With the nut polished and glued in place it's time to tweak the nut slots down to their final depth. The final depth depends on the guitar and the players requirements and is measured as the clearance between the bottom of the string and the top of the 1st fret. I was taught to start with a 0.5mm gap for the bass E string and a 0.4mm gap for the treble E string and then work down from there. When starting out it's best to use a set of feeler gauges to measure the gap. Once you are at that point you can go lower but it's a matter of playing it by ear. Pop the string you are working on out of the nut slot and make a couple of gentle strokes with the saw or file. Then pop the string back into the slot and test. When the string buzzes against the top of the 1st fret, you've gone too far. At that point you can either start again with a new bone blank, or you can learn the art of filling the slot and re-cutting it ;)
Having cut hundreds of nuts I no longer use feeler gauges, I do it all by eye now. And one handy hint is to polish the 1st fret so it clearly reflects the string. I watch the gap between the bottom of the string and it's reflection as I work lower. That gives a very good visual guide. I also pluck the open string and watch it's reflection in the 1st fret as that gives the best visual indication that you are approaching the point where it might start buzzing. I also fret the string behind the first fret to see how it feels. It should be no harder to press down than the same string barred at, say, the 5th fret and then fretted at the 6th fret.
Getting the nut slot depth right is really important. Most mass produced guitars leave the factory with the 1st fret action too high. The reason is simple economics, the skilled labour and time it takes to do it right cost too much and when it's done by semi-skilled labour the cost of removing and replacing the nuts where they cut too deep is even higher. The factories operate with such small margins that the cost of replacing a badly cut nut could be more than they make in profit on a guitar.
Now, once the slots are at their final depth it's time to finish the job properly. And that's what I meant by The Holy Grail. Far too often people don't pay enough attention to the cutting and finishing of the slots. I've drawn a few examples on a whiteboard and photographed it, as it was a lot quicker than trying to do the same on PC...
A/ shows a slot cut flat. The problem here is that when the strings vibrates up it lifts away from the bottom of the slot and buzzes in the slot.
B/ shows the slot cut to match the break angle to the machine head. While it solves the problem in A/, it introduces problems of it's own that are less obvious but just as difficult. The first problem is that you can't guarantee the break angle to the machine head, if the string doesn't wind down the post low enough it no longer touches the slot at the back and can buzz in the slot again. It also puts a lot of pressure on the front edge and over time it can wear it down. That will lower the action further until it starts buzzing on the 1st fret.
C/ is pretty much ideal, it ensures that the string is pulled into the slot at the front and rear avoiding buzzing and also minimising the risk of wear on the front edge lowering the action.
In D/ I've tried to show how the bottom of the nut slot is still rough after you finish with the saw or file when you examine it closely.
That roughness is a cause of friction and can lead to the string sticking in the nut even though the slots are at the correct width. Regrettably, a lot of techs will cut the slots with saws and files and leave them exactly as the tool finished them. However, a decent luthier will go a stage further and finish the slots properly. The most common way is to use a piece of very fine wet & dry paper folded in half. The fold is inserted into the slot so that some of the grit polishes the bottom of the slot. But that is far from ideal, and the paper is too thick when folded to use in the plain string slots.
About 10 years ago I was studying jewellery and silversmithing and came across the Jewellers Mop made from a piece of string.
It's simply a piece of string doubled over, tied in a stopper knot just below the bend and with the individual threads teased out below the knot. Jewellers use this with abrasive paste, like rouge, to smooth and polish fine fretwork (small holes cut into metal).
I put 2 and 2 together and started using a mop with Brasso to polish fret slots so that they are nice and smooth.
Just like that. And you can see from the earlier pics how well Brasso polishes bone.
This is something I've kept to myself for a long time, but it is why I have so often been insistent that a well cut bone nut is as good as anything else available. And in the last few years StewMac have started to sell abrasive cord
for exactly the same purpose. However, the cord is nowhere near as versatile as the jewellers mop. You can load a mop with whatever abrasive you need and it will go far finer than the cord too.
So, there you have it. How to make and replace a bone nut in next to no time. It's as good as any alternative material in performance and sounds better. And my personal record for making and replacing a bone nut for a strat is 15 minutes (that was with the use of my disk and belt bench sander). And yes Jef, I was thinking of trying to beat that record at the next Analog to Digital ;)
Any questions, please feel free to ask.