Sandwich construction assembly table top???

[ian maybury]

Hi guys. I need to make an assembly table, and have seen the various pieces on using ply or MDF to make honeycomb assembly table tops.

Which is fine, except there's a fair amount of labour involved, and some risk that it won't be accurately flat if the webs are not cut exactly parallel.

I'm wondering if anybody may have instead tried sandwich construction - that is the laminating of ply or MDF top and bottom skins over a core of a fairly high compression strength structural foam?

I've some experience of doing very lightweight glass and kevlar skinned sandwich mouldings over a purpose made structural foam core which proved incredibly strong, stiff and light.

An assembly or router table would need thick enough MDF/ply skins for ding resistance, and foam of sufficiently high compression strength not to crush under any foreseeable load. Laminate facing would be useful in the case of the router table too.

I haven't looked at the numbers yet, so I don't know if rigid polyurethane insulation boards would have high enough compression strength to do the job - what's needed is determined in quite a large part by how stiff the ply/MDF skin is as a stiffer skin will spread a point load over a greater area of foam core.

There are fancy core materials like aluminium honeycomb and Nomex as used in the high performance composites business, but that's probably getting more expensive than is necessary.

I don't know either how flat or regular in thickness this foam board is, and it'd take a bit of figuring to decide a good way to press up the lamination. (a flat bench top would be very helpful) Epoxy is probably a good adhesive option, although PVA might do the business too.

I'm guessing it should stay flat/be stable, but it would end up quite a thick compared to say a solid MDF router table.

Thoughts/experience anybody?

ian

[John Coloccia]

How will you keep it dead flat during the glueup?

[glenn bradley]

some risk that it won't be accurately flat if the webs are not cut exactly parallel.

That's what tablesaws do. I see most torsion box designs use MDF. The reason here is that the flattest plywood is a BMX track compared to MDF. If a sheet of plywood is flat enough to meet your needs, I would not bother with a torsion box.

If you find a reasonably flat surface to use as a reference during your glue up I think you will get what you are after. Folks who go the torsion box route are after a dead-flat surface and are willing to go through some effort now to have the benefit for years to come.

Just as I do not use walnut for my shop cabinets, I would not build a torsion box if I did not really need it. JMHO.

[ian maybury]

I guess I'd be shooting for accurately flat all right, and take the point on the flatness of MDF vs. ply Glenn - it's just that I'm not that experienced, and have not yet run done a job that showed up flatness problems with birch ply.

To yours on flat surfaces John I guess one option is carefully straightened, shimmed and leveled battens on a decent floor or bench as is often suggested when making a torsion box table. That method possibly has room for errors though - maybe a good reference surface would make it easier?

One option might be a large chunk of polished stone, it might be possible to get access to a large piece for overnight use. (I'm think of a local cutting and polishing operation)

ian

[Erik Christensen]

I use a veneering vacuum bag, rip my web parts on a table saw, clamp them together then dado the slots where they cross with my sliding table. Use 2x solid wood on the edge bands (webs can be whatever ply/mdf you have handy) and just glue it up with enough 23 guage pins to keep things from moving around while you slide it into the bag.

30 minutes of vacuum & you are done. I make the box about an inch oversize & trim to finish dimension so all is nice & square.

[ian maybury]

Thanks Erik. It starts to look like the torsion box construction may be more straightforward - in that there's uncertainties to resolve with the sandwich approach. Which could lead to wasted work and materials if it goes wrong.

I've a soft spot for sandwich construction though, and was hoping that somebody might pipe up to say 'I've done that, but you need to watch out for this, this and this..... etc'


ian

[Ben Hatcher]

Being so close to Thanksgiving, I was kind of hoping for a different kind of sandwich construction thread.

I've used portions of a hollow core door for webbing on a torsion box. I used a thin layer of yellow glue to attach 1/2" MDF to each side and wrapped the edges with hardwood. The foam is an interesting idea. I don't see any reason why that wouldn't work. Finding an adhesive that is thin enough not to lump, bonds to both and doesn't melt the foam could be a challenge.

[ian maybury]

:-) Could turn out to be a bit of turkey sandwich/sandwich turkey if it went wrong Ben....

[glenn bradley]

I've a soft spot for sandwich construction though, and was hoping that somebody might pipe up to say 'I've done that, but you need to watch out for this, this and this..... etc'

My workbench top is sandwiched 3/4" MDF. My tablesaw/RT surface was large enough to provide a reference surface (27" x 105" for my 30" x 94" glue-up). I cut one piece to the exact size I was after and the next layers about 1/4" larger in width and length.

I positioned the second layer so that all edges had an overhang and put in a few screws to assure alignment. I then removed the second layer, applied the glue like roll-painting a wall, replaced the layer and drove in the screws to act as clamps. I also piled gallon paint cans and free-weights spread out over the surface to apply pressure.

After setting overnight, I pulled the screws and used a flush trim router bit to bring the over-sized second layer into identical dimension of the first layer. I then repeated this process to get a 3" thick top for my workbench. The weight would be out of line for an assembly table but, the bench sure stays put ;-)

[Chris Fournier]

I have a 4' X 8' assembly table that I made for my shop. It has a metal frame base which I welded up containing bins, drawers and a cubby. The table is on casters and has levelers on the four corners should I want to lock it in place.

The top is a 4" thick torsion box that uses 13/16" particle board to make up the 6" grid core with 1/4" MDF "X"s inside each grid cell. The skins are 5/8" MDF, with laminate applied to all faces. Two guys grunted and groaned to flip it about.

I made the metal base first and took great care to make sure that it was as flat as possible and "locked down" before I began to build the torsion box on top of it.

The outcome is a very flat assembly table that can be moved about and take a real beating. Woodworking, metal fabrication/welding and motorcycle wrenching all take place on this bench. An 1/8" piece of masonite protects the top from the potentially nasty stuff keeping the bench fresh for woodworking.

No need to notch the gridwork for the core, just glue and nail your way across the width of the table - fast and plenty strong.

[Philip Rodriquez]

Okay, so I use a 3'x4'x6", grade A, granite inspection plate... It weighs over 1100 lbs and it is flat to something like 0.0001”. If you have access to a machine shop, they may have one you can use for the glue up.

If not, take a look at http://thewoodwhisperer.com/episode-18-assembly-table-torsion-box/ (http://thewoodwhisperer.com/episode-18-assembly-table-torsion-box/)

It will give you a good starting point for your adventures.

[ian maybury]

I don't have access to a big machinist's surface table Philip, but can probably manage something close at a nearby stone cutting operation.

Picking up on your mentions of weight Chris and Glenn. MDF sure is heavy stuff. Weight is a significant factor in what I want to do in that space is tight, and I need something I can haul out when needed, and put away again when finished without risking a hernia. So I was thinking that the lightweight construction that would results from a sandwich construction top would be useful.

One angle in making it work is to find a rigid foam that as well as being flat and cheap has enough compression strength. Foam offers the advantage of supporting the surface skins all over (while they have to bridge between webs in a torsion box) - so if the foam has enough crush resistance then the skins might not need to be very thick.

I'm looking at two options - either drop the resulting flat top over the router table I have (which would be higher than is ideal), or go for a trestle/support system made of rectangular section MDF boxes or similar that give variable heights depending on which sides they are placed on.

Maybe I should just buy a honeycomb core door and slap a sheet of MDF on both sides....

It'll be a while until I get that far as I have to finish the shop re-layout (wiring and ducting) first...

ian