Truss Rod Replacement
A big job. . .
This fine mandolin was made in 1977, and, apart from a seriously warped neck, it's in great shape after 25 years.In a misguided attempt to straighten the neck, the fingerboard had been planed quite thin at the nut as the instrument was refretted. While it did probably result in a straight, level fingerboard and frets, the process left the neck with significantly less rigidity. That, combined with a truss rod that was seriously damaged from over tightening, resulted in more forward bend, rendering the mandolin very difficult to play.
The fingerboard had been thinned to a point where the neck became very flexible and the truss rod had no effect at all, no matter how tightly the nut was adjusted. In fact, the peghead veneer had buckled at the end of the truss rod nut pocket from the compression of the maple neck wood. This baby had been tightened REALLY hard!
It's hard to say what caused the neck to bend forward in the first place. I think it was most likely a result of heat exposure that allowed the fingerboard glue joint to slip a bit under the immense tension of eight strings tuned to pitch. The fingerboard had originally been glued with aliphatic resin, and that glue can creep even at relatively low temperatures, and slip like crazy above 150 degrees Fahrenheit in the heat of a parked car.
My job was clear. I needed to remove and replace the fingerboard, excavate and remove the truss rod, and replace it with one set a bit deeper in the neck so it would be more positive in its action. A few years ago, Don MacRostie gave me this neat little notched aluminum caul for removing mandolin fingerboards. Here was my first chance to put it in action.
I stuck the aluminum on my hot plate and got it up to temperature, and placed it on the fingerboard.
I used a second heated aluminum block on top to keep the temperature up a bit longer. You can see my contact thermometer reads 400 degrees, and that's about as hot as I'd care to go. Immediately the rig starts to drop in temperature, so if I get distracted and forget the whole business, it will simply cool off safely without starting a fire. . .
Testing the joint from time to time, I waited until my putty knife would slip right in there. I was able to lift the fingerboard off without damage.
Now you can see what a "set" the neck had taken. I don't want to plane it for fear of losing thickness and uniformity. I'll clamp it to the new fingerboard to straighten and glue it in position.
Using my funny little homemade circular backsaw, I cut along the glue joints of the truss rod filler strip.
Then I was able to chip the maple strip out with a narrow chisel.
Here, you can see where the truss rod sat a bit high in the neck near the 5th fret. This is the position Gibson used for truss rods in the 1920s but these days we tend to set them lower for a more positive action. For more on how truss rods work, take a look at the truss rod article.
The original rod was buried very deep at the other end, and terminated at the end of the dovetail. Clearly, it was installed before the neck was glued in place. I had no hope of getting that terminal nut out without messing up the dovetail joint, so I decided to leave it in place.
The truss rod was stainless steel which is pretty tough stuff, so I chose a solid carbide straight flute drill (3/16" diameter, the same as the truss rod) to cut through the rod at its end. I made a little sleeve of brass tubing to keep the drill from wandering and cutting on the edges.
Then, I was able to chuck it up in my Makita and drill straight through the rod
After I had severed the rod, I could tighten the truss rod nut to withdraw the rod a short ways.
Once the end was free, I slipped the rod out through the nut adjusting pocket.
Here's that cut end.
I set up my old Dremel with its router base and a 1/8" router bit to deepen the truss rod channel.
Since I would never be able to get out the original end nut and since it was buried very deep in the heel, I simply filled the space above it with some new maple.
Cutting the maple plug flush with the neck surface.
My new rod has a 3/8" cylindrical anchor, so I used a Forstner bit and drilled a hole for the anchor to the appropriate depth.
The hole was far enough toward the end of the dovetail to avoid weakening the joint, and I chipped out the rest of my little filler plug.
The rod itself is a simple compression rod of 3/16" diameter steel. All that excavation had left the truss rod channel a bit worn and slightly wider than the original 3/16" so I took advantage of the extra width by adding some heat shrink tubing to the truss rod. The resilient tubing would keep the rod from rattling, and would keep any glue from causing it to seize.
I gave the rod a bend to conform to the bottom of the truss rod channel.
The rod slipped in under the peghead veneer and I was able to plug the anchor into its hole.
I made up a little torture rack so I could stress the neck backward when I glued the rod in place. Clamping the peghead and end block, I was able to bend the neck quit straight.
Then, my new ebony filler strip would help hold the neck in the correct shape after the glue set.
All trussed up.
After the glue dried, I chiseled off the excess filler strip,
and scraped the neck gluing surface very clean to prepare it for the new fingerboard. I used hide glue for this job, so it was important for me to remove any traces of contamination or oxidation so the glue would stick as well as possible.
Using the old board as a template, I traced out the profile and cut my new 3/16" thick fingerboard to the same outline.
Now I wished I had a pin router in the shop, but, as usual, it was time to improvise. In order to cut the binding recess around the fingerboard, I chose a 3/8" spiral cut end mill. Subtracting double the thickness of the binding, I picked the appropriate drill bit to use as a guide.
I clamped a piece of scrap plywood to my drill press table, and used that drill bit to punch a hole. If I didn't move the plywood, my hole would remain centered under the drill press chuck, of course.
I took the drill from the chuck, and pushed it right down through the hole until only a tiny part of the end of the shank stuck out. I'd previously ground that end flat for a little extra clearance. A drop of cyanoacrylate held the bit in place.
Now, with the end mill chucked and lowered, I had a perfect little over arm router.
Cranked up to full speed, the bit cut a nice little rabbet all along and around the fingerboard.
Heated over the electric element of my hotplate, I bent the new ivoroid binding to conform to the end of the fingerboard.
I used Weld-On #16 to glue the binding in place and wound up rubber bands to hold it overnight.
Celluloid edge dots completed the binding operation.
I center marked the inlay dot positions,
and drilled 1/4" flat bottom holes with a brad point drill. I'd inlay the dots after the fingerboard was installed and leveled.
A few strips of 1/4" wide masking tape gave me confidence that the clamping pressure would be extra strong at the edges of the fingerboard. Hide glue contains a lot of water, so it's important to realize that a fingerboard will tend to curl at the edges. Clamping only at the edges gives plenty of pressure for the whole job and takes advantage of that curling action as the center presses downward on its own.
I have a heavy piece of very straight cold rolled steel that makes a great clamping caul for fingerboards. It's entirely rigid, so when I clamped the mandolin neck to it, everything was straight and solid. Hide glue has the most amazing "quick grab" when parts are clamped together, so there was no need for any kind of locating pins or fixtures. Just feeling with my fingers, I was able to align the fingerboard on the neck accurately.
After leveling the fingerboard preparatory to fretting, I inlaid the round dots with some regular Titebond
Using an acrylic caul, I drove the dots in tight and flush with the surface. Since they will not have been heavily sanded, it's unlikely that the dots will become thinned out in future refrettings.
A few passes with a glass slide scraper cleaned up the edge and leveled the binding to the neighboring original finish. Aided by a bit of extra luck, I was able to mate both surfaces without scratching through the colored stain on the neck.
I'd put the fingerboard on full thickness along its entire length, even though the owner of the mandolin wanted to have the tongue "scooped" and left without any frets. I cranked up my old Dremel once again, and used that same router bit to lower the surface of the fingerboard in that area.
After roughing off the majority of the area, I switched to a round bur to give a more gentle transition to the last fret position.
Here's a favorite trick. I covered half of a 3/8" diameter chainsaw file with masking tape.
Then, sliding the tape against the new lowered surface, I was able to round into the end of the higher level without scratching the new flat area.
A bit of block sanding gave the scooped end a nice look. And, since I started with a fingerboard that had no frets in that area, it didn't look "modified.
Installing the frets, I was nearing the end of the job, but it would be a while before I could send this baby home.
I brushed new aged amber lacquer all along the fingerboard binding to match the original aged look of the body and peghead binding. I allowed some of the lacquer to flow onto the neck finish to fill and blend the area where I'd been working all this time.
The owner of this instrument is an expatriate Canadian, and he asked me if it would be possible to inlay a maple leaf in the bare tongue of the fingerboard. So, I called up my old friend, Judy Threet, a recently naturalized Canadian expatriate Californian, and asked her for a bit of help. Instead of simply sending me the drawing of a maple leaf I'd asked her for, she cut and sent me this wonderful green abalone inlay. What a pal!
I used my trusty Stanley #5 sanding block to level the frets with some 600 grit waterproof sandpaper.
And, I rounded the ends with my trusty cant saw file.
A final polishing with Micro Mesh, and the fret job was complete.
Here are a few shots of the final product, set up and playing well again:
