neck block size

[James Sanford]

I have two instruments with which I have the same problem. I cannot get a neck to remain glued in place with either.

One of these is mandolin which was built long ago by a friend. The neck failed soon after he finished it and it was a wall hanger for years. I decided to resurrect it and put a new neck into it with upto date methods. I did not change the neck block which is spruce. The neck failed again. It is now again a wall queen.

The neck block has two problems I believe. Spruce as the material and it is too small based upon what is considered standard for an F5 now.

I believe I understand the mandolin based on the spruce and the size of the block.

The second is a mandola of my own design (therein lies the problem I believe )

I have renecked this mandola 3 times and it still will not withstand string pull so I think that the neck block is too small which allows the entire neckblock and rib area to deflect and pull loose at the back plate neck button. The neck is not seperating from the ribs where they join the neck on the sides. The neck block is laminated cherry.

Other than copying something from, for example an H5 neck block, is there any way to accurately design the neck block when building a non-standard design? I suppose that you could do the overkill thing and really beef it up but that adds weight which I want to keep down.

Really frustrating because the mandola sounded really good and played well.

[Marty Jacobson]

If you have renecked it three times there is probably more to it than just the size of the neck block. I mean, Orville Gibson's first mandolins didn't have neck or heel blocks at all, and they could withstand string tension (not perfect, but OK). There are a lot of other things that could cause this... how is the neck joint failing? Is the neck block splitting, is the neck itself splitting at the joint, is the glue failing because of inadequate clamping or poor fit? We just don't have enough to go on to give you good advice. Pictures would help.

[sunburst]

A head block needs to be stable, relatively strong (mostly split resistant), and I like them light weight (for sound and balance).
The pull of the strings puts a considerable torsional load on the head block, "trying to" rotate it so that the peghead moves up in the direction of the strings. If we understand that torsion that needs to be controlled, we see that there needs to be a good area of glue surface (and a good fit) between the head block and the rim, and a good area of glue surface (and fit) between the head block and the back. Also, it helps if the design is done such that the structure of the instrument helps control that torque.
The neck joint needs to be chosen and executed so that it is plenty strong and at the same time doesn't unduly weaken the block or remove undue amounts of gluing surface between the block the rim and the back.
It's a bit of a balancing act, like instrument building in general, where we need to find the best compromise between mass and strength; light enough to be responsive yet strong enough to withstand bumps and bangs, the stresses of humidity changes and the stresses of general use while constantly under the full tension of a set of strings up to pitch.
So how big should the block be? Just big enough to withstand those bumps, stresses and string tension in the specific instrument in question. The structure has a big effect, the materials, the amount of string tension... it's a complicated equation. A little over-built is obviously better than a little under-built, but ultimately enough force will break it no matter how strong, so we might as well not go overboard with over-building hoping to make a "bullet proof" design.
Having done repairs on instruments for quite a few years I've seen failures and I've seen successes. I make head blocks similar to the successful ones I've seen and avoid the failed designs I've seen. So, it's back to the normal way of lutherie that has worked for centuries; use tried and true designs and don't deviate too much at one time. In other words, to answer the question; "Other than copying something from, for example an H5 neck block, is there any way to accurately design the neck block when building a non-standard design?"; we can engineer the whole thing if we have the training, know our materials and their strengths, know our glues and adhesives and their strengths, know or can measure the tensions, torques, stresses and strains that the design must withstand. I, for one, can't do that, and many other builders don't have that training and skill, so it sure makes sense to stick to standard designs that have been "reverse engineered" for us through the years. It also gives a sort of data base from which to guess what to do when coming up with non-standard designs.

So anyway, all that is to say; gosh, I don't know how big to make the head block, but those are the bits of info I'd use to guide me in guessing.

[Marty Jacobson]

Thorough as always, John. Lots of good thoughts there.
I still wonder if it's the design of the joint, or the execution of the joint which is causing the problem.

[the padma]

Ok...
Ya take a long rod, weld on a 1/4 inch drill bit on one end and a Phillips head on the other, and drill a hole into the tail block, right down to the neck block and into the heal...
then you slip a big screw in thru the sound hole and use the other end of the rod to screw it through the whole body, block and all right into the neck. Be sure to slather on a lot of that there cheap 5 min epoxy before screwin it all up.

Or before you go and screw it all up....I mean together,
you could take it to one of us loofers to get it fixed professionally ...but that would cost BIG bucks and wtf, if its only a wall flower queenie...just screw it.



Blessings
duh Padma


Ps . Ya ... you could do the quick and dirty fix as well ... just screw or nail the neck to the body...Hey, don't laugh, how do you think Stradivarius attached his necks. Spit and chewy gum? Not.

This repair tip brought to you courtesy of duh Padma~pa School of Looering Around.

Donations accepted.

[belbein]

John: What effect does OVERbuilding the neck block have? There's the added weight, but other than that, I mean?

[sunburst]

Isn't that enough?

All other things being equal, lighter is better. "In theory" we want all components of an instrument to be of minimal mass for best sound. In other words, we assume (from evidence gathered from observation and experimentation) that the limited amount of energy delivered to the instrument body from the strings, the only energy source the body has to work with to make sound, will make more sound if the body has less mass for the strings to set in motion. That principle is used to it's best effect when applied to the parts of the instrument that do the most work producing sound, the bridge/top system and the back, but for the best instrument we can make, we want to carry it over to all parts of the instrument, making instrument parts that are minimal in their mass while being strong enough to do their structural job. Architecture and engineering use similar approaches, and in each case a certain amount of overbuilding is figured in, more for things like bridges and less for things like space station components.
There are those who like massive rims, including head and tail blocks, to act as ballast or anchors (more or less) for the top and back, believing that a rigid, massive mount (like a speaker cabinet) is good for the top and back, but the work of those who have done experimentation (including Dave Cohen who shows up around here from time to time) indicate that overall, lighter is better.

[Tavy]

Isn't that enough?

Never from you John

There are those who like massive rims, including head and tail blocks, to act as ballast or anchors (more or less) for the top and back, believing that a rigid, massive mount (like a speaker cabinet) is good for the top and back, but the work of those who have done experimentation (including Dave Cohen who shows up around here from time to time) indicate that overall, lighter is better.

I wish someone would tell that to the electric guitar makers... Oh and the those banjo builders!

[sunburst]

I started to add an exclusion in there for amplified instruments and instruments with "skin" heads. There are good arguments in favor of mass in those cases, unfortunately...

[Marty Jacobson]

I started to add an exclusion in there for amplified instruments and instruments with "skin" heads. There are good arguments in favor of mass in those cases, unfortunately...

And my 1931 Duolian that had a solid machined aluminum neck. That thing was amazing. It fell on me when I was in bed one night during an earthquake (the only one I've ever heard of in Georgia, go figure). It definitely woke me up.

[James Sanford]

Thorough as always, John. Lots of good thoughts there.
I still wonder if it's the design of the joint, or the execution of the joint which is causing the problem.

Thanks to all who have responded.

Just to comment on the quote from Martin:

These two instruments have caused me to tear my hair and fingernails etc. I have built 13 from scratch and this mandola is the only one I have not been able to staighten out. I believe it is the tailblock size that is causing the problem and I intend to fix it when I get time. I built a new neck for it with probably as good a fit as I have ever had, used HHG, and paid particular attention to the fit of the back plate to the neck heel. When glued it all looked very good. The joint is a dovetail joint. From every thing I can see the joint did not move in the rib set but pulled loose from the back plate button and rotated upward from string pull.

So, operator error is not totally out of the realm of possibilty but I believe there is something else going on.

[Rob Grant]

"Insurance"...

[belbein]

LOVE the look of that dark heel cap and the dark binding. Very pretty.

[James Sanford]

"Insurance"...

Rob, I may just do what you have shown in this photo plus a very large screw through the heel button.

Just joking about the screw but when I redo the neck block I may very well remove a section of the back and make it more sturdy in that area.



Here are a couple of pictures from the original neck installation. I believe that the neck block is too narrow and short for the string pull. This is built in the violin style with the top and back overhanging the ribs so the width through the neck joint is more narrow than is obvious. It may be a design flaw which cannot be rectified easily.

[Rob Grant]

James, Using a screw might not be a joke in all cases. I've had a couple of repairs of cheap, dowel pinned mandolin necks where I have used that heel button cap to cover up a stainless steel screw used in conjunction with a ebony tenon. This screw not only reinforced the neck attachment, but held the cheap, multiple laminated heel together.

The below photo shows one of these mandolins (of Korean manufacture) before I added an extended heel button to the "void." The piece shown sitting on top of the job is what remains of the original, poorly designed heel button. It wasn't even part of the back plate, but rather a separate "add-on" for looks alone! That little old heel button, that is usually a part of the back plate, definitely serves a purpose in securing the neck in quality instruments. With my mandolins, I use it mainly as a bit of "eye candy."

[James Sanford]

I have rebuilt the mandola as follows:

Extended the neck block by approx. 1"
Removed a section from the back plate and increased the thickness to approx 3/16" at the heel button as well as increased the thickness over the neckblock glue area.
Added a tenon between the end of the neck heel and the neck block.
Added a small screw through the heel button into the neck heel.

Let's see what happens as I have restrung the mandola and left it to break or stay together.

Adding some photos.



I didn't notice any change in the voice of the mandola due to the changes made so here's hoping........

[James Sanford]

OK, just to close the book on this thread....

I am now 6 days into the "new" neck set with the modifications mentioned above and the mandola is staying together. Played it in a jam session last night and I was pleased with the result.

Now, if I can just figure out which key I am playing in.............................