you had a lever arm to jump on (which was very stiff), then you would not be comparing the torsional performance which I would expect to be worse for the I beam.

[wiremonkey]

Thanks for sharing your info. Here is a great paper on torsion of beams. The clear winner is closed shapes, tubes, of all shapes: circular, rectangular. https://www.slideshare.net/edwinrami...-torsion-guide
I'm in the process of beefing up my aluminum extrusion CNC to be a dual-purpose wood/metal machine and have so much to learn. This is a great forum! If any of you would care to view this video and lend some advice, I'd sure appreciate it.
https://youtu.be/2QTZa2Ge9A0
Cheers,
Tyler

[routercnc]

Hi Tyler

Welcome to the forum. You may find the picture build menu interesting
http://www.mycncuk.com/threads/10066...s-picture-menu

Note the suggested rules of no replies just to keep that thread clean, but see if there is something there. The posters are regulars here so start a new thread with questions for more info on any of the machines.

Thanks

[Voicecoil]

OK, putting the gantry C of G between the "X" bearings is obviously a good thing. In my waters I feel that trying to get the cutting tool axis as close as possible to the gantry (making the Z-axis assembly as thin as poss.) is also a good thing, any comments on this please? Obviously in terms of leverage it will reduce the twisting moment on the gantry a bit, but if you look at it in terms of purely lateral (cutting) force, then it shouldn't make much difference??

[jhobson]

I'm completely new to CNC and should be careful about posting in a 'sticky' thread but I didn't notice discussion about diagonal bracing for torsion. I don't mean to tech how to suck eggs.
Torsion results in diagonal forces so you want material on the diagonal. Where you see flat plates braced together with diagonal webs, then the webs greatly help with torsional rigidity - e.g you might see this in machine tool castings where it provides torsion stiffness but allows for simple moulding without need for cores. If your design lacks torsion stiffness then look to add material on the diagonal.
I remember this from some lecture almost 40 years ago. I also remember that it is much more complicated to calculate the stiffness compared to closed sections so a good reason to avoid in designs! I 'remember', but it might be an over-simplification, that non closed sections all have similar torsional stiffness as flat plate. So a tube with a slit down the length is a flat plate, not a circular tube, The detail of the end attachment may complicate this in reality but as a rule of thumb...