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Technically… this wouldn’t be a true compound radius. The best description I’ve heard was when someone called it a “Conical trim”. I’ve done the math on this and with (for example) a 12” radius, it’s 12 @ the nut and gradually decreases until it becomes maybe 11.5 somewhere around the 8th or 9th fret and then back to 12 by the time you get to the end of the extension area. I haven’t looked at the effect on too many radii but I suspect the smaller the radius, the more it changes at fret 8-9. This is of course a theoretical, mathematical assessment of the result but I have found them to be 90% (or more) true when put into practice.

I have never understood people that claim you have to keep the beam parallel to center with a cylindrical radius. The string will never be parallel so why would you do that?

This info came from a forum post a friend of mine directed me to on Talkbass years ago when I was making my first compound fretboard. Since we’ve gone deep, I figure it’s appropriate to share.

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Bruce Johnson writes -

I think this discussion is getting confusing because you guys are missing one element. I'll try my hand at explaining the geometry here:

First, a normal, typical bass fingerboard is cylindrical. That is, it has the same radius at the nut as at the heel.

However, as described above, the strings aren't parallel. They are closer together at the nut than they are at the bridge. Even though the radius of the undersides of the strings may be the same at the nut as at the bridge, the underside of the strings actually describe a slightly different shape than a cylinder.

The strings going right down the center of the fingerboard will be true. That is, if you lay a straightedge down the centerline of a normal cylindrical fingerboard, it will lay flat. However, as you move the straightedge off to the side at an angle to the centerline, following the actual paths of the outboard strings, you'll find that the center will be high. The straightedge will rock on a high spot in the center, as if the neck were backbowed. The further off-angle to the centerline you go, the worse this condition will be. Also, the smaller the fingerboard radius (rounder) is, the worse it will be. If you cut the fingerboard to a pure cylinder, you'll end up with high-spot buzzing on the outboard strings, but not the center strings.

So how do you trim the surface of a cylindrical fingerboard to correct for this problem? The process is just like Musiclogic described above; you level-file right along the actual string paths, blending in between them. This process is often called "conical filing", but it isn't really forming a cone shape. That's what gets confusing. It's actually forming a slight "hourglass" shape. The radiuses at either end of the fingerboard are untouched. You are trimming away wood at the middle of the fingerboard (lengthwise), but only on either side of the centerline. So, the radiuses at the nut and the heel may be 12", but the radius at the 7th fret will be slightly less, like 11 3/4". I prefer to call this "hourglass filing" to minimize the confusion.

On a fingerboard that has been properly "hourglass filed" like this, a straightedge placed along all of the string paths will be dead flat. When the straightedge is placed parallel to the centerline, but off to either side of center, there will be a slight gap under the middle, a "relief". But, down the center, there's no gap. This is where owners often get really confused when trying to check the relief and adjust the truss rod. A high quality, hourglass-filed fingerboard can give you confusing relief readings if you don't understand what you are looking for.

If you want to get really technical, the real mathematical description of this hourglass-filed fingerboard is.....wait for it.....an offset hyperbolic paraboloid! I'm sure that brings back some frightening memories from your school days. Picture an hourglass, where you take the narrow waist and push it off to the side enough that one side becomes straight. That's what an "hourglass-filed" fingerboard looks like. The centerline of the fingerboard is right on that straight side. The further off to either side you go, the more hourglass-shaped it becomes. Try not to get a headache.

So, how does this relate to compound radius fingerboards? A "compound radius" fingerboard is just another term for a conical-shaped fingerboard. That is, the radius is larger at the heel than it is at the nut. The surface is a section of a cone. Basically, the more conical you make the fingerboard shape, the less need there will be for the "hourglass filing" correction. There is a point where the surface becomes conical enough that no hourglass filing is needed. However, most commercial compound radius fingerboards aren't that radical, and still need a little bit of correction to make them flat along the string paths.

What confuses things even more is that instruments built with compound radius fingerboards often also use a flatter radius on the bridge than on the nut. So, the underside of the strings are also on a conical shape, but they still aren't parallel, so the fingerboard surface still ends up needing some hourglass-filing correction. Don't hurt yourself picturing that one.

How does this relate to frets? Exactly the same way. A really good fretjob has this hourglass-filing correction as part of it. The typical process involves using a straight file or diamond stick or oilstone, working along the string paths and blending in between. Leveling frets with sandpaper on a radius block is only a way of roughing them in. It takes an extra step to make them really true for the strings.

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"I'm not going to say that perfection has never been achieved. However, if it has, it probably went unnoticed due to it's lack of character."

I've been skimming this thread and the real gem hidden here is Hesh's description of tweaking the headstock during fret leveling to create more relief on the bass side. I got a chance to try it out yesterday on a Martin D-16GT that came in for a refret. It has a plastic (Richlite) fretboard and bridge which was a bit of a surprise, but turned out to be fairly easy to work on. Anyway, I did the little dance with the headstock and the final relief was .003" on the treble and .008" on the bass side. The setup turned out great and it plays like a dream. Thanks to Hesh for helping an old dog learn a new trick.

Barry, love the padded string winder. Although I never thought about doing that, I completely understand why you did.

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"I'm not going to say that perfection has never been achieved. However, if it has, it probably went unnoticed due to it's lack of character."

You are very welcome Barry, it's a Dave Collins innovation I just spilled the beans at my ole Lutherie alma mater forum. Good term too it is a "dance" of sorts and kind of cool moving across the board transitioning from a gentle pull to a gentle push of the head stock. I call it the touch as well.

It reminded me of that old child stumper, "pat your head with one hand while you rub your tummy with the other".

If a guitar player has got to a point where the preferred string guages are known and the fine points of setup are coming into play, I like to tune the strings down one to three frets. Then when I play just a bit harder than normal I want all the notes on every string to fret buzz equally. (With a Gibson style electric bridge I lower the bridge slightly and/or tune down.)

The notes that buzz too easily are on a low fret that is being "shadowed" by higher frets up the neck. I go through each string using one color of marker on the low frets, another color on the high. The dots of color show areas that need to be filed and not filed. Then I carefully use my leveling tool to remove just the high markings. Like SlightReturn says, the less metal removal the better. It gets tiring doing this over and over, but the guitar ends up more even from note to note.

Hi Leonard - We have a completely different approach and philosophy to truing up/leveling a fret plane.

First we only address the fret plane as a whole and never go after individual frets. Using bluing we level until all frets are in the same plane and the bluing disappears everywhere at the same time. Then we further impart shapes we want, relief and how much, fall-away etc.

You sound like you find the high spots and take them out. It's one approach but we could not get the precision that some of our clients demand your way and I speak of the very low action players. I'll add that using string lash as you do as a tell tale where we use ink is problematic because not all frets are equal distant from the end points of a specific string. Or, more specifically you will receive almost no string lash in frets 1 - 4 but much more string lash beyond that with the most being in the center of the speaking length of the string. This makes for some high spots not getting noticed and others being amplified to be higher than they actually may be if they are under the middle of the speaking length.

It may work for you and the results that accept but with no disrespect intended my friend this would never provide the results that our clients demand and pay for. We pride ourselves on our precision and are well known for our fret work which we have taught to others including well established professional Luthiers with hundreds of builds. They tell us our method upped their fretting game considerably.

We also can shape in relief with our method and fall away which can be important to many players, not all.

You also said what you do get tiring doing it over and over. We never have to revisit an operation in our method and we get to move on, finish it, next. I can use our methods and usually do a fret dress and full set-up in an hour and a half.

With all this said we do not recommend a piece meal approach to fret leveling and instead favor always viewing and working on the fret plane as a whole exactly as the strings see it.

100% agreement. Once I have a flat fret plane I take care to work each fret equally and to remove as little material as possible to prevent over-working individual frets. This not only prevents uneven frets but it also goes really quick.