Gluing down 2nd sheet ?

[dubliner]

I have a 10x5 table with one sheet of mdf bolted to steel table, now comes gluing the second /plenum sheet. As you know a 10x5 sheet of mdf is very heavy & I'm working alone so that & cure time are critical. Do I need to have a perfect film of glue on both sheets or could I prop the top sheet up on strips ( as used in positioning laminate ), put glue is as best I can & caul/ clamp the sheets also would you recommend countersunk screws from underside?

[henrik_o]

Are you sure you can't get someone to help you just with the insertion/positioning part?

We were worried about slight uneveness on the bottom board, so we used liberal amounts of extra-filler dowel glue, but only on the bottom board. This is mdf face-to-face, there should be no need for glue on both faces.

We put on a couple of hundred strips of laminate flooring that we had laying around to act as a clamping force evenly distributed over the table.

As long as the bottom board is reasonably even, and you can apply relatively even pressure overnight, I think you're good to go. I would not screw through the bottom; the clamping pressure will be very small per screw and likely cause glue to shift away from those areas making for an uneven glue joint.

All in all, and again this is not a shopbot guru talking, try to get it as even as you can, but no need to overdo it. It's the bleeder that needs to be exactly level, and unless you plan on using a bleeder without levelling it under vacuum pressure, small deviations in the supporting substrates will be taken care of when levelling the bleeder.

The best part still awaits you, machining the plenum grid and then the painstaking process of sealing the grid. Man, that does take some time. I started out with polyurethane, did two zones out of four, realized I could be putting on layer after layer after layer without being completely sealed, then switched over to PVAC glue for the two remaining zones. Sure enough, after two easy rubbings those two zones were all done with a great plastic film, while I put on the final coat of poly on the two first zones this friday, more than two weeks after the PVAC zones were done. This is probably due to me using a less than perfect poly, admittedly, but that's how it played out.

[Brady Watson]

Nevile,
SLATHER wood glue ALL over the bottom sheet. Place 3/4" part of 5X10 top sheet on table supports and lean top of sheet against the wall to hold it in place until you get the bottom sheet all glued. Then let gravity help you - just ease that puppy down onto the glue, positioning the front edge (where 0,0 would be) making the long X-edge the most important part & then throw some weight in the middle of the table. If you have a choice, put the crown of the sheet DOWN towards the glue. Then start clamping all around the edges and throwing weight on the table.

As long as you don't have a hammer laying in between the sheets...it's pretty hard to mess up. Slap on some glue (work quickly & use spreader sticks) and flap the top sheet down. The glue will do all the work & a flattening routine will make it perfectly flat.

-B

[srwtlc]

You should have seen mine after the top sheet was down. I had six junk 50lb 7.5 hp motor stators, two large tractor tire chains, a couple of small chains, two 75lb lawn tractor wheel weights, a couple 5 gallon pails of conversion varnish and a few other miscellaneous items along with clamps around the edge.

It looked more like a trailer full of junk than a ShopBot table!

[knight_toolworks]

one way to do it by yourself after you apply the glue put down strips of wood or anything that will go clear across the bed. every 4 feet or so put the top layer on that then pull the strips out.

[beacon14]

You guys are making it harder on yourself than it needs to be. Before you glue anything just lay the eventual top sheet on the table and rout a shallow grid maybe 1/8" deep using a 3/16 or 1/4 straight bit. Grooves every 2"-3" should be plenty. The grooves should be no closer than 2" or so to the edges of the sheet. Rout a hole in the base layer sized for whatever shopvac hose you have handy. Make sure the hole will end up directly under at least one of the grooves.

Now do as Brady says and stand the sheet up, cover the table with white woodworking glue which has plenty of open time, lay the MDF down on the glue (grooves down), and turn on the shop vac. Cover the whole thing with a piece of melamine or plastic sheeting.

The improvised vacuum system will hold the sheets together much more tightly and uniformly than any amount of weight or screws you could use, and the grid will forever be encapsulated in your plenum board.

[billp]

I thought I had posted something about the glue to use awhile back, but I can't seem to find it through the search engines.
Tite Bond makes a version of their woodworking aliphatic resin (wood glue) called "Extend". It claims an "open" time of 45 minutes, and has the same holding power as their standard Tite Bond #1,2,& 3. Not all stores carry it, you may need to get it online.
If you REALLY want to hold that second sheet in place, do as David suggests above, but take some two part epoxy (MAS, WEST, System 3, etc.) and put it in the fridge for a few hours. That will slow down the curing time appreciably ( being thermo reactive,epoxy doubles it's curing time for every 7 degrees you add/subtract, so you can tailor it to any season). Even the "slow"hardeners are sometimes too fast in warmer months...Then you can roll a coat of epoxy on both surfaces and pile the rest of your shop on top of the table....., AND use the vacuum...
Either way this takes the "rush factor" out of getting the second board installed...

[dewey_dog]

David,Thanks for the tip I just came in from the shop and used the method you described and it worked well. The only problem I encountered was leakage through the sides of the MDF. My son made a keen sugestion to put a layer of electrical tape around the perimiter and this worked to seal in enough vac to get a good draw.

[bill_lumley]

As I anxiously await the arrival of my PRS this is one step I worry about because mine will be setup in my garage in December where temperatures can dip below 32 degrees fahrenheit at night . Any tips on how I can adjust the process of getting the plenum board down in that climate . I could maybe cut the plenum on a friends bot 'inside' and seal with PVA in a controlled environment but this means the top of my plenum may not be flat as the surfacing routines will need to wait till the bleeder is in place .

Bill - could I use the Epoxy method to fix the plenum in place and then after surfacing and cutting my pattern in the plenum use the epoxy to method to seal the plenum board ?

Thanks Bill

[billp]

Bill,
Yes you can, but you might want to brush the epoxy on in thin coats. If not you could wind up with "lumps"that will be a bear to sand down. Rolling it on the "flats"will work fine, but in your channels/grooves, etc. you'll need to brush it.
I don't know what your current temperatures are in Ontario, but re-read the post above where I point out the need to work with the thermometer in your favor. If it's getting too cool your epoxy could take a very long time to cure, so you could put your containers in tubs of warm water before mixing your batch to speed things up. If your shop can't be heated well for a few hours in December then the warm epoxy may do the trick... It should also make it flow better for brushing...

[knight_toolworks]

it would not be hard to keep the bot table warm cover it with a couple layers of plastic and put a few light bulbs under it. or a small heater. since there is so much free room a fire would not be an issue. this should keep the temp up enough for the glue to cure.
I used to do this when I had to glue things and it got cold overnight.