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Carbon Fiber Rods in Neck: Best Practices?

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I know that some folks have been regularly been putting carbon fiber rods in the necks of their instruments, especially cellos. I am curious about people's shop procedures for this. What shape of rod have you been using? What size? What kind of glue? Putting a veneer over it? Any experience with violas or violins? Etc.

p.s. My thanks in advance!

p.p.s.  I am not interested in another debate about the pros, cons, or whys…but I also know this is maestronet and I probably can't stop y’all!

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I think square/rectangular in cross section is what is typically used. A lot of guys rout the channel while the neck is till a block for ease. 

I'm not sure if a veneer is used. I don't see why one would be, though. 

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Here is how I do it.  I use the Stewart Macdonald .200" rod and their bit to match.  I insert the bit in a router table and make the cut in several passed.  The channel exits out the root of the neck and stops just under the nut.  If a future repair person removes the nut I want them to see the rod.  The rod is glued in with epoxy and capped by a 1mm piece of maple.  When gluing, the cap of maple is about 8mm or so to act like a caul.  After the glue is set I plane the protruding bit away so that the cap is flush with the neck.  The line that's just adjacent to the chin is the cutting limit of the bit.  You can see a line on the fence.  When the limit line hits the fence line I know that's the limit of my cut and Ill slowly raise the piece off the bit.  I have many years of experience using this type of setup and I'm confident that I will not hurt the work or myself but lifting the piece off the bit can be dangerous so please proceed with caution!

 

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13 hours ago, Thomas Coleman said:

 The line that's just adjacent to the chin is the cutting limit of the bit.  You can see a line on the fence.  When the limit line hits the fence line I know that's the limit of my cut and Ill slowly raise the piece off the bit.   

but lifting the piece off the bit can be dangerous so please proceed with caution!

Could you show a closer photo and explain again this cutting limit line?  It is an important step to know that can go awry for others who may choose to follow you and your router method - which is a good way, by the way.

Making sure I have enough heel stock height [unprofiled], I just go back the other way without lifting up the neck after a pass to the line and then shut off power to the router to make another depth cut adjustment. 

I'll reiterate for cleanliness and for safety - make several passes to get to the desired depth without losing digits/fingers in the process.  Eye protection and ear protection will enable better concentration.      B   E         V  E  R  Y         C   A  R  E  F  U  L  L.

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1 hour ago, uncle duke said:

Could you show a closer photo and explain again this cutting limit line?  It is an important step to know that can go awry for others who may choose to follow you and your router method - which is a good way, by the way.

Making sure I have enough heel stock height [unprofiled], I just go back the other way without lifting up the neck after a pass to the line and then shut off power to the router to make another depth cut adjustment. 

I'll reiterate for cleanliness and for safety - make several passes to get to the desired depth without losing digits/fingers in the process.  Eye protection and ear protection will enable better concentration.      B   E         V  E  R  Y         C   A  R  E  F  U  L  L.

Unfortunately I don't have a better photo of it.  I can try and explain a little better.  With the router and bit all set up, unplugged, I set a square on the table one edge against the fence.  While rotating the bit with one hand I slowly push the square towards the bit until in just barely kisses the bit at the apex of it's cut ( don't dull the bit!).  Then I transfer that position up the side of the fence.  This is the limit of the cutting edge of the bit.  Since my CF rod channel is blind ( it doesn't pass all the way through) this helps me to know where/when to stop feeding the piece into the cutting bit.  Since I want the cut to end under the nut,  using a square I transfer that to the side of the neck block and even around to the opposite side, the side facing up during the cut ( but just a little tick mark though).  As I'm cutting, when the tick mark touches the mark on the fence, I know exactly where my cut ends.  At this point, it's fine to just back out of the cut a few mm and turn the router off, waiting for complete stop before removing the neck block.   Hopefully this explanation is clear and doesn't sow confusion! 

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16 hours ago, Nick Allen said:

I think square/rectangular in cross section is what is typically used. A lot of guys rout the channel while the neck is till a block for ease. 

I'm not sure if a veneer is used. I don't see why one would be, though. 

The veneer is to allow future planing and truing of the neck for fb replacements etc.  Block planes will balk at CF :D

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Here's my jig, it is simple and works flawlessly.

I put them in after the fiddle is finished and the board is going on the final time.

I use a dremel with a router bit in it, to run in the jig, it is designed perfectly so no mistakes are possible.

I apply tape to the neck to eliminate glue contamination, with an extra layer of tape over the carbon fiber to allow a tiny gap between the carbon fiber and the fingerboard. In the event of leveling if necessary, which it is usually not, I don't want it to contact my tools.

I glue in the CF using a flat steel plate equipped with a vent to allow excess epoxy to escape. I've got it down to a non messy, simple, fool proof procedure.

Works for me.

Did I just call myself a fool?

better me than thee!

Evan

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Hi Thomas & Evan

Thanks for those pics.

I'll use a combination of Evan's jig (with the router table parallel to the bottom of the neck, so that the c/f bar lies within 3mm of the bottom surface of the neck) and a maple spline a la Thomas to keep it honest. :-)

Second thoughts - maybe I'll machine a mould in HDPE and lay-up an L-shaped c/f insert. That way it'll stiffen both the neck and the heel. A width of 4 mm and a depth of ~ 8mm should add oodles of stiffness*. Make the maple splines from neck off-cuts for a near perfect grain match.

* measured in Spaghettis/squ. mm.

cheers edi

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18 minutes ago, edi malinaric said:

Hi Thomas & Evan

Thanks for those pics.

I'll use a combination of Evan's jig (with the router table parallel to the bottom of the neck, so that the c/f bar lies within 3mm of the bottom surface of the neck) and a maple spline a la Thomas to keep it honest. :-)

Second thoughts - maybe I'll machine a mould in HDPE and lay-up an L-shaped c/f insert. That way it'll stiffen both the neck and the heel. A width of 4 mm and a depth of ~ 8mm should add oodles of stiffness*. Make the maple splines from neck off-cuts for a near perfect grain match.

* measured in Spaghettis/squ. mm.

cheers edi

Do violin neck heels even need stiffening? Obviously cellos, perhaps even large violas. 

For sure basses, too. But violins?

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Probably not if only being concerned with the strength.

But I've heard mentioned before about the effect that humidity can have on the stability of the end grain on the heel before,, and it does make me wonder about it.

If it were a piece of wood laying there 1 inch thick with the end grain situated on the face, it would visibly change shape if one side got wet.

 

I don't know?

Does it matter?

Does prolonged dryness raise the neck,

Does prolonged humidity drop it?

I have kept a wet rag directly on the heel of a strung up fiddle for days, seems like it might have come down, just maybe a tiny bit, but I don't exactly remember.

Didn't make a big impression if it did.

I should give it another go.

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10 hours ago, Nick Allen said:

Do violin neck heels even need stiffening? Obviously cellos, perhaps even large violas. 

For sure basses, too. But violins?

Hi Nick - I've had a few broken cello heels pass over the bench. The interesting ones were those that had been previously repaired with threaded rod and nuts, lag bolts, wood screws and a variety of glues. It's easy to do better after seeing what hasn't worked. Being an engineer and accustomed to visualising how stresses work in the material also helps.

My "dowelling" answer to failures in highly-stressed, wrong-direction-for-stress wood began when the top shop in town called me and asked me to look at a broken bow head. It was a gold mounted bow that had come in for a re-hair. After the re-hair the hairs were slightly wetted, half tightened and the bow was placed in a corner to dry out.

This was done on a Sat. afternoon after the shop had closed - all quite routine.

Comes Monday morning and HORRORS - the head of the bow had parted company with the stick! OK - it was one of the hottest weekends on record - but...

The customer demanded a equivalent replacement. The shop owner  pointed out the discoloration of the broken surfaces indicated the presence of a existing crack and that failure had been unavoidable. He offered to repair it at no charge -  but only as a measure of goodwill - he flatly refused to accept responsibility for the bows failure.

After a ping-pong of "your fault" and steadfast "denials" the bow owner grudgingly agreed to try the repair - but only if the repair was totally invisible.

I had previously shown the shop owner my cello - after I had re-bushed the  peg-holes, sleeved the peg shafts, bushed the end-pin hole and fitted my home made c/f endpin. This resulted in a "friendly" discussion about traditional practices and modern materials. I must have left a favourable impression because he contacted me and, after relating the story of the bow, asked how I would go about doing an invisible repair.

I concurred with the "existing crack theory" and because of the clean break, I suggested that we glue the head together using instant glue (water thin and only 60 seconds of "hand-clamping" to "grip") to hold the parts together, remove the lining and insert two carbon fibre pins as reinforcement.

The job went off surprisingly quickly - the longest part of the work was removing the lining and drilling the two holes for the 1.6mm dia, c/f reinforcing rods.

After buffing the head it proved impossible to say with certainty where it had failed.

The bow owner returned, closely examine the stick, played it. He wasn't happy - but had to agree that it was an invisible repair.

I was next called in to work my magic on a previously repaired cello neck...

I did a couple of calcs and settled on a 16mm dia. maple dowel/epoxy "fix".

Word escaped and I have had a quiet flow of broken instruments cellos,violins, double basses  and bows pass over my bench.

I grabbed the chance to build an instrument under Brian Lisus' guidance and spent many happy hours in his workshop.

I built my first violin - I didn't hesitate - the heel was doweled with an 8mm dowel before it had a chance to break.

Discussions followed about the latest trend of inserting a c/f bar into a neck - I did a couple of calcs and couldn't convince myself of any structural benefit. Most of the string load is expended in trying to compress the ebony fingerboard.

Still - that's theory. I'll try out a c/f angle-rod as described in my previous post and see if it makes a difference in real life.

cheers edi

 

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11 hours ago, Evan Smith said:

Probably not if only being concerned with the strength.

But I've heard mentioned before about the effect that humidity can have on the stability of the end grain on the heel before,, and it does make me wonder about it.

If it were a piece of wood laying there 1 inch thick with the end grain situated on the face, it would visibly change shape if one side got wet. 

 

I don't know?

Does it matter?

Does prolonged dryness raise the neck,

Does prolonged humidity drop it?

I have kept a wet rag directly on the heel of a strung up fiddle for days, seems like it might have come down, just maybe a tiny bit, but I don't exactly remember.

Didn't make a big impression if it did.

I should give it another go.

 

Hi Evan - I suspect that the initial drop in projection on stringing up a new instrument is the result of strain (percentage change in length of the material due to applied stress) of the heel, This is greater on the tension side of the heel than on the compression side by a factor of about 5. (Wood can withstand higher loads in compression than in tension).

By inserting a dowel you change to properties of that part of the heel that is subjected to a load in tension and reduced the amount it will strain. i.e. the fingerboard doesn't drop as much.

On moisture:

Since the dowel is encapsulated by a film of epoxy resin there will be neglible change in moisture content and hence dimensions. The heel has a film of varnish that would reduce changes in moisture content of the neck.

The presence of the the dowel with the grain running at right angles to the heel of the neck will resist any swelling of the heel due to moisture change. 

D%^& it!

I must stop just talking about this and knock up a VSO made from a piece of 2x4 hardwood, complete with endpin and bridge and run some experiments using the same neck and fingerboard.

Cheers edi

cheers edi

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2 years ago at Joe Thrift's workshop he recommended putting in a carbon rod to add stiffness and protection from warping. So, I set up my CNC to mill in the channel 6.35 mm x 5 mm deep to accept the bar shown in the photo. Joe preferred having the bar below the surface to which the fingerboard is glued. I guess that would be good for future repair work. But I wanted the fiber bar to make contact with the FB, believing that vibrations would be better served. 

The channel is a snug fit and slow-setting epoxy fills in the end voids and locks in the bar. There is also a hollow round rod running vertically in the heel from the fiber bar down to the button surface. This is 5 mm dia with a 2.5 mm hollow that keeps weight down.

I forget the suppliers of the bar and rod I found by googling. It was not Stew-Mac.

Warning: Wear gloves to avoid fiberglass splinters. 

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24 minutes ago, Michael_Molnar said:

2 years ago at Joe Thrift's workshop he recommended putting in a carbon rod to add stiffness and protection from warping. So, I set up my CNC to mill in the channel 6.35 mm x 5 mm deep to accept the bar shown in the photo. Joe preferred having the bar below the surface to which the fingerboard is glued. I guess that would be good for future repair work. But I wanted the fiber bar to make contact with the FB, believing that vibrations would be better served. 

The channel is a snug fit and slow-setting epoxy fills in the end voids and locks in the bar. There is also a hollow round rod running vertically in the heel from the fiber bar down to the button surface. This is 5 mm dia with a 2.5 mm hollow that keeps weight down.

I forget the suppliers of the bar and rod I found by googling. It was not Stew-Mac.

Warning: Wear gloves to avoid fiberglass splinters. 

Mike, I take it that you do not put a slight hollow along the length of the gluing surface of the fingerboard on the center-line? 

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42 minutes ago, Jim Bress said:

Mike, I take it that you do not put a slight hollow along the length of the gluing surface of the fingerboard on the center-line? 

Right. This means that all surfaces must be flat and stable. 

 

 

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On 5/15/2019 at 11:06 AM, Thomas Coleman said:

The veneer is to allow future planing and truing of the neck for fb replacements etc.  Block planes will balk at CF :D

This is a serious issue. Carbon fiber will trash a plane blade in one stroke.

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jedidiah-- I have never tooled up for a router and jig, and i've put a few dozen CF bars into necks. I mark out the channel, drill the length with a depth-stopped bit in a hand-drill, slice the sides with my cello bridge knife, and excavate with a chisel the same width of the channel and a light mallet. /only takes me about fifteen minutes for a cello neck after a bunch of them. I usually have the neck clamped into my patternmaker's vise throughout the process.

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3 hours ago, David Burgess said:

This is a serious issue. Carbon fiber will trash a plane blade in one stroke.

 

'Zactly!  I want my CF to cause the least problems to a future repair person as possible.

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4 hours ago, David Burgess said:

This is a serious issue. Carbon fiber will trash a plane blade in one stroke.

There are bigger issues. A plane can cut the bar along the fiber grain, but not across the grain. The issue for me is the razor sharp shavings that will stab like a knife and draw blood. Leather gloves are needed. If it is sanded, face masks should be used.

I don't want to become an apologist for carbon fiber problems, but it is no problem for me to insert fiber bars and rods now that I learned what to do. Maybe on future necks I will lower the bar and cement a wood strip over the channel. It's easy to do.

Stay tuned.

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Thank you for your sharing and for your feedback! I really appreciate it when Maestronet works well.

Chris— I was thinking of getting a router plane, like the small one lee valley makes. Power tools are a road I am not eager to go down.

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For the necks of violins and especially violas, I just use very high-modulus maple, which pretty much means high density.  To keep the mass down on violas, I might graft on a lower density scroll.

If I ever made cellos (which I never will), I would look into the L-shaped composite reinforcement like the one on this site, developed by Jim Ham.  It reinforces the heel as well as the usual neck reinforcement, although there is still the concern about differential expansion/shrinkage of the wood and carbon in the heel.

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1 hour ago, Don Noon said:

For the necks of violins and especially violas, I just use very high-modulus maple, which pretty much means high density.  To keep the mass down on violas, I might graft on a lower density scroll.

If I ever made cellos (which I never will), I would look into the L-shaped composite reinforcement like the one on this site, developed by Jim Ham.  It reinforces the heel as well as the usual neck reinforcement, although there is still the concern about differential expansion/shrinkage of the wood and carbon in the heel.

 Hi Don - thanks for that link.

cheers edi

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