Iron vs. Wood - which is better?

[sonicwonder2000]

Hello to the forum!! Been lurking here for the past couple of months and am in the final stages of nailing down a design for my first cnc...

My hobbycnc motors (305oz/in.) and controller board are already completed and working well after some awesome support from Dave at hobbycnc (thanks!!).

I'll be building a Solsylva belt-driven table modified to handle a 4'x6' cut area. The gantry on this table is made completely of aluminum. I have been debating whether to use metal or wood for the frame of the cnc. I will be using the standard skate bearing rail design; Given that the rails themselves are not precision, will there be any benefit in making the frame metal? On a related note, will there be any benefit in using cold rolled steel shafts as opposed to gas-pipe for the rails?

I am hoping to do some detailed 3-D work in wood, plastic, and soft aluminum. The cost of a wooden frame is cheaper than the metal option by about $200; will I see a commensurate improvement in performance by going all-metal?

[Splint]

The main draw back of wood is that it is prone to warpage and shrinkage over time and due to climatic conditions. I personally would be going with steel. In the grand scheme of things the extra $200 isn't such a big deal.

Cheers
Splint

[ahren]

The other (I would argue larger) benefit of steel over wood is increased stiffness, or resistance to deflection. Steel is roughly 20 times stiffer than wood, and 3 times stiffer than aluminum for the same geometry. In some cases you can overcome this advantage just by using bigger pieces of material. Take bending in a solid beam, for example. This is a function of the material thickness cubed, so you could achieve the same deflection under load as steel by using a piece of wood 2.75 times as thick. Note that this is a comparison of solid steel vs wood only. A steel tube would require an even bigger geometric advantage to create in wood.

Your machine will be only as stiff as your weakest interfaces. When building with wood, you will need to take special care that your joints are strong, whereas a bolted steel frame is a little more forgiving. One material isn't necessarily "better" than another, but the material characteristics of steel make it potentially superior for creating a stiffer frame.

Ahren
www.cncrouterparts.com

[thegimpster]

I built my router out of about $160 worth of steel for the whole thing and its surdy enough to hold my 250lb butt with no problems or flexing. I have a pic of me standing on it in my build post. I'd go with the steel.

[sonicwonder2000]

Okay, metal it is!!

I drafted together a design using 10x10' pieces of unistrut. The green segments will be welded to the adjacent members, and the rest of the unit will bolt together using normal 3/8" bolts and washers.

One concern is getting the unistrut cut accurately, especially the rail support members. These need to be EXACTLY the same height to ensure that my rails are equidistant from the work bed. Unfortunately, I only have a harbor freight piece-o-crap cutoff saw; I dunno if it's going to be accurate enough? Anyone else have any suggestions for getting these pieces cut accurately?

I plan on welding the members on a granite slab which should be fairly flat. I am also going to use a right angle clamp to hold the members while welding.

Anyone have any suggestions on improving the structure or getting this thing welded up squarely?? I'm gonna start a log soon ...

[thegimpster]

Ya I have that same peice o crap harbor freight chop saw. Cut one peice the legnth you want, then put the other peice in the vice. Lay the first peice ontop of the other peice with the ends flush. Push the 2 peices towards where they should be cut. Lower the blade and push the 2 peices till the end of the cut peice sits flush w/ the side of the blade. Make sure you are not bending the blade. This should get them accurate w/ in 1/32 or so, if not right on. Takes a bit of pratice but you can get good results w/ it.

[cncspear]

As for welding, put down a few tacks and check alignment constantly between tack welds. Then lay down permanent weld beads in alternating segments.

As for the machine frame, it could probably use some gusseting or truss elements (diagonals) to stiffen it up.

Very good decision to go with metal instead of wood.

Gil

[sonicwonder2000]

Originally Posted by cncspear As for the machine frame, it could probably use some gusseting or truss elements (diagonals) to stiffen it up. Gil

It occurred to me to use a truss structure, but I wanted to keep the frame unboltable for transport. Additionally, I wanted to use regular bolts as opposed to the uni-strut special fixtures which would drive the cost up significantly. To accomplish both these criteria, I would have to weld the cross-trusses, which would make the structure too unweildy to transport.

Hopefully, the horizontal crossbrace on the legs tied to the vertical bed support will serve as sufficient stiffening. If I find the frame wobbly, I suppose I could add another horizontal member through the center of the frame, or bite the bullet and buy some 45 degree unistrut connectors ...

[Oldmanandhistoy]

Hi,

What’s the wall thickness of that unistrut? Just wondering how stiff it will be with all those holes.

John

[X-29]

Originally Posted by sonicwonder2000 Okay, metal it is!! Anyone have any suggestions on improving the structure? I'm gonna start a log soon ...

A tube with a square cross-section have much more torsional stiffness than an U-section. And of course resist bending better too.

[Mr.Chips]

For my 2 Cents worth, I would go with a completely bolted unistrut unit. Take a look at Joe’s Hybrid, it's the latest hot building method, and looks to be a very good performer both in design and strength.

With welding no matter how careful you may be there is always the possibility of warping, and once warped a devil to fix.

With bolting, the unit becomes totally adjustable and it is easy to make changes. And making changes is what hobby builders do as new tips and ideas come around.

Diagonal bracing is a must both for the vertical and horizontal planes. What with the constant starting and stopping of all that mass you will quickly see the effect of no diagonal bracing.

Anyway my 2 cents worth. Good luck and post lots of pictures.
Hager

[wcarrothers1]

Well I like the overall design aproch since heck. It looks darn similar to mine. And everyone knows I love my machine.. see http://www.cnczone.com/forums/showthread.php?t=26768

Anyway that said I have a few things you should think about.

1. C channel isn't that ridgid. if you notice.. I have square tube welded to my C channel for my X axis on my machine. And that is backed up/supported by some 2x2 square tubing.

2. C channel is not always very streight. So if getting from the local store or what ever.. Be very carefull looking at how streight each one is as you choose them from the shelf..


Like the design. But think it could be better using square tube then c channel. And I don't think the square tube would be that much more if you managed to find a local place near you willing to sell you some. (try places that build custom trailers. They use lots of square type tube for those)..

Anywho.

b./