Floating Shelves - Torsion box

[Aidex]

Hi Everyone!

I have to make a couple of floating shelves for an alcove. They'll be about 350mm deep and 1200mm wide. I'd be grateful for some advice on what materials to use for the torsion box.

Top and bottom will be 9mm MR MDF. Can I use 18 or 25mm mdf cut into 30mm strips for the torsion box? Or is it best that I use softwood? For the front face, can I use 9m MR MDF or should I use something thicker?

Many thanks for all your help!!

Aidex

 

[Steve Maskery]

Hi Aidex
I am about to do the same.

9mm skins are plenty, I intend to use 6mm. If you go much below that, there is the chance of the internal structure ghosting through.

As for that internal structure though, you choice of material is way too thick. The whole point of torsion boxes is to get strength with minimal weight, which 18mm MDF is not, let alone 25mm.

I suggest that you use 3mm MDF and use a 1/8" kerf TS blade to cut the halving joints. Clamp all the strips together and cut them as one, that way they all line up.

For the edging, make a frame the finished size and glue on the bottom skin, slightly oversized. Use a roller to glue the webs in place (you need to glue just the webs, not the whole internal surface of the box). When that has gone off (how are you going to clamp it? I have a vac press. If you don't, you can improvise for a job like this with one of those vac bags for storing duvets and the like), flush trim the bottom skin. Then you can glue on the top skin and trim that in the same way. Finally glue on your show strip to the front edge. Job done.

 

[Setch]

I successfully used 9mm mrmdf with 20mm PAR softwood as the mounting battens and front rail between the skins.

I covered the front with 3mm MDF, my reasoning being that there world be less visible side face on the top and bottom of the shelf. It worked well, though it was a bit time consuming. As the facts had to be glued clamped and rather than pinned on, as is worried the 3mm MDF might curl and show gaps if it wasn't securely clamped whilst the glue dried.

 

[paulrockliffe]

I thought the point of the torsion box was so you couldn't see any fixings, they don't need to be light and or an efficient structural design in this use.

I think Steve is talking about a construction where you're sliding a box over the frame attached to the wall. I did mine the same way as Setch, though I used glue only to hold the panels to the frames and a trolleyjack or two to apply the pressure while the glue set. I used a thick piece of ash for decorative purposes at the front, but also to stiffen up the weakest bit of the structure.

 

[will1983]

I've just done one for my mum.
I used 6mm MDF skins with 38x25mm soft wood for the internal framework and a strip of 6mm MDF for the front face.

Final dimensions were 1300mm x 400mm x 50mm. The finished assembly was fairly light, no measurable sag under a 1200mm straight edge and no hint of ghosting once painted.

Needless to say I'm in the Mother's good books for a little while longer.

 

[Steve Maskery]

I thought the point of the torsion box was so you couldn't see any fixings, they don't need to be light and or an efficient structural design in this use.

That's not my understanding, Paul. A torsion box is to provide great strength and stiffness, it has nothing to do with whether or not you can see the fixings.

My outfeed table to my TS is a torsion box and there is a wedding table in a very posh restaurant in Nottingham which I made in the same way. I once made a boardroom table that way too. No idea who it was for, somewhere in London, I was just subbied to make it. Actually I got the job because I did know how to make it. The guy who designed had a client who wanted a table with no aprons, and so the designer was scratching his head, given that the aprons hold the legs together as well as support the top. So I asked him if, if I told him how to do it, he would give me the job. I did and he did. I'm seriously thinking of making my next dining table in the same way.

I think Steve is talking about a construction where you're sliding a box over the frame attached to the wall.

No I wasn't, but that is certainly something that you can do with them.

 

[Aidex]

Thank you all for your help and advice!

Steve is the whole torsion box made of 3mm mdf? If so, how do you suggest fixing it to the wall? Couldn't the weight applied to the shelves cause the screw heads/washers to rip through 3mm back and sides?

Many thanks!

Aidex

 

[Steve Maskery]

No. The perimeter will be softwood, 20mm thick. Front and back can be 25mm wide or so, ends will be 70mm or so wide, as I need to cut 45mm notches out without cutting into the interior. This is going inside a cupboard. It's only the internal ribs that are 3mm. But they do have to be glued in place, not just a loose array. They are stopping one skin from bending independently of the other.

I'm also planning the sort of floating shelf arrangement you are suggesting. For that I probably will use 9mm skins. I shall have support battens screwed to the walls over which the shelf slides. The potential snag with this is that my walls are not straight, flat or vertical. It's not so much of a problem if the alcove is wider at the front than the back, but it makes it more difficult to do a clean invisible job if it is wider at the back.