Part 6: Compound Radius Fretboard
Thor’s fretboard has compound radius, with a radius of 304 mm (12″) at the nut and 456 mm (18″) at the 24th fret. Typical fretboards have a uniform radius, which is less than ideal for low action because the strings must be parallel to each other for consistent string height. But they are not. The fretboard is tapered. The strings are closer together at the nut and wider apart at towards bridge.
Compound radius fretboards follow a conical surface, as opposed to cylindrical. This allows for lowest action without string buzz. This article on Stewmac goes into greater detail: Compound Radius: Explained.
Of course, we made our own jig for precise compound radius shaping of the fretboard. The jig saves time and ensures consistent results without the need for tedious sanding with a sanding block.
A router guide (top) swivels along an axis supported by two pillow blocks, one end 304 mm (12′′) below the fretboard’s top and the other 456 mm (18′′).
Here’s a quick video of the jig in action:
And the result, after fret and nut slotting (coming up next!):
Whats your method for leveling frets on a conical fretboard?
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We use a Fret Press Caul with progressively increasing radii inserts (https://bit.ly/3O5G1NU)
The idea is to precisely seat the frets such that you do not have to level them. Instead, fine tune individual fret for level using a fret rocker to make sure everything is straight and file of (or even re-hammer) ONLY the high spots.
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Hmm….I’ve explored and had a hard time with this approach. IDing the first set of high frets and leveling them individually often just leads to adjacent frets being high after correcting the first set. Have you found a way to avoid this?
Precision is the key. I think the crucial step here is precisely seating the frets in the first place such that they are more or less leveled already with only a few high spots. That is evident in the video. It requires properly radiusing the frets prior to seating and applying constant downward force. Probably also does not work on softer fretboards? Obviously, it also requires precisely manufactured fretboards. Oh and it probably also requires precisely manufactured fretwires. Perhaps some fretwires are not manufactured consistently with varying tolerances? Jescar SS fretwires are very consistent in that regard. Finally, I think leveling goes from low to high, instead of up and down the fretboard.
I use jescar SS, CNC my boards and sand them with a matching radius aluminum block, ridiculously carefully – I even rigged up my CNC to do the sanding for me to ensure perfect alignment and avoid any unwanted influence from the mechanics of the human body. Unfortunately it just took way too long so I had to try to go back to manual sanding to save time.
I get my fretwire from a supplier that pre-radiuses for me, maybe they’re not doing an accurate job with the radiusing….Ill have to look into it.
Its funny, I refused to level for years trying to make this approach work, then gave up thinking my results were subpar without leveling, now you’ve got me headed back down that rabbit hole again, ha!
This video is more instructive with tips:
I’d add to that one thing I think is also crucial: sand as little as possible or not at all. Sanding, esp. hand sanding is another potentially aggressive step that can destroy precision. We do very little sanding — just enough to perfectly smooth the surface.