Added steel channel to X-axis c-beam

[wiremonkey]

So, after much deliberation, I've decided to go with a 2" x 6" x 3/16" x ~750mm long piece of mild steel for the X beam. I found a local machine shop willing to flatten the face where the linear rails will go and make the bottom perpendicular along with plates that I'll weld onto the ends for mounting. I found a good source on eBay for the steel. The reason I opted for 2" x 6" x 3/16" was both budgetary and design based. I couldn't source the size tube I wanted locally so shipping was prohibitively expensive after a certain wight/size point, massive jump in price. Literally from around $36 shipping to well over $100! And, since I'm doing this upgrade in increments, I'll eventually switch over to a fixed gantry with Y table movement, I want to keep the weight somewhat reasonable. This beam is 9.42lbs per foot making the total weight around 23.5lbs. That's a goodly chunk of steel to be throwing around along with the whole X/Z assemblies and router! Ooftah. Let's see what these poly carb wheels can take (on the gantry plates). The total upgrade is going to be close to $500, not including all the swearing I will do. I'm teaching myself to weld with a pitiful little DC arc welder I picked up, and off to the races! Wish me luck! Video to come.

[wiremonkey]

So, here is my current plan for the X beam, 2" x 6" x 3/16" steel beam with linear rails. I'll weld at least .25" thick plate onto the ends for mounting to the gantries. Maybe thicker? Not sure if it's necessary, will just add more weight.
Over on the Openbuilds forum, M90Ranger suggested I split up the rails and use a design like this: Certainly two .25" wall 2" or 3" square tubing would be cheaper than the single 2" x 6" I purchased. Somehow I like the overall feel of the single rectangular tube. Which do you think would be more rigid?

[magicniner]

Two tubes could be almost but not quite as good if the end plates were of infinite stiffness, the guy who suggested that obviously isn't aware of the concept of Triangulation in structural design or the properties of different cross sections in torsional load.
I don't find that surprising any more though ;-)

[wiremonkey]

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!

[magicniner]

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

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

[routercnc]

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.

[wiremonkey]

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

[wiremonkey]

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.

VS.

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

[Doddy]

Linear rail, and epoxy?

[Clive S]

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

Yes what Doddy said