Material Choice - 8020 vs steel

[Hack]

I am looking at building my 4th router. This one will be a 4' x 8' machine and it's primary use will be cutting cabinet parts out of MDF, Melamine, Plywood, etc as well as doing a bit of Solid Surface Material and Solid wood. Sheet Goods Primarily. Will be cutting for myself as well as others, so I condiser this a serious hobby machine. My goal is to route sheets in a single pass at good speeds - perhaps comparable to a Shop Bot.

I have drawn a machine up in CAD made with 8020 extusions. The X and Y axis to be supported by 3060 extrusion, the legs out of 3030, and the rest out of 1530 and solid aluminum. Drives will be Rack and Pinion.

My questions is, how solid is this design and could someone critique it? In the back of my mind I am thinking I may want to go with steel instead of extrusion for the vibration dampening ability and sheer mass.

My problem with this is I cannot weld, do not have the tooling to cut it, and would have to farm this out at additional cost.

With the cost of extrusion being so high, I was considering steel, but with the soutsourcing needed to cut weld, paint, etc the cost is closer to what the extrusion would be.

So I come back to design. Is ther a big benefit either way?

Thanks

Dan

[LeeWay]

I think steel might be more rigid of course, depending on the sizes and types used. However, I think that either material is well suited to do what you want.
I built my first small router using steel tube. It is rigid. I didn't weld anything but the lower shelf onto the legs. The rest was all either drilled and tapped or through bolted. You could easily cut all your parts with a hand held bandsaw. They are cheap enough.

I built a small mill using the 3060 as a base and column. It is very rigid as well. It is much easier to modify an 8020 design than a steel one, but given the steels strength, I would mark them as near equals with the edge going to steel.

You can save a lot of money on your design as well. Use only 3060 for the gantry and the sides of the table. Use steel or aluminum tube for the rest. Make your own corner braces and brackets using steel or aluminum.
You can make it incredibly strong and cheaper this way. You would be able to over brace for less money.

[jhowelb]

Check these guys.
http://www.faztek.net/

[dertsap]

nothing says steel has to be welded , why not simply concider bolting the steel frame together, the extrusion uses brackets and is bolted together
all you should need for cutting the steel is an angle grinder and a handfull of cutting discs ,or you can buy a chopsaw cheap enough
brackets can be made from chunks of angle iron and or pieces of plate , once again you'll only need 2 tools , an angle grinder and a drill

[ger21]

You should look at building a Mechmate. www.mechmate.com

[Oldmanandhistoy]

Originally Posted by dertsap nothing says steel has to be welded , why not simply concider bolting the steel frame together, the extrusion uses brackets and is bolted together all you should need for cutting the steel is an angle grinder and a handfull of cutting discs ,or you can buy a chopsaw cheap enough brackets can be made from chunks of angle iron and or pieces of plate , once again you'll only need 2 tools , an angle grinder and a drill

What would you guys recommend as the best type as in cold/hot rolled steel to use in a frame construction?

John

[dertsap]

i would say structural steel channel and tubing ,which for the most part would be hot roll

i agree with Ger , the mechmate is a nice looking setup

[Oldmanandhistoy]

Originally Posted by dertsap i would say structural steel channel and tubing ,which for the most part would be hot roll

Thanks dertsap

[Dougal]

Steel is three times stiffer than aluminium and also three times heavier. I'd make it from steel.

Regarding the sections, the geometry usually dictates that and gives you a few choices on size. Hot vs cold rolled isn't going to matter as they only affect the strength where your frame will be mostly concerned with higher stiffness and low stress.
You can't use higher strength materials without deflecting the steel a lot, so even pig iron will do the job well.

[Hack]

Now if I were to substitute the base for steel and still use the 8020 to mount the rails on the x and y axis, would this be any better or will I still be limited by the 8020 gantry? Kind of the chain is as only strong as it's weekest link principle. Would the gantry limit the strength and vibration dapening properties? I am considering this in order to save on drilling all the holes for the linear railes as the t-slot seems to be much easier.

I had a previous version drawn up in cad that I had considered building. It is based mainly on c-channel and a bolt together principle. Also, how rigid will bolting get me vs welding?

I had given up on this design as well as a shopbot or mechmate design as I do not want the X rails above the table height. I would rally like to be able to slide the material on / off from the sides without fighting the rails.

Thanks again.

[Oldmanandhistoy]

Here's a nice machine that might give you some inspiration

http://www.cnczone.com/forums/showthread.php?t=30751

john

[Dougal]

Originally Posted by Hack Now if I were to substitute the base for steel and still use the 8020 to mount the rails on the x and y axis, would this be any better or will I still be limited by the 8020 gantry? Kind of the chain is as only strong as it's weekest link principle. Would the gantry limit the strength and vibration dapening properties? I am considering this in order to save on drilling all the holes for the linear railes as the t-slot seems to be much easier.

Stiffness and strength are treated quite differently.
While a chain may only be as strong as it's weakest link, it's stiffness is the result of the combined flex in each link.

It's difficult for me to comment on the stiffness required because I don't know what forces are encountered at the cutting tip and I don't know how much flex is tolerable.
It looks like the load from the cutter is taken from the gantry down almost directly into the rails and table edges. In which case the design of the gantry itself is going to be your biggest concern. You'll also gain/lose far more in each rail than you will in the table itself.
Bolting will be ample for joins. If you don't think a particular joint is stiff enough then just add more bolts.