Tuesday, August 23, 2011

The Morin Khuur Part 2




Here's the assembled rim structure.  For reference sake, it's about 12" (300mm) long, and almost as wide. I fashioned the end blocks from Douglas fir and you can see I've drilled them out to reduce weight. It may be difficult to see, but they are also curved slightly to reduce the possibility of standing waves in the finished box. (Caused when sound bounces repeatedly between two parallel surfaces.)
I rarely use my adjustable spherical workboard for guitars, but this seemed a good opportunity to put it to use. It's made from plywood and 1/4" hardboard tensioned with a machine screw. By cranking down the screw the radius is increased.  I went ahead and pushed it to the limit - it's capable of a 10' radius, which would be extreme on a guitar.  I'm thinking of this Morin Khuur as more of a flat-topped viola.
After scribing the contour of the form on the ribs, I went planed it down to an approximate fit.

The fit is perfected by sanding. I marked the surface to be refined with some graphite and spent about ten minutes a side working it down until the joining surfaces were smooth and clean.

Later in the day I glued the bass-bar into position using Go-bars and the spherical form as a caul. The violin makers are laughing and pointing.  No. The photo isn't reversed. The stringing on this instrument is actually opposite to that of a violin with the top string on the left hand side when looking at it from the front. Hence the bass bar is on the "wrong" side.  Seriously. I'm not joking. It's not a mistake!
Using spool clamps seemed the way to go.  Given that this is a flattop, I decided to splay the bar out to cross more grain and resist the downward tension. There will be a soundpost installed as well. The top thickness is 3mm (.0120") 
I decided to attach the neck using machine bolts and inserts and I've predrilled for those holes.




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