Fretboard shaping jig

[Mike Trobaugh]

The router jig is as stiff as if it was solid
Moves smoothly across the rails
Next up for this jig is to make a set of rails to cut the truss rod slot.

[Kevin Proaper]

I'm getting ready to build one in the morning, thanks for the info

[Greg Martin]

The only thing that has kept from building this type of jig is not knowing the formula/math to end up with an exact 12 " radius how did you tackle this ?

[David King]

Greg, what's so special about 12"?

[Mike Trobaugh]

Greg
The rails are made to the radius you want
There is no math or voodoo involved.
As shown in my pics,the radius will be whatever the rails are made too
Hope this helps

[ronald bakker]

i wonder how you can make a compound radius neck

i was thinking more toward having the neck hanging from a fixture across a long sandingbelt running the length of the neck
changing the sandingbelt with a fine metalsandingpaper would take care of flattening the freds too but with the radius used it would become a pretty large machine for just the one job it has to do

[Gordon Bellerose]

After seeing this post I went out to the shop and built one for myself.
I have used it several times, and it works very well. There are pics of me using it in a thread called "Neck-Thru Build".
Thanks for sharing.

[Hans Bezemer]

i wonder how you can make a compound radius neck

Google on "compound radius fretboard jig" and you will find a lot of examples, some simple, some more sophisticated.

[Donl Mathis]

One approach to a compound radius jig would be to replace the curved rails with an inverted "V", making a long compass, and then having the tubes closer together at one end than the other. One tricky bit is that the router platform would tilt down toward the narrow end, so you'd have to compensate somehow. It shouldn't be too hard if you're permanently establishing the radii, but if you want adjustable radii (by changing the angle between the pipes and/or the angle of the "V"), the tilt would also have to be adjustable.

Such a jig would make a true conical surface, which I *think* is the Right Answer.

(By the way, I notice that Wikipedia's page on "Inscribed Angle" has a nice animated sketch showing how a fixed "V" angle fits inside a circle, showing how a long compass works. It has a steep angle, and we'd want a much flatter one, but the idea is the same.)

[David King]

Wouldn't parallel rails with adjustable compass angles be an easier way to keep track of your actual radii. Calibration would be useful especially when it comes to setting the end to end heights to achieve a constant fingerboard thickness.
Seems to me that if you have a long bed belt sander you can't find a simpler, more reliable system than the Fox radius jig.

[Andy Birko]

One approach to a compound radius jig would be to replace the curved rails with an inverted "V", making a long compass, and then having the tubes closer together at one end than the other.

This doesn't work. A given long compass (moving part) will make the same radius irrespective of how far apart the rails are. Moving the rails closer together will affect the thickness of the fretboard, but not the radius.

Think about it.

[Donl Mathis]

Long compasses are a bit twilight-zone, aren't they?

Try this: Draw a circle with a given radius, pick a point for the vertex of the V, and two other points on the circle for the pivots of the long compass. Draw the V between those points.

Now, try to find a way to draw two pivot points, on the circle, closer together, using that same angle.

Can't do it, eh?

The only way to put two pivot points on a circle, closer together, using that same vertex angle, is to make a smaller circle.

[Donl Mathis]

Wouldn't parallel rails with adjustable compass angles be an easier way to keep track of your actual radii. Calibration would be useful especially when it comes to setting the end to end heights to achieve a constant fingerboard thickness.

Only if you want to make the sled full-length. The radii is set at the pivots -- where the slide meets the rails. So as you slide a "short" sled along, it has the same radius on the left and right edges at every position, and the router bit will move at (roughly) the average of those two radii, at every position. Not what we want.

Seems to me that if you have a long bed belt sander you can't find a simpler, more reliable system than the Fox radius jig.

I try not to argue with reality. So be it!

I like to think about theory, though.

If you image a cone, with the smaller and larger radius you've chose for your compound radius fretboard, at the proper distance apart, you can envision two disks of different sizes at those positions. If you were to attach an arm perpendicular to the axis of the cone, so that it sweeps out that circle, you could make a jig to do the job. The fretboard lives on a "section" of the surface of the cone while the work is being done. It sits at the cone's angle between those two disks -- in other words, the junction of the swinging arm and the fretboard is not a right angle, but rather, the right angle is at the central pivot. For this jig, there would be an inclined axis, with swinging arms at right-angles to that inclined axis, of the proper length, fastened to the fretboard platform at the proper angle.

If you alter this mental drawing a little, so the disk is perpendicular to the fretboard (i.e., the cone's surface), you have to turn the disk into an ellipse. Since a jig made this way won't have arms that change length as they swing (to track the shape of the ellipse), the overall motion has to get a bit wonky, requiring a sort of a twisting motion at each end of the fretboard. This is how pretty much all the swinging-arm (or rocking) jigs are made; at least the ones I've seen. The fretboard surface is ultimately affected by this elliptical element of the geometry, and the interaction between the two different ellipses.

These fretboards are obviously perfectly fine in Real Life, and I'm not trying to take anything away from them, or change anybody's approach! For me, I tend to choose certain areas of life where I'm seeking theoretical perfection. (That's why I never finish anything.) If I were designing and making a jig, I'd go for a purely-conical approach, with the pivots on the conical axis, because it seems "good" to me somehow. Simple, pure, in the theoretical sense, and then the goal is to find a real-world implementation that is also simple and pure.

But then again, I never finish anything, which seems bad.

[Andy Birko]

Long compasses are a bit twilight-zone, aren't they?

Completely.

I'll stay out of it. I got into this on another forum making the same suggestion as you did and was proven wrong. Now I'm somehow unable to prove you wrong on this one so it seems that the rules governing long compasses change depending on what side I'm on.

Here's the last time I got into it....

http://www.luthiersforum.com/forum/view ... ss#p479727

[Jason Rodgers]

I made a mini-mock-up of the long compass with adjustable rails idea that we're talking about here (no router or cutting done). It would, obviously, change the thickness of cut (by a factor that could probably be predicted with the maths), but I didn't confirm a change in radius, only assumed it.

[Hans Bezemer]

I've remembered reading an article a while ago where a compound radius jig was described and this involves make a pivoting router sled (the right word for it?) with to radii.
http://www.cambrianguitars.com/compradjig.html

[Donl Mathis]

I'll stay out of it. I got into this on another forum making the same suggestion as you did and was proven wrong. Now I'm somehow unable to prove you wrong on this one so it seems that the rules governing long compasses change depending on what side I'm on.

That's too bad, because you were right! I think Chris went off track when he started thinking too much about the sagitta. I draw the reader's attention to the left part of the diagram. The red and green lines are the same long compass -- the vertex angle is the same for both of them, and that's the (inarguable!) geometric rule that makes a long compass work. It doesn't matter where you "rock" the thing side to side, that V will always be the same angle, somewhere on the circle, with the two pivot points fixed on the circle.

Now, the "height" of the vertex above the two pivot points changes the whole time. When it's in the center, it's the sagitta, but that's the only time the sagitta is even vaguely interesting, I think. Don't even go there. I'm not going to try to re-understand all of the logical argument in those postings in order to "fix it", but my gut instinct is that there is a factor of two or some such thing that got lost somewhere.

The right part of the diagram shows two different sized circles made by the same "V" angle, simply by moving the pivot points closer together. It's the only way to make everything fit.

[Donl Mathis]

Okay… did I get my diagram attached this time?

[Donl Mathis]

By the way, if anyone is actually thinking about the long-compass approach, here's an idea to address the issue of the height changing as the rail separation changes.

Instead of just moving side to side, the rails ride on a V whose angle matches that of the long-compass sled. So… when they get closer together, they also get higher. You can move them to any separation and the height of the sled always stays the same.

[Andy Birko]

That's too bad, because you were right!

Yea, I think there's some incantation that was missed or something. Obviously Trevor got something working.

At the end of the day this is just academic for me. When I need a compound radius fretboard, I simply make the right entries in a spreadsheet, rebuild my 3D model and have my CNC cut a perfect one every time