Compound Radius fretboard jig

Ive been thinking about this for a while, looking at different designs on the internet. Finally decided on a design concept. This is the first run, it is still a work in progress. The way it is set right now, it is not adjustable, it is set for a 12" radius at the nut, and a 14" radius at the end of the board. I wanted to feel it in action before I went any further. It works very well. I ran 2 pieces of wood through it, the first one was 0.030" different from one edge to the other, due to me not locating the true center of the face of the fret board plate. The second board, with a little adjustment and more care, was within 0.010" throughout, meaning that the edges are the same on bopth sides, and at both ends. The fret board face of both boards is excellent.. The radius of both is very good, and a precision straight edge held on the board shows no light. The radius matches the stew mac radius guage, at least the 12" end does. The 14" end looks like a 14" radius when compared with the radius guage. I was working with 120 grit.

The feel with the sander is smooth, easy, feels right, plenty of control.

The plate that holds the fret board is made from Richlite, and has a wide dado, giving some ribs to hold the board in place in case of disaster. 2 sided tape holds the board in place, and showed no signs of weakness. It would be easy to rig this for vacuum clamping.

I am very pleased with this. It looks plenty accurate, and the adjustable bits to allow greater accuracy are not even in place. I saw some funky looking rigs on youtube, and elsewhere on the web, to accomplish the same result. This ended up being much simpler than I expected. I like how compact this is.

When this is finished, the radius will be fully adjustable.

Any ideas or critique would be greatly appreciated!

Cheers! and thanks to you all..

Looks pretty cool, Chris. Makes me wish I had an edge sander.

Good design... It should work OK..how are the boards held in place?

Ingenious, Chris. Now I've got to figure out how to make it work with a 6x48 belt sander.

Thank you. The boards are held in place with double sided tape. Yes, sounds scary, but, I saw someone doing it on youtube, tried it with a router jig for a similar purpose, and it holds fine. This relies on smooth surfaces, and high quality, permanent double sided tape. It does not budge, and requires a little effort to pop the thing off after.. I did think that in the long run, a vacuum clamping setup will be better. There is a wide, smooth dado which contains the fretboard blank. With care, this could be made to a standard fret board blank width, and then it is super consistent. Double sided tape works.

Saltytri, if your platen is at least 12" it can work. Otherwise, I saw people using the same concept with a router, lots of different ideas to achieve a compound radius. I did not find any ideas that were clearly shared though. It was a confusing time, at first, trying to figure this out. I can already think of ideas to use your mill to accomplish this....



All the parts came from McMaster Carr. The bearings are the extreme offset type. They are decent quality, made in Germany, plastic bushing, plastic housing. They have a tiny bit of play, but not excessive. Certainly better bearings could be had. The shaft is 10mm hardened steel. It could be a little beefier. Total cost for all parts, shipped, was under $50. (except the scrap plywood)

Weaknesses are: shaft could be beefier. I did not see flex, but can flex it a little if I apply pressure in the unsupported middle. I can feel the table slightly moving in relation to the sander, barely, but I will make some legs to tie it into the ground and give much grerater support. Not a big deal. The swing arms are made from 25mm Baltic Birch, perhaps the bracing that joins with the Richlite piece that is what the board tapes to, peraps it could come all the way up close to the shaft for greater stiffness. (overall, it is stiff enough, and accurate enough to do a decent job) Precision location of the centerline of the bearing shaft, and the centerline of the board holder, both these matching, is one of the most important relationships going on. After looking on the web, I like how small and simple this is.

The second test I did last night, had it been slotted, it could bave been a keeper. It is plenty accurate. It's surface feels very good under the fingers.

I like radiused fretboards, aesthetically, and from a feeling standpoint. I am an absolute beginner on the uke, but see how it is not necessary, but also how it might be of benefit to certain playing styles more than others. I will do 2 or 3 radiused boards in this current group of tenors I am building. I look forward to learning the more complex setup that will be required.

One question that crossed my mind this morning, is that the saddle will have a crown to match the radius. This means that the 2 middle strings will be higher than the 2 end strings, not by a lot, but some. The string break angle will be greater on the C and A string. I think the C string can be kind of a dominant force in moving the top, maybe it does not need any extra help, I wonder if a radiused board on an uke might be moving in a contrary direction to balanced sound, or how to offset this? Maybe with a low G it will be a better match? Not an issue? At this point, lots of questions, few answers, and 'eyes open'...

Due to too many projects, and too little time, this is done for a while. It will do a fine job of my immediate needs, several tenor boards as it sits.

also... belt sanders are only as acurate as the (graphite) platen beneath the belt... I regularly check, and surface mine

A flat board has zero difference between the crown and the shoulders. A 12" radius board has 42 thou difference and a 14" radius board has 36 thou difference between the crown and the shoulders. Anyone who has played a 12" radius fretboard uke can feel the 42 thou difference to a flat fret board.

So down the length of the compound fretboard the difference of the crown to shoulders at the nut to the end of the board is about 6 thou. Were talking a rate of change of about 160 feet per inch. Can you feel that?

**PS. Can someone please check my maths.

What kind of permanent double sided tape are you using?
I’ve tried the scotch brand, but it didn’t really seem appropriate for shop use and didn’t perform that well for me, maybe I got a bum roll? I don’t think I would have relied on it for such an operation.

Assuming the nut is 1.5" wide and the other end is 2" wide ..I make the 12" rad sagitta .0235"...and the 14" rad @ .0358"

Cheers Ken. I used 2" for both ends.

Very cool!

liam_fnq said:

So down the length of the compound fretboard the difference of the crown to shoulders at the nut to the end of the board is about 6 thou. Were talking a rate of change of about 160 feet per inch. Can you feel that?

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if a 0.001 shim is laid on a flat surface, and you run your finger across it, you will feel it. Fingers can be very sensitive. I am not sure if I can feel the difference down the length of the board, but I think I can. I can definitely see the difference if I look. How some people can read (feel) Braille is beyond me, but they can.

Chris_H said:

I can already think of ideas to use your mill to accomplish this....

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Hmmm.... Trying to get my mind around this before the full effect of the coffee kicks in. Are all radiused boards done with compound radius or is this an extra flourish? Will a simple radius do?

AlaskaTheo said:

What kind of permanent double sided tape are you using?
I’ve tried the scotch brand, but it didn’t really seem appropriate for shop use and didn’t perform that well for me, maybe I got a bum roll? I don’t think I would have relied on it for such an operation.

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It is the Scotch brand, their 'permanent' tape. Yes, I was skeptical too, at first. I saw someone else doing it on youtube. With smooth mating surfaces, and 3 full length strips, it has plenty of holding power, no detectable play. At the end, the piece requires some effort to even budge. flat clean, smooth surfaces, which the underside should be anyway, and push it down hard.

saltytri said:

Hmmm.... Trying to get my mind around this before the full effect of the coffee kicks in. Are all radiused boards done with compound radius or is this an extra flourish? Will a simple radius do?

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No, some boards are done with the same radius the entire length.

thistle3585 said:

No, some boards are done with the same radius the entire length.

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At a guess, I'd say MOST boards are done this way. That's why stewmac sell the tools for it!!

most uke boards..