Long desk and lightweight panels

[gaius]

hi all,
For some time now I've been wanting to make a fairly large desk, 2800mm across. to be lacquered in a solid colour.
Since sagging would be a problem, in principle, mdf would be a good candidate, since it's easy to finish and tends to be straighter than even good plywood. But for thicknesses of 18mm+, this would be very heavy.
Birch plywood would eventually sag, unless I were to somehow reinforce it.

But how about lightweight panels (honeycomb sandwiched between two panels)? I've never used them, but from what I've read they seem to be amenable to normal woodworking tools. The only problem seems to be the edging.

The way I see it, after cutting/sawing, the inner honeycomb material becomes exposed, which leaves a very irregular pattern. To complicate matters, I would like to smooth the edge with an ovolo bit. This could work if I could fill the exposed core with a filler/paste which I would then route after drying. Would wood filler be sufficiently hard to allow it to be routed, adhere well and remain stable with time? In principle this would entail a strip of filler, almost 10mm thick, around the entire perimeter of the desk.

thanks in advance for your suggestions.

cheers,
Gaius

 

[Jojo]

Hi

If I were to use this material I would cut away the honeycomb material for about 25mm inside the edge and rebate both sides off a strip of timber and glue it into the board sides, this would allow you to get a good machine finish.

John

 

[Mike.S]

+1 for what Jojo said.

I'm just finishing a project involving honeycomb cores - see this post. I built the cores and included a frame around the panels' perimeters - so none of the honeycomb core was visible. This would allow you to router the edges.

However, I suspect you're thinking of buying the ready made panels. If so, then Jojo's advice is good and I recall is exactly what the panel manufacturers suggest be done. I had to repair one of my panels and found it was easy to use a small router (with straight bit) to remove the honeycomb material between the sides and glue in a wood (MDF in my case) insert. A quick plane, fill and sand and it was ready for finishing....or routing.

My longest panel is around 2.6m long and I believe that for a desk of 2.8m sagging would be a problem unless you use very thick (100mm+) panels or intermediate leg supports.

 

[Steve Maskery]

Make it yourself.
This is called torsion box construction and it is quite straightforward. You can make the honeycomb centre stuff out of 2mm backing board used for picture frames, or thicker stuff like 6mm if you really want to, but it is not necessary.
Start with, say, a 6mm base layer. Glue a softwood strip to the face all the way round, say 25mm thick and set in a couple of mm. Flush-trim the ply to the strip. Make the honeycomb filling and drop it into place. It must be perfectly flush with your softwood strip. If you need any anchor points for fixings, make sure they get glued into place.
Glue the top layer over the lot and trim it flush.
Lip the edging with the timber of your choice and trim flush.

A note on gluing and clamping:
You are not going to be able to clamp this by conventional means. A vac bag is the answer. You don't need a posh AirPress thingy, you can make one with a roll of polythene (painter's dustsheet) and a tube of mastic. You can cannibalise the valve from one of those cheap clothes storage bags. You don't need lots of pressure, it just needs to be applied all over. Indeed, too much pressure can cause the structure to implode. DAMHIKT.
You may find it a good idea to drill an airhole in every cell of the honeycomb, and in the edge of the assembly, so that air can get out.
I suggest you have a trial go with something smaller than 2.8m long, but it is not that complicated.
HTH
Steve

 

[gaius]

To be honest, I never considered building my own lightweight panel, but now that you mentioned it, perhaps it's a good option, as it would allow me to adapt it. I'll look at other posts about that subject on this forum.

As to the suggestion of inserting a rebated strip along the cut edge of a ready-made panel, I agree that that would be a great option if the edge were straight. But the contour is very curvy. Here's the shape I'm after:

The edges are to be rounded over with an ovolo bit.
Looking at it, do you think that lightweight panel is the way to go?

 

[Mike.S]

Having seen the shape my main query would be how's it be supported? Drawer units under edge and/or legs or ...? Can you source 2.8m long sheet materials (max length in UK is usually 2.4m - except by special order)? If not a join will be necessary - adds to potential strength/support concern, especially as so much of the depth (front to back) is to be removed.

It might be that if you were to include an apron as a support - particularly under the longest section - you could get sufficient rigidity to avoid the need for a torsion box. But without knowing more (e.g. desk to be free standing (on legs/supports) or fixed to wall on one or edges?) I'm struggling to be very helpful.

 

[Steve Maskery]

Regardless of whether the top is a torsion box or solid MDF, how do you propose to edge it?
I think we need some more detailed design concept details.
S

 

[houtslager]

I made a simerler sized boardroom table 3m x 1,3m and that had a leg on each corner, I made it from 12mm MDF with a torsion box constructed with 9mm MDF on 150mm centres in both directions. I built in a camber of 10mm over the length. So, on installation it settled to a shallow curve of 3 - 5 mm over the length, so when I stood on it, [ 90 kg ] it went perfectly flat

I used cascamite [ as it was sold then :/ ] no vacuum bag then, so I used lots of clamps and go bars.

hth,

K

 

[gaius]

forgot to mention the support.
Here's my plan. The dark red structure comprises 5 feet - one on each corner and another in the middle, with 3 square (hollow) steel beams connecting them.

Nothing else under the desk, or attached to it.

The point about sheet goods being hard to source over 2.5m is a good one. I can get regular 2.5mx1.25m birch plywood sheets and moisture resistant MDF in 2440x1830. But it is also possible to order MDF in sheets measuring 3660x1830, plywood in 3050x1500 (I'm not sure about the exact values here). The plywood boards have to be made to order with a minimum order of 2. The MDF I'm not sure about the minimum order requirements.

The ready-made lightweight panels (by Egger) measure 2800 x 2070. and a 50mm thick panel costs about 60% more than 18mm birch plywood in regular panel sizes. Probably a special order plywood size would even eliminate the price gap.
So, all things considered, I thought that if I could find a way to fill the outer 10-30mm of the curved edge, the ready-made lightweight panel would save me some labour and money.
Of course, I could route a 30mm replica of the curved edge in wood with sufficient thickness to fill the honeycomb gap, but that is probably a bit much, no? Would the filler be a decent replacement?

 

[Mike.S]

You seem to have the support issue all figured, so I guess it's returning to your original query - filler or wood?

If it was me I'd use wood: insert wider cut-offs (say 50mm+) so that it protruded beyond the final shape then use a jigsaw to trim it. The edge could then be routed to the desired final shape. However, I'd also think seriously about whether I could avoid the routed edge. If so, the board could then just be edged e.g. with Egger's 2mm ABS edging.

Your idea to fill and route may work - I guess you could first try it on a sample/offcut - but I fear the filler would crack, later if not immediately.

Good luck and do post an update - whatever you decide.

Mike