Thursday, April 26, 2012

Five-Course Mandolin Part 4

I apologize for the delay between posts. I've been enjoying an experience with kidney stones.  Anyhow, I have been fitting the interior tone bars. To do this I've prepared a couple of sticks of high-quality spruce and held them in the appropriate positions while scribing the interior undulation using a small washer to hold the pencil in position against the brace. I trimmed down to the lines with a knife and you can see I've come pretty close on this brace. The fit is important. I want perfect contact along the entire length and width without having to use pressure to close the gap. The bar in violin family instruments is occasionally "sprung", being glued under tension. This can have the effect of kick-starting the instrument's response, hot-rodding it as it were. The effect is temporary in bowed instruments and it's not generally seen as beneficial for plucked ones like the mandolin or archtop guitar.
To refine the fit I've placed some 120 grit garnet paper in position and will sand judiciously. The movement is very small. Broad sweeping strokes will act to average out the inconsistencies in the curve rather than mate the two surfaces.
Traditionally chalk was dusted on the plate surface and the brace pressed into place. I'm using carbon paper instead. It's slightly less messy and makes reading the transferred areas easy. You can see I've carved the f-holes in a previous session I neglected to document.

Here's a high spot. It's only thousandths of an inch proud so I'll use my carving knife held perpendicular to the surface as a scraper. Soon the entire surface is grey, and a final gentle scrape leaves the surface ready for gluing.
To glue these bars down I relied on a heavy strip of cork, 1/4" (6mm) thick on the outside surface of the plate to absorb the clamping pressure from several C-clamps. Traditional violin construction sometimes involves casting a negative mold using plaster of Paris, especially if the bar is to be sprung. These braces fit perfectly and the hot hide glue pulls the pieces together as it dries. 
To finish things off the ends of the braces are tapered off and feathered into the surrounding surface. The bars are trimmed to final height and rounded.  You can see that the bars aren't symmetrically placed. The bass-side bar is slightly shorter. The treble bar is narrower, and is at a more acute angle to the centre-line of the plate.

Saturday, April 7, 2012

Five-Course Mandolin Part 3

Carving the plates requires a certain kind of determination. This is the back, and I have used a router to remove a series of rings of material at varying depths to provide a guideline for carving. It's like a topographic map in maple. I'll use a small plane to take down the pointed areas of the terraces followed by a scraper.
You can see traces of the stepped profile remain as I further refine the curve. Still a lot of lumps and bumps. I've also started to carve in an indication of the "recurve", the area next to the edge of the plate that dips before rising again to resolve at the binding.

More scraping. It's easy to start chasing the lumps around. They disappear in one area only to pop up again in another.

Ready for hollowing!  I'll hold off on carving the recurve to its final depth until after the mandolin is assembled. The recurve of the back has a major effect on the tone of a carved instrument and it's best to sneak up on the optimal flexibility.

I embedded a dowel in the surface of my drill table that is centred on the bit and marked out some depth notations on the inner surface of the plate. By setting the drill stop at various depths I will give myself a visual aid in the hollowing process. To make measuring between bit and dowel easier I made up a little wedge of hardwood with increments of 1/2mm marked on it.  I left a generous amount for waste.

This is a quick-and-dirty carving cradle I whipped up. It's made from 3/4" (19mm) ply that is sawn out to a profile that supports the plate around its edge. I lined it with some scrap foam I had sitting around. This allows me to clamp the work in position. Carving involves some muscle and the movements are repetitive. It's especially hard work in maple, though spruce furnishes its own set of challenges. It's much easier to tear the grain of softwoods. In either case most of the rough carving is done across the grain. 
Like so.  I'm using a small convex-soled plane and various shallow gouges. The thickness profile was taken from a smashed 1950's Gibson I ran into quite a long time ago.  It's thickest right in the center and thins toward the edges, but not in a strictly concentric way. The area near the neck end is quite thick and it extends forward .  When I reach the bottom of the guide holes I start to pay very close attention to what is happening, making frequent checks with my thickness caliper.
I even got out my baby Ibex plane!  Isn't this ridiculous? It works well though, even if it is hard to hold on to. Curved scrapers finished the graduation and then a quick sanding with a random orbit sander.

Nice. It took some doing. The curved profile of the hollow extends all the way out to where the lining meets the back.  The top was carved using the same method.