torsion box

[No skills]

Hi folks.

I might need to make a torsion box type bench top as an assembly/clamping surface. I would like to use an 18mm top and 12mm material for the rest (all plywood). Is it a problem having a thicker surface on one side? I was going to make the horizontal strips around 100mm deep and space them around 300mm centres.

Opinions welcome. ..

 

[marcros]

From my OU Engineering summer school some years ago, i don't see that having a thicker surface one side than the other will make any difference.

 

[marcus]

Searching my memory banks from the research I did for one I made recently, the only issue I can see from having one face thinner is that the stiffness of the box is mainly a function of the thickness of the faces combined with the depth of the core, so making one surface thinner would compromise the stiffness a bit. Should still be pretty stiff at the dimensions your are talking about though.

 

[No skills]

Many thanks for the swift replies. Cant see it having any heavy use but thought I'd ask, I'll prolly drill the top mft style and cut a few access holes in the base when I do it.

Cheers.

8)

 

[marcros]

i seem to remember that the glue bond to the top and bottom is quite important- so make sure the strips are the same width.

 

[yetloh]

I agree that different thickness faces should not be a problem, although, looking to the future, using 18mm for both faces would mean that you can simply turn the top over when the top surface gets badly spoilt which is pretty likely over the course of time.

A couple more points that might be worth considering. First, I don't know how you intenfd to make the internal lattice, but simple butt joints reinforced with epoxy/filler fillets remove the need for half lap joints and also the need for great accuracy in the making while greatly speed ing up the process. The technique is explained in the West System user guides which you can find here http://www.wessex-resins.com/westsystem/bonding-gluing-clamping.html.

Secondly, I assume you will be fixing the top down by screwing from underneath. If so, It would be a good idea to include some solid wood sections in the latice in the areas where the fixings will go, particularly if you go with the 12 mm face underneath.

Jim

 

[Jacob]

A good design for an assembly/clamping surface is the trad assembly bench with a top like a cattle grid - parallel rails say 2x3" hardwood with say 3" gaps between.
Easy to construct and keep flat. Debris drops through the gaps. Well oiled so that glue can be removed. Rounded arrises to avoid marking. G clamps can be fixed all over the top or sash clamps dropped into the gaps.
I've never understood this concern with rigid boxes for assembly, still less why they are called "torsion" boxes. It's a good way to make light weight but stiff shelves or table tops but there's nothing inherently flat about the construction. Stiff/strong yes, but stable/flat not necessarily. You quite likely would still need to flatten the surface as it curls as the glue dries or with variations in humidity etc. It's compared to aircraft wing construction but wings are never flat and are always flexible.

 

[RogerS]

Any good this link ?

http://www.thewoodwhisperer.com/videos/ ... rsion-box/