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.

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