veneer project- please critique my glue-up plans

[Lynn Kasdorf]

I brought this up a while back and got some great advice. I now have the contract to build this thing, and I have the veneer, and I have the vacuum press and bag.

I'm now trying to solidify my glue-up strategy for the veneer. The glue will be urea-formaldehyde resin type.

Here is (sorta) what the piece will look like:
http://barncattech.com/projects/coffeetable/boke_table_1a.jpg

Here is a cross-section of how I plan to lay it all up in the vacuum bag:

http://barncattech.com/projects/coffeetable/coffee_table_cross_section.gif

As indicated, I want to have the veneer on the apron all done and trimmed before I glue-up the top and backer. *maybe* it makes sense to glue the apron veneer in the vacuum bag, but the piece is large, and I figure I can simply use clamps and cauls.

For the substrate- likely MDF, but if I can find some really nice 13 layer plywood, I'll try that.

I realize that I need to slip shims or something between the spacer cauls and the apron in order to keep the vacuum bag from bending/breaking the apron.

A different approach I have thought of is this. Rather than the 3 spacer cauls, I'd make 4 wedge-shaped cauls that will ride up against the inside of the apron. The bag would press down on the middle of the backer and the wedge cauls would ease the transition to the apron.

Opinions?

[Scott Rollins]

Looks well thought out. My experience says that glue will squeeze out on to the face veneer of the apron. I would blue tape all seams which will show. The MDF spacers will work as well as the wedges. Just make it a tight fit to handle the inevitable crush forces from the sides.
One other option is to simply adhere the backer veneer and face veneer first then apply the aprons. I did a similar table last December this way, I applied the veneers to the front and back routed the veneer off the back where the apron would glue to and adhered the veneer to the aprons with cauls and clamps. After doing the glue up I realized I could have made a veneered torsion box just as easily and for less money. I live and learn.

[Lynn Kasdorf]

One other option is to simply adhere the backer veneer and face veneer first then apply the aprons. I did a similar table last December this way, I applied the veneers to the front and back routed the veneer off the back where the apron would glue to and adhered the veneer to the aprons with cauls and clamps. After doing the glue up I realized I could have made a veneered torsion box just as easily and for less money. I live and learn.

I considered veneering the face and backer first, then attaching the aprons. BUT- I want the top veneer to overlap the apron veneer, to minimize catches and for the optimal appearance. So, I need to veneer the aprons first, I believe.

I constructed a sample in which I did the top first, then the aprons. It is subtle, but since the edge of the apron veneer is visible, it is more fragile.

Regarding the wedges vs the multiple flat cauls, my thinking with the wedges is that there would not be so much lateral crush force with wedges. My thinking is like this: imagine no spacer cauls at all. The bag would press down on the backer, and do its best to press down into the corners. The wedges would ease the transition and allow pressure along the inside corners. I'm new to vacuum pressing, but it seems that there wouldn't be much lateral force with the wedges- am I wrong?

Torsion box- I thought of that. If I made a torsion box, could I avoid the backer?

[David DeCristoforo]

I would take a slightly different approach. I would veneer the back first. Then attach the aprons and apply the apron veneers with clamps and cauls. Finally, I would apply the top veneer with the top facing up. I would use the "spacer cauls" simply to keep the aprons from collapsing.

[Matt Zettl]

I agree with Scott on the torsion box. It would be much stronger, even though the ultimate strength in the finished piece is not an issue. It would make the glue up of the sides considerably easier. Once the torsion box is made, glue the veneer to the sides with clamps/cauls. Then use the vacuum bag to do the top and the bottom. The torsion box also gives you a better platform to fix to the base. Additional material cost is negligible.

Matt

[Scott Rollins]

Regarding the wedges vs the multiple flat cauls, my thinking with the wedges is that there would not be so much lateral crush force with wedges. My thinking is like this: imagine no spacer cauls at all. The bag would press down on the backer, and do its best to press down into the corners. The wedges would ease the transition and allow pressure along the inside corners. I'm new to vacuum pressing, but it seems that there wouldn't be much lateral force with the wedges- am I wrong?

The forces from the surrounding air will be uniform in every direction-including up. Think of it like a diver going underwater. The water will press on every surface even the feet. Air is no different. Once you have evacuted the air from the bag all the air from the atmosphere is pressing at about 14 PSI in every direction on the contents of the bag. A little long winded, but you will need to protect those aprons on both sides with a caul.

Torsion box- I thought of that. If I made a torsion box, could I avoid the backer?[/QUOTE]

If you used 1/2" ply for the top and bottom I would think you could get away without using a backer on the unseen bottom.

[Lynn Kasdorf]

Ah- I'm getting it now. So, when you guys say "torsion box", you are suggesting a hollow box with top and bottom panels, and a matrix of ribs on the inside. It wasn't until just now that I realized that you are talking about a panel on the bottom, making it hollow.

This sounds like a great idea. I just need to decide what material to use. If I'm doing a torsion box, I think a good quality 1/2" ply would be fine. I would probably make the aprons out of 2 layers of 1/2" or maybe 1 layer of 3/4" MDF.

For attaching to the base, my plan is to use brass inserts that have machine screw threads. Then, I'll runs machine screws up from the bottom.

Good idea- thanks!

[Thomas Knapp]

I would take a slightly different approach. I would veneer the back first. Then attach the aprons and apply the apron veneers with clamps and cauls. Finally, I would apply the top veneer with the top facing up. I would use the "spacer cauls" simply to keep the aprons from collapsing.
Me too! I think it is easiest and would work well.

[David DeCristoforo]

About a torsion box: If you are going to put it in a vacuum press you need to be sure that the "skins" are able to resist the one atmosphere of pressure you are going to be applying to it. If the "skins" are too thin or the grid members too far apart (or both), you could have real problems when you put it in a vacuum....

[Chris Padilla]

I like the torsion box idea. I attached a quick Sketch-Up for you.

With the torsion box, I actually think you could get away without having to put a back veneer on the bottom! Torsion boxes are extremely ridgid and so the top veneer wouldn't be able to pull the top out of flatness at all. (I would still veneer the bottom to be more complete and to completely "seal" the MDF or plywood.)

In fact, folks use torsion box cauls to clamps veneers all the time.

As David said, just make sure the grids have a good spacing such that the skins of the torsion box have adequate support to handle 10-15 psi. Frankly, I think one sheet of 3/4" MDF would work just fine. I made a workbench out of 3/4" MDF that included a torsion box and it is very solid and VERY heavy.

As to the legs of the table, just plan ahead and glue in a few scraps of plywood so that you have something to bolt to.

You could also veneer up a smaller panel and then slice it into aprons. I'm not sure I'd bother with the cross-grain second veneer on the aprons. It sounds like you are using a good hard drying glue so there should be no need, I think...especially if you use MDF which has no grain and is extremely stable and flat.

[Lynn Kasdorf]

I like the torsion box idea. I attached a quick Sketch-Up for you.

With the torsion box, I actually think you could get away without having to put a back veneer on the bottom! Torsion boxes are extremely ridgid and so the top veneer wouldn't be able to pull the top out of flatness at all. (I would still veneer the bottom to be more complete and to completely "seal" the MDF or plywood.)

In fact, folks use torsion box cauls to clamps veneers all the time.

As David said, just make sure the grids have a good spacing such that the skins of the torsion box have adequate support to handle 10-15 psi. Frankly, I think one sheet of 3/4" MDF would work just fine. I made a workbench out of 3/4" MDF that included a torsion box and it is very solid and VERY heavy.

As to the legs of the table, just plan ahead and glue in a few scraps of plywood so that you have something to bolt to.

You could also veneer up a smaller panel and then slice it into aprons. I'm not sure I'd bother with the cross-grain second veneer on the aprons. It sounds like you are using a good hard drying glue so there should be no need, I think...especially if you use MDF which has no grain and is extremely stable and flat.

I'm down to one of the following:
1. torsion box with top and bottom of 1/2" MDF or 1/2" baltic birch. Ribs and aprons of MDF. Veneer top and bottom.

2. torsion box with top of 3/4" MDF, ribs and aprons 3/4" MDF, no bottom, no backer.

BTW- I can get plenty of "junk" veneer from a friend. He has a bunch of really weird stuff that is man made and dyed blue and pink and such. So that will work for my backer.

Does anybody recall whether Home Despot carries 1/2" MDF? I was there yesterday and I think I saw that.

[Chris Padilla]

My local HD has 3/8", 1/2", 5/8", and 3/4". Frankly, I was shocked at the selection. I used the 5/8" for my Tansu to get the panel thickness my wife liked.

If it were me, I'd build the whole top out of MDF or plywood...and not mix the two. Leave solid wood for your legs. I think 1/2" or 3/4" MDF (my preferred choice) would be fine. 1/2" could possibly be lighter but you'd want a tighter grid over 3/4", I think.

How did you like the veneer book I suggested to you?

[David Keller NC]

"Does anybody recall whether Home Despot carries 1/2" MDF? I was there yesterday and I think I saw that.

If you're making this on commission, make sure that what you get from Home Despot is not made in China. There has been a lot of discussion about poor quality plywood coming in through the ports, and there are now a fair number of lawsuits over contaminated drywall produced there. Generally speaking, the biggest issue that might arise from mdf manufactured overseas is the possibility of formaldehyde out-gassing that's way over US limits.

You -might- be fine with mdf manufactered there, but then again you might not, and for a commission, it's not worth taking the chance, IMO.

[Lynn Kasdorf]

Man, given all the thought time I'm spending on this project, I'll be making about $0.50/hour if I'm lucky...

Ok- here is the question. Well, a couple actually...
1. using urea-formaldehyde glue, which will experience more pull - 1/2" baltic birch or 1/2 MDF?

2. If I build a torsion box, will I need backer veneer on the underside of the top skin?

Here are my steps:
1. veneer backer on underside of top skin in vac bag
2. construct torsion box - 1/4" ply grid on 3" centers, bottom skin 1/2" ply, glue up in vac bag
3. attach veneer to sides with clamps/cauls and trim flush
4. veneer face veneer in vac bag

As long as the backer on the underside of the top skin won't deform the top during steps 2 & 3, this will work, I think.

So, what do you think of this plan, and what materials? Given that baltic birch ply has a modulus of elasticity about 3X that of MDF, I think that I'll be safest using bb ply for the top, so I don't experience warp in steps 2&3.

Chris- thanks for the book suggestion- very good. I also got the "Foundation Course" book by Benton, as I recall. Somewhat different thinking about things, but I really like it. Both are good and I've learned a lot from them.

BTW I have my vac press and bag all working now. This is so cool! Can't wait to build this sucker!

[Earl Kelly]

Lynn,

I second David's thoughts. Also, in your first drawing your platen is upside down. You need to be able to evacuate all the air from under your panel or caul. That's what the grooves are for.

Unless you're trying to get a continuous grain effect it's very easy to "Let" in a 1/8-1/4" solid wood to protect the veneer edges.

The way I would build it is to glue up the top as a separate panel and the sides in one sheet. And cut them on the 45 and glue up. Then come back and rabbet in a 1/8" wood trim flushed off and rounded ever so slightly.

I also like the idea of a torsion box, just be very careful to make it strong enough if your putting it in a vacuum press. A hollow core door can be crushed and I've built forms in a rush, taking shortcuts(not putting enough braces inside) and promptly deformed it with the press.

Edit: The platen in your drawing would work, the way it's drawn, if it's only slightly larger than your workpiece. I use a 4x8' platen with cauls sized for the workpiece.


Earl

[Lynn Kasdorf]

Lynn,

Unless you're trying to get a continuous grain effect it's very easy to "Let" in a 1/8-1/4" solid wood to protect the veneer edges.
l

The veneer is macassar ebony. Can't get solid pieces that are big enough or will match the veneer. Otherwise, I'd love to use solid wood for sides. Client wants "wrap around" grain effect.

Regarding the grooves in the platen, I got my info from Joe, and a couple books I have.

http://www.joewoodworker.com/veneering/platenscauls.htm

[Earl Kelly]

Lynn,

I'm assuming your vacuum nipple is coming in from the top. And that's why you use the evacunet material. My vacuum comes in from the bottom and I don't use any Evacunet, but use cauls from 1/4" melamine which I spray with Topcoat that prevents them from sticking to any squeezeout or bleed thru.

Here's a link - http://earlkelly.com/CurvedVeneerPanel.htm - to a curved panel of many I've made that shows how my setup works. I've been using this same setup since '93. This allows me to glueup any size panel from 4x8' to 1x3' with only changing the cauls to fit the panel.

Earl

[David DeCristoforo]

"I'll be making about $0.50/hour if I'm lucky..."

Woah... You're doing good dude! If you want to be a woodworker, you are just going to have to get used to making big bucks like that on your projects. Why do you think there are so many of us doing it? Just accept it and don't let yourself start feeling guilty about making so much money.

Seriously though, when you do custom projects like this for the first time, you can spend most of your "profit" on the R&D. The hope is that the next time you make a similar piece, you will do better because you will have worked out most of the "bugs".