Neck dovetail and truss rod barrel nut

[putnamm]

I'm using plans based on a 1920s Loar F5. This includes a curved truss rod, which I'm replicating. The heel end of the truss rod I am securing with a barrel nut, which is different than the Loar plans I have that simply show a regular nut securing the heel end of the truss rod. However, those plans do show a circular recess drilled in to the heel end to accept that nut.

I am cutting a dovetail joint to join my necks to my bodies. Thus, the circular recess for the barrel nut is actually cut IN to the dovetail on the heel end of the neck.

So here is my question: Does having the circular recess in the dovetail compromise the integrity of the joint? I assume so... But I'm not sure how else to secure the barrel nut in the neck.

I will try to post a picture if I can.

Thanks,
Mark

[putnamm]

(Please note, I did NOT cut the dovetail with the truss rod or barrel nut in place... )

[Dick Hutchings]

All of the stress of a dovetail starts at the bottom where you have removed most of the dovetail. Is it bad, I don't know.

[putnamm]

I guess my question is: If anchoring the truss rod with a nut at the heel end and also cutting a dovetail, where does one place the space for the anchor so as not to compromise the dovetail? Or am I just doing it all incorrectly?

[SammyV]

I guess my question is: If anchoring the truss rod with a nut at the heel end and also cutting a dovetail, where does one place the space for the anchor so as not to compromise the dovetail? Or am I just doing it all incorrectly?

Perhaps orienting the barrel nut vertically and chiseling a mortise rather than a circular hole would work? The nut would be prevented from spinning and the dovetail would retain much more material.

[Dick Hutchings]

I would say behind the dovetail and plug that hole but I've never installed one of these. I just bend them like old Gibson's. No additional hardware and they never twist or come loose.

[putnamm]

Perhaps orienting the barrel nut vertically and chiseling a mortise rather than a circular hole would work? The nut would be prevented from spinning and the dovetail would retain much more material.

Thanks, Sammy. I will give that some thought.

I also considered drilling a cavity downward in to the neck at the top (i.e. where the fretboard will sit) but behind where the dovetail will be. I just wasn't sure if that would provide enough length of truss rod to do any good.

[j. condino]

The whole purpose of the barrel nut is so you can drill a vertical hole that the barrel slips into, thus not allowing any rotational movement. They clearly show that on the installation instructions....

[putnamm]

The whole purpose of the barrel nut is so you can drill a vertical hole that the barrel slips into, thus not allowing any rotational movement. They clearly show that on the installation instructions....

I don't have any instructions for these. I made them myself.

[putnamm]

I would say behind the dovetail and plug that hole but I've never installed one of these. I just bend them like old Gibson's. No additional hardware and they never twist or come loose.

Thanks, Dick. Gladly, I won't need to plug anything. I cut this and a few other neck blanks strictly for testing out execution and fit of truss rod installation. But I get your point and appreciate the suggestion.

[SammyV]

The whole purpose of the barrel nut is so you can drill a vertical hole that the barrel slips into, thus not allowing any rotational movement. They clearly show that on the installation instructions....

That makes perfect sense.

[Dick Hutchings]

Provided the barrel is locked on somehow.

[Richard500]

Provided the barrel is locked on somehow.

I’d assume that any screwed-on anchor (what S-M. sells), would be peened on or thread-locked by careful builders, but that’s just an assumption. What I don’t get is why the cylindrical anchor is placed perfectly to split the neck if maladjusted and sturdy. I’d think it would best have a flat side in a flat mortise. And yes, I do know that knockdown furniture uses round ones. But beyond that, wouldn’t it make sense to bring a truss rod all the way through the neck block where it might help hold said part on, and of course lower than indicated?

[Dick Hutchings]

You make a good argument but in reality, i doubt this has ever happened. If it has, I'm sure someone with more experience than me will chime in.

[sunburst]

...I’d think it would best have a flat side in a flat mortise. And yes, I do know that knockdown furniture uses round ones...

Simply, anyone can drill a round hole using common shop tools. It takes relatively special tooling for a flat mortise.

The old Gibson curved truss rods had a nut and washer at each end. No peening, no locking mechanism of any kind. The rods did not spin in the neck because of the curve, so nothing other than a nut (and washer) was needed. The drilled hole is near the center of the dovetail and doesn't detract from the strength of the neck joint enough to cause failures of the tenon. When the neck joints are broken, it is nearly always the head block that splits, not the neck.

Somewhere I have a picture of a Gibson neck heel with the inset truss rod nut, but I haven't been able to find it. I can find these pictures from the process of a replica neck that I made, and they show the configuration, although not very well.

I pulled the rod out of the original neck and used it in the replica.

[Dick Hutchings]

John, that's not the one I was talking about. I make the one from Rogers book where the end is bent ~90 and then curves down into the neck and straight out to the peghead. I can't find a picture of it.

[putnamm]

Simply, anyone can drill a round hole using common shop tools. It takes relatively special tooling for a flat mortise.

The old Gibson curved truss rods had a nut and washer at each end. No peening, no locking mechanism of any kind. The rods did not spin in the neck because of the curve, so nothing other than a nut (and washer) was needed. The drilled hole is near the center of the dovetail and doesn't detract from the strength of the neck joint enough to cause failures of the tenon. When the neck joints are broken, it is nearly always the head block that splits, not the neck.

Somewhere I have a picture of a Gibson neck heel with the inset truss rod nut, but I haven't been able to find it. I can find these pictures from the process of a replica neck that I made, and they show the configuration, although not very well.

I pulled the rod out of the original neck and used it in the replica.

John, these photos are extremely instructive. Thank you for sharing.

[putnamm]

So in referring back to my original image and how I'm approaching this now it seems that using a barrel nut is unnecessary and also compromises the dovetail. I should just use a regular nut and a smaller washer to limit the diameter of the hole. Thanks.

[Dale Ludewig]

There's no need to the truss rod to go the the end of the neck. Once the neck hits the body joint, the truss rod isn't going to do anything anyway. If you're using a bent truss rod, it's most effective at it's midpoint. And as the neck gets thicker toward the neck joint, it's less effective and probably less useful.

[j. condino]

Vectored force conversations aside, I've only had to replace three broken truss rods in about 25 years of mandolin work. All three were by top shelf builders and every one broke at the right angle bend that was then stuffed into the heel...

[Larry Simonson]

I wish someone would write a paragraph or two that explains the physics of truss rods. It seems to me that the string tension is trying to bow the neck upward and the truss rod is installed to counter that force. If that is true then l have some difficulty with a truss rod that starts near the heel, goes upward toward the finger board, then away from the finger board. Wouldn't putting tension on such tend to straighten the rod thereby also bowing the neck upward?

[sunburst]

...Wouldn't putting tension on such tend to straighten the rod thereby also bowing the neck upward?

There is a very small amount of that, but it is completely overpowered by the compressive force of the rod below (away from the fingerboard) the neutral axis of the neck.

[j. condino]

I wish someone would write a paragraph or two that explains the physics of truss rods. It seems to me that the string tension is trying to bow the neck upward and the truss rod is installed to counter that force. If that is true then l have some difficulty with a truss rod that starts near the heel, goes upward toward the finger board, then away from the finger board. Wouldn't putting tension on such tend to straighten the rod thereby also bowing the neck upward?

About 8 years ago I went on a pretty heavy truss rod quest. I contacted about a dozen of the best mandolin builders in the world hoping for some serious insight. What I got was 20 different answers all contradicting each other. Frustrated, I made up ten different necks and installed ten different truss rod designs, fully built up and fretted. Guess what- all ten worked almost identical in real world application. So I use the one that is the easiest to install- a simple straight rod with a bullet nut on one end, very similar to what Stew Mac produces and shown in my earlier diagram. The only design change I use is that I make my own rods & bullet nut out of titanium and have another shop weld them in position for me. I also like the small profile LMII allen nuts rather than the larger old Gibson style that requires more of the neck wood to be removed. Here are some I posted a while back for guitar, but the basic design is similar, just a shorter version.

Also remember that a simple single action rod becomes a double action rod if you tension load it and then level the neck surface before you install the fingerboard. No need for a big heavy double rod design for a little mandolin...

[Richard500]

...20 different answers...
I’ve learned that quite a few highly-skilled people do exceptional things without being intellectually aware of how they do it. First saw this when I learned to ski (that’s on snow for you southerners). High skill, no ability to describe or understand what was involved. Silly books on technique.
In my trade, which is physics, things not subject to measurement or very difficult to model are not useful, so it is hard sometimes to appreciate the value of human skills that are complex. But we must. A wonderful instrument is it’s own proof, even if its builder isn’t very clear on the engineering.

[Dick Hutchings]

About 8 years ago I went on a pretty heavy truss rod quest. I contacted about a dozen of the best mandolin builders in the world hoping for some serious insight. What I got was 20 different answers all contradicting each other. Frustrated, I made up ten different necks and installed ten different truss rod designs, fully built up and fretted. Guess what- all ten worked almost identical in real world application. So I use the one that is the easiest to install- a simple straight rod with a bullet nut on one end, very similar to what Stew Mac produces and shown in my earlier diagram. The only design change I use is that I make my own rods & bullet nut out of titanium and have another shop weld them in position for me. I also like the small profile LMII allen nuts rather than the larger old Gibson style that requires more of the neck wood to be removed. Here are some I posted a while back for guitar, but the basic design is similar, just a shorter version.

Also remember that a simple single action rod becomes a double action rod if you tension load it and then level the neck surface before you install the fingerboard. No need for a big heavy double rod design for a little mandolin...

This sure looks like a very easy way to do it. From what you're showing, it looks like you will have to also route or carve the headblock as well, is that right?