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conference table substrate7/5
we have a decent size conference table to build and i want to double-check my plan before we run into trouble. i haven't built one in a long time and am trying something that is new to me.
we finish at 2" thick so i'm thinking 3/4" particleboard on top with 1/2" and 3/4" baltic birch for the substrate. pb on top for flatness and not telegraphing anything through the veneer and bb to get us to thickness and provide rigidity and strength. sound good to those who have done something similar? we're also planning on grooving the substrate to accept a 2" square hardwood strip along the perimeter before we veneer on top of this (this t-molding for a visual).
any thoughts would be greatly appreciated.
I would not use baltic birch underneath particleboard. The additional cost will benefit you nearly nothing and potentially cause you trouble. Veneer core plywood will move with moisture/temp differently than the particleboard which may cause you headaches later one.
If I were building a conference table 2" thick I would only use particleboard, and I would definitely not build the whole thing 2" thick. Only the outer 4" would be built up 2" thick and perhaps sufficient crossbands to support the legs, and perhaps a few more for rigidity's sake.
thanks for your reply.
the top is 180" x 48" with three 6" x 24" x 28" legs. the leg placement is called for to start 34" off either end. this is a lot of unsupported material hanging off into space. i was planning on using some 1/2" thick steel plate to support this and the other 4+' of gap between the center and outer legs. i was thinking bb and solid 2" to help it resist sagging over time. solid banding should help as well.
would i be better to make the whole substrate out of bb? i would have liked to sub this whole part out but the deadline got moved up and we are going to have to build it ourselves.
i'm not opposed to building it as you have suggested but i cannot see how .75" or 1" thick pb will not sag with those spans.
Have you ever heard of a torsion box? The perfect way to build that table top. A fraction of the weight and incredibly strong. With all that particleboard and 1/2" steel plate, will you have a crane available for install? I hope you don't have to turn any corners or have narrow hallways to go down with a table that long!
Torsion box is where it’s at. Cut your weight by 2/3 or at least half. Veneer the plywood with a cross band if needed.
We build our torsion boxes with balanced components so they stay flat.
We make large tables and large flush doors with torsion boxes. 1/4” MDF, face veneers, and an ‘egg crate’ framework to make space. Half-lapped hardwood, 1/4” to 3/4” x whatever is needed to get us to thickness.
You do not want a 4” thick table. 2” max, with added thickness at the edge. People like to cross legs at a table, and a 4” top will prevent that.
Paul Downs, the King of Conference tables may have something to add. Some makers use all foam and veneer, little or no hardwood or solid anything. Think airplanes. Imagine your 48 x 180 weighing less than 100 lbs. People would complain.
MDF is a better choice for flatness and span IMO of course balance the top.
Maybe the torsion box would be lighter but I wonder about flatness
You might send a message to Paul Downs, he will make it for you as well.
The guys are right about a torsion box, in my mind the 'crossbands' would be serving in similar (but lesser) fashion to a torsion box.
We have several assembly tables that are torsion boxes. You could build this using 1/2" MDF but the hard part is finding a 180" long surface that is flat enough to leave it sitting until the glue cures. At 180" long and only 2" thick, even a torsion box will start to warp if you don't have it sitting flat.
thanks for the replies.
i thought about torsion box but my previous experience (first work table i ever build) didn't turn out as flat as i would have liked. i made it 4" thick with 1/2" mdf skins. i used a glued mdf grid for the interior.
180" will be divided in two, so i only need a 90" x 48" flat surface to glue up each half.
what about using 1" thick rigid foam for the core with 1/2" mdf skins on either side? i'll lay it up in my veneer press.
what glue to use to glue the foam to the mdf? also, do you think i'll need any metal to help stiffen it up? i'm worried about this sandwich combination keeping these table flat with the amount of overhangs. but, the metal wouldn't work as well if mortised into the above sandwich.
Steel plate doesn't stay flat either. If you think making a wood torsion box is hard to keep flat, try it with a slab of foam! When I did a 4'x12' conference table, I made a tall grid system of particle board across the shop floor. It was 16" rips and bridged over any irregularities with ease. We shimmed under gaps, laid up the torsion box, then wedged in 1x4s up to the concrete ceiling above us for clamping. Kind of a "where there's a will there's a way" job!
rich - i was thinking about doing it similarly except we have a vacuum bag, so no stick clamps against the ceiling.
1/2" plate doesn't tend to sag and isn't likely to bend with the spans we're talking about.
what kind of glue did you use to glue the mdf to foam?
I worked with a lot of rigid urethane foam when I was a model maker, we used hot glue or yellow glue with that. Never used it in a torsion box. If you are considering a really lightweight table top, I'd look at cardboard honeycomb.
If you use metal to make up for anticipated shortcomings of wood, then use it on edge. A 1/8" x 3" on edge will be very stiff. Flat, it will sag almost as fast and bad as MDF.
I would consider the use of metal a fault, or response to, of bad design. Wood is a wondrous material, and I suspect you are not utilizing a perfectly good technique because of your one bad experience.
Start flat to stay flat is a given. Almost no clamping is needed beyond a dialed down vacuum press. No glue on the egg crate members at their half laps. Glue on the egg crate edges only.
please elaborate on your glue schedule. i don't understand your half-laps comment and gluing only at the edges doesn't jive with my thinking of having glue across all surfaces.
i picked up some 1" thick, carbon core scrim honeycomb from a marine supplier yesterday. he demoed a 1/2" thick sample glued to fiberglass for me by standing on it. it barely flexed across 2'.
my plan is to use west systems epoxy (what the marine supplier recommended) to glue the honeycomb to the mdf. i was planning on spreading it everywhere on both surfaces.