BYO: (Build Your Own): Forming a pliage has had me buffaloed. I avoided it last time by doming the top with a sanding bowl, but that doesn't get the dome in the right place, under the bridge. This time, I was determined to make a true pliage. Pliage means fold and is clearly shown in the Francois Charle plans as a 6 degree bend (fold) in the top, laterally immediately below the bridge.
I knew of two methods suggested for the pliage. One is to bend it over a hot edge or pipe. The other, as shown in Collins' book, is to use a forming jig consisting of a shaped box and heat lamp. I didn't want to go to the trouble of yet another jig, so I tried the hot pipe method. Didn't have much luck. Despite preheating, good hot pipe and lots of moisture, all I really accomplished was a 3 degree bend and a fair amount of warping of the lower part. Surely this is the fault of my technique, but I'm a slow learner and I didn't want to learn at the expense of a nice top. It is amazing how resilient a 2mm piece of spruce can be.
So I changed gears. I dampened and heated the pliage area well by slapping it on the top of my wood stove (sizzle, sizzle, an electric steam iron would probably work too). Quickly transferred the top to the workbench, exterior face down and clamped a stiff piece of wood at the pliage line. Before tightening the clamps, I slipped another piece of wood under the lower portion. Tightened the clamps. I was going for a sharp pliage line, so I clamped all the way to the bench. If you want a more gradual line, round the edge of the pliage board and maybe don't clamp all the way to the bench. I ran a heat gun (hair dryer might work) over the pliage ara for about 5 minutes to cook it and let it cool to room temp. Voila
There is a fair amount of bounce back when released. I used roughly 15 degrees in the clamp up to get 6 degrees upon release. Surely this will vary with the rigidity and thickness of your particular top and the subtleties of your technique. So saying exactly what degree to set the clamp up is not possible. The good news is adjustments are easy. If you don't get enough angle, just crank the jig up and try again. I over-bent the first piece and it was fairly simple to heat it up with the heat gun and straighten it a little.
I like this system for several reasons. 1) very simple, no permanent jigs to construct or store. 2) the steady controlled pressure of tightening clamps is a lot less scary then bending a $60 top by hand over a pipe. It is probably my bad technique, but the amount of pressure I was applying was substantial and I was on the verge of having to throw my weight into it, something that I could neither control well or maintain long enough for the wood to set. 3) It worked well; really well. Here is a sketch of the clamp set up.