Rough carving the inside of the back plates

Shaping the outside faces of the tops and backs involved terrain mapping the wood blanks to a given shape, through the contour templates I made. Now that the outside faces are carved and smoothed, I can use these faces as a reference for carving the opposite (inside) sides.

The backs are supposed to be carved to a consistent 3/16." The tops, however, vary in thickness from 1/8 near the rims to about 3/16" in the center, under the bridge. To complicate this, the tops need a fair curve along the longitudinal axis for the parallel bracing.

To carve the backs to a consistent 3/16", I started by hogging out a lot of waste with this drill press jig. Basically, I made a soft (cork) topped 1" pin directly under a 3/4" Forstner bit which I reground to have no center point, and on which I extended the cutting blades into the middle. I set the distance between the cork and the bit at a hair over 1/4". and made as many holes as I could without overlap (which might have made the work slip). 

The result was a large number of reasonably flat bottomed holes, the bottoms of which were reasonably parallel to the opposite side of the back plate.

Then I made another pass at the drill press, this time with a larger bit, removing as much of the "honeycomb" between the 3/4" holes as I felt comfortable with. This would save me a lot of gouging in the next step.

Next job was to connect the bottoms of the holes with a sharp shallow gouge and that neat little double convex plane I made.  Took about an hour per piece, but was satisfying work.

After the backs were gouged, planed, and smoother a little more with rough (60 grit) paper and a hand block, I made "scraping maps" with my depth tool, to show me where hand scraping was needed. In the photo above, the center cross-hatched areas need about 1/16 off, while the other outline needs from 0 to 1/32. "  I went through this process of scraping and measuring about 4 times with each piece, until I was satisfied that it was "good enough" - by which I mean a fair, smooth-feeling curve on both sides, pretty close to the dictated thickness. 

Mapping thickness for scraping, using the depth caliper. The tool is on a high spot, which needs a lot of wood removed to get to 3/16.