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Trenchworker

cello neck crack

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A 4/4 size student cello has come into my shop to fix a loose fingerboard.  I noticed that there is a jagged somewhat-more-than-hairline crack running along the neck heel, about midway up the heel, and extending from one rib side to the other.  Except for the loose fingerboard, everything is tight and sturdy.  I was asked only to fix the fingerboard.  Should I leave well enough alone until the instrument comes back with a broken neck, or should I try to convince the teacher that the neck should be attended to now?  She would not be eager to pay the price of broken neck repair, nor am I too eager to attempt the repair unless there is an easy way that I don't know about

 

Thanks for any feedback.

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TW,

If the cello was left for anytime in your shop you have no choice at this point. If the neck has been repaired, she may wonder why you did not notice. If it has not been repaired, it will most likely come out soon with the change of season, and you will be the last person to have your hands on it. She may even notice it for the first time when she picks it up without you mentioning it...dropped off no crack - picked up with crack. Honesty works well, point it out without doom and gloom or estimate, then when it lets go you will be on the right side of the issue. You can even mention that you were torn about saying something and consulted your colleagues on maestronet!

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I agree with Jerry.

 

It seems to me that now would be the best time to do the neck repair, while the fingerboard is loose. And on a cheap cello, you'll hardly be doing a complicated or expensive repair.

 

I've had good success using a long screw to fix heel cracks. You can either glue the crack first, and then put in the screw, or use the screw to clamp the crack. First make sure that everything is lined up and drill a hole down past the crack. Countersink the hole to take the screw head and a washer. File off all the threads that will be above the crack. put a washer on the screw and drive it in just shy of the crack. Now glue everything with good strong glue and drive the screw home quick with an electric screwdriver.The washer will let the screw head turn freely, and the screw will act as a powerful clamp. Now put the fingerboard back on.

 

Not something I'd like to see done on a Strad, but for a cheap student cello it'll work, and be done and dusted in less than an hour.

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Conner's solution is what I would do as well with just the additional note to use a pan head screw instead of a wood screw.

reese

 

the shaft above the crack should be just larger than the screw diameter so the two sections are pulled together otherwise they will work apart over time. 

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I've had good luck with a screw too. To be clear though, this isn't just a matter of running a screw in there. Like Connor and Reese mentioned, the screw is installed in such a way that it will keep the broken area in compression. It's like having a permanent clamp holding the joint together.

 

If I were to do that repair again today, I think I'd experiment with putting a strong spring-washer under the head of the screw. That way, if the wood contracted a little over time, or because of reduced humidity, the internal clamp would still be applying pressure.

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Hi All  - negative, negative, negative on that screw(y) solution.

 

Twice I've had to repair a second-time-round broken neck where the second break was just below the end of the screw.

 

The first-time repair held OK,

 

I sat and studied it and finally satisfied myself that I understood what was happening.

 

Yes - the screw is pulling the original break together - BUT - immediately at the end of the screw it is doing its best to pull the heel fibres apart!

 

The original failure pointed out that the strength of that particular piece of wood was marginal for its job - that still remains the case in the wood just below the screw.

 

Same advice on using a thru-bolt washer and nut. All too often in wooden gliders we have found that over-tightened through bolts have compressed the wood  beyond its yield point and the attachment becomes loose. (Also applies to wooden propellers!)

 

A far better approach is to drill parallel to the outer edge of the heel ( I aim for 3mm in from the outer surface but keep chickening out and add a couple of millimetres of safety margin) and insert a 16mm dia maple dowel. 

 

Just to make sure that the fingerboard isn't riding high on the end of the dowel (if you sand the gluing surface you'll find that the dowel will be sitting high) use a gouge and sneak off a shaving to dimple the top of the dowel.

 

Use the browser - I've detailed the repair step-by-step a couple of times before.

 

cheers edi

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Hi All  - negative, negative, negative on that screw(y) solution.

 

Twice I've had to repair a second-time-round broken neck where the second break was just below the end of the screw.

 

The first-time repair held OK,

 

I sat and studied it and finally satisfied myself that I understood what was happening.

 

Yes - the screw is pulling the original break together - BUT - immediately at the end of the screw it is doing its best to pull the heel fibres apart!

 

The original failure pointed out that the strength of that particular piece of wood was marginal for its job - that still remains the case in the wood just below the screw.

These breaks are usually the result of an impact (such as the case falling over backwards), so consider another scenario for the failed repair:

The repaired neck receives another major impact. Part of the heel is reinforced with a screw, so that is the strongest area, making the heel most likely to break somewhere else. Force and leverage on the heel from the impact will decrease as you go down towards the button, so the new break occurs at the highest un-reinforced spot, which happens to be near the lower end of the screw.

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Thank you, everyone. 

 

Regarding letting the teacher know --- she knows, as right from the start I pointed out the crack, but admitted I didn't know exactly what to do yet, but would find out.  She said it had been that way as long as she could remember, so just take care of the loose fingerboard.

I thought perhaps the repair had already been made, as there was a circular patch on the fingerboard that looked like it was fill over a hole.  When I separated the fingerboard from the neck, there was nothing underneath, no connection between the patched hole and the heel.  I concluded that a repair to the heel crack had not yet been done.  (Heaven knows what that filled fingerboard hole was --- a guess is that it was an old fingerboard originally screwed into an old neck and patched up to fit this cello.  Any other guesses?).

 

Re the heel repair:  I will inform her of your opinions, take what she can pay me (probably $40-$50, the cello belongs to the local public school district), and then think about all your anwers, check some books, and then going ahead with it.

 

Thanks again.  It's really great to have serious backup when one is a sole proprietor.

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I misread the post so I deleted them.(Thanks JP)

 

Regarding the hair line crack,there is  actually preventive repair.

 

If any one is interested,I'll post what can be done.

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Since there's already a hole in the fingerboard, and since you'll only be paid to put it back on, I think you should do jut that, and then if the neck fails, just do the repair without removing the fingerboard again.

 

I've never seen a screw repair fail in the way Ed predicts. The screw spreads the load over an inch or more of wood, and there should be no concentrated force pulling just below it.

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snip

 

I've never seen a screw repair fail in the way Ed predicts. The screw spreads the load over an inch or more of wood, and there should be no concentrated force pulling just below it.

 

Hi Conor - not a prediction - seen it, had to repair it. Guess that puts me two up on you then. :-)

 

Think about it for a moment - when you tighten the screw the head tries to pull the top piece down while the tip of the screw is trying to pull the wood upwards - from head to tip of screw the wood is subjected to a compressive stress - no problem.

 

However immediately beyong the tip-of-screw there will be a tensile stress.

 

Now add some extra load due to the scroll hitting a wall on its way to the floor and the stress at that point becomes too much.

 

Hi David - you wrote...

 

"Force and leverage on the heel from the impact will decrease as you go down towards the button, so the new break occurs at the highest un-reinforced spot, which happens to be near the lower end of the screw."

 

Two things here...

 

i) that statement would be true for a heel of constant cross-section.However, since the area and the depth of heel are both decreasing towards the button, there isn't as much change in the stress as one might think. (Years back I sketched three cross sections of the heel on graph paper and counted little squares until I was cross-eyed)

 

ii) add to that the effect of the extra stress caused by the screw pulling the wood upwards at the tip of the screw...

 

Given that the wood is just marginal in strength to begin with - you have a repeat break.

 

cheers edi

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ii) add to that the effect of the extra stress caused by the screw pulling the wood upwards at the tip of the screw...

 

Force from the screw isn't concentrated at the tip of the screw. It would be spread along the length of the threaded portion. The tip of a wood screw has relatively no holding power, in a straight pre-drilled hole.

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Force from the screw isn't concentrated at the tip of the screw. It would be spread along the length of the threaded portion. The tip of a wood screw has relatively no holding power, in a straight pre-drilled hole.

 

Hi David - you're quite correct about the force being spread along the length of the screw thread. However at the end there is a discontinuity. The tip-end of screw thread had better be trying to pull the wood towards the crack or it's not doing its job - and when it does this - at the point where the screw thread stops there will be a tensile stress in the wood that didn't exist before.

 

This is easily demonstrated if one models it in perspex and views it under polarized light.

 

cheers edi

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In my opinion, the screw (lag bolt in my terminology) should not be over tightened. I have seen dowels loosen over time because of differential shrink/swell, then there was no reinforcement.

 

Hi captainhook - screw/lag bolt are technically the same animal - a threaded fastener. A lag bolt usually has a larger diameter and a coarser pitch and would normally be used in softwood.

 

As to dowels coming loose - for this repair I use a structural epoxy for the following reasons...

 

I) the bond strength is greater than the strength of the wood

 

ii) it has gap-filling  properties that makes for a stress free joint

 

iii) it is water-proof and thus seals the dowel from  moisture-driven movement

 

iv) it has a long working time compared to hide glue - so there is no chance of exceeding the gel time

 

v) it is a repair that never has to be opened again - but could be drilled out if necessary

 

vi) the 16mm dia dowel was calculated to give a very low-stressed glue join.

 

I have been using epoxy glue and resins for the better part of 50 years on boats and aircraft - with not a single failure.

 

cheers edi

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Hi David - you're quite correct about the force being spread along the length of the screw thread. However at the end there is a discontinuity. The tip-end of screw thread had better be trying to pull the wood towards the crack or it's not doing its job - and when it does this - at the point where the screw thread stops there will be a tensile stress in the wood that didn't exist before.

 

In my method, the screw is threaded into a pre-drilled hole, so the tapered tip serves only to help form concentric compressed threads, and serves no holding function.

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I misread the post so I deleted them.(Thanks JP)

 

Regarding the hair line crack,there is  actually preventive repair.

 

If any one is interested,I'll post what can be done.

Yes please, I am aware of the level of your work and would be interested in any information you would so humbly impart.

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In my method, the screw is threaded into a pre-drilled hole, so the tapered tip serves only to help form concentric compressed threads, and serves no holding function.

 

Hi David - agreed - the tapered tip, the part of the screw that is in the hole, does nothing.

 

However the last full thread is still pulling the wood towards the head of the screw and away from the wood beyond the end of the screw and - Voila! - there is a stress field beyond the end of the screw.

 

cheers edi

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I am leaning toward using a wood dowel rather than a screw, on the basis of no data whatsoever except  gut instinct (no pun intended) that wood may be more "forgiving" than metal, and less likely to add interfering vibrations to the overall sound.  However, seeing the carbon screws posted by Jessupe interests me and I ask the same question: has anyone worked with carbon screws in fixing a broken neck?

 

Thanks, all.

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TW,

Dowels are not a good idea to fix this. The expansion and contraction from humidity will work constantly to break apart your repair.

Edi,

If in fact the screw in the heel was causing the new crack wouldn't we see evidence in other contexts? Cracks in furniture, cracks in every 2x4 in a wallboard wall, or frogs that have under slides screwed on?

You may have an argument if the end of the screw was dull and bottoming out in the pilot hole.

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I am leaning toward using a wood dowel...[beacuse]...wood may be more "forgiving" than metal...

 

...Dowels are not a good idea to fix this. The expansion and contraction from humidity will work constantly to break apart your repair...

 

I agree that a dowel is not a good idea here

 

Wood is more forgiving than metal, but only if the grains of the separate pieces are parallel.  If the grains are running at right angles to each other, as they would be in a doweled neck foot, some very unforgiving stresses are the result. 

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