Some thoughts on cnc machine design principles. . .

[williamturner1]

By popular demand I've updated the spreadsheet to include a range of sections for the gantry sides in bending.

As expected the channel is similar to the I beam for torsion (i.e poor), but redeems itself in the bending load case.

Great work! Thanks for sharing.

I really am surprised at the figures for IBeam. With industrial stock ... eg 100mm Width / 200mm Height / 10mm thickness upper + lower plates, 7mm thickness of upright.
Comparing it to 12mm thick Plate, 200mm height
and RHS 120 x 80 x 3

Ive placed 120x80x3 2m length between two RSJs and stood on its middle and bounced, observing its considerable flex.
I've done the same with a 2.2m RSJ (100x200x10) and done my best tigger impression but observed little if any movement.

but the spreadsheet indicates the opposite should be observed:
DEFLECTION: 31micros vs 2micros (RSJ vs RHS)
TORSIONAL STIFFNESS: 28,000 vs 1,800,000 (RSJ vs RHS)

[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??