Some thoughts on cnc machine design principles. . .

[routercnc]

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

I've also added the 'channel' section to the torsion page. I referred to this as C section before, which is not quite correct, since C section is channel with small return flanges on the short pieces (like serifs).

Anyway, see post#2 above for the updates.

As expected the channel is similar to the I beam for torsion (i.e poor), but redeems itself in the bending load case. Overall though, closed sections are good all round choices. The flat plate option for the sides of the gantry looks less good that the other sections, but to get the advantages of the other sections you do need to get the load into them. The easy method of bolting through the flat outer surface is likely to distort the section locally before the rest of it gives you the benefit. A flat plate is likely to be fine for many applications, but if you can get a good connection to a sectioned part then it will be better.

If you can afford the profile and carraige style on the X axis, the moment reaction these provide during Y cutting gives great benefits vs open bearings on supported rails (which rotate!), although they do have a max moment (check the bearing spec).

Enjoy . . .

Ah, another thing. The X axis bearing close together bit. I was surprised at this myself but the maths points to it if:
You are machining thin 2D type parts whereby the tool is in about the same Z location during cutting, and you stick to the equation about relative position of parts (if you are machining deeper parts, then the tool is at different heights, moments creep in, and the geometry is no longer ideal, and you might want to not put the bearings so close)

You are using modest acceleration rates - probably hobby rates would be OK. At very high production type rates the gantry inertia would start to have an effect and again you might not want them so close due to the moment.

My next machine, or future mod to this one, will put them closer together, but I know why I'm doing it and what I want to achieve. You'll have to decide if it works for your application. If you're not sure, go with Ross and space them out a bit.

[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??