Adjustable Neck

The whole area of how the neck of a steel stringed instrument joins and interacts with the body is of crucial interest to the guitar maker. Under string pressure a steel string instrument is trying it's best to fold up into the soundhole, and the builder is always striving to make an instrument that is light enough to deliver the best possible sound, with the best playing action and that mantains this stability over time.

 

From my earliest building I have used bolt on necks as this seemed the most practical to me in terms of easy maintenance and repair of the instrument. Also from very early on I have built my instruments with free floating fingerboard extensions rather than gluing them to the instrument's top.

 

I consider that the body and neck of the guitar are related but have two different tasks to perform. The body of the guitar needs to be built so as to make the best use of the strings vibrational energy to translate this into the sound and responsiveness that the player desires. The neck needs to keep the energy in the strings for transfer to the body via the bridge/saddle and provide a playing surface for the player to fret the required notes in a way that frees up their "flow and creativity" to making music rather than having to think about physically manipulating the instrument.

 

With this in mind I have found it impossible to reconcile how to achieve the optimum from neck and body when the neck is constrained on both the vertical (neck block at the sides) and horizontal (fingerboard extension) planes of the body - not enough degrees of freedom as I would say in my Mathematics days. In my view the compromise of having to heavily brace the upper bout and neckblock area to maintain neck/body stability restricts the sound potential of the upper part of the guitar body. I don't believe that this area is tonally inactive - only if you brace it to death. Even  building a fingerboard extension into the neck block and bolting the fingerboard to this I think is a compromise.

 

I have been influenced by the building ideas of Mike Doolin, Rick Turner and Howard Klepper (my thanks to them all) and now build in the following way:

  • using carbon fibre flying buttress braces connected from the sides to the neck block to provide a unified stiff box structure for minimum weight that provides resistance to the string pull of the neck flattening the back arch in time.
  • building the neck as a unified structure up to the top's transverse brace
  • connecting the neck to the neck block by bolts on the body's vertical axis and leaving it free floating on the horizontal axis
  • building a pivoting adjustment in the neck joint to allow for different action settings and fine adjustments as the guitar settles under string tension

I am testing out an adjustable neck joint based on Mike Doolin's in my guitars and guitar bouzooki/citterns and I have documented it's construction here from the build of my cedar/maple Samhain guitar:

The neckblock is supported with the carbon-fibre flying buttress braces. The channel in the top of the neckblock is 40mm wide by 20mm deep and this is where the free-floating neck extension sits. The difference from my normal neck block is that there are three extra holes - one on each side of the top neck bolt hole and one below the bottom neck bolt hole. These contain the pivot srews (top) and adjusting screw (bottom).

Here is a front view. The sides are covering the 2 top pivot bolts and will be revealed when the neck pocket is routed later

These are the pivot/adjusting screws. They are 1" long 5/16" set screws and are screwed into 9mm holes creating their own thread.

The neck supporting the fingerboard goes up to around the 18th fret and so the neck shaft is made 20mm deep. The neck joint is scarfed and I do a stacked heel.

For this guitar I wanted a 5 piece English sycamore, maple, Cuban mahogany, maple, English sycamore stripe and so I have to do the stackings carefully to get the centre line looking continuous. Each stack is aligned with dowels and then glued up. Here is the finished stack. The tenon is then cut.

The heel is then aligned, dowelled and glued to the neck shaft.

The neck is then routed for truss rod and carbon-fibre stiffners. Both of these can extend almost the whole length of the fingerboard.

Here's the neck with the CF rods, truss rod and truss rod cap fitted. The fingerboard extension has been cut to 40mm width.

Another view.

Now the neck pocket has to be routed that the heel sits in as it pivots in and out. I routed this about 5mm deep.

Now you can see the top 2 pivot screws. Note that they are retracted so that the router bit doesn't catch them. Don't ask me how I know this!!

Next the neck heel is marked where the 3 pivot screws will bear and drilled for ¼" brass rod inserts.

The brass rod inserts are epoxied in

When the epoxy is dry the brass inserts are sanded flush

Then the neck is drilled for the neck bolts as usual.

Here is the finished neck with fingerboard attatched.

For extra interest I bound the sides of the neck pocket with koa matching the bindings. I have left a slight gap between the bottom of the heel and the bottom binding to allow for the neck moving in and out as it is adjusted.

The top 2 pivot bolts can be used for fine tuning the neck straightness with the body centreline, and also for fine tuning intonation. Most of the adjustment is via the bottom screw

 

 

Clockwise pushes the bottom of the heel out, reducing the neck angle and raisung string action. Anticlockwise and the neck angle increases, decreasing string action. For the adjustment the strings are loosened and the bottom neck bolt loosened. Very little andjustment is required to get a movement of the neck. Finally the bottom neck bolt is re-tightened.