Cutting fanned fret slots...

[Beate Ritzert]

... into existing fingerboard which alread has a radius.

It is this project: https://www.mimf.com/phpbb/viewtopic.php?p=66356#p66356
Actually it is my 2nd attempt on a fanned fret fingerboard, and it is the 2nd time i have to cut the slots into an already finished fretless fingerboard, which has been radiused. Remembering all the trouble i have had to get this sufficiently right i would like to learn about ideas to maintain the necessary procision.

Currently the positions of the frets have been marked with a knife (as lines), and i could use either my Kugihiki or my fret saw (or both starting with the Kugihiki in order to get the initial line more easily) to do the cuts free handed. Mhmm...

[Bob Gramann]

You might try to figure out how to clamp guides to the neck on each side of the saw blade so that the blade has no choice but to cut where you want. I know freehand is tempting, but that blade wants to dance and it will get what it wants sooner or later. If I were doing it, it would be worth my while to figure out a jig to force an accurate cut even if I had to make several for the different angles. Good luck.

[Beate Ritzert]

I know freehand is tempting, but that blade wants to dance and it will get what it wants sooner or later.

That's why i was asking

[Alan Carruth]

One person slots a narrow board of some hard wood that is thicker than the fingerboard they're making for each scale. Then they cut off a strip from each, I'd guess about 3mm wide, and stick them onto the sides of the board to be slotted. This gives a guide along each side for the saw. I have yet to try it out myself, but I can't see why it would not work with care.

[Marshall Dixon]

I can't cut frets freehand consistently. I keep the saw sharp which is a must, but without a guide it's too easy for me to go off vertical or bounce around.

I use a fret saw that cuts on the pull stroke with 6mm aluminum bar guides at the bottom. I adjust the depth with a countersunk set screw at either end. I cut the holes for set screws slightly larger in diameter than the set screw to be able to adjust for depth.

The wider footprint of the aluminum pieces minimizes the marring effect when it hits the wood.

I have a home-made jig that holds every thing secure and the side of the guide bars are about 25 mm wide so they give the saw good vertical support. A little paraffin wax helps to keep the aluminum from binding up.

I put a radius on the boards before fretting. Just pull the saw over in an arc and feel when the cut is complete. I usually cut fret slots before tapering the board, but when I have had to cut a finished board I prevent tear-out by simply cutting with the back stroke at the exit point of the board until it's a millimeter or so in from the edge, then complete the cut correctly.

What I'm thinking is to have a padded wood support on the back side of the neck. Make it so that it sticks out over the sides to accommodate clamps.

For saw guides on the top clamp two lengths aluminum bar stock at just the right angle of the fret. Make the bars long enough to be adjustable. The aluminum will bend slightly to conform to the neck's radius. If you already have a fret saw with guides you could make it work with that.

When you get to the body, though, it won't work. Maybe double sided tape to hold guides in position.

Maybe you just need one guide for the saw and can devise something adjustable that lays on the fingerboard securely enough. A parallelogram type of thing. I use double sided tape for stuff like this. Carefully though as I've had it pull up wood when removing.

[Beate Ritzert]

My "solution" was to use a rest of alder left after cutting a solid body guitar. Pretty mach improvised, but feels a lot better than the more or less free handed attempt on the sharkfin bass. Partially with slighly narrower radius than the fingerboard and sufficiently elastic to be clamped to the neck.
Today i am going to do only the easy part of the neck. I am looking forward to be able to use it above the body if i shorten it accordingly.

[Beate Ritzert]

Roughly successful - i had to redo "only" 3 fret slots. The wooden "jig" was a bit to slippery.

[Jarno Verhoeven]

Ouchie, are you going to remove a piece of fretboard, glue in a new piece and then redo the fret slots? Or just fill the erroneous slots with veneer?

[Alan Carruth]

Over the years I worked out a system for ensuring accuracy when marking fret slots by eye. The old adage says 'measure twice, cut once', but I realized that there was an issue with that; basically, if i mis-read the rule once, it's likely that I'll mis-read it again in the same way. The trick is to use two different sets of numbers for the layout to avoid that.

Since I work in metric, and calculate the fret locations using my own program, it was easy enough to set it up to give readout in terms both of nut-to-fret and saddle-to-fret distances. I mark the slot locations using the nut-to-fret distances along the side of the fretboard, with the scale clamped in three places (so I can shift a clamp without any risk of the rule moving), and mark them with a knife. Then I scribe the lines across. The scale is reversed, and the saddle-to-fret numbers are used to check the scribe lines. Any line that's around .2mm off gets marked with an arrow to show which side of the line to cut on: normally I center the cut on the line. Anything that's further off than that get re-scribed. With multi-scale fretboards, of course, you have to do each side separately, but otherwise it's pretty straightforward. I use a number of different non-commercial woods for fretboards, and I've checked my slots against available pre-slotted boards for accuracy, so I'm pretty confident. This has really cut down on the number of 'whoops' moments when I look at the finished job.

[Beate Ritzert]

Thanks Alan, again, for Your great hints.

@jarno: These are just shavings glued with fish glue. As the repair is mostly covered by the fret wire i ommitted the last step - cutting deep slots parallel to the grain and filling these with matching shavings as well. That leads to almost invisibility of the repair. I learned that a while ago from this community.