[mikeysp]

I have some free 3" C-Channel to make my CNC Plasma table. However, since it will have a water table and removable/replaceable slats, it won't have much connectivity from lomg axis to long axis like a router table. How do I deal with my concern for flex and holding it true. The image shows my concern. I will be using 1.5" x 1/8" angle iron for rails, so they will not offer much help. Do I need to find another material and leave this for a router build where the table has many side to side connections and look for square tubing instead? Or is there a simple solution I am missing?

[Jim Dawson]

It seems a little light, but probably OK depending on the supporting structure under the water tray. I'm guessing that support structure will tie the sides together, some large gussets in that area should keep it from flexing..

I would build the whole structure out of 2x4x1/4 wall tubing, but I tend to over build

[mikeysp]

Jim, thank you. I do have access to 4" c channel also, so maybe I should use it. I would use 2x4 too if it were free like the C-channel is. I will have to figure out my water table design and see how I can tie them together. Thanks.

[pirate323i]

I would also be concerned about its torsional stiffness. So the supporting structure will need to address this.
Also i would be welding in gussets at regular intervals.

Alternatively you could box it in with some flatbar...

I would definitely use it if it is free! 👍

[mikeysp]

I understand boxing it in; but, I don't have that material. What do you mean by gussets? Is it what this updated drawing shows in orange? If yes, what does that do for the channel? If not, can you explain or link to an image so I can understand what you mean? The C Channel is pretty thick stuff. Seems to be about a 1/4" thick and a pound per foot for the 3". Thanks, -Mike

[pirate323i]

I understand boxing it in; but, I don't have that material. What do you mean by gussets? Is it what this updated drawing shows in orange? If yes, what does that do for the channel? ... ... The C Channel is pretty thick stuff. Seems to be about a 1/4" thick and a pound per foot for the 3". Thanks, -Mike

Yes, that is what i meant about the gussets.
They will increase the stiffness of the beam and go somewhat towards counteracting the torsional stresses. (It stops the "c" from opening up)
I agree it is a fairly heavy section and I am guessing the gantry wont be too heavy as it is only a plasma table (?)

Good luck! 😊

[mikeysp]

got it. thanks. The gantry is going to be very light weight. Probably aluminum or thin rectangular tubing. I have the aluminum plate 3/8" for free, but cutting it all out with our taig mill will be tedious, so I will use rectangular thin gauge, if I come across some for free.

[pirate323i]

got it. thanks. The gantry is going to be very light weight. Probably aluminum or thin rectangular tubing. I have the aluminum plate 3/8" for free, but cutting it all out with our taig mill will be tedious, so I will use rectangular thin gauge, if I come across some for free.

Not sure if you are aware, but you can cut Aluminium with a circular saw/table saw (ideally with an Al blade, but can be done with any Tungsten carbide blade) make sure you use the wax lubricant with it also. AND use hearing protection!!!

[Eldon_Joh]

to increase torsional stiffness the gussets need to be in a triangular pattern. in fact they don't need to be welded to the web of the C channel, though that would help. you could take a strip of 1/8th by 1 inch steel, bend it like this /\/\/\/\/\/\/\/ and weld it at each point where it contacts the flange of the C channel.

[hanermo]

Short A.
No, the C channel is not the right choice.

It is very, very, very flexible, over any length at all, and plasma tables ten to be big.
BUT..
Use the bigger 4" size, sand the flat, and glue, with industrial epoxy, a 4x2" box, sanded on contact side, to it.
10 minutes per beam, if you have a big belt sander (I use a Festool 105, grin.)
And yes, epoxy is much more than strong enough.

Hint!
Hang them vertical before clamping, so they are relaxed.
If you have anything precision, clamp them first to that.

If not, buy one piece of flat ground stock as a precision reference.
IF $ is an issue, you can use 4 sides milled vs flat ground stock, as it is cheaper.

[C0matoes]

Yes and no. First you're undersizing your material. I've read alot of replies to this thread, some good, and some, while not wrong, not so good. Here's the thing. Any time you use mild hot roll steel for a project expect some difficulty due to how the steel is manufactured. Channel, pipe, solid bar, and tubing all have their own process but the idea for most is similar, molten steel into a shape. The stuff is rolling out of the mold hot, rolls down through processing, goes to storage, gets put on a truck by the cheapest guy we could find, gets removed at the shop by also the cheapest guy we could find, then we get to work around its deficiencies.

C channel is just as good a material as the rest for fabrication. That being said, none of it is straight or the same from different sources so get all the material from one spot, at the same time if possible. Yes, they all will flex and yes they will all have their individual problems but it is entirely possible to build a 20'x6' cut table, with c channel. I just built one within .02" so I'm fairly certain here. I would suggest 6" channel, not for straightness but ridigity. 3" just doesn't handle things well and is rather flexible for a long table and you won't need 100 gusset pieces to make it happen with 6".

Accept from the start that it's not straight, stop looking for a way to straighten it because it won't happen. Look for a way around it. Inspect it when it gets off the truck carefully and send back anything that's not useable. You ordered it, it follows an astm spec, and they will take it back if it's out of specification. Now that we have accepted that our material is close but no cigar we have to figure out how we're going to get this crooked metal to fit the project.

I've been a fabricator of steel/aluminum concrete forms for over 20 years. I've built boxes up to 16'x8'x7' within .025" using every material available. I assure you, channel can work if it's what you have to work with. Weld up what you can on as flat, and level a surface as you can come up with and keep it sqaure by clamping it hard. Assemble the table level, plumb, and square and you'll be close. Make you're axis rail is made out of something you can adjust to fit to compensate for descrepancies in the material. If you run into something you can't figure out, give a shout and maybe I can help you figure it out. It's a plasma torch and that in itself limits it to a certain attainable tolerance. Keep that in mind but keep it simple. If you're using a THC your long rails could technically handle 1/8th" or more out vertically so don't spend a buttload of time trying to compensate for vertical (up and down) problems if you're using one. To be clear, when I say compensate I do mean less than .0625".