Wednesday, January 13, 2016

Irish Bouzouki - Fun With Moulds

Making jigs and moulds is a messy and time-consuming affair. There are definitely more enjoyable ways to spend one's time, so I figure it's worthwhile investing a little extra effort and expense to get it right the first time.

Half-body and full-body templates
Half-body and full-body templates, with headstock template

While I wait for suitable weather and a break in my IT work long enough to afford me the opportunity to spray finish coats on the four guitars I've been constructing (seemingly forever!), I'm taking the first steps in building an Irish bouzouki - a first for me. After deliberating for some time where design and dimensions are concerned, fabrication of an outside body mould and templates for the bridge and headstock have been my focus in spare moments over the past couple of weeks.

Where construction of the body mould was concerned, my first task was to decide on the overall dimensions in the absence of any universal standards. In the guitar world, the iconic factory models manufactured by Martin and Gibson for many decades have at least served as a loose benchmark for the independent builder. However, as the Irish bouzouki was appropriated to a large extent from the Greek instrument and randomly modified to suit the requirements of a motley band of Irish folk musicians, its evolution appears to have been driven less by tradition than by the whims of the builders and musicians who have adopted it. It seems there are any number of combinations of scale length, body shape and body dimensions in common use, and I finally decided that in the absence of any agreed formula I'd develop something largely on what appeals to my aesthetic tastes. I was influenced a little too by the length of the truss rods I have on hand, my available fretting templates, and my concerns that an extreme scale length and a body join at the 17th fret - as is common practice - would result in a cumbersome, neck-heavy instrument. For better or worse, I've decided on a more conservative 632mm (24.9") scale length, a body width of ~350mm (14"), and a neck to body join at the 15th fret.

Using a set of French curves and a flexible ruler, I tidied up the half-body outline I'd drawn freehand on paper so that the curves transitioned and flowed smoothly. I traced the final outline onto a piece of baking paper, then transferred the outline to a piece of 6mm (1/4") MDF using transfer paper sourced from an art supplies shop.

I jig-sawed the MDF just inside the body outline I'd transferred from the tracing paper, then carefully removed the rest of the waste up to the line using a spindle sander. This yielded me a half-body template that would guarantee symmetry when used as a routing master. To fabricate the full-body master template, I aligned the centreline of the half-body template to a line I'd ruled on a second piece of 6mm MDF, traced the body outline and jig-sawed within the line, then clamped the two pieces together and routed to the line with a flush-cut router bit riding along the inner edge of the half-body template. Flipping the half-body template over and again carefully aligning the centrelines, I routed the other half in a similar fashion. The half-body and full-body templates are shown above.

I know from experience what a dusty mess MDF makes when machined, and of its tendency to deform and deteriorate over time, and made the decision to spend a little more money and fabricate the outside body mould from ply. To do so, I traced inside my full-body template onto a piece of 18mm ply, then jig-sawed inside the line leaving a small margin. Once again, having carefully aligned and clamped the full-body template in place, a flush-cut router bit removed the remaining waste.

I've struggled in the past to accurately align the individual layers when fabricating new moulds, and the solution - now that it's dawned on me - seems embarrassingly obvious. On the second layer of ply, I once again jig-sawed inside the pencilled body outline transferred from the full-body master template, then aligned and glued the two layers of ply together. It was then a matter of routing the waste to the line on the second layer of ply using the first layer itself as the bearing surface for the flush-cut router bit. The process was repeated for the third and fourth layers, with the final mould therefore comprised of four perfectly aligned layers of ply yielding a total thickness of 72mm.

Useful links:
Graham McDonald's, "The Bouzouki Book"
Nigel Forster Guitars - Irish bouzouki page