Making A Jazz Bass
I made three templates for the machining of the body, a body template proper a cavities template and a body relief template. Making these was a large part of the work of making the body. The machining itself was straightforward, However, because of the "organic" nature of the body shape, a good deal of hand work was necessary, much of it sanding.
First I made individual pickup and control cavity templates. I could have marked out and cut directly on the cavities template but the risk was that I would make a mistake that would mean starting again, wasting a good deal of time and material. In the event, I did not find it straightforward making accurate pickup templates. The method that worked to my satisfaction was as follows: Drill two 13mm holes in the MDF template material at the spacing of the pickup fastening holes. Saw along the line between the holes and plane the sawn edges until the half-holes become 5mm recesses, measuring these with vernier callipers so as to get them accurate; a delicate operation.
Then plane the outer edges parallel to the inner. The width does not matter but they should be parallel as they will be positioned with a square against an outer edge. Make a strip of MDF 21mm wide and accurately parallel. Squared one end on the shooting board and cut off a 30mm length, repeating four times. These became the spacers to be glued in position. Two templates are required as the two pickups are different sizes.
A cavities template board was prepared with two planed edges accurately at right angles. These edges, along with a centre line would serve as references for all cavities and the bridge positioning. Note,: it is quite difficult to align components perpendicular to the centre line without this edge.The pickup cavity templates were attached with a few dabs of super-glue.
Placing thin card beneath the plan and using a sewing needle, I pricked through the drawing to transfer the front body shape to the card. Then I cut it out with scissors. I was slightly disappointed to discover that the card, when laid on the drawing of the back, did not match exactly, though close enough, I suppose. Placing the card template on 10mm MDF I drew round it and bandsawed the outline creating a rough sawn template.From the band sawn offcuts, I made numerous radiused sanding cauls, internal and external. These were used to refine the shape. Whilst absolute accuracy is not essential, it is necessary that the curves are smooth and flowing. There should be no straight lines. The control cavities were also transferred to the outline template.
Machining the Body
I bought the body blank as a three piece set of light American ash which had to be glued together. The Ebay seller said that they could be glued together as supplied; they could have been but the joints would have been poor. I planed the edges flat using a very finely set smoothing plane then, choosing the best grain for the front, glued up the blank, clamping with sash-cramps. When the glue was dry, I cleaned up the back and front with a smoothing plane, checking for flatness and twist against a straight edge. I did loose a couple of millimetres in thickness in so doing.
I marked both sides and ends of the body blank with a centre-line. I then took some time in deciding where to place the body template so that any blemishes would be concealed. When I was happy, I drew round it. Then I bandsawed the outline, taking great care to remain outside the line. A point to note here is that the bottom of the cut can wander, relative to the top, where tight bends are negotiated. Beware. I used small screws, one in the upper body relief area and one where the neck plate would go to attach the template to the bottom of the blank.
I transferred the body to the pin router, fitted with a 1/2 inch spiral router and an oversize pin: The actual pin is 1/4 inch diameter and I have a number of bushes that can be installed over that to allow for differing router bits and various depths of cut.
I routed the body outline in stages, both horizontally, by using different pin bushes and vertically by setting the bit height. Here it is, firstly routed to half depth and finished, complete with cavities but not the neck cavity yet. The neck cavity would wait until I had a neck plan template.
I guess the router cutter was not as good as it could be. Shown here is some tearing of the end grain. It is not too deep and will easily sand out but I had hoped for better.
Shaping The Body Reliefs
I made a further cardboard template for the relief positions and transferred them to the body as pencil lines. The arm rest relief was made by plaining a flat with a smoothing plane. The 'stomach' relief was made with a small plane with a curved sole. It was rather too small for the job, I had made it some years ago for carving the plates on my Jazz Guitar, but using it was less time consuming than making a larger one so, use it, I did.
The plan was not too clear about the depth of these two reliefs so I gave it my a best guess, erring on the shallow side expecting that I would make them deeper after the first pass of rounding over the edges was done.
The main work of rounding was done using a router. I have a home made router jig that was made for cutting the binding channels on acoustic guitars. It holds the router vertical whilst allowing it to slide up and down. The vertical depth of cut is set by an annular guide that surrounds the cutter whilst the horizontal depth of cut is set by a cutter bearing. I modified this machine to take a 3/8 inch radius rounding over cutter. Such a device does not work for the areas of body relief so they had to be done by hand using sanding sticks and templates.