Official Luthiers Forum!

Hi everyone:
Now that some of my instruments are getting to be several years old I'm finding that some of them are developing a problem. The juncture of the neck to the body is a large mass of wood and it seems that as the rest of the guitar shrinks and moves the 14th fret hump rears its ugly head. The fret boards are straight on delivery but time does funny things to wood. I'm wondering if a heavier UTB's might be the long term cure? In my thought train, if the neck block is stationary and you can crank the truss rod around then if the transverse brace is solid then there shouldn't be any unsolveable problems (???????) Does anyone else have to pull the upper frets and shave the hump after a year or two? TIA Mikey

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The Biggest Little City, Nevada
www.hammondguitars.com
I love building guitars!

That happens. It happens most if the inside block has the grain oriented up and down. A lot of builders use a plywood block, either store bought or home made, which I suspect would contribute to similar problems, but I do not have enough knowledge to really assert that. There are other causes, too, I suppose. Imagine the external heel shrinking. It's grain is horizontal. If the internal attachment has the grain oriented vertically, instead of horizontally, it will not shrink as much. The truss rod cannot compensate for that. Some may argue that well aged wood that is stable at the build humidity will not have differental shrinkage if the post build environment is kept at exactly the original humidity--but that is almost impossible to maintain, especially if you buy a guitar built by other than yourself.

Bump

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The Biggest Little City, Nevada
www.hammondguitars.com
I love building guitars!

hi novice here thats workin on cheapies so dont listen to me
just wondering how much clearence was there
in between the fb and top when you set your neck

that hump happened to me before after a neck reset or two
trying to make the fb fit flush on the top
to the point of me taking the whole fb off and sanding into the top
trying to get it level ....i wont be doing that again

since then i dont make the gap so tight
and adding a shim under the fb
just wanted to know your thoughts on anything ive said
and if mabye this helps fight the hump
thanks

In my experience in repair and building I find strong evidence that the neck block,
over time, wants to rotate forward by pulling the back flat and diving at the neck to top
junction as it journeys toward the bridge. I think this is why Martin put in the Popsicle brace and others used a double transverse brace approach.
In the newer Martins they run a couple of braces right into the tongue of the neck block. All of these actions can be construed as attempting to minimize the forward movement of the top of the neck block.
The back being pulled flat seems to me is a smaller part of the problem but shouldn’t be ignored. Some builders beef up the back a lot to prevent this flattening.
I personally prefer the flying buttress made with hollow carbon fiber rods running from the top of the neck block to the bottom of the waist, and braces from there to the bottom of the neck block forming triangles. I only have about a years worth of experience with this my latest approach, but the stiffness in the upper bout is remarkable to say the least.
A case can be made that the stiff upper bout and neck junction puts more string energy into the top, just a side benefit.

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Keith MacKenzie
Location: Florida
"To err is human to really screw up you need tools"

Keith:
The guitars I have been having trouble with do not have the carbon arrow shafts in them. All of my newer stuff does and they are holding fine. The other style has been moving around on me. Do the older, lighter built guitars have this problem with time or is it just my stuff? Mikey

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The Biggest Little City, Nevada
www.hammondguitars.com
I love building guitars!

Mikey,
You've got a lot of company. Most old guitars move in this way, some a little some a lot.
I too am having much better results since I started beefing up the upper bout.

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Keith MacKenzie
Location: Florida
"To err is human to really screw up you need tools"

can someone explain for a newb what you mean by using carbon arrow shafts?
Thanks
matt

Someone will tell you more, but in the meantime I'll offer this. Archery arrows can have shafts made of hollow graphite/epoxy tubes. You can use these to imbed in a neck similarly to a graphite/epoxy rectangle. The archery shafts can be purchased cut to length and without a point or feathers--just the shaft. You can get these online.

Ordering arrow shafts online is certainly less expensive, if you only need one, go to a sporting goods store and buy one arrow. When cut in half, it will re-inforce the neck block of a guitar for just a few dollars. Your choice........ Mikey

P.S. search the archives for pictures of Rick Turners neck block bracing method, brilliant!

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The Biggest Little City, Nevada
www.hammondguitars.com
I love building guitars!

Thanks, I didn't know you guys meant an actual arrow. Seems like a great idea. I tried searching every way I could think of but was unable to locate the rick tuners meathod.

When you guys say imbed in the neck or neck block do you mean the head block or the actual neck?

They are referring to embedding them in the block in the body. They are making a buttress bracing on each side of the block that attaches to the rim assembly (typically into the lining) somewhere near the waist to give resistance against the block rotating forward.

Yea, Prairie State (Larson) used to run a compression metal tube from neck block to tail block on top and a tension rod at the bottom. I remember having a Fender 12 that had a tube also.

I think you mean torsion rod at the bottom. The tube at the top set the top position and the torsion rod at the bottom was adjusted to counter act the rotational force.

Tension, torsion, whatever...we called it a tension rod because it was put under tension to counteract the torsional forces of the neck block. The top tube was called compression because it was placed under compression to counteract the torsional forces of the neck block...

Matt:
Here is one way of installing the carbon buttress system:
viewtopic.php?f=10102&t=14541&st=0&sk=t&sd=a&hilit=carbon+fiber
Just look around in the archives, keywords: carbon fiber, flying buttress, etc....... Mikey

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The Biggest Little City, Nevada
www.hammondguitars.com
I love building guitars!

Actually Prairie State called the lower rod a "Straining rod". Here's the way I've done it although on the latest 12 string I've moved the buttresses close to the tailblock and added some wings to the neck block .

I've also added the straining tube at the bottom. I tried the single rod through the middle of the soundhole on the 6, and then realized that I couldn't get my hand inside the instrument...not good. Don't much care if the tubes show from the soundhole as long as I can get mu hand in there.
I thought of that other buttressing system tied to the waist, but elected to keep the tube straight and in one piece. I don't know what they weigh, but it certainly doesn't bother the tone of the 12;s!

Thanks That was the help I was looking for.

This is what I'm doing at the moment.

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Keith MacKenzie
Location: Florida
"To err is human to really screw up you need tools"

How do you set the rough angle on the block under the fretboard? I'm guessing 1 1/2 degrees?, just eyeball? By the way, thats a stout lookin' boat.......... TIA
Mikey

_________________
The Biggest Little City, Nevada
www.hammondguitars.com
I love building guitars!

"How do you set the rough angle on the block under the fretboard? I'm guessing 1 1/2 degrees?, just eyeball? By the way, thats a stout lookin' boat.......... "

Sandpaper in a 28' dish.
The top of my head block is a layer of spruce so it sands along with the linings.
That gets me very close and I do the final angle after the top is on using the sanding block referenced to the
top of the bridge technique.

Except for the upper bout, the back bracing is off the L00 plan.
My top will be thin and bracing will be a very light lattice so I need the boat to be stable.
My last guitar used this approach and it was very pleasing. Not yet a "McKnight"
but getting closer. I took the Somogyi class a year and a half ago so I am following the basics of:
Strong upper bout.
Stiff rims.
Lightly braced tops of known deflection.
In my attempt to isolate the forces on the top due to boat failure, I may be a bit overboard.
I should mention that I don't sell my guitars. These experiments are fun but I have a large respect for
the relentless pull of the strings over time and wouldn't presume to know what will happen long term.

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Keith MacKenzie
Location: Florida
"To err is human to really screw up you need tools"

A little thicker headblock would help without having to go aerospace. That really bothers me in concept and appearance. Cant we find a woodworking solution?

I have had pretty good luck thus far but putting a tongue on the top of the neck block. It extends about and 1-1/4" along the top. I could be wrong but I feel the 'hump' is caused by string tension pulling the top toward the neck block.

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Joe Beaver
Maker of Sawdust

Sounds good to me. Go ahead and extend the tongue to the UTB, add a slipperfoot, and I think the problem goes away. I mostly build classicals, so what do I know? But the traditional steel string neck block would make any structural engineer scratch his head, "What were they thinking"?

I have made buttress braces from spruce that were about 3/8" by 3/4" in cross section. Wood is very strong in compression. My experience with this type of bracing is that it does help the structure, but there can be a loss of deep bass due to the stiffening of the box.
I build with some radius on the UTB....just enough to produce a straight fretboard with the 1.5 degree neck pitch.
You must distinguish between a true hump and fingerboard tongue drop-off. If you have a combination of drop-off and neck bow, the result is a hump, but it has nothing to do with neck block swelling.
Swelling of the neck block is pretty rare if the neck block is well-seasoned mahogany with the grain running parallel with the sides.
A rotated neck block (which the buttress bracing is intended to prevent), does not generally produce a hump at neck to body joint....but it can produce a rising fretboard tongue.

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John