Compound radius fretboard question

[Humphrey9999]

I'm making a compound radius fretboard, 9.5" to 12", for my CBG.

I just made a 4" x 2 3/4" x 12" radius sanding block and will now make a 9.5" one. Is that enough, or should I also make a 10.75" block as well, for a smoother transition through the middle of the fretboard?

Any thoughts welcome.

N.B. The second reason I'm making the blocks 4" long is so I can shape them on my 4" oscillating spindle sander. It does a good job very quickly. The first reason is that I figured 4" would be best since I'm only doing part of the fretboard at a time.
I should also mention that this is only a short 16 fret board that ends at the body. Scale is 25.34".

[Fret Less]

I should point out that I'm not qualified to give you the answer you want, but I just wanted to share some thoughts which may help or possibly hinder.

I was going to do a similar thing and started designing a jig based on the offset conical section described by the strings. I was going to use dowels or something like that and have an arch at each end of the fretboard and sand underneath the dowels. As you can guess, I haven't fully experimented and thought it through enough to put it into practise, but the idea was to get to a perfect conical section so that I wouldn't have to blend in different radii.

Another idea I had was again with dowels, but having one go in a diagonal from the side of one radius-arch to the other, then swap it to the other side. Then put the dowel straight down the middle of the fretboard and finish it off.

There's also the thought of trying a thumb-plane and again going down the length using a similar conical jig. Then sanding it smooth. Probably useless to you, but just some thoughts that might help in getting those transitions to work right.

[Humphrey9999]

I should point out that I'm not qualified to give you the answer you want, but I just wanted to share some thoughts which may help or possibly hinder.

I was going to do a similar thing and started designing a jig based on the offset conical section described by the strings. I was going to use dowels or something like that and have an arch at each end of the fretboard and sand underneath the dowels. As you can guess, I haven't fully experimented and thought it through enough to put it into practise, but the idea was to get to a perfect conical section so that I wouldn't have to blend in different radii.

Another idea I had was again with dowels, but having one go in a diagonal from the side of one radius-arch to the other, then swap it to the other side. Then put the dowel straight down the middle of the fretboard and finish it off.

There's also the thought of trying a thumb-plane and again going down the length using a similar conical jig. Then sanding it smooth. Probably useless to you, but just some thoughts that might help in getting those transitions to work right.

Hello Fret Less. Thanks for the response.
Like you, I considered other methods and did heaps of Googling for methods of fingerboard radiussing, but settled on the simplest in the end.
I also considered a constant radius - this would have been easiest since I could have used a jig with my wide drum sander, but I really want the compound radius for a nice light, low action. (Small hands)
Also considered using my Dremel with a 1/4" straight cut bit, with the Dremel mounted on a pendulum with 12", 9.5" etc between bit tip and pivot to make the blocks. Started looking at ways of doing that this morning, but it seemed over complicated just for one fingerboard, so I used the OSS method instead, sanding to a radiussed scalpel cut with a 90 degree jig to keep it even over the length.
I made the 9.5" block earlier, but so far I've started sanding with the 12" block to get the feel. Going fine so far.

When I look at the actual curves of 9.5" and 12", there's very little difference, only about 1/4mm to 1/2mm, judged by eye, in curve depth across a 70mm block, so unless someone says otherwise, I'm going ahead with just the two blocks. After all, it is only a cigar-box guitar, but since it's a 6-string, I added an LMI 2-way truss rod and decided on this compound radius for playability. (Is that a word?)

I saw your fingerboard thread, but couldn't suggest anything useful. (My fingerboard is Rosewood, too. NGR)

[Fret Less]

Sounds like your on to a winner. Can you post some pics when it's done?

I was starting to make a short scale bass, but the thought of a compound radius and trying to get everything 'perfect', actually ended up with me buying my current bass instead. So I know what you mean, it's only a CBG, but you want to be proud of it nonetheless.

[Humphrey9999]

Sounds like your on to a winner. Can you post some pics when it's done?

I was starting to make a short scale bass, but the thought of a compound radius and trying to get everything 'perfect', actually ended up with me buying my current bass instead. So I know what you mean, it's only a CBG, but you want to be proud of it nonetheless.

It's actually a sort of hybrid, based on the appearance of a CBG. A neck-thru design, but I've allowed 3mm clearance between the neck and soundboard so the thru-neck doesn't hinder the soundboard vibrations. I'm not using a cigar-box, but instead making a box-like body with a slight (5mm) arch-top, 'f' holes, 6 strings + truss rod as mentioned. A vintage cigar box label for a touch of authenticity. I'll fit an under-saddle pickup, but I mainly want to use it as a quiet accoustic, for lounge-room playing.
Body is Merbau back and sides with a Camphor Laurel soundboard, Merbau neck and NGR fingerboard. 6-in-line Grover machine-heads, through body string mounts, (accessed by opening the hinged back of the box/body).
All my own design, so God only knows how it'll turn out, but coming along great so far.
I will post pics of the finished product. Would have done a full WIP, but I'm not confident enough yet.
So far, the fingerboard is coming along well with just the two radius blocks. Fingers crossed.

[Humphrey9999]

All done. Came up OK, I reckon.

Fretboard Radiussed 9_5 to 12.jpg


Now onto the fret slot cutting. Should be fun.

[Humphrey9999]

Now onto the fret slot cutting. Should be fun.

Well, that was much easier than I thought it would be. Ready for tapering:
Fret Slotting Finished.JPG

[Fret Less]

Well, that was much easier than I thought it would be. Ready for tapering:
Fret Slotting Finished.JPG

Looking really good. I'm surprised it was that easy. This thread is now my tutorial for trying it myself one day. Do hope there aren't any nasty surprises when it's fully strung up.

[Humphrey9999]

Looking really good. I'm surprised it was that easy.

Yeah, no apparent problems at all. Out of interest, I made a 10.75" cardboard template and tested the middle of the fretboard - as close to perfect as I could ask for.

This thread is now my tutorial for trying it myself one day. Do hope there aren't any nasty surprises when it's fully strung up.

I hope not. As mentioned, it seems OK, so I don't think I need to worry.
For fret slotting, I made a jig that keeps the (StewMac) saw at 90 degrees in 2 planes, with the original depth stop for depth. Worked out really well.
I'd seen lots of jigs for saws without a depth stop, which incorporated the stop into the jig, but this was simpler for a one-off. I made it for an unradiussed fretboard, though, so had to put a thin spacer under the fretboard to allow for the angle on the edge of the radius. No dramas.
A couple of pics. (That's not a real fretboard in the second pic, just my practice piece.): -

Fret Jig.jpgFret Jig with Saw.jpg