Making A Jazz Bass
There are numerous choices to be made in the construction of a neck. Availability of materials, ease or difficulty of manufacture and personal preference all play their parts. Below are my choices along with the reasoning behind each.
For the neck wood, whilst I should have liked to use hard maple, I could not find any from the many tonewood suppliers on the Internet. Those that did advertise it were out of stock. So, I reverted to what I had in stock, a board of old mahogany recovered from an ancient bookcase. I call it mahogany, it may be Brazilian mahogany it could be one of many of the woods that go under the mahogany heading. Nevertheless it is hard and reasonably straight grained but not quarter-sawn. It turned out to be easy to work, so that was a bonus.
Partly because the neck wood was less than ideal, I decided to add reinforcement in the form of carbon fibre bars. These I did find on the Net from a company called Easy Composites. I purchased 11mm by 2mm bar which I doubled up and and machined to 10mm by 4mm. I installed one each side of the truss rod.
The truss rod was the next decision. The Fender installation method is not straightforward for the amateur. It involves a slot routed from the back of the neck for part of the way, then drilling from each end to meet up with that slot, maintaining the correct truss rod profile throughout. The drilling, I thought was fought with problems. I chose, therefore, to install it from the front, using a routing guide to define the slot profile. The adjuster was to be at the body-end of the neck rather the at the headstock.
The neck profile was to be elliptical (I can se no reason to make it otherwise) with the major axis equal to the fretboard width and the minor axis equal to twice the distance from the back of the neck to half-way up the fretboard.
Fretboard material was to be ebony. I prefer it to rosewood for its stark blackness. I found some African ebony blanks on Ebay. I would have preferred Indian but could not find it. It turned out to be very similar to the Indian variety.
Fretboard radius concerned me for a little while. I appears obvious that a compound radius would provide the ideal of consistent string-fret clearance throughout the length of the neck but is it worth the trouble? Reading a couple of discussions on the Internet is was said to be both spectacularly good and of little value. Receiving no guidance, I did my own investigation. Using simple trigonometry and a spreadsheet, I calculated the worst-case inaccuracy found on a Jazz Bass neck having a 9.5 inch fretboard radius. In detail, I calculated the deviation from a straight line between the nut and the last fret at the very edges of the fretboard. This would always be greater than the deviation seen by any of the strings. It was just less that 0.1 mm or 4 thousandths of an inch. It is all in the same direction, equivalent to a back bow of the neck so it could be expressed and a a deviation of 0.05 mm or 2 thou. from the best fit straight line. However expressed, it is in my opinion negligible and almost certainly less than other inaccuracies, especially as the deviation is progressive, changing gradually along the fretboard length. Had an error of this magnitude existed between adjacent frets it would definitely cause buzzing. I conclude that compound radiused fretboards are more a marketing ploy than a real advantage to the player. It would therefore choose a simple radius of 10 inches: 10 inches because I have a fret caul of that radius. Just one further note before I leave the subject: The Jazz Bass fretboard probably has a greater taper (width change) than any other Fender guitar. It therefore has the most be be gained from compound radiusing, even so, it is of little or no value.
Four more templates were made for the build, a plan template for routing the neck outline, a truss rod routing template to conform to the required curvature and two neck shaping templates.
Here is the truss rod template. In use, the neck blank sits on the baseboard with the reference edge against the near rail, clamped to the two cross braces, beyond the route. The hand-held router fence is set so that the 5mm cutter runs in the centre of the blank. The longitudinal alignment matters and nut position is marked on the rails.
The neck plan template is made from 6mm thickness hardwood and is straightforward except that its accuracy is critical, particularly in the heel area that mates with the body. I achieved better the 0.005 inch accuracy in the critical areas. A painstaking job. Having done so, I revisited the body cavities template, (I had knowingly left the neck pocket cut-out under size) and adjusted it to be a truly good fit to the neck template.
Two neck profile templates were made from 3mm clear perspex, one is for the 1st fret position and the other for the twelfth. I drew the profiles in a free drawing package. The shape is elliptical with a minor axis of 2*19mm and major axis of the neck width at each position. The neck thickness is 16mm plus the fretboard, so the curve of the neck extends 3mm into the fretboard. I printed out the resulting drawing and glued it to the perspex as my marking out. I did take measurement of the print-out to ensure that it was not scaled in any way. As with all templates, accuracy is important. I took my time fret-sawing and filing the profiles.
Assembling The Neck Blank
The neck blank was prepared, 24 * 80 * 900mm, all square and parallel, not wide enough to accommodate the headstock so 'wings' would be added later. It was routed for the truss-rod, using the truss-rod routing guide. The channel was stopped about 60mm from the bridge end of the neck and a 5.5mm hole was drilled from the bridge end of the neck to meet it, following and extrapolating the curve of the truss-rod channel. I counter bored the hole 10mm dia. by 10mm deep to take the truss-rod adjusting nut. The truss rod was made from 5mm dia. bright drawn mild steel, threaded both ends. The anchor, at the head end, would be an 8mm square bar about 18mm long, drilled and tapped. The rod was screwed into the anchor with a little Loctite to prevent it coming unscrewed. I opened up the truss-rod slot to accept the anchor. It extended to a point under the nut. I made a filler piece for the truss-rod slot from sycamore 5 * 8mm. The truss rod was wrapped in PTFE plumbers tape to prevent glue sticking to it and it was installed in its slot. Epoxy resin was used to glue in the anchor and PVA for the filler piece. Clamps were used to hold down the filler strip using sufficient pressure to make the truss-rod conform to the curved slot but not so much that it would be bound solidly. When the glue had dried, the filler strip was planed down flush with the neck blank.
The rebates for the carbon fibre reinforcing were next. Their centres are 8mm either side of the blank centre line and they are 10.5mm deep by 4mm wide. The carbon fibre strips were glued in place with epoxy resin. They sit just below the surface so that, if the neck shrinks, they don't lift the fret board.
I cut the fret slots using the fretting jig before glueing the fretboard. I also cut the nut slot. The datum for the frets is the bridge side of the nut. This corresponds to zero on the fret cutting jig. The fret slot saw cuts 0.023 inch slots (0.58mm) and the nut is 3mm wide, so the centres for the nut saw cuts are -0.29, and -2.71mm. The jig will not measure on the negative side (note to myself, modify the fretting jig) so a little ingenuity was required with the callipers to cut the nut slot. That accomplished, the fret slots were cut, making sure that they were less deep than the required final depth. The slots would be deepened later, when the depth gauge could be fitted to the fret cutting saw. During this project, I modified the fret slot saw to make it more useable. Fet saw modification
The slotted fretboard was placed on the neck blank, taking care to align the centres of each, and clamped in place. small holes were drilled in fret slots 1 an 19 to accommodate sections of cocktail stick which locate the fretboard. Do not attempt to glue the fretboard in position without these pins. It will slide about and will surely end up in the wrong position. Believe me, I've done it.
The fretboard was glued on, applying glue, PVA, to one side and clamping with a stout, flat board on the fretboard side and plenty of clamps. They should not be tightened too tightly.
This photo shows the wings being glued in place. \you can also see the cocktail stick locating pin at the first fret. Note that it avoids the reinforcement bars. When the glue was dry, the bottom surface was planed flat.