Mandolin neck repair: the final solution?

[Bernie Daniel]

This is an idea to make a final attempt to repair a neck break that has failed several times. This is a pretty "hard core" repair and if it fails again the neck will next stop at the nearest trash can for incineration.

The fret board was taken off and the truss rod removed in this thread.

The diagram shows the plan. Basically the empty truss rod channel will be partially filled with a 0.2X0.5X10 (inches) carbon fiber bar which will be epoxied in to the bottom of the channel using StewMac clear, slow setting epoxy.

The bar will extend from the dove tail joint, run under the nut and be embedded into the head stock up to near the end of the truss rod pocket. Thus the CF bar will completely cross the the neck break line.

To provide additional support on the top or the head stock the truss rod pocket will be filled with epoxy and a truss rod cover made of aluminum flat stock -- which will run all the way to the nut (and butt up against it) will be epoxied into place. Thus the TRC becomes a structural component of the neck repair. Obviously that TRC will never be opened again.

The CF bar will be topped with a maple bar that glued in place with Titebond and will be leveled so that the fret board can be re glued to the neck.

Clearly this is either the last neck repair for this mandolin or it is the last time this neck will be in place on the mando!

Any other thoughts on this fix?

[George R. Lane]

Bernie,

I know nothing about this type of repair, but your plan seems to be well thought out. I say go with it. I am sure there will be somebody that knows this kind of repair will chime in.

[ProfChris]

I'm not an experienced repairer, but ...

Looking at the diagram, most of the strength is in the epoxy bond to the CF bar, and primarily along the back of the neck side from the break until the end of the bar. That is what resists the string tension, along with some compression in the aluminium truss rod cover.

Would a backstrap, bridging the neck break, be a useful addition? That would give you wood in tension along the grain, which would be pretty strong and also not subject to the leverage pressure on the CF bar/neck glue join.

Or is that either overkill, or too much of a change to the originality of the neck?

[John Arnold]

The backstrap repair has worked every time for me.........even after multiple previous failed attempts.

[HoGo]

The backstrap repair has worked every time for me.........even after multiple previous failed attempts.

+1. Backstrap adds more strength where it is needed than all the epoxy/carbon/aluminum together. I think 1/8" thick maple piece would be more than enough.

[Bernie Daniel]

+1. Backstrap adds more strength where it is needed than all the epoxy/carbon/aluminum together. I think 1/8" thick maple piece would be more than enough.

Thanks! But the truss rod is already out so that train has left the station? I think anyway?

[Joe Mendel]

I'd go with the backstrap anyway, even with the truss rod gone. It will add a lot of strength.

[burt50]

What is the back strap repair?

[Bernie Daniel]

I'd go with the backstrap anyway, even with the truss rod gone. It will add a lot of strength.

Seems like a prudent idea.

[Bernie Daniel]

So the consensus is no one but me is excited about carbon fiber.

I realize that the key to "the fix" that I propose is getting a strong bond between maple floor of t he truss rod channel and the carbon fiber bar. My research lead me to believe that a strong bond between epoxy and CR is possible as it is used in both auto racing repairs and the aircraft industry and of course epoxy does bond strongly to wood.

However I am not locked into filling the truss rod channel with carbon fiber. Here is an alternative idea.

Instead of filling the channel with CF suppose I fill it with precisely shaped a bar of hard maple or some other wood that that has a very high bending modulus of elasticity -- e.g., birch, hickory or black locust. That way for sure I can get a very strong bond between the bottom of the channel and the wood re-enforcement bar?

Thus I effectively am converting the neck to a solid wood -- mandolins were made for years that way and did just fine? In fact going to a truss rod weakened mandolin necks IMHO?

[Bernie Daniel]

What is the back strap repair?

Essentially cutting out wood on either side of the break and, gluing in fresh wood and then reshaping it.

[multidon]

I think you are on the right track with your repair plan. Since the joint has already failed more than once, I would go with your original plan, and add the backstrap as well. With all three things working for you, the rod, the backstrap, and the aluminum plate, you are throwing everything you have at it. Hard to see how a repair like that would ever fail. The key is going to be to use a really high quality marine grade epoxy like West system or similar. Hardware store epoxy is not as strong as it could be due to the addition of unnessesary filler. They do this to dumb it down for the average joe. Hardener is actually a tiny amount when compared with resin, but the hardware store versions add filler to the hardener so “equal amounts” are used and it’s easier to measure, but the resulting product is weaker.

[Drew Streip]

My very amateur thought is that there are four surfaces of the crack: outside back, inside bottom of truss rod channel, inside "roof" of truss rod channel, and outside front of peghead.

Your CF rod fixes / reinforces inside bottom and inside roof; and your epoxied metal truss rod cover reinforces the front of the peghead.

So 3/4 of the crack points would be fixed, but the back of the neck could still be vulnerable to separating with another jolt plus the string tension. The backstrap would reinforce that weak spot. Then you'd be positive that the neck was stronger than original.

[Jeff Mando]

Bernie, extending the carbon fiber rod into the break area is a good idea, as well as using the new aluminum truss rod cover as a "mini front strap" to add strength, but the success of the repair is in keeping the break area from bending again under string tension, since the break has basically become the fulcrum of that force. A back strap may help since it counteracts the string force.

Even if looks were not an issue, this would still be a difficult and iffy repair, IMHO -- especially considering the previous repair failures. However, I have seen this type of repair done sucessfully with two scarf joints cut for maximum surface area for glue and adding a new section of neck wood basically going from around the 3-4 fret area and extending up into the tuner area, replacing the break area. Obviously, a major repair -- but you could add a new truss rod this way, your peghead overlay would hide the front scarf joint, the back of the neck and peghead would show the new scarf joints, which could be hidden with touch-up and shading. This is probably MORE work than actually making a new neck, so you have to decide how much you love that neck.

On the brighter side, you seem to like working on instruments and this mandolin has provided you with much experience and practice!

[Jeff Mando]

So the consensus is no one but me is excited about carbon fiber.

Thus I effectively am converting the neck to a solid wood -- mandolins were made for years that way and did just fine? In fact going to a truss rod weakened mandolin necks IMHO?

In my way of thinking, going from a truss rod to no truss rod is a major "demotion" for lack of a better word -- carbon fiber or not......

[Bernie Daniel]

In my way of thinking, going from a truss rod to no truss rod is a major "demotion" for lack of a better word -- carbon fiber or not......

Thanks for the comment and thoughts. Funny you should say that I was over at Will Kimble's shop this morning looking at his new mandolins and talking about this repair. Basically he said the same thing you just said. He had a period early in his building when he experimented with a no-truss, carbon fiber bar and eventually abandoned the idea.

Another thought would be to get my hands one some of the metal T-bar that guitar companies used to use before the days of truss rods. Some mandolin builders used it too?

One thing I have not puzzled through yet and why am surprised by the doubts expressed here by you and others who have posted is that I looked a bit into possible materials to use in stiffening the neck.

If you consider a property like Young's Modulus (aka the Modulus of Elasticity) which is a parmeter that quantifies the stiffness of an elastic material. You find the following (all units in 10^9 N/m2 or Gpas (GigaPascals):

Pine = 9
Oak = 11
Aluminum = 69
Carbon Fiber = 181
Steel = 200

So there you see that CF is over 10 times as "stiff" or resistant to bending as something like a good hard wood (maple is a little, circa 10% more stiff then oak?).

What am I missing here? The key is a good bond between the bottom of the truss channel and the CF - if I get that I don't see why I am not "golden"????

[pops1]

I think you would need a good bond not only on the bottom of the carbon, but the sides as well. I was going to suggest the same repair as Jeff, but didn't get around to it and am glad someone suggested it. Adding the carbon is a good start, but I would also agree to adding a backstrap to ensure the repair holds. If you drill some divots in the sides of the carbon it will give the epoxy something more to hold onto when bonding to the wood and carbon.

[Jeff Mando]

I think the carbon fiber rod will do a good job to stiffen the neck and keep it front bending. That is not your problem here. You have to prevent the peghead from bending under string tension. You can't get the cf rod to extend past the break enough to eliminate that possiblility, IMHO.

You've already removed the fingerboard and the truss rod, PLUS the peghead is already broken, so if it were me I'd go ahead and try the scarf joint and add some new wood. If you don't want to deal with two splices, you could just cut around frets 3-4 and add a new neck section and peghead. That way the scarf joint would be bolstered by the fretboard and you can cut a new truss rod channel in the new neck section, THEN decide if you want to go with cf or a truss rod.

Just my 2 cents, but you might want to watch some scarf joint repair videos, before you try to repair what you've got.

[Bernie Daniel]

I think the carbon fiber rod will do a good job to stiffen the neck and keep it front bending. That is not your problem here. You have to prevent the peghead from bending under string tension. You can't get the cf rod to extend past the break enough to eliminate that possiblility, IMHO.

You've already removed the fingerboard and the truss rod, PLUS the peghead is already broken, so if it were me I'd go ahead and try the scarf joint and add some new wood. If you don't want to deal with two splices, you could just cut around frets 3-4 and add a new neck section and peghead. That way the scarf joint would be bolstered by the fretboard and you can cut a new truss rod channel in the new neck section, THEN decide if you want to go with cf or a truss rod.

Just my 2 cents, but you might want to watch some scarf joint repair videos, before you try to repair what you've got.

I think you would need a good bond not only on the bottom of the carbon, but the sides as well. I was going to suggest the same repair as Jeff, but didn't get around to it and am glad someone suggested it. Adding the carbon is a good start, but I would also agree to adding a backstrap to ensure the repair holds. If you drill some divots in the sides of the carbon it will give the epoxy something more to hold onto when bonding to the wood and carbon.

Thanks for the thoughts -- every bit of additional perspective is helpful!

I'll reply to both comments as similar thoughts were expressed.

Yes I agree that getting a very strong bond between the maple channel and the CF bar is the key to getting any benefit out of the bar. I am pretty confident that should be possible. In addition to looking at resistance to bending I also did some looking into epoxy adhesives. It has already been mentioned in the thread that one should not use commercial epoxies as on those the polyamine catalyst is usually diluted to a concentration that will permit it and epoxide binding agent to be at the same 1:1 molar ratio when mixed equal volume. I think Timbo mentioned the West System products.

The West System products see very good and their Six10 product has huge bonding capability for both wood and CS. In additon it dries hard and has a high modulus of elasticity -- so it seems to touch all the bases. There is another very similar product called Thiox2:1 that is a little cheaper.

Also agree on scaring up the CF bar. The tech support from one of the epoxy companies I talked to suggested scaring the CF with very course grade sand paper -- the kind used for floor sanders.

I'm not at all afraid of doing a conventional back strap. Whether I want to get into trying to match up two scarf joints so as to end up with exactly the right length of neck is something I would need to think about.

[HoGo]

Looking purely from static viewpoint, the string tension wants to bend the neck and headstock the bending creates tension on the outside of the bend and tension on the inside with neutral axis somewhere between (depending on exact crossection and materials) basicly anything that is close to neutral axis has very little effect, the further from neutral axis the more effect the material has. your CF ir right at the axis but the backstrap is as far as can be so it's effect is larger because that distance, not strength of wood. Straight grained wood strap is more than enough strong for this application and going to exotic numbers of CF is not necessary. On the opposite end the compression strength of the glued break is not much smaller than that of unbroken headstok so there is no real need for additional reinforcement.
If I were repairing that I would just glue it back together and add 1/8" backstrap going at least 2" in each direction from break. (tapering to nothing on both ends)

[Bernie Daniel]

Looking purely from static viewpoint, the string tension wants to bend the neck and headstock the bending creates tension on the outside of the bend and tension on the inside with neutral axis somewhere between (depending on exact crossection and materials) basicly anything that is close to neutral axis has very little effect, the further from neutral axis the more effect the material has. your CF ir right at the axis but the backstrap is as far as can be so it's effect is larger because that distance, not strength of wood. Straight grained wood strap is more than enough strong for this application and going to exotic numbers of CF is not necessary. On the opposite end the compression strength of the glued break is not much smaller than that of unbroken headstok so there is no real need for additional reinforcement.
If I were repairing that I would just glue it back together and add 1/8" backstrap going at least 2" in each direction from break. (tapering to nothing on both ends)

OK understood.

So would you go with option A equal strap on both sides of the break or option B putting more glue in play above the break?

[rcc56]

I would extend the backstrap at least 1" above the upper edge of the old truss rod pocket.
My point of view is that I would want the backstrap to extend over plenty of solid, undamaged wood in both directions.

[HoGo]

I would keep the lines smooth and use bent piece of maple (straight grained). You can extend the backstrap all along the headstock to make its end less visible there (Frank Ford has such guitar repair on his site) That would be easiest to do if you have drum sander - just add shim at the end of headstock to keep taper (if present) and sand away 1/8" from rear of headstock and then smooth the transition into neck at least 2" past the break. Bend long piece of maple rib stock to fit the curve and glue it in place with strong fresh HHG and flexible cauls. Trim excess wood and done.

[Tom Haywood]

These computer drawings are great. What are you guys using?

[HoGo]

Bernie's drawings are photo of the Siminoff drawings with lines added (I have no ide what program he used). I saved one of his, removed lines in Photoshop and drew in new curves in Photoshop as well. But for drawings I use combination of Photoshop and Illustrator or Rhino for 3D modelling.