Torsion boxes

[chippy1970]

I am just about to build a small 1m x 1.8m x 75mm torsion box bench and was thinking of using 12mm mdf. When building the honeycomb framework do you think it would be ok to slot the mdf together rather than just butt it up.

Would this weaken the construction.

Also whats the thinnest mdf I could use on one that size.

 

[engineer one]

that goes back to the old fashioned "egg crate " design, may well depend on how you make the slots.

depending upon your spacing you could go down to 6mmon edge.

have you checked out the latest issue of the american mag woodworkers journal, there is a whole section about torsion boxes. if you are in london, then borders sell the mag.
otherwise it might be on line
paul :wink:

 

[Steve Maskery]

I've just built an outfeed table this way. You don't need 12mm ndf. The whole point of a torsion box is to get strength at zero weight, or as close to it as you can.

The strength of a torsion box is due to the separation of the top and bottom skins, and derives from the fact that plywood, or mdf, does not stretch. In order for the box to sag, the bottom skin must stretch mucch more than the top, and the greater the separation, the more difficult this becomes, so at 75mm you could have the Dragoons tap-dancing on it and it would hold.

Make it all out of 6mm MDF, with 30mm edging and you will be fine.

S

 

[chippy1970]

Yeah I did think maybe 6mm would be ok but thought 12 or 9 mm would be better for the framework as its easier to nail the skins to.

When you say 30mm edge do you mean to make the torsion box 30mm thick and not 75mm or are you saying to use some 30mm edging to protect the edges.

 

[engineer one]

i guess the answer would be why make the box 75 thick.

as steve suggested 30 should be adequate.

paul :wink:

 

[Steve Maskery]

I mean use timber 30mm thick and63mm wide to make the edging. With two 6mm skind you will get a 75mm thick structure.

I wouldn't rely on nails (except perhaps to keep the skins in place while the glue sets).

As Paul suggests, 75mm will hold up a house.

Cheers
Steve

 

[chippy1970]

I would never rely just on nails sorry I should have gone into detail I have a compressor with several nail and pin guns so I will use pva glue and small brad nails just to tack it while the glue goes off.

As regards the thickness Paul is saying that you are saying to make it 30mm thick but then you have comeback saying 30 x 63 plus two 6mm skins = 75mm so you are still saying 75mm as I said in the first place.

By the way this top is going to be used on top of two metal saw horses for dimensioning 8 x 4 sheet material with my Festool TS55 I was going to put small strips of mdf tacked to the top to stop the saw blade damaging the torsion box.

It will most likely get used for a bench too just for putting stuff together etc.

 

[Steve Maskery]

Hi Chip
I'm not disagreeing with you. I'm just saying that if you make it 75mm it will hold up a house, I'm not saying it must be 75mm, I do think that Pauls 30mm is a little on the light side, if I'm honest. It depends on what load you are planning.

I can confirm, however that a torsion box 900x600mm, with 30mm thick edges, made from 6mm ply up to a total thickness of 50mm (6+38+6) will hold 102kg without any noticeable complaint!

Cheers
Steve

 

[engineer one]

steve, i have referred again to the Ian Kirby article in woodworkers journal, june 2007 issue.

he talks about the strength of a 600x600 box which is 25mm thick, and its total gluing surface which he suggests is the idea of why it is so strong.
he has set the uprights at 100mm centres gives 320 square inches of glue surface,(sorry forgot the conversion factor so its a 24x24 x1/2 core plus 2 1/4 surfaces.) and he looks heavier than you steve :?

he suggests using staples to hold the core pieces together, and what is interesting is that he only joins the core with staples, and uses the glue only on attaching the top and bottom surfaces. :?

as for steve's dimensions i think it depends on how you lay things out, hxdxw or another way.

paul :wink:

 

[chippy1970]

Yeah I imagine you would only need to glue the skins to the framework I suppose thats where the strength its like when you cut into a hollow core door its only wavey lengths of card that are only glued on the edge to the skins.