Compound Radius Conversion
 

So I have a soundgear sr505 sitting in my closet collecting dust, not a terrible bass, but I honestly dont understand why so many dudes play these things. Sounds like crap, anyhow, I figured I might as well convert to fretless. I watched a series of very in depth videos on the topic and having some lutherie experience, I decided to try it out. I spoke to a luthier friend and he suggested I do a compound radius on an epoxied fretboard. sounds like a lot of work. But I'm wondering what are some advantages/disadvantages to this idea. It's not an upright, so Im curious if someone could shed some light on the topic or if if should just get a sanding block and do the whole thing at a 14" radius sans epoxy.

i see zero point in flattening the radius in this case, because as a fretless you're not gonna get your vibrato by pushing the string across the fretboard anyway (which is the advantage of flatter boards with guitars, being able to do big bends with low action).

As you probably know, the purpose of a compound (or conical) fretboard is to provide
straight fret paths beneath each string. The path beneath the central strings will be
virtually perfect on a cylindrical (standard single radius) fretboard, because they are
parallel to the fretboard axis. The outer strings are skewed though, and not parallel
to the axis. Therefore the fret will be just a little higher midway along the neck.
The question is how much higher is it, actually?

With this jazz bass:

34" scale
1.15" string spacing at nut
2.3" sring spacing at bridge
14" radius

the extra height at mid fretboard, beneath the outer strings, is:

0.0007"

In other words, that standard radius fretpath is flat to less than a thousandth of an inch.
Your leveling beam isn't going to be that flat. The 0.0000" error that a compound radius
would provide is unattainable in practice, anyway.

^^^^

I'm wid him.

Well, call me crazy but I don't see the point in putting a bunch of work into a bass that "sounds like crap."
:D

I wouldn't bother with compound radius at 14".
It could have some advantage with a guitar at 7.5" radius if fretting out was occurring.
But the advantage there would be from the larger radius at the high fret, not from the
compound radius itself.

Consider this:

Take a small straightedge and lie it along a beer or soda can, or other cylindrical object,
parallel to the can's axis. It should sit on there perfectly. The path along the surface of the
cylinder, just beneath the straightedge is perfectly straight.
Now twist the straightedge so that it is at an angle to the can's axis. The straightedge will
now rock end to end on a high spot near the middle. The path along the surface beneath
the straightedge is no longer straight but bows up in the middle.

If you were to use a cone instead of a cylinder, you would find that there are paths that are
angled from each other, they fan outward like the strings on a bass, and these paths are all
straight. The straightedge will lie along any of them without rocking.
This the purpose of the compound radius, which forms the fretboard into the surface of a
cone section. Each end of the fretboard needs to be set to a different radius, with a smooth
progression from one radius to the other. If you take the two different radius gauges and set
each one at each end of the fretbaord, you can see the fairly large amount of material that
must be removed in order to reshape the fretboard into a compound radius.

I would have to question the wisdom of removing that much wood, when the original error
from perfection was less the 1/1000 of an inch to begin with, on a single radius 14" neck.

Another way to view this error is in a neck with some relief. Suppose you set the relief to
.012" at the outer strings. The inner strings would be .0127". You are going to have many
imperfections all over the fretboard that are larger than that.
A 24" precision straightedge from StewMac is only accurate to .003" over the distance in
question. That is 4 times worse than the error in a 14" single radius neck.

Now imagine a perfect 14" single radius fretboard. Set a perfect leveling beam right down
the center of the board. It will sit perfectly flat. A string in the center would see a perfectly
straight fret path below itself. Now move the beam off to the edge, along the same path that
the outer string would have. The beam will now rock on a high spot that is less than .001" high.
In the normal process of leveling, if the beam were angled just as the strings are, that high spot
would not only quickly dissappear, it probably would not have even been noticed to begin with.

thank you very much guys, I think i've got all the info I need.

Also remember that most fingerboard inlays are very thin and changing the radius of the fingerboard will sand through some of the inlays...assuming the fingerboard is of sufficient thickness to begin with. Further, sanding that much material off the fingerboard usually means a neck shim will be necessary unless your bridge saddles have lots of adjustment left.

So here's the workaround: if you defret the bass and level the fingerboard, use a flat sanding beam for the leveling instead of a radius block. This automatically flattens the radius of the fingerboard but not to any dramatic extent.

there you go;

if you sand with this long beam "in the string lie", so that the outer areas are angled out a bit just like the strings are, you'll naturally create a slight compounding.

the mathematically miniscule tradeoff is that while the strip of board area under each string will be straight (important), the actual radius at each point up the neck might not be perfectly circular anymore (not important).