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Neck reinforcement?


RedneckEngineer Fred
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I've recently repaired a couple of Srentor cellos with broken necks for a local music store.  They were not broken completely off, but were retained by a small rectangular piece of wood inserted vertically through the neck.  Made it easy to locate and glue.  Easy repair with a couple of dowels.

I am building a violin and wondering about drilling the neck at the heel and installing a dowel as reinforcement, possibly preventing a future break.

Maybe that has been done before.  Maybe it's overkill.  What do you think?

Thank you!

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I wouldn't bother on a violin.  David Burgess reinforces his cello necks, but I'm thinking it's more for stability (keeping the neck angle). No need on violins (IMHO). Not to mention I'm sure you will use  better quality maple on your violin neck than Stentor uses........................

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21 hours ago, RedneckEngineer Fred said:

I've recently repaired a couple of Srentor cellos with broken necks for a local music store.  They were not broken completely off, but were retained by a small rectangular piece of wood inserted vertically through the neck.  Made it easy to locate and glue.  Easy repair with a couple of dowels.

I am building a violin and wondering about drilling the neck at the heel and installing a dowel as reinforcement, possibly preventing a future break.

Maybe that has been done before.  Maybe it's overkill.  What do you think?

Thank you!

Hi Fred - affirmative for inserting dowels in both violin and cello necks during the building stages.

The cello neck because of the greater chance of it breaking and the violin to reduce the fingerboard projection changing in its first few months of life.

The easiest way is to insert them 100% of the way - before the neck has even been completed. When carrying  out a repair I try for at least 2 diameters below the break.

I use an 8mm dia. dowel in violins and a 16mm dia, dowel for cellos. These sizes give lots of glueing area and keep the glue line stresses well within the  working limits of of the epoxy. 

Use maple for the dowels with the grain running vertically through the heel of the neck.

I try drill the hole parallel and within 3mm of the outer surface of he neck. (One day I'll succeed - I suspect that I'm averaging around 4.5mm to date.)

The dowel end to be conical and the same included angle as the drill point.

Avoid a too-tight fit - "slip fit" is a good description. A "rattling good fit" is a bit on the loose side. :-)

Cover surfaces that you want to remain glue-free with "Magic Tape" (Scotch Brand)

File a flat along the length of the dowel to allow air to escape during the gluing operation.

I use a structural epoxy - ABE 372

http://abe.co.za/wp-content/uploads/2015/08/abe-construction-chemicals-ds_epidermix372.pdf

(It's used in the construction world to glue in steel reinforcing bars. Given the correct gluing depth the steel rebar will fail before the concrete. Works just fine with wood)

Mix the glue, trowel it (it's a paste - rather like butter) into the hole and spin in the dowel while slowly pressing it down. Gentle pressure allowing the trapped air to escape via that flat on the side of the dowel. (It makes a disgusting noise as it escapes - can't be helped. Just grin and be thankful that it is odourless)

Spinning the dowel all the way home ensures that you have thoroughly mixed any dust from sanding/drilling  into the epoxy paste and will also ensure that all surfaces are properly "wet out".

Don't drive it home  with a clamp - you'll create a "hydraulic lock" and blow out the side of the neck.

Leave lightly clamped overnight. Then trim the dowel flush. I also recess the dowel by about 0.15mm using a scraper -  just to make sure that it remains clear of the fingerboard.

I guess that about covers it.

cheers edi

 

 

 

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Wow, Edi, that is very thoughtful.  Didn't occur to me that a dowel in the violin neck would reduce the probability of fingerboard projection changing, but that makes sense.

I've been using the bandsaw to cut a spiral groove in the dowels to allow excess glue to escape, and can see that filing a flat or a channel would also work.

I like your idea of using tape to protect surfaces from unwanted glue, easier than cleaning it up afterwards.

Thank you again for sharing.  I am 75, and that's not too old to learn!

Best regards, Fred

 

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10 hours ago, RedneckEngineer Fred said:

Wow, Edi, that is very thoughtful.  Didn't occur to me that a dowel in the violin neck would reduce the probability of fingerboard projection changing, but that makes sense.

I've been using the bandsaw to cut a spiral groove in the dowels to allow excess glue to escape, and can see that filing a flat or a channel would also work.

I like your idea of using tape to protect surfaces from unwanted glue, easier than cleaning it up afterwards.

Thank you again for sharing.  I am 75, and that's not too old to learn!

Best regards, Fred

 

Hi Fred

i) dowel & stability - can confirm that it works in a violin. When I make my cello I'll report back.

ii) bandsaw and spiral grooves - show-off :-) Flat is quicker to make and it's easier for the air to escape. Less of a stress riser too.

iii) tape - just me being engineer lazy. Learnt the benefits while doing 400 hour repairs to fibreglas gliders that landed wheels up.

iv) 75 years old - stop complaining - try 79 - with wood for 4 cellos, a viola and about 25 violins in the workshop! All bought in anticipation of retirement. Unfortunately got hauled back into harness at 68 for a further 9 year spell at the workface. Retired a year back - I think.

Cheers edi

 

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I do pin cello necks for stability, but not violin. A couple of years ago I must have been asleep, because I managed to do one too close to the throat, and it appeared when I was cutting the neck. I had to graft the thing even before it was varnished!

On a wooden dowel for repair, its the one place youll see a dowel sheared straight across if the neck breaks again. The dowel really has no function unless the joint fails, and then it has to bear the whole load, whereupon, because it's held tightly it can shear off. Please Edi! I'm not trying to be contrary - I've seen it!

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3 hours ago, Conor Russell said:

I do pin cello necks for stability, but not violin. A couple of years ago I must have been asleep, because I managed to do one too close to the throat, and it appeared when I was cutting the neck. I had to graft the thing even before it was varnished!

On a wooden dowel for repair, its the one place youll see a dowel sheared straight across if the neck breaks again. The dowel really has no function unless the joint fails, and then it has to bear the whole load, whereupon, because it's held tightly it can shear off. Please Edi! I'm not trying to be contrary - I've seen it!

Hi Conor - yup - been there as well.

However I'll dispute "it has no function" - the stress in the dowel reached failure point - and dutifully failed. The glue had been transferring the stress from maple to dowel all along. First step in these investigations of material failure is to determine the properties of the failed materials and then examine the loads applied and the stresses that develop as a result of the loads. Other factors creep in later - temperature, vibration, impact, wear, operator error....

For instruments-in-build I have only placed dowels in almost fully carved necks - safer. 

In my "failure" case it was a 1840 Henry Thompson cello. It was the second broken cello neck that came onto my workbench. The neck had been previously repaired and the previous dowel was as you described - "sheared straight across". Although to be technically correct - it had failed in tension as shown by the "stalactites and stalagmites" of wood fibre.

I "mmmmed", sat down and put on my Engineer's cap and worried about my first repair that I had done 2 years earlier.

The dowel was about 10mm dia.-  a softwood - not quite balsa but the chisel said "softer than spruce". No clear grain lines. Maybe Poplar. Probably a dowel bought at the local grocery store.

On the compression side of the heel there was a 80mm long No 10 brass screw. Neatly countersunk - with the slot lined up on the centre-line of the cello - nice touch.

The screw had bent and it was fun "clapping" the open V of the break like a pair of castanets.

I pounded the keys of my calculator, thought of my first cello neck repair, obliquely asked the luthier for whom I had done the first job how it was playing. 

He reported it was doing just fine - and I found out that he'd recommended me to do the repair.

 

OK - so a 12mm maple dowel installed with structural epoxy was holding.

The difference between Poplar and Maple to failure in tension is roughly 1 to 1.5. So I was 50% better off using maple for the dowel.

The difference in diameters 10mm vs 12mm told me that my repair was stronger by 100 to 144. I was 44%  better off.

So - just considering wood failure - my repair was at least twice as strong. I began to feel less concerned.

The larger dowel had a greater gluing area so the stresses in the glue line were lower and nowhere near the failure point. Another plus.

I wanted to be sure of removing all of the 10mm dowel and improve the odds still further in my  favour - so I drilled out the old dowel and replaced it with a 16mm dia. maple dowel. This reduced the glue-line stresses still further and creep due to sustained loading would never be a factor.

Also I now had a joint that was about 4 times stronger than the failed joint and twice as strong as my first repair - that is now "19 years and still going strong" (with acknowledgment to Johnny Walker Whisky).

The brass screw was replaced by a 8mm dia. maple dowel. No calculations were done as that area is under compression and, after 150 years, I was confident that any strain due to the string tension had already occurred and no further movement could be expected.  

I did make one mistake. I used epoxy for gluing the two bits of heel together. I had everything neatly  lined up and clamped - an hour later they were still lined up and I went to sleep. However in the morning there was signs of movement and the break was clear to see. However the luthier for whom I had done the repair is a dab hand with varnishes and after sanding it smooth she made it disappear. (Two different luthiers in this tale. L1 of my first repair passed up on the work and recommended me to L2).

Today I use instant glue to immobilize the break. It's easier to hold things aligned by hand for the 60 seconds necessary than fiddle with clamps. Dowels and structural epoxy then handle the stresses.

About "contrary" - contrary is OK by me. Dad used to call me "my argument". Apparently I endlessly questioned everything. It's even rumoured that my first words were "Why am I here?"

cheers edi

 

 

 

 

 

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3 hours ago, Conor Russell said:

On a wooden dowel for repair, its the one place youll see a dowel sheared straight across if the neck breaks again. The dowel really has no function unless the joint fails, and then it has to bear the whole load, whereupon, because it's held tightly it can shear off. Please Edi! I'm not trying to be contrary - I've seen it!

I have seen it as well, it is extremely common.  I bet everyone that has done it thinks they know how to do it better.....until the next time it fails.

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3 hours ago, Jim Bress said:

Is the dowel simply to stabilize the neck against projection drop?  How are the necks broken? By the instrument being knocked over or someone tripping over a cello laying on it's side and falling on it?

Hi Jim - we are looking at two things here.

1.0 Projection Drop

When you string up an instrument for the first time you are dialing in stresses while tensioning the strings. Projection drop occurs and new owners are urged to return in a couple of months for a check up (and maybe a new bridge).

After seeing this a couple of times I gave it some thought and realised that the culprit might be the extension of the rear of the heel due to the tensile stresses that occurred there. I inserted a dowel into a new neck that was then  built into a violin. The luthier reported that the fingerboard had still dropped - but much less that what he considered "normal".

I sat back - feeling quite virtuous - happy that inserting a dowel at the back of the heel altered the properties of the heel in a favourable way - but slightly disappointed that there was still some drop.

More thought -aah - what about the "crushing" due to the compression stresses at that part of the violin that was glued into the body of the violin? The difficulty here was that the neck was glued to the end-block (of the right oriented grain direction) and the ribs (thin, long and with a slenderness ration that guaranteed buckling!)

The case for the violin proved to be inconsequential.

For the cello not so. The higher upstand did actually introduce movement. So in my cello I will insert a 16mm dia. dowel in the back heel of the neck and put in two 10mm dia. dowels at the front end. One of our luthiers already does this. (I tip my cap to him.)

2.0 Failure

The heel and the bow tip failure are similar cases.

The wood where the break occurs is simply not up to the job. That is all there is to it.

It can be...

i) "not enough wood" - an example would be the early Tourte bow heads, referred to an earlier post, that were too small.

ii) wrong orientation of the grain - applying tensile loads perpendicular to the grain - your typical bow-head, bow-shaft and  neck-heel failure.

Remedies that are available to us are...

i) make the bow-head a separate item that has the grain running vertically w.r.t. the stick. Drill a hole thought it push the stick through the hole and glue it in. (it has been done - could someone possibly turn up the picture of it - it was among some pictures from a baroque bow maker. Could do double duty as a bed-bug smasher)

ii) modify the shape of the head to have more wood at the leading edge and so reduce the stresses to a safe limit.

Here we have two approaches...

- a broader head - look at some of the battle-axes out there. The archetier was probably making allowances for the quality of the wood he had to work with...

and

- an approach that I have only seen a picture of - a bow-maker "Ayre", or something near to that, who runs the upper flat of an octagon stick all the way over the front of the head and down into the tip. This is touch of genius - he has added two little triangles of wood on either side of that knife edge down the front of the head. A knife edge where the stresses peak and the failure begins. Very neat solution - looks a little strange at first sight but it grows on one. (Another tip of the hat)

iii) Introduce something to improve the properties of the head.

These are a spline or pin -- both do exactly the same job - successfully if done properly or not if done poorly.

Another possibility that I have been mulling over is to modify the wood by introducing a polymer into the head by vacuum and curing it with RF radiation. This will add weight to the head - but might just make it strong enough to allow one to carve a delightfully sleek and petite head - only limited by the necessity of the "hair-hole".

cheers edi

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17 minutes ago, edi malinaric said:

 

Another possibility that I have been mulling over is to modify the wood by introducing a polymer into the head by vacuum and curing it with RF radiation. This will add weight to the head - but might just make it strong enough to allow one to carve a delightfully sleek and petite head - only limited by the necessity of the "hair-hole".

cheers edi

Interestingly enough, this was done c.1840 and many still exist today....I will let you guess how.:D

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1 hour ago, Jerry Pasewicz said:

Interestingly enough, this was done c.1840 and many still exist today....I will let you guess how.:D

Hi Jerry - no need to guess - I am a slightly familiar with what you are referring to. The 1840 processes were all attempts to protect wood against deterioration - moulds, beetles etc.

For altering wood properties you have to move forward into the early 20th century ~ 1920/30s when the first manufactured resins were formulated.

In the Munich Science Museum you can see laminated wooden propeller blades that were transmitting upwards of 700 HP. There are some WW2 Spitfires flying - still with original wooden laminated propellers.  (wooden props are still being manufactured today but the horsepower limits are now approaching 1200 HP/blade. Good stuff compressed. laminated birch + plastic)

Wooden laminates proved superior to forged aluminium blades - less vibration, longer fatigue life (near infinite), better able to withstand combat damage and lower gyroscopic forces.

 

In the 80s I was handling some "wooden" blocks - infused and irradiated - interesting product. Might have had some use in engineering but, fairly typical in new products, the maximum size of product was limited to the size of the irradiation chamber - so it just faded away.

cheers edi

 

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Actually Edi I was referring to the bold and underlined part of your text "but might just make it strong enough to allow one to carve a delightfully sleek and petite head - only limited by the necessity of the "hair-hole".

Of course the answer being a head spline.

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25 minutes ago, Jerry Pasewicz said:

Actually Edi I was referring to the bold and underlined part of your text "but might just make it strong enough to allow one to carve a delightfully sleek and petite head - only limited by the necessity of the "hair-hole".

Of course the answer being a head spline.

Hi Jerry - my apologies.

Lesson 1 for writing exams. Read the question, then read it again and then twice more  before even thinking about trying to answer it.

I did make reference to changing the grain direction for the tip.

i) make the bow-head a separate item that has the grain running vertically w.r.t. the stick. Drill a hole thought it push the stick through the hole and glue it in. (it has been done - could someone possibly turn up the picture of it - it was among some pictures from a baroque bow maker. Could do double duty as a bed-bug smasher)

I'll freely admit that I have a thing about splines.

In the early 60s I was asked if I could repair a broken squash raquet handle. The preparation went well, good fits. Unfortunately my urea-formaldehyde glue was slightly beyond its sell by date.

I handed it over to the owner and went along with her to the squash court to see how it held up. A couple of soft pitty-patts - the stick held. The game warmed up. On the first full-blooded smash the head of the racket took off. It might have cost the player a couple of glass wall sheets as it ricochet about. Strong glass though.

Examination showed a clean failure in the glue.

Always use fresh glue.

cheers edi

 

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1 hour ago, Jerry Pasewicz said:

There are some interesting epoxy compounds out there with suspended carbon fiber strands in them as well.  I am not against new materials or uses, just rather work on problems that have not yet been solved.

Have you seen Raymond Schryer's article in Best of Trade Secrets 4 where he uses an L shaped CF bar down the heel and along the length of the neck to support cello necks?

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1 hour ago, Jim Bress said:

Have you seen Raymond Schryer's article in Best of Trade Secrets 4 where he uses an L shaped CF bar down the heel and along the length of the neck to support cello necks?

Yes, there are quite a few makers doing something similar.  In fact, I think a similar L shaped bar is available commercially if I am not mistaken.

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