Replies

1. 
   Jamie_Buxton | Apr 05, 2003 12:39am | #1

   Yes, sort of.   You can indeed make 3/4" ply from thinner sheets, but you do have to be able to provide clamping pressure over the area of the sheets.  If you don't have a set-up for this, making it for one job is probably more trouble than ordering the plywood you want.   Even more serious, lauan is not mahogany.  It is sometimes called phillipine mahogany, but it is unrelated botanically.   If you want the beautiful color and grain of mahogany, lauan will not give it to you.

1. 
   BG | Apr 05, 2003 12:57am | #2

   Jamie,

   That is good info, thanks(especially the correct spelling). I'm building a two draw lateral file for  the CFO and it will be kept in dining room with the mahogany furniture.  I may just make the carcass and draw fronts out of solid wood..it may be the easiest alternative. 

   I have cauls and clamps...also about 1000 pounds of weight lifting plates..would that be sufficient for pressure?  She actually wants to play down the grain in the wood...that is why I am thinking about the glue up

1. 
   Jamie_Buxton | Apr 05, 2003 03:29am | #3

   A half ton of weights?  Yikes.  Depending on how big a sheet you're trying to laminate, that might big good enough.   The issue is to get pressure pretty much everyplace.   For instance, if you're laminating two 4x8 sheets and you pile all your 1000 pounds in the middle, you'd easily be able to stick your finger between the sheets near the corners.   With your situation -- 1/2" and 1/4" ply --  I'd put the 1/2" on top and spread the weights as uniformly as I could.   If I had a sheet of 3/4 hanging around, I'd use it as a caul between the weights and the 1/2".

   If you make the carcass entirely out of solid wood, remember to orient the grains the same at the corners.   Often that means the sides' grain goes up-down, and the top's and bottom's go across the piece, not front-back. 

1. 
   BG | Apr 05, 2003 03:55am | #4

   Jamie,

   Can I use yellow glue if I do the laminate? Would it be better to pre cut the 4x8 into smaller equal squares and stack with the 3/4 on top and the weights on top of that?

   Edited 4/4/2003 9:01:05 PM ET by BG

1. 
   Jamie_Buxton | Apr 05, 2003 07:00am | #5

   Yes, yellow glue works fine for laminating flat work like this.   I step up to urea formaldehyde only for bent lamination, or really compicated pieces.

   Unless I really needed a full 4x8 sheet, I wouldn't try to laminate the whole thing.   Instead I'd glue up panels a little larger than the finished size, and trim afterwards.  That way, you get to use all your weights on the one panel, not spread them all over the 4x8 sheet.

2. 
   Armin110 | Apr 05, 2003 02:27pm | #6

   What you are doing is essentelly veneering. A very important rule to remember is you must balance out the stress on both sides of the substrate panel. Also keep in mind yellow glue is different than the glue used in the mfg. of the plywood. This, I have found, in some cases to cause seasonal expansion problems. I would recommend you forget about using the 1/2 core and use 3 layers of luan instead of glueing one layer onto a totally different type of plywood. Glueing different thickness & different species to only one face spells trouble. In most cases the finished product will bow, sometimes to the point of rendering the new panel useless. http://www.northernsunwoodworks.com

3. 
   BG | Apr 05, 2003 04:01pm | #7

   AKA,

   I hate cutting 1/4" ply on the saw, all my near disasters have happened with 1/4" ply.  Yes, the 1/4" ply is usually warped to begin with and may render a totally usless piece when glued.  Sigh...I better find a source of 3/4 mahogany ply or go with solid wood.  thanks 

2. 
   HowardAcheson | Apr 05, 2003 07:41pm | #9

   Jamie, according to the Forest Products Handbook of the US Forest Service and "Understanding Wood" by Hoadley, secure gluing of hardwoods requires 200 psi of pressure. For a 48x96 sheet of plywood, 460 tons of pressure would be required. Even given that maximum strength is not required for this situation, even 25 psi requires 58 tons evenly spread over the whole surface. A perfect vacuum press will give 14 psi which would be the absolute minimum to get a flat, secure bond. That is the reason plywood manufacturers have such large hydraulic presses.

   My recommendation for in he wants to create his own panels is to use a solvent based contact cement. It will be plenty strong for the application and he doesn't have to find an M1A2 tank to sit on top of his glue up.

   Personally, if I was making something for my CFO, I would order real mahogany in the thickness I neeeded.

1. 
   Jamie_Buxton | Apr 05, 2003 08:14pm | #10

   Howie --

      Consider a wiped glue joint.   The old-timers used to use this technique for glue blocks and other small thing.  You take two pieces of wood, put a little glue between them, and wipe them against each other for a couple seconds.  Most of the glue gets squished out of the edge, leaving a thin layer of glue between the blocks.  Set the blocks aside to cure -- without clamps --- and you'll find you've got a solid glue bond between them.   There's no clamping pressure at all.

      In gluing wood, clamping pressure does three things.  First, if the joinery isn't perfect, the pressure may bend the wood to make a thinner glue line.   Second, it moves the applied glue layer around to make a thin uniform thickness --  mostly by squishing it out the sides of the joint.   Third, for thin wood, the pressure may prevent the wood from temporarilly changing shape due to water absorbtion from the glue.  If you have perfectly flat and unchanging glue faces, and apply a uniform thin glue layer, you wouldn't need gluing pressure, let alone 200 psi.   Of course, in the real shop, perfectly flat glue faces may not exist, so clamps are a part of almost all gluing operations.   However, you're greatly overstating the necessary pressure.   For instance, as you say, the 14 psi of a vacuum press does a fine job of veneering and laminating; in fact, I generally run my press at 10 psi or so, and the bond is stronger than the wood.

      Jamie

2. 
   HowardAcheson | Apr 06, 2003 12:41am | #12

   Jamie,

   The oldtimers were using a hide glue which actually contracts as it sets which primarily accounts for not needing clamps. But, you are correct, you can do the same with some of today's adhesives. It will work with PVA's and urea formadehyde if the parts are worked against each other to fully wet the two surfaces and to leave only a microscopic thickness of adhesive. The thicker the glueline, the less strong the joint will be.

   However, for the size of the sheet of plywood, it is impossible to both get a complete wetting of the surfaces and a microscopic glue line unless you used the large hydraulic presses used to manufacture plywood. Forturnately, ultimate strength is not needed for most lamination or veneering applications. Vacuum presses or other minimal pressure clamping is fine for these minimal strength applications. Using these types of clamping pressures for a laminated, weight bearing beam would be foolish.

   IMO, using solvent based contact cement will give a perfectly adequate strength for laminating two thicknesses of plywood. It will also get the job done much faster than trying to use other adhesives.

3. 
   Jamie_Buxton | Apr 06, 2003 06:09pm | #13

   Howie --

      So you're arguing against conventional glue because small-shop gluing technique is inadequate to build structural beams with it, and then you're advocating contact cement -- presumably because that is used for structural bonds?

4. 
   Armin110 | Apr 07, 2003 12:11am | #14

   Unless you are doing formica counter tops forget the contact cement. I can't think of a worse glue to use for woodworking. http://www.northernsunwoodworks.com

5. 
   HowardAcheson | Apr 07, 2003 01:28am | #15

   Jamie, I suspect you mis-read my postings or I didn't make myself clear. My point is simply that solvent based contact cement is more than adequate for laminating multiple thicknesses of plywood used in non-stress or structural applications. It is faster and doesn't present clamping challenges. I suspect that it is as strong as a lightly clamped PVA adhesive joint.

   In the shop I was involved with, we made over a hundred multiple layer plywood and composite panels a year. They were used for countertops in kitchens and for bank teller counters. The teller counters were faced with 1/4" walnut ply in many cases where we didn't use pre-veneered materials. All these were laminated with solvent based contact cement. Other shops that did similar work constructed their panels the same way. Even today with vacuum bagging, it would still be faster to use contact cement.

   All I am trying to do is provide an alternative that IMO is a valid approach. There is more than one way to skin the cat.

6. 
   MIKEBUETTNER | Apr 07, 2003 07:38pm | #16

   I still have a work bench I made almost 20 years ago that is 2 layers of 3/4 ply and 1 layer of tempered masonite. All laminated with contact cement. It has held up just find and is as solid as the day I built it.

   bit

7. 
   BG | Apr 06, 2003 12:10am | #11

   Personally, if I was making something for my CFO, I would order real mahogany in the thickness I neeeded

   Now you know its never that simple.  She wanted to buy either the $79 or $139 lateral file from Staples....you can imagine the quality. The issue is she dosen't want me spending $400 for the supplies either. 

2. 
   nigelUsa | Apr 05, 2003 04:15pm | #8

   Check with your local "formica" supplier. 4410 is a real veneer in the wood you want.