Added steel channel to X-axis c-beam

Well, to be fair, most of us building our own CNC mills are hobbyists, not engineers. So, the closed rectangular tube is acting as a triangle, in a sense? What if the walls on the smaller set of two square tubes were thicker, say .25" in stead of my 2" x 6", which has 3/16" walls? The weight would be about the same, within a pound or two. I'm leaning toward the singular rectangular tube for sure. Although my hunch is that they would be similar in rigidity due to the fact that the linear rails would ensure that torsion would be applied to both rails evenly, no?
Thanks for the input magicniner!

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Originally Posted by wiremonkey

What if the walls on the smaller set of two square tubes were thicker

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You get less rigidity and torsional strength for your weight than with one larger box.

Here is the Taig in action. I mean, those are some seriously deep cuts! I'm not affiliated with them, I just like the design. https://www.youtube.com/watch?v=_9y-_9SKJ8s

The split tubes have the advantage that the Y axis can be very close to the gantry as it does not need to be offset to accommodate the ballscrew behind it which usually takes up more space than the rail and carriage.

But it is nowhere near as stiff a shape as a single rectangular gantry. In vertical bending the 2 tubes are only twice as stiff as one of the tubes on its own. But when joined as a single large tube they are 2^3 times stiffer (8 times) for example.
As mentioned by magicniner there are similar benefits for torsion for similar reasons.

That is why when you see 2 tube designs with the screw in the middle the builder often plates them together at the back to try and gain some of this advantage back.

If you go with the single large rectangle then the rails can be spaced out on thick solid bars welded to the section to give the ballscrew clearance. Sure, this puts a moment on the section but you can afford a bit of this loss as you have so much to start with.

Another variation is to have a tall more slender section vertically and have another horizontal section behind in a reverse L shape. This is popular when used with aluminium extrusions and the ballscrew can sit behind the section driven off a good bracket that often houses the Z stepper too then the rails can be tight against the section again.

I’d favour the large rectangle as per your blue drawing but the above gives you some options and reasons.

Thanks for sticking with me routercnc! I plan on putting the lead screw on the face of the X beam, like Wade'o has in his design. In fact, when I can afford it, I intend to switch to a fixed gantry and Y table. Take a peek: https://www.youtube.com/watch?v=herMY_2WowM

OK, so I'm solid on using the 2" x 6" steel tube for the X beam! I have a few more questions for you good folks.

- 1/4" inch steel plates for the end caps on the beam? Or should I go thicker? 1/2"?

- 20mm round supported rails and bearings ~$80, or spend the extra dough and get linear rails and bearings ~$240!
The other factor on the square rails is that I'll have to have the steel beam milled, which will probably cost $200 or more, whereas the supported round rail is far more forgiving when it comes to alignment. The rectangular steel tube has a round crest to it, but it is fairly consistent along the length of the beam. I'm thinking that I can screw the supported round rails right to the beam, maybe a little shim stock if necessary, easy peasy.

Attachment 24471 VS. Attachment 24472

I'd love to hear your thoughts on these two issues.
Cheers!

Linear rail, and epoxy?

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I'd love to hear your thoughts on these two issues.
Cheers!

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Yes what Doddy said:applouse:

Supported rail will be better than the wheels you have now and my previous machine ran on those for a few years. But there can be some maintenance such as sometimes the bearing coming loose in the housing plus there was some wear on my set that I noticed when I took them apart and was the likely cause of it getting worse for chatter.
So if you stay in this hobby you will get linear profile rails in the end.
As for the gantry sides - looks like they are the drop down style to raise the gantry. So they need to be quite thick. If they are aluminium then they should ideally be 20mm. But if they are steel they can be thinner. Steel is 3 times stiffer than aluminium, just in terms of material properties, but then when you go thinner you quickly loose out due to geometry (which is independent of material) so . . . go for 12.5 mm over 6.35 mm.
Better still if doing a really good upgrade raise the bed sides so the gantry doesn’t have to drop down and you will gain much more than trying to get stiffness back in the side plates.

Linear rails are far superior to round supported rails.