That's a long title but, for those of you not aware, a guitar fingerboard is typically not flat but has a radius (except classical guitars and those are most often completely flat). Electric guitars typically have a smaller radius fingerboard and acoustic guitars have a slightly larger radius.
So how do you get that radius? Most people just sand the radius because it's a slight amount. Some people use a flat block and sand until it 'feels' right, others rig up something to use a router or sander, but I have feeling most people use a defined radius block with sandpaper attached. That way you know that if you want a 12" radius then your fingerboard will end up with exactly that radius.
I am at the point in my build where I need to radius the fingerboard but I don't want to spend $20 for a sanding block or $75 for a set of five blocks, especially since the set at StewMac only has one block I would use, the rest are too small. And since I plan on building a lot of guitars and figure I will get requests for all sorts of radii for fingerboards then another fixture was in order. Btw, I don't really build guitars... I only build fixtures, or so it seems.
This is an idea gleaned from YouTube, so this isn't completely original. Some facets of it are but basically I saw a few things I liked and just winged it from scraps yesterday. Now I can cut any radius I want and make the blocks as long or short as I desire.
You'll note the pivot point is the exact distance to the end of the router bit so I have marked 12" to 20" in one inch increments. For this build I plan on doing a conical radius, often called compound or multi-radius, and it will be 14" at the nut (headstock end) and slowly transition to 18" somewhere in the middle on to the soundhole end.
To use the fixture the block is pushed from front to back going under the bit, then turn the crank about half a turn and run it through again. After a couple of minutes there is a constant radius arc cut into the block. I will glue sandpaper to the block and use that side to sand the fingerboard to the desired radius. Simple, really. And it took a lot longer to type all this than it does to cut one block, fwiw.
Fingerboard sanding block fixture -
Adjustment mechanism -
Pivot hole increment marks -
14" and 18" radius sanding blocks -