Repairing a Broken Neck – Carbon Fiber
Some builders will tell you that it is not worth the effort to repair a cracked neck – that it is more cost effective to simply replace it. My first two or three attempts at neck repair were not successful so I can understand their doubt and frustration.
That said, I have not had to redo any warranty work when I have used Carbon fiber in new construction or to reinforce a repair. I think this method is one of the quickest and least invasive ways to repair a broken neck. It is frequently used in pedal harp restorations as the best insurance for prolonging the life of a renovated instrument.
Photo-The underside of a harp neck reinforced with a carbon fiber patch. Some builders cut a recess with an inlay bit and cover it with wood veneer to hide it. I have to get below the harp to even see it.
So, while we have not had the 20 years needed to verify whether this technique will stand the test of time (and there are precious few builders that are willing to guarantee their NEW harps for that long!), I think it is showing a lot more promise than splines, re-laminating, dowel pins, biscuits or the other techniques I have seen or attempted.
Basically, there are 8 steps
1. Remove the strings, and strip off the hardware near the break
2. Build a jig to effectively clamp the break
3. Glue the break
4. Scrape the underside of the neck
5. Apply carbon fiber in Epoxy to the bottom of the neck
6. Re-drill pin and screw holes
8. Re-mount hardware and re-string
On average, this repair will take about 15 hours of labor and less than $20 in materials.
The typical break looks something like this:
Strip all the pins and levers from the first two octave of strings. I usually write the lever number on the bottom of the base with a Sharpie® pen so that I can put the lever back where it came from. In a perfect world, the neck would be removed from the sound box. Some builders glue their neck (along with the pillar) to the sound box though. It is a bit more difficult to hold the crack apart and apply glue, so it is not absolutely necessary to remove the neck.
Breaks rarely fall along lines that are convenient to clamp. The best orientation for clamping would be perpendicular to the break, centered right over the break area:
First cut four small 4x6” rectangles of scrap plywood. ¼” BC Fir or cheap Luan will work fine. Place them under the neck as shown. I trace the outline of the neck onto the plywood and use a center punch to mark the holes for the tuning pins onto the plywood underneath (the two pins are marked with an X in this drawing). I also put two pieces of tape centered over and perpendicular to the break. This helps me visualize where the jaws of the clamps will be and where the pressure is should be centered as the jigs are built.
Next, I cut two blocks. The blocks are just a bit thicker than the neck are cut to the same contour of the neck. The blocks will be glued between two squares of plywood to make a sandwich. A tuning pin will pass through the outer layers of the plywood and through one of the tuning pin holes in the neck to keep the jig from sliding as the clamping pressure is applied.
At this point, I also stack each piece of ply together with its mate and drill a 3/16” holes through both at the same time so that they will be aligned.
Side View of the “sandwich” with the pin in place:
Then I clamp the sandwich together and ream the holes for the tuning pin. Finally, I glue the sandwich together with the tuning pin in place. It doesn’t hurt to back up the glue with a few sheet rock screws.
Just about ready to glue the crack together - This is what it should look like with the jigs and pins in place.
To minimize damage to the finish on the neck, cut some padding and place it between the block and neck. A few layers of cloth from an old cotton T-shirt, even a folded sheet of paper towel will help.
One nice feature about the jigs is that they can be built to provide access to clean up glue that will squeeze out of the joint or into the pin a screw holes of the neck
Before applying glue, do a dry run and to make sure the joint fits tightly. Sometimes it is necessary to pick off a few broken wood splinters. Mask off the neck to keep epoxy drips and fingerprints from the finish.
I use 2:1 epoxy (See end of article for epoxy sources) to glue the crack and later to laminate the carbon fiber. Mix up a 15ml batch of epoxy and dab it onto both surfaces of the crack Then mix fine sawdust (wood flour) into the remainder of the batch so it has the consistency of Sunday gravy. Spread that on the surface to be joined. Clamp the joint together and clean up the excess epoxy. Use a tooth pick or thin stick of wood to remove the gobs of epoxy that are pushed into the holes for the tuning pins and screws for the levers.
I leave the extra epoxy in the mixing cup. That way, when I am tempted to touch the joint to see if it has cured, I can touch and flex the epoxy In the cup instead of messing with the joint. After the epoxy is has cured hard, (5-15 hours). I use remove the clamps and clamping blocks and use chisel and scraper to carve the excess epoxy away and clean up the glue line.
Use the scraper to remove the finish from the underside of the neck. You will want to bond the carbon fiber to bare wood.
I cut a 1.5” x 14 inch strip of unidirectional carbon fiber ribbon with a sharp pair of Fiskars (source for carbon fiber listed at the end of the article). I try not to handle the CF too much – the fiberglass binding weft starts to unravel.
It helps to have gravity working with you to pull the carbon fiber down onto the neck and to wick the epoxy into the fibers. Get the bottom of the neck facing up.
Mask the sides of the neck to keep drips off of the finish. Mix up about 25 ml of epoxy and spread a coat of epoxy on the bare wood. Place the Carbon fiber into the wet epoxy. The carbon fiber will still slide around, but the epoxy wicking up from underneath with helps hold it onto the neck. Spread more epoxy onto the top of the Carbon Fiber tape. At this point, don’t fret to much about how it looks. Focus on making sure all the fibers are wet out with epoxy.
If the curve on the underside of the neck is tighter than a ¾” radius, the carbon fiber may be stiff enough that it wants to lift off. If this is a problem, clamp a scrap of wood or dowel to push the carbon fiber against the neck. Cover the scrap of wood or dowel with packing tape. The epoxy does not stick very well to the packing tape, and you can usually remove the it in an hour or two as the epoxy begins to gel.
After the epoxy has cured, remove the masking and scrape epoxy globs off so the underside of the neck is smooth. The carbon fiber comes out a nice jet black and is fairly inconspicuous on the underside of the neck.
If you remembered to clean these holes out when you glued the neck together, you will find this fairly easy to do, and the bits will stay centered in the holes.
Follow the regular procedure here. With careful masking, the area to refinish is small, only ¼” or so on either side of the crack.
Sources for epoxy and Carbon Fiber:
Your local hardware emporium will sell general purpose epoxy in tiny tubes. Typically it is thick and does not work well for laminating on Fiberglass or carbon fiber. If you use it, try to find the high strength (slow cure) formulation. System Three, Raka and West started as mail order suppliers to the boat building trade. Their epoxy is thinner is specially formulated to work with wood. The builder can control the consistency of the glue by adding fillers and thickeners.
I like to use RAKA products. They are the most reasonably priced of the three, their service is excellent, and they carry carbon fiber, Kevlar and a wide range of fiberglass cloths and tapes. For harp necks, I use their unidirectional wide warp 5.2 oz carbon tape. It comes in a 6 inch width, so a single yard will be enough to reinforce 6 harp necks, which is a pretty good deal for $7.75.
Raka’s smallest lot of epoxy is 1½ Quarts ($28) which is overkill if you are just doing a single neck repair. For smaller lots, System Three offers a much better deal. Their $20 trial kit includes 12 oz of epoxy, gloves, measuring cups, additives and a $10 certificate towards a future purchase.