This project calls for a large (55" wide x 12" tall x 7" deep) box made from Birds' Eye Maple to serve as a Mantel piece over a fireplace. Due to construction and installation concerns, we decided that the best way to go about the piece would be to veneer the entire thing. Using MDF as a substrate under the veneer means that the piece will be absolutely stable - in other words, no wood movement to worry about. In this case, the piece will be constructed from one sheet of 3/4" MDF, and veneered on both the inside and outside of the box.
The first step is to rip the 8'x4' sheet into manageable strips. As with any project, I like to cut each part oversize, then come back later and trim them down to final length and width before assembly. In this case, I cut each part (the front, top and bottom, and two sides) about an inch longer in each direction.
With the MDF cut, I cut each sheet of veneer about 1/4" longer and wider than the substrate. This allows some play if the veneer happens to shift during glue-up.
I prefer to cut veneer with a knife and scissors, rather than a veneer saw. I find it much faster, and because I don't need to join the sheets along a seam, the edge quality is not of utmost importance. Besides, everything is going to get trimmed down to correct size later, anyway.
The next step is to make the platens, which ensure even clamping pressure once the veneer/MDF "sandwich" goes into the vacuum bag. In this picture I am rounding over the edge of one of the patens. This will keep the vacuum bag from tearing once the pump is turned on.
Filing down the corners of one of the platens, again to keep the bag from tearing.
Glue-up, using Unibond-800 (a kind of urea-formaldehyde glue) and a cheap foam roller. At Darryl Keil's advice, I decided to veneer to one side of each piece of the box at a time. Once the poplar "backing" veneer is applied and pressed, the sides will be cut to correct size, and the box will be constructed and glued with the poplar veneer facing the inside of the box. The Birds'-Eye Maple veneer will then be applied to the outside of the box, one side at a time.
The vacuum bag, which works by sucking all the air out of the vinyl envelope-style bag, exerts an enormous amount of pressure on the parts inside. A common misconception is that the bag is "pulling" down on the the parts. This is not so. Rather, the lack of air within the bag (the vacuum) allows the atmosphere itself to "press down" on the bag. That may not seem like much, but there is actually over 2,000 pounds of pressure per square foot being applied to the bag. If you look closely you can see the bag stretching over the platens on top of each stack.