Torsion box - how thick?

[MixedHerbs]

Hello,

I am going to make an 8ft American pool table. The slates have arrived and I wish to rest them on a table made from a torsion box so as to keep them flat. The torsion box will be built on the leveled slates that are presently lying on my workshop floor. There are three slates each of about 150 pounds.

My question is, what is the minimum thickness I should make the torsion box to hold such a weight? Are there any calculations or "rules of thumb" that can be applied? I am expecting to use 6mm plywood for the skins and 10mm (or thereabouts) softwood (at 100mm centres) for the internal structure.

Since, in theory, a torsion box is rigid, I will only need four legs. Assuming the legs are also rigid, how can I attach them directly to the torsion box so as to create a rigid structure?

Regards, Peter.

 

[DomValente]

I'm not convinced that box will support that weight because the slate is in three pieces, I would have a frame around the box as well.

Dom

 

[Scrit]

I'd agree with Dom. The skins themselves need be only 3 or 4mm thick, but your torsion box frame would probably be better made from 70 to 100mm wide timber - and hardwood or plywood would be more rigid than softwood. 10mm isn't really a torsion box at all and I reckon your table would flex badly.

Scrit

 

[OLD]

http://thewoodwhisperer.com/?cat=3
The above shows a torsion box project some of the info may be useful.

 

[MixedHerbs]

Apologies for the confusion. By 10mm, I meant the thickness of the wood used for the internal frame.

Anyway, by the looks of things, a box made with 6mm plywood skins and a frame made from 9mm plywood cut in 100mm strips should do the deed.

Regards, Peter.

 

[DomValente]

Apologies for the confusion. By 10mm, I meant the thickness of the wood used for the internal frame.

Anyway, by the looks of things, a box made with 6mm plywood skins and a frame made from 9mm plywood cut in 100mm strips should do the deed.

Regards, Peter.

Hmmm, Good luck.

Dom

 

[MixedHerbs]

"Hmmm, Good luck."

Thank you. I understand your mis-givings. Your "Hmmm" is not out of place.

I will build a 8ft by 4ft torsion box, place it on some low trestles, load it with a great deal of rock, and see what happens. If it bows or breaks, I will beef it up. If all else fails, I will add more legs.

Regards, Peter.

 

[Shultzy]

Peter, you might want to look at this site for info on building the table.

http://www.bestbilliard.com/resources/buildtable.cfm

 

[lemonjeff]

Peter,
This site http://www.tjupholstery.com/jwnixon/index.htm?http&&&www.tjupholstery.com/jwnixon/PoolTable.htm has WIP videos of the guy building a pool table,
he hasn't got to the table top yet but you can see his design which does'nt look like it's going to be a torsion box.
However he does use torsion box design for the leg supports.
If your going to make the torsion box the same size as the slate, cutting out pockets holes will weaken the box.

Jeff

 

[MixedHerbs]

Jeff,

Thank you for finding that site. I thought I'd found all the self-build pool table sites, but that's a new one to add to my list.

Yes, cutting slots out of a torsion box is not a good idea.

Regards, Peter.

 

[Sawdust]

Peter,

I have a 9ft snooker table with slate beds (it came with the house) The slates are only supported on the edges on a very heavy frame. I'm at work now but could take some pics tonight and post them if that would help.

I don't think you need to support the slate ove its whole area.

Cheers
Mike

 

[engineer one]

the most recently available issue of woodworkers journal, (an american mag) has a great deal of detail about building torsion boxes, by Ian Kirby.

the big thing seems to be the centres of the internal pieces, ie the framework, and for what you are suggesting he thinks about 10cm centres.

i think though that you will actually need 6 legs , not least to help with the support round the centre pockets.

paul :wink:

 

[inventor]

I suspect this would horrify traditionalists, but, were I building a snooker or pool table, I'd use a single piece of glass for the bed, rather than slate. Many years ago, I built a large flatbed pen plotter using a sheet of glass, and it was easy to support it flat. All of which may not be much use if the slate has already arrived!

What is the mass of your slate? What is the maximum deflection you want for the table?

Does anyone know why these tables have legs at the corners? To produce minimum deflection in a uniformly loaded beam with two supports, place the supports some way in (I seem to remember one seventh of the length) from the ends. Why don't snooker tables work like that?

 

[engineer one]

at a guess, snooker type tables were designed arouond solid wooden frames and made originally in the indian market, so it became the style

if you think that the legs have to support the cushioned edges and sides, plus the thick rest side, plus allow for the openings for the pockets, it makes sense in fact to tie things off legs.

i would guess since it has taken the time it has to allow women to referee, and they still use dinner jackets, it is much to do with the traditions for match play, and if you are buying that's what you want.
:roll:
paul :wink:

 

[MixedHerbs]

Thanks for the offer of pictures, but there are enough on the web for my purposes. I do not want a heavy-looking frame, however, if I accidentally bump into the table, I want it to feel as immovable as mountain.

I would guess the weight of the slates is about 230kgs. You are probably right about requiring additional legs, however I want to see how far I can go before giving in to the idea.

There is a problem with the idea of using glass as a bed for a pool table - you would need to cut/drill the pocket-holes. Each hole is about 4.5 inches in diameter.

Regards, Peter.