Woodworking Tips, Tricks and Techniques

 

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Vacuum Systems

    

Vacuum Systems For Woodworking 

Part Four

Vacuum Bags For Woodworking

 One of our most frequent uses for the vacuum systems are the vacuum bags they typically come with. (or are ordered separately). The sizes range from 2' x 2' up to 5' x 12', or even larger by special order. You can even order the raw materials, and make the bags yourself. Our largest size is 5' x 12', and for our purposes, it's fine.The bags are available in a couple of different materials, and in varying weights. Industrial quality vacuum bags are much heavier and puncture resistant than their counterparts, which are designed for occasional use. A great deal of pressure is applied to these materials, and sooner or later they'll fail.

 
 
 
 

After numerous uses the heavier bags are still able to withstand the immense pressure they are subjected to. This is assuming care is taken while using them. The vacuum bags are normally used for gluing up flat panels, and require a platen placed above, and below the work piece, sandwiching the work between them. If any sharp corners are remaining on the platens, the bags will self destruct. We make our bottom platens out of 3/4" melamine with saw kerfs cut every 12" or so in both directions, forming squares, on the top side only. These saw kerfs are about 1/8" deep, and allow the vacuum to have full access to all areas of the platen. We then take a 1/4 round bit and "soften" all edges on the top and bottom. We also round over the corners with a rasp. The work piece is placed in the bag, with another platen on the top of the work. This top platen, which only needs to be 1/8" to 1/4" thick. Again, we use melamine, which prevents the glue from sticking to it. It wants to be about a 1/4" to 1/2" larger overall, than the work, and we tape it in place to prevent it from moving.

Since the veneer is typically larger than the substrate, to allow trimming to size once the glue up is complete, the top platen prevents the veneer from being crushed down at the edges. Again, we use a rigid glue, such as Unibond 800, ( a urea resin glue), applied by foam roller, and place the workpiece and top platen in the bag. We then start the vacuum, and keep an eye out for the veneer staying put. It will tend to slide during this process.

The reason for a foam roller is it seems to apply the right amount glue. Too much glue will actually bleed through the pores of the veneer, and make quite a mess, at best. Too, little glue, will result in hollow spots, which will show as bubbles in your work. This can be fixed by injecting glue into the empty area, while at the same time, using a second needle to suck the air out of the hollow. ( No fun, but it can be done) The ideal result, when removing your work from the bag, is some glue squeeze out, as this shows complete coverage, and adhesion. This is easily removed with a cabinet scraper.

All important to a successful vacuum application, is some obvious basics. And some not so obvious. First thing to do is put the bottom platen in the bag, seal it up, ( this is done quickly with the plastic clamping system, supplied with the bag). Now turn on the system, and let it pull a vacuum. After you've applied the glue is a poor time to find a problem with the system, or a leak in the bag! Next, is make sure all parts are ready. The following photo's show a table ready for glue up.

         Laid up veneer           Dining table frame work             Parts ready for bag

The first shows the back, or glue side, of the "laid up" veneer. The other side is covered with veneer tape at all seams. (Behind the table, you can see the upper platen, ready for use). The second are the parts to be included in this particular project. This table has a honey comb core, for weight cutting properties, and for stability, as this is very strong. The honey comb is actually cardboard, coated with resin. (this is also available as raw, or uncoated, but for our purposes, the coated is best). We also have hardwood cross pieces to fasten the legs to. On top of this is 1/4" M.D.F. (medium density fiberboard). This is a perfect surface to veneer to, as it's very stable, and very smooth, and uniform.

Next step, seen in photo 4, to is mix up the glue and get started. In this case, we liberally applied the glue to the recess in the table top. (photo 2) This will lock the honeycomb in place, as well as the cross pieces. The next step was to glue the back side of the M.D.F. and set in the recess of the table frame.  Then lay the top platen on, and tape it in place with blue painter's tape. Now it's just a matter of sliding the whole thing in the bag and sealing it up. As the vacuum is being pulled, keep an eye out for things sliding around. If it does, just turn off the pump and fix it. And begin again. After this glue up is complete, fill any voids between the M.D.F. and the table frame. After this has dried and been sanded, the veneer can be glued on, in the same fashion.  Do not put the blue tape directly on the veneer, and put it in the bag. The blue tape will crush the veneer! Blue tape as seen in photo 5 is fine to use in veneer layup, in fact it's perfect for that, but once the veneer tape is dry, remove all the blue tape. In the fifth photo, it's in the bag, and the vacuum has been pulled!

         Mixing the unibond 800        Vacuum bags are easy to use           Out of the bag


When the project is removed from the bag it should look like the final picture. The splotches, or dark spots is just about the perfect amount of glue squeeze out. A cabinet scraper makes quick work of removing this glue. The veneer tape has already been removed, by wetting it, and peeling it off with a putty knife. Note: Do not sand the table for a couple of days after this, to allow the moisture to evaporate.

As you can see, vacuum systems are a very handy piece of equipment to have in your shop, and there are units available at very affordable prices. There are basically two types of systems, one which uses an electric pump, and one that uses an air compressor. A little research on your part, based on your needs will help in determining which one is  right for you.

 

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