Compound radius jig

[Jim Hilburn]

When I built this jig a couple of years ago I posted photo's of it but not in the context of how it works and I recall it only made for more questions than it answered. I was doing some boards today and took a few more shots to show how it works.
Here is the fingerboard stick-taped to the platform.

[Jim Hilburn]

Here's the whole rig on the height-adjustable table on the sander. Grizzly sells this sander with a radius jig included but it was expensive. This sander like so many of their tools was very reasonable although the table isn't exactly rock solid.

[Jim Hilburn]

You can see the different heights of the swingarms which is what makes it cut a compound radius. I put a bock of wood to hold it in it's extreme swing so you can see how it makes a tight arc at the nut end and a wide sweep at the tail.
I made it completely adjustable but I've left it just like this since I made it.

[Jim Hilburn]

Here's the fingerboard right off of the jig. 7.25" at the nut.

[Jim Hilburn]

Here's 9.5" between the 8th and 9th fret.

[Jim Hilburn]

And 12" at the end of the board.
This allows for a flatter radius at the bridge so your not picking into a strong radius. Also seems to just feel good to play.

[mythicfish]

"I see." said the blind man, as he pick up his hammer and saw.
Good on you! And your friend, Euclid too.

Curt

[Jim Garber]

Jim:
You definitely have to have some sort of warped...er...radiused mind to create that jig. Very impressive. I would love to see and play the result of that compound in person one day. It sounds like it works very well.

Jim

[amowry]

Very nice, Jim. Not to hijack your thread, but you inspired me to share the solution that I decided on after talking to you about your jig last summer. This was based on a jig someone had posted photos of on mimf.com, although the original was built to work with a large edge sander. Mine works vertically with my little cheapo Delta 4' belt/6" disc sander, which is just large enough for a mandolin board.

The heart of the jig is a rigid plywood support frame that bolts to holes tapped in the far side of the sander body. The large cherry block in the center has replaceable radiused inserts on each end that ride on the little tongues on the plywood frame. You can use a different insert on each end for compound radii, or you can use two with the same radius. You take into consideration the height of the insert above the sanding belt when you make it, so for a 12" radius fretboard I would make an insert with roughly an 11 1/4" radius.

In use, you double-sided tape the fretboard to the bottom of the block, put on whatever inserts you like (adjusted to the corect height to give you the proper final fretboard thickness, drop the block into the vertical grooves, and rock it back and forth until it stops removing wood. I've been surprised how well this thing works, and I'll probably adapt to fit a bigger sander when my Delta finally kicks it.

[amowry]

Here's the end of the block, showing the fretboard and the radiused insert.

You can't see it, but the back of the plywood support frame is a torsion box, to make it plenty rigid.

[brunello97]

Nice jigs guys! Thanks for posting such clear photos.

Andrew: do the plywood 'tongues'-as you call them-cantilever out over the sander or are they supported on both sides?

Mick

[amowry]

Mick-- they just cantilever out, which is why I wanted to make the back portion of the support plenty strong. There isn't any noticeable flex. Now that I think about it, I suppose they could be bolted to the front of the sander by an additional support, as well.