Anatomy of a Neck Joint

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There is more than one way to construct a mandolin neck joint. These include, but are not limited to: the dovetail joint, “V”-shaped and pinned joint, straight-slotted joint, and a bolt-on neck. Each method is invisible from outside the mandolin and each provides a unique process and different level of craftsmanship for attaching the neck to the mandolin’s body. Here is some background on each of the neck joint methods along with some features, benefits, and pitfalls of each:

The Dovetail Joint

Neck Dovetail, exampleDovetail being cut

A fitted dovetail joint including the headblock cap (l). Cutting the neck’s dovetail in a fixture (r).

The “dovetail” dates back to man’s earliest construction methods for securely joining two objects when working in either wood or stone. Luthiers began using dovetail joints in the 1500′s (although most viol-family makers preferred a slotted joint similar to the “straight-slot” method shown below). The dovetail is considered by many to be the most secure means for joining the neck to the headblock. However, since the mandolin’s dovetail joint requires mating the curvature of the neck to the curvature of the headblock as well as fitting the male and female mating faces of the joint itself, the dovetail is a tricky and complex joint to construct, especially for first-time luthiers. The process is further complicated by the need to achieve a 6° neck-pitch angle (F5 and A5, 4° neck-pitch angle for the F4) as well as to maintain a centerline-axis alignment of the neck to the body. Lastly, the luthier must ensure that the neck is not rotated so that the frets are level to a theoretically level bridge. One benefit of the dovetail joint is that wood shims can be used to ensure a tight, locking connection. As described in The Ultimate Bluegrass Mandolin Construction Manual, the dovetail joint is cut with the aid of a fixture (above, right) that supports the neck at the proper neck-pitch angle. But fitting of this complex joint still needs to be done by hand and suffice it to say that dovetail joints require a little bit of patience and experience. With the dovetail joint, the final fitting must be done after the rib is attached to the headblock.

The “V”-shaped and Pinned Joint

Components of the "V" joint methodDrilling for dowels to lock neck

Components of the “V” joint method (l). Drilling for dowels to lock neck (r).

The “V”-shaped and pinned joint is a method I developed back in the early 70′s and then described in my first instruction manual Constructing a Bluegrass Mandolin. This neck joint was intended for first- and second-time builders to facilitate the process of joining the neck to the body, locking it in place, achieving the correct neck-to-headblock fit, and providing an easy way to arrive at the correct neck pitch. In this method, the “V” joint is cut into the headblock after the soundboard and rim are attached. Then the neck heel is fitted to the joint, followed by the neck being glued in place in a fixture that ensures centerline alignment and neck-pitch angle in one step. Lastly, the bottom of the neck heel is sanded flush to the headblock and rim and two holes are drilled for pins to be inserted between the neck heel and headblock to lock the neck in place and prevent movement. The pinned joint is very strong, cosmetically pleasing, mechanically stable, and the entire connection is comparatively easy to do for either those just getting started in luthierie, or advanced luthiers as well. An added feature of the “V” joint is that during gluing, it allows more final alignment of the centerline axis and neck angle than the dovetail joint does. The final joint is very secure.

Straight-slot Joint

Gibson's straight-slotted neckThe rabbited joint in the body

Gibson’s straight-slotted neck (l). The rabbited joint in the body (r).

In the early 70′s, while Gibson was still building mandolins in Kalamazoo, the Company developed a straight-slot joint in which a square stub on the bottom of the neck was fitted into a rabbited joint in the mandolin’s body. The development of this joint demonstrated Gibson’s superior woodworking capabilities along with its inventive methods for facilitating production. As the photo on the left demonstrates, the fretboard extender was an integral part of the machined neck heel (i.e. the neck and fretboard extender were one piece). Through several ingenious machining steps, the fretboard extender was cut and shaped, the 15th fret crosspiece notches were made, the neck heel boss was machined, and a contouring tool was used to gouge the neck heel’s curvature for mating to the headblock. The entire assembly was then joined to the body in one step. (This method also eliminated the need for a separate headblock “cap.”) While this connection was frowned upon by some luthiers, it has held up quite well in thousands of mandolins. (Over the past many years, in a move initiated by Charlie Derrington, Gibson has switched back to using the dovetail joint.)

Bolt-on Neck Joint

Banjo lag screws can be used hold the neckBanjo lag screws can be used hold the neck

Banjo lag screws can be used hold the neck (l). The assembled bolt-on neck and block (r).

Some luthiers have chosen to use a bolt-on neck as it is the easiest of all connections to make. In this method, the neck’s heel is shaped to match the curvature of the headblock by first rough cutting, and then mating the neck and headblock by fixture-shaping or using sandpaper in a similar fashion to fitting the mandolin’s bridge feet to the soundboard. Then, one (see photo above) or two lag screws (similar to those used on banjo necks) are fitted into the neck heel. As the neck is being glued to the headblock, the neck is secured and pulled flush by tightening the nut(s) inside the mandolin’s body using a long socket driver with access through an enlarged end-pin hole. The neck can also be attached before the backboard is glued on and the tightening nut could be easily secured with an open-end wrench. While this method is simple and fast, its structural integrity is questionable since the gluing surface is minimal and the lag screws can cause the comparatively small neck heel to split. On the positive side, it does offer a little bit more latitude for achieving the correct centerline alignment of the neck by permitting the neck to swing around the headblock slightly before the nut is tightened. While I have not used this method, it does appear to be somewhat plausible. (As an alternate, some luthiers have selected to bolt on the mandolin’s neck using bolts that go through the backboard, similar to solid body guitar construction.)

Other Methods

I know of at least one luthier who reported using a curved neck heel similar to the bolt-on joint above, but used two dowels straight into the neck heel and into the headblock to secure the neck in place as is often done in cabinetry. This is the least desirable of all the connections described here, and while I felt it was worth mentioning, I do not recommend this method for mandolin neck connections.

Note: Although this “Anatomy of a Neck Joint” describes several neck attachment techniques, only the first two methods shown above are recommended; you can draw your own conclusions on the rest.

Comparisons (dovetail to “V” joint)

Stewart MacDonald dovetail neck A partially completed Stewart MacDonald dovetail neck joint. While the front curved part of the neck heel is nicely routed to the shape of the rib (rim), there is a significant gap between the neck heel and the headblock that requires being filled with shims.

 

dovetail joint on a 2006 Gibson F5G This is the dovetail joint on a 2006 Gibson F5G. Rather than a curved mating surface, straight cuts were made into the neck where the neck’s heel mates to the rib (rim). The resulting gap was filled with an adhesive. Note the significant gap between the neck heel and the headblock.

 

Siminoff V-joint provides for more neck wood in the jointAlthough our dovetail joint uses a wide dovetail cut in the headblock, the fit of the dovetail joint still requires working five mating surfaces.






The Siminoff V-joint provides for more neck wood in the joint, two locking dowels that go straight into the neckblock The Siminoff V-joint provides for more neck wood in the joint, two locking dowels that go straight into the neckblock (and pass at an angle from the headblock to the neck), and 100% mating of the sides and the back of the neck to the headblock. Note that from the outside, they all look the same.

 

Gibson dovetail joint from 1915Here is a top view of a Gibson dovetail joint from 1915. Note that the neck is only secured on four mating surfaces (two sides of dovetail, and two rear faces of neck). Also note shims that were needed. Our dovetail (below) uses the same 14° dovetail and same .550″ width at the dovetail’s narrowest point in the neck, but we extend .125″ further into the headblock and fit all five surfaces.

While many pros can get a dovetail joint to fit right, a properly fitting dovetail is more difficult to manage for the average luthier. Compared to the V-joint’s three mating surfaces (two sides of the neck and the butt of the neck heel), the dovetail has five mating surfaces. Ideally, on the dovetail, the curved cuts of the neck heel should mate to the rib at the same time the butt end of the neck heel mates to the headblock. But getting this to happen accurately is a nightmare. Since the curved parts of the neck have to mate to the rib for cosmetic reasons, considerations for fit are given there first, and gaps are often found between the butt of the neck and the headblock (as shown above). Many builders will fill this gap with shims and glue. With the V-joint, the neck is pulled back into the headblock, mating the butt and sides of the neck to the headblock in one operation. A solid connection to the headblock is an important consideration to ensure that the neck stays in place under string load, will not rotate, and never needs to be reset.


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