Wood Screws or Glue?

For most guitar necks, this is the question that has to be answered early on. Am I going to build a guitar with a set neck (glued in) or a bolt-on neck (held on by 4 wood screws). Which one is "better"?

To answer that question, we have to define "better".

This is where is starts to get a bit tricky, and where it becomes a perfect decision to apply the engineering analysis that we love here at VRC Guitars. During the brief time that VRC Guitars was Via Real Customs, I researched this issue, because I loved the manufacturing ease of bolt on necks made famous by the guitar engineering master, Leo Fender, but have always hated using wood screws for critical joints. Let's be honest, how many stripped wood screws have we all encountered? Furniture, cabinets, drawers...

from Neal Moser Guitars

Of course there is the third option, skip the attachment of the neck to the body completely and use neck-through construction. This option is hard to argue since it completely eliminates the connection question, but it's also a major manufacturing challenge, and one I prefer to leave to the masters (like Neal Moser Guitars). So back to the original question: to glue or screw? Time to jump into the research, which is surprisingly lacking - the good news there is that means it was easy to dig into. A little digging brought me straight to a German company called NecTite, and several blog posts from "The Mad Electric Guitar Professor"...perfect! Here is the Professor's opening statement:

Since the dawn of the electric guitar era, a holy war has raged in forums, workshops, and music shops worldwide: does a glued (set-in) neck produce more sustain than a bolted-on one? Disciples of Gibson swear by the dovetail. Fender faithful worship the four-screw plate. And the neck-through crowd watches from the sidelines, smugly convinced they’ve transcended the debate entirely.

He goes on to declair that only one person has bothered to measure this using actual engineering princciples, and my research didn't uncover any additional research than what he cited, which are studies condutcted by luthier and engeering researcher R.M. Mottola, whose findings were publshed in the journal American Lutherie in 2007 ("Sustain and Electric Guitar Neck Joint Type").

Between the link to the article by the Mad Electric Guitar Professor and the link to the original publication by Mottola, there is plenty to read if you want to dig into the science and engineering, but here are the conclusions:

1. A "neck-through" has no joint at all — maximum coupling. The wood is continuous. Vibration

   transfer is perfect. Used in high end models like those from the B.C. Rich Custom Shop (built by Neal Moser), as well as Schecter and Ibanez on some of their models.

2. The "set-neck" is the artisan's choice, long prefered by Gibson. It uses a tightly fitted mortise-and-tenon or dovetail joint, secured with wood glue. When the glue cures, the bond is permanent. In theory, its as rigid as if the two pieces had grown from the same tree.

3. The "bolt-on" neck was pioneered by Leo Fender and is considered by many to be a manufacturing compromise. Most notably, it doesn't actually use bolts. It uses wood screws. Its fast, cheap, and (mostly) effective.

Traditional guitar wisdom also ranks quality (defined by better sustain), in that same order - neck-through, set-neck, then bolt-on. The crazy thing is that the only research that seems to have actually measured this using sound engineering principles, and eliminating all the other variables, concludes that this is all wrong:

There was no measurable correlation between neck joint type and sustain duration.

It turns out that the joint type doesn't directly determine sustain, coupling stiffness does. If you think about it, that makes sense. In a guitar, we have two vibrating elements - the body and the neck. When they are connected, energy transfers between them. The efficiency of that transfer depends on how rigid that connection is. The implication here is that a poorly executed glue joint could result in less energy transfer than a well-executed bolt-on joint.

The problem with the traditional bolt-on joint lies in the fact that it doesn't use bolts at all. It uses those dreaded wood screws. It relies on the screw threading into the wood of the neck and compressing the wood fibers while the threads of the screw bite into the wood. There are several problems with that. One problem seen in some cheaper mass-produced guitars is that the screw holes in the body are not large enough and the screw threads bite into the wood in the guitar body. When the screw bottoms out on the body (or neck plate), it stops pulling the neck tighter to the body, resulting in reduced clamping force. Even when the holes are larger and allow the screw to be turned freely in the body, the clamping forces can only go so high before the wood fibers in the neck are stripped. Even done well, the result is a joint in which clamping forces will change over time, and capability is reduced every time you remove and replace a wood screw.

Enter the threaded insert solution. The treaded insert system selected for use in the VRC Neck is actually a true "bolt-on" system. It uses specially designed high strength carbon steel inserts with knife edge threads that are permanently installed into our roasted maple necks. These are M4 threaded inserts, paired to M4 machine screws that spin freely in the holes in the guitar body. The neck is attached to the body with 4 machine screws that thread into the metal inserts rather than into the wood itself. These can be torqued to a measurable, repeatable, force. They can be removed and replaced without damaging the wood fibers in the neck. The result is superior clamping force that results in the coupling stiffness we need to maximize the sustain by efficiently transferring the energy between the body and the neck.

We are certainly not alone in recognizing the superiority of this type of neck to body connection. Several high-end electric guitar manufacturers use this connection, and Tylor acoustic guitars have used a true bolt on neck system since 1999.

We went through several types of threaded inserts trying to perfect the installation into the wood we use. It's a tiny little part, but it turns out that not all threaded inserts are created equal. German engineering won out here. The zinc coated, carbon steel inserts behave exactly the way we want them to in our roasted maple necks and allow us to get maximum clamping force and coupling stiffness when we bolt these necks onto our mahogany guitar bodies.

"What matters is coupling stiffness, surface contact, and clamping force. A masterfully built bolt-on will outperform a sloppy set-neck every day of the week. A tight neck-through will behave nearly identically to a tight set-neck. The joint type is a manufacturing decision, not an acoustic one."

Quote from the Mad Electric Guitat Professor

from research conducted by R.M. Mottola

Click on the links in this article if you want to dig into the science and the research. Incidentally, we choose this same threaded insert concept to attach the custom VRC Pickguard to the guitar body instead of wood screws. The screws don't come loose, and you can remove the pickguard to access the control cavity without fear of stripping out the wood when you put it back.

Being able to torque our aluminum and carbon fiber pickguards tightly down to the body also means that we don't have to worry about unwanted vibration when the screws inevitably come loose.