Replacing a Guitar Nut
This Gibson 335 guitar came to me because it had a few problems, "It just does not play right", was the cry. A quick look revealed some buzzing on open strings and a bowed neck. I measured the string clearances at fret 1 as: 7, 12, 7, 12, 15 and 17 thousandths of an inch, string 1 to 6. Clearly a little erratic. The clearances at the 12th fret were around 70 thou' across the board. I adjusted the truss rod and, thankfully, the neck straightened nicely. I had to raise the bridge to restore the 12th fret clearances then I re-measured the first fret clearances, they had not changed. I diagnosed a worn nut. It would be replaced.
Now, replacing a nut is not without risk and the risks come from the removal of the old one. It can cause damage to the lacquer or worse, splintering of the wood adjacent to the nut slot. This is a valuable instrument, cherished by its owner, so I would approach the job with great care and some trepidation. I planned, first to try to extract it using pincers, as in a tooth extraction but if it refused to budge with moderate force, I would saw it out.
Removing the Nut
The nut had been installed before the instrument was lacquered so, to avoid chipping it around the nut slot, it was scraped off both ends of the nut using a razor blade.
I grabbed the nut firmly with the fret-cutting pliers and gently rocked the nut back and forth. I heard the glue give way, then I lifted the nut from its slot. To my relief, it came out cleanly, causing no damage whatever. See below.
I made some sanding sticks, one for the bottom of the slot and one for the sides and I cleaned off the adhesive residue.
The bottom is being cleaned here. Great care was excercised so as not to round the slot bottom or sides.
Making The New Nut
Here is the raw material and a nut blank sawn from it. Cow bone can be obtained from the butcher for mere pence. This came complete with a joint at each end. These were sawn off and the remaining piece was boiled for about 1 hour in a water and washing-up liquid to remove the grease and other nasties, then it was sawn on the band-saw. I could equally well have been sawn by hand, it is just more time consuming. Hardness varies throughout the bone, so it is important to select the piece carefully.
The nut blank was sanded flat and square on two sides by rubbing it on a sanding block. Note; the 240 grit paper is glued to the block s that it presents a flat sanding surface.
It was then marked at the required thickness, 3/16 inch in this case and filed and sanded until it was exactly right. As the final size was approached, I measured with a micrometer and sanded the high spots off and gradually arrived at a parallel nut of the correct thickness. I managed to get it within 2 to 3 thousandths of an inch.
Here it is in the nut slot. It is a good sliding fit. It was not however, fully down in the slot. I had to remove a little material from the slot bottom corners with a small triangular file to get it fully down.
The next stage was to make a special nut-curvature marking tool. I have made no end of these and I can never find one when I want it. It is a pencil with a flat planed on one side. The flat has to be long enough to reach from the nut to fret two.
And here it is in action.
The ends were also marked and then cut off using a fine tooth saw. Bone is very easily chipped, especially in the final stoke of cutting through the section. It is extremely annoying to get this far and have to start all over again so I saw half-way from each side. The ends were sanded on the block, testing the fit for width in the slot repeatedly.
Here is the nut blank ready to be filed to it curved profile. The waste material above the line was removed with a file, leaving all of the line showing. Better to leave too much rather than too little but leaving too much height makes cutting the string slots more difficult.
Before sanding the slope at the back of the nut, the slot positions were transferred from the old nut to the new. I made sure that the orientation was right, so that the 6th string position on the original transferred to the 6th string position on the new nit ( as opposed to the 1st ). I used nut files to mark the positions. Only then did I file and sand the slope at the back of the nut.
Sanding the slope profile.
The nut then fitted in it slot with two spots of superglue applied to the bottom of the slot. I wanted it to be easy for some future repairer to remove it, should he have to. I stress that the glue was applied to the bottom of the slot. In the past, I have made the error of applying it to the sides of the slot and when I have pushed in the new nut, it has set before the nut was properly postioned. Beware! I put the strings back on the to hold down the nut, then set about deepening the string slots. The clearance required should be written down before starting. You have to know where you are going. I chose: 12, 12, 12, 15, 16 and 18 thou' from first to sixth strings.
Filing the string slots has to be approached in a calm, unhurried manner. The accuracy required is high and it is all too easy to cut one slot too deep and have to start all over again. So the order of the day is take frequent measurements and remove a small amount of material at a time.The slot bottoms should have a radius that is slightly greater than that of the string and they should have a profile that eases the string from the head angle to that of the fretboard. It should be a curve.
The first thing to do is to roughly tune the instrument then to adjust the truss rod and set the bridge height. The bridge height affects the clearance at the first fret to some degree. The string to be worked on is de-tuned a little and lifted out of its slot and parked anywhere it will stay put. Then filing with the right size nut file in a stroke that starts at the angle of the headstock and ends just short of parallel to the fretboard, the first string slot is filed. Measuring by eye at first but switching to more exact methods (more later) as the destination approaches. It is quite difficult to judge by eye, so it is best to measure sooner than later. I do no more than three stroke between measurements once I get close. I proceeded thus until all of the strings were within a a couple of thou' of the required clearance.
Here is my method of measuring the string clearance. It consists of a Dial Test Indicator (DTI) in a block of wood that is slotted lengthwise to go over the strings and crosswise to clear the fret. It sits on the fretboard and the DTI has a brass foot that presses on the string. The wires connect to a buzzer, one wire in the DTI and the other on a string and when the DTI foot contacts the string, the buzzer sounds. To make a measurement, the DTI is placed over the string and the button at the top is pressed, forcing the string against the fret. The dial is set to zero at the pointer then the button at the top is slowly released until the buzzer stops. The pointer then reads the value of string clearance in thousandths of an inch. The measurement is accurate and repeatable. In my estimation this is the only way to take this critical measurement. Before I made this device, I used feeler gauges but the difficulty is in judging when the string is just touching the gauge.
I checked the clearances again and discovered I had missed the 6th string. So did that, then I sanded a little off the height of the nut so that the strings were not buried deep in the nut. I like the wound strings to be proud of the nut and the pain strings to be just below the surface. That meant sanding the back relief and polishing the nut again. Only then did I put on the new strings and check the clearances again. A slight adjustment was made to the first string.
It is best to perform the nut-filing using the old strings as they are tightened and slackened so many times that they are likely to break. I did break the first string in this instance and that is why I had to make a small adjustment later.
Here is the final check on neck straightness. It shows my method of measuring. I think the picture speaks for itself, except to say that the device has to be checked against a straight edge before using it. The neck should be adjusted to be slightly concave. A reading of 8 thou' on my device does it.
Job Done! It now plays without buzzing and is very light on the fingers. I reckon it took me about 8 hours. I' should starve if I had to do it for a living. I hope it owner likes it as much as I do.