Thread: MDF or plywood for torsion box assembly table.

Go back a bit. Why do you need a torsion box? What advantage does it bring to an assembly table?
If it's an old dining table then surely it already has a top, no?

DaveVman,I think that if you make it out of MDF you may never be able to lift it,so ply is my choice

The tabletop is made of some very soft wood. It is solid wood but unusually soft. I don't know what it is but it is softer than pinus radiata. The smallest thing creates great dents in it. Hence it needs to be covered with something or it will be in very poor shape within no time.
It also lacks rigidity. It's is not designed to have heavy things on it so I need to strengthen it.
I am in the middle of some renovations at the moment but once I get some time, I'll try to figure out if it is flat. The floor of my workshop is not flat so I'm not sure how I am going to determine that to be honest.
Plus a torsion box is a nice "little" project.

3mm mdf to cover the top and a rail between each leg for support ? Even another leg if needed

Originally Posted by Sam

3mm mdf to cover the top and a rail between each leg for support ? Even another leg if needed

Yes that (plus some casters) was my initial plan. Especially if I don't care about it being completely flat. However that was before I read about torsion boxes. The torsion box, leg rails and casters total about $100 worth of materials. It's not a problem.

The thing I'm not too sure about is MDF or ply.
It is at least 3 sheets of MDF (probably 4) which as Sam points out, is a lot of weight.
I wonder what is a good thickness of MDF for this? If I made it from MDF, the sheets would not have to be as thick as the ply I would likely use, which might mean it was not too extreme weight wise.

Originally Posted by Sam

3mm mdf to cover the top and a rail between each leg for support ? Even another leg if needed

or a sheet of masonite

Originally Posted by DaveVman

The thing I'm not too sure about is MDF or ply.
It is at least 3 sheets of MDF (probably 4)

without doing a sketch, I'm thinking 1800 x 900 should be doable from just 2 sheets.
the whole torsion box shouldn't need to be more than about 60mm thick, using 40mm high ribs.

I'm also thinking 9mm, or if you're laying out is good 6mm MDF

Originally Posted by nrb

DaveVman,I think that if you make it out of MDF you may never be able to lift it,so ply is my choice

I have recently made a torsion box 3.6 metres long out of 6mm MDF and two people could lift it no problem. It is sitting on a ledger along one edge and the front is unsupported loaded with four sewing machines and it has not bent.

Originally Posted by Chris Parks

I have recently made a torsion box 3.6 metres long out of 6mm MDF and two people could lift it no problem. It is sitting on a ledger along one edge and the front is unsupported loaded with four sewing machines and it has not bent.

yeah

people sometimes forget that the strength in a torsion box comes from the top and bottom skin.

how thick is your box and what spacing did you use for the ribs?

Originally Posted by ian

yeah

people sometimes forget that the strength in a torsion box comes from the top and bottom skin.

how thick is your box and what spacing did you use for the ribs?

I will measure it tomorrow. My understanding and experience is that the depth of the ribs is the main strength factor.

Originally Posted by Chris Parks

My understanding and experience is that the depth of the ribs is the main strength factor.

you're right.
Torsion boxes are a bit like a steel I-beam. The bending strength and stiffness of an I-beam principally comes from how far apart the top and bottom flanges are -- i.e. how deep is the web. There's a bunch of other factors related to flange and web buckling that engineers need to consider, but as a first approximation, for a given flange thickness, the strength of a beam goes up as the cube(?) of the section depth.

So with a torsion box, the depth of the ribs largely controls the strength and stiffness of the box, which means that the top and bottom skins can be fairly thin, provided they are firmly attached to the ribs. Also because the ribs are there to hold the skins apart, they don't need to be all that thick. But I'm not sure I'd try to build a box with ribs only 3 mm thick, as I think they might be just a bit too flexible, but depending on the expected abuse, I might use 3 mm skins.

I made one with 3 ply skins which was incredibly light and used as a ramp to load a golf buggy into a car. The whole concept is to build as light as you can with sufficient strength for the job at hand. if I was building an assembly table it would be 3mm ribs and 6mm skins and the only reason I would use 6mm on the skins would be to stop breakage if something was dropped on it. The biggest issue with torsion boxes is being accurate when cutting the ribs so that they all have the same height for gluing to. Gang cutting is the go when doing the ribs to minimise the problem.

I once made a torsion box out of MDF and that was because I had it for free. If I was going to do it again I would use plywood.

I will also state that as you already have a table with a top use the existing one and add Masonite to the top. Once this becomes roughed up modify accordingly.

I promised Ian I would get back with some details, so here they are.

The top to bottom depth is 60mm and the internal ribs are 54mm deep spaced from memory at about 100mm square and the bench is 600mm deep in total. I made a mistake in the post above, it is made from 3mm MDF not 6mm which is why it is so light. The back edge is supported on a french cleat style ledger and the front all round is totally unsupported, the cabinets underneath do not touch the underside at all. The pine edging is machined in T and it is about 40mm deep on the T. The biggest issue I had was cutting the ribs accurately, even gang cutting did not get them all the same height but I suspect that was because it is only 3mm thick and being soft it moves with the blade. I tried to get clever and run the ribs through the helical head thicknesser to get them all spot on but being so thin the outer ones in the bundle basically delaminated to some extent but as they were not seen I used those anyway. It was dead cheap to build and I am really pleased at the result even if it took two weeks out of my life. The bench usually has another two machines on it and all those machines are damned heavy.

To build it I needed a flat surface that was dead level so DaveTTC helped me, no I helped him build four beams out of 18mm MDF and we used a laser level to set them up on saw horses, basically shimming them to height and this worked well. The same could be done with a beam level or even a water level but the Laser is so much easier to work with, thanks Dave. I have since bought my own and threats of death might persuade it out of my hands but I doubt it. Escuse the mess behind the pic of the beam as I am doing a refurbishment of my workshop at the moment and the whole thing is a mess.

What has surprised me the most is that where I jouned the return onto the main bench, even unsupported it has not dropped as I was sure it would after a few months of being installed.