Torsion box vs Laminating MDF.

Hurro...

In the planning / sketching stage for a new router, and wondering about the large torsion boxes folks build up...

Is the background to this keeping the weight down?

Without putting much through into the process I would have thought that a few bits of 19mm mdf laminated together would have similar strength/rigidity as a torsion box with substantially less effort. The down side being the weight of the beast, but you could cut 'holes' out of the middle layer bits of MDF to keep that in check a bit.

Just curious, as I'm looking at a composite MDF/Alloy build by the looks of it, and I don't have the hand-tool skill to build a large torsion box accurately and my current CNC router is no where near big enough to make the parts for one!

Cheers, Me.

Solids get their torsional and bending rigidity from the cross-sections, and so for the same weight, you can build a hollow box with a much thicker cross section and stability. You can see this in engineering formulae for deflections : compare the bends for a solid bar of material to a hollow beam of the same weight :the difference is enormous.
You have seen how you can bend a horizontal sheet of MDF by picking it up from one edge. Laminated sheets will reduce this in proportion to the number of layers, but it will remain bendy.
I have never built a torsion box, but I don't think that the accuracy is an issue if you are building on a reliable flat surface using a table saw to cut the material; you are going to skim the top anyway to make sure it is flat to the CNC.

You can easily build a torsion box with nothing more than a router. Use pre-dimensioned lumber for the webs, as it's available in consistent widths. Lay out the grid on a sheet of MDF, and shim it until it's perfectly flat. Attach the grid, then the other side. Use the router with a flush trimming bit to trim the skins to the web.

With a torsion box, the thicker it is, the stronger it is. 2 laminated sheets is basically a torsion box with a 0 thickness between them.

My torsion box is two sheets of 1/2" (12mm) MDF, with 3" (76mm) high frame. It's 60" (1500mm) long, and deflects less than .01" (.25mm) with 200lbs (me) on it.

Making it lighter is not really the primary objective. Making it stronger is. Weight is actually you're friend, as it can reduce vibrations.

Think about why cardboard boxes are made the way they are. The corregated pattern between the two outside layers increase the strength by a factor that combining several layers of plain paper together can never give you. Torsion box is the only way to go, in my opinion.

I think that a stack of laminated mdf would be equivalent to a torsion box IF the stack was as thick as the torsion box. However, to get a stack of mdf several inches thick would cost a lot and it would weigh a whole lot. Also, deep cuts through the top layer would negate the torsion box effect of the mdf stack!

I just want to thank you guys for giving a name to what I've been building.. Didn't really think about the pros or cons... or ask anyone if that's the way to go... just did it instinctually.. maybe it's the 35+ years of construction experience.. but your comments made me feel good about the decision...

I'm sure there are better materials to build it out of than particle board.. but I already had three sheets of it.. and considering that my prototype base table of 2x4's and plywood is already kinda flimsy... soooo..

http://www.r8rphan.com/cnc/pix/cnc_build_77.jpg
http://www.r8rphan.com/cnc/pix/cnc_build_84.jpg
http://www.r8rphan.com/cnc/pix/cnc_build_86.jpg
http://www.r8rphan.com/cnc/pix/cnc_build_91.jpg

Wow Montaincraft, that's one big-arse torsion box!

This basically confirms what I was thinking.. It's a weight/strength trade off.

My thinking is that for a reasonable sized machine (up to 1/4 sheet?) laminating up some full sheets of MDF for the base is probably not to much of a silly idea, as the added weight will help with vibration damping, and for the tool challenged like myself it looks easier. :-).

For a gantry on the other hand, particularly if it's not fixed, you're on a hiding to nowhere if you use 4 layers of 20mm MDF, it'll weigh 4x as much and make the whole thing difficult to work with.

Thanks folks, clears the thoughts and hopefully will help someone else when they Google the title. :--)