neckangle

The illustration above shows what we are trying to accomplish. The neck cheek/fingerboard
extension angle, perfectly matches the rim/top angle. The fatal error that I have seen time
and time again is where the cheeks of the neck heel have been trimmed to get the neck into
the body mortises. Doing that changes the neck angle and disregards all the design
parameters --- when that occurs, we are in a major REPAIR MODE --- nothing matches.

The first consideration may seem obvious, but is often overlooked. The combination of “A”
+ “B” is a straight line. If the neck angle is 88.5 degrees and we are trying to match it with a
body designed for a neck with an angle of 89 degrees --- it will not work since the combined
angles do not equal 180 degrees. In the following example, I will use 89 degrees as the neck
angle design criteria. We will explore the relationship of the various top components, their
configuration and how it relates to the neck angle.

I have seen much written regarding the benefits of a radius or contoured top on an acoustic
guitar – dome strength, is one I question, since dome strength relates to the forces from the
outside. The reality is the strings are pulling the instrument apart from inside out. Another, is
sound quality, perhaps? However, I have seen and heard guitars with flat and in some
cases, classic and vintage guitars with concave tops. To me they were so loud and clear
one would swear they were electronically processed. Those discussions will be never
ending. To be sure, the radius on the X brace has a distinct role relative to the neck set
angle. Using the “rim perimeter edge plane” as the reference. One can visualize how the
contour of the brace actually creates a dome and raises the soundboard area were the
bridge is placed.

The following is geared toward the initial construction of the guitar. While aspects of the
information are certainly useful in the repair arena, those corrective procedures will not be
covered at this time.

Because I get inquires, view the various guitar builders forums and fix newly built guitars it
became apparent to me that there is a general mis-understanding regarding the concept
of the neck angle set. For the most part, the problems that are encountered are do to the
fact that the builder has used the terms “neck fit” and “neck set” interchangeably. In other
words, when the necks were installed to the body, most all the surfaces on the neck joint
have been trimmed to make the connection cosmetically acceptable. Unfortunately,
doing this completely disregards all the mathematical requirements needed to build the
instruments as designed.

I hope that the following will clarify why certain dimensions must be held to very close
tolerances during the construction processes. The proper connection between the neck
and body is not by accident --- there is no “magic fairy dust” used by the seasoned luthier
to get this correct. The key is understanding the design criteria, good planning and
accurate measuring and execution in the initial building stages.

Even though we are attempting to impart the “hand made” appeal to our works, trying to
emulate some of the mass production mentality can only lead to better quality and
improved efficiency. The example I will sight is the fact that the necks and the guitar
bodies for the same finished instruments are built by different individuals. In fact, in
different locations in the factories. Nevertheless, when the time comes the necks and
bodies are assembled perfectly in a relatively short period. The reason this is possible is
that the parts “were made for one another”.


		Kenneth Michael Guitars est.1978

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	Fitting the neck to the body --- The Basics -- Parts that match




During construction there are several absolutes that affect the fit of the parts at angles “A” + “B”. I found it helpful to be able to visualize the various angles, contours and elevations as they relate to the plane made by the top outer edge of the rim.

Think of the “rim perimeter edge plane” as this imaginary area. It is the flat surface that is the base for the various angles and projections.





The 89-degree neck angle in my view, works out ideally. Based on measurements I have taken off Martin factory produced parts, I believe the majority of their products are built using the same specifications.




In our example of the 89 degrees neck set, we can see how a 40’ radius “X” brace raises the bridge location to an ideal level. The “top of the fingerboard plane” projects just above the bridge – string, saddle are at the height required to achieve proper action etc. Note the effects of a smaller radius “X” brace. These increase the height of the dome raising the level of the bridge and thus compromising the relationship with the “top of the fingerboard plane”. Although this is not an article about repair – if I were to encounter this type of mismatch, my first corrective action would be to make the bridge thinner – I would not try to adjust the neck angle.


Notice how the “X” brace pattern actually imparts the contour to the soundboard. It is primarily across the width. A slight amount contour lengthwise and nothing at all in the (appropriately designated) gray area. The shoulder brace under the fingerboard is designed with a taper on each end. Because the fingerboard must fit flush to the soundboard, the center section of the shoulder brace is flat.


The braces in the pictures below were supplied by C.F. Martin, note the gentle curvature on the top side of the long "X" brace. The raised center portion of the shoulder brace is clearly visible. The lines indicate the level where each brace will intersect the rim.





The “gray area” is perhaps the biggest mystery to guitar builders, new and old. It is easy to see that the “gray area” must be angled so that it makes a perfect match with the bottom of the fingerboard. This is NOT an adjustment issue. The design of the guitar calls for an 89- degree neck heel/fingerboard angle and a 91-degree rim/ to soundboard angle. Steps must be taken to build the neck and body to these specifications. What is often overlooked is the need to raise, or rather, have a rise in the shoulder brace in the center relative to the “plane of the rim perimeter edge”. If this brace is flat all the way across, the soundboard will be lowered to the level of the perimeter edge. Consequently, a gap will occur because the “top to rim” angle has been lost.

With this basic understanding, a builder should be able plan ahead with the goal to keep tight tolerances relative to the items mentioned above. Particular attention should be placed on getting the neck block in perfect relationship to the “rim perimeter edge plane”. It is of value to know “Exactly” the angle of the neck set machined onto the neck you have purchased. Even more importantly – before gluing the shoulder brace to the top you must know how high it rises under the fingerboard area and what angle, that rise creates back to the neck block at the rim. I have found that calculating the neck angle in terms of rise per inch is helpful. Actually, I use rise per 3 inches. To me this is more useful than dealing with tiny increments of a one-degree angle. In future installments I will detail some of the actual methods used to assure proper neck angle set. Elsewhere on the site is a technique for preparing a completed guitar body assembly for a perfect neck set. I would like to reiterate that a problem is created when the cheeks of the neck heel are altered in the effort to get the neck tenon into the block mortise. Carving should only be done on the tenon. In addition, if the cap created on the mortise by the glued on sides is not properly and complete removed one cannot expect the neck to fit correctly.

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