Planning a twisted neck

[TFM94]

Yes, I was thinking the same. I will cut the truss rod channel vertically so that it is perpendicular with the top surface around the 8th fret. Then cut a little deeper than usual, say, 11mm and fill the top of the rod with a strip of wood after the truss rod is in place.

The truss rod is a bit of a compromise here, because if it needs to be tightened a lot in either direction, it will likely change the twist. I will put in two carbon rods, one on either side with the same technique (straight slot, fill any voids on top of the rod with wood), probably 6x6mm each. The plan is to minimize the need for adjustment in the first place, which is a bit of a challenge here in Finland with pretty cold winters. Minor adjustments should be similar to straight neck adjustment.

At first I considered laminating the carbon fiber in a twisted slot, but I'm not sure if it's worth the effort.

 

[TFM94]

I'm sorry that I kept you waiting for a while, but as I already said, this is gonna be a slow one

After some more planning and math, I finally built the router rails/jig. I went with 9mm MDF as aluminum tubes in a sufficent length (>1m) were ridiculously expensive at my local hardware store. I beveled the top of the rails on the outside, so that there is a well defined contact point between the router and the rail, and I am not changing from the outside to the inside edge of the MDF rail when I pass the neutral point. Hard to describe, but easy to understand with the pictures.

The router itself will sit on an extended base made out of MDF, which has three straight MDF strips at the bottom (ca. 1cm thick) to rout the neck surface, and these strips will be replaced by concave ones of the desired fretboard radius (probably 16") later to radius the fretboard.

 

[TFM94]

The jig is relatively wide by design. I made it this way so that I can also rout the body wings to blend them in with the twist of the neck.

 

[TFM94]

And here's the rest of the jig. The router base is MDF with three rails, two on one side, one on the other. The single rail is beveled on both sides so that it can slide over the fences at a tilt. I glued two leftover carbon strips to the bottom, because I noticed some flexing of the base due to router weight.

With the fences being quite thin at the top, is it possible to harden the MDF edges with superglue to avoid dents? Will this cause too much friction?

 

[Bruce Johnson]

With the fences being quite thin at the top, is it possible to harden the MDF edges with superglue to avoid dents? Will this cause too much friction?

You can harden the edges of MDF with epoxy, wood glue, or polyurethane finish. You could use CA glue, but that would take a lot of glue! With any of those, build up a little thickness on the surface, then sand the edges smooth. Then wipe a little wax on them. Smooth sliding, no grinding.

 

[Bruce Johnson]

And here's the rest of the jig. The router base is MDF with three rails, two on one side, one on the other. The single rail is beveled on both sides so that it can slide over the fences at a tilt. I glued two leftover carbon strips to the bottom, because I noticed some flexing of the base due to router weight.

With the fences being quite thin at the top, is it possible to harden the MDF edges with superglue to avoid dents? Will this cause too much friction?View attachment 4333613
View attachment 4333614

Overall, the fixture looks good. That should give you a consistent twisted surface on whatever parts you put in there.

 

[Max Bogosity]

The fretboard could either be routed as a twisted piece from a thicker block, or it could be laminated from 2-3 veneers (e.g. 2mm) using the neck blank as a mold.

Option 3: How about a Fender-style one piece neck with the truss rod installed from the back? Not having a separate fingerboard would completely avoid the problem of trying to join the fingerboard to a twisted neck surface. The neck wouldn't actually have to be a single piece like old Fender necks; having the neck laminations visible on the fingerboard surface might be visually interesting.

 

[TFM94]

Option 3: How about a Fender-style one piece neck with the truss rod installed from the back? Not having a separate fingerboard would completely avoid the problem of trying to join the fingerboard to a twisted neck surface. The neck wouldn't actually have to be a single piece like old Fender necks; having the neck laminations visible on the fingerboard surface might be visually interesting.

From an engineering perspective, that's not a bad idea! Unfortunately, I am not a big fan of maple fretboards (my neck blank is maple), but I might give this a try with my next build. I think a one-piece wenge neck would look amazing!

 

[TFM94]

It sure has been a while. After finishing another build, I can finally get back to the twisted neck bass. I put the blank into the jig today and routed the twist. First I routed with a large 25mm tenon cutter bit to remove most of the material, followed by many shallow passes with a rounded bit to get a cleaner surface, and then finished up with a scraper.

The result shows one major problem. The rails of the jig are parallel, but the string spacing is of course tapered towards the nut. Bridge spacing is 16mm and nut spacing is 11mm, so the strings fan out. I drew the string path onto the routed neck blank and realized that the outermost strings paths have a 3mm bump/backbow on the bass side and a 3 mm gap/upbow on the treble side. The center string path is perfectly straight. Now that I am thinking about it, it makes perfect sense. I should have made the jig with non-parallel rails with a distance ratio of 11/16 at the nut vs bridge.

Unfortunately, I screwed and glued the rails to the baseplate of the jig to make it rigid. I will try to remove them and re-glue them close together at the nut, but maybe this will damage the jig. We'll see. In the worst case I'll have to rebuild the jig. The neck blank is plenty thick, so there is enough material left to re-rout it with the new jig.

Lesson learned, I guess!

 

[TFM94]

@MPU , did your jig have parallel or fanned/tapered rails?

 

[MPU]

I don’t remember for sure but I think they were parallel.

 

[TFM94]

I don’t remember for sure but I think they were parallel.

Ok, that's good to know. Of course it depends on the amount of twist. My jig is now for -15/+20 deg, so 35 deg twist in total. For less twist it might be less noticable and still be possible to remove this issue while levelling frets.

 

[TFM94]

It's been a long day in the workshop and I got exactly as far I thought I was this morning, but I am happy to report that there's a new jig. It's now tapered towards the nut, and that required a change in rail height and slope as well, so that the angles at the nut and bridge are the same as before.

I was telling myself that the distance between the rails doesn't matter for a steady, continuous twist, but it does. Here's proof:
The angle at the nut and bridge is the same as on the neck blank, which was routed with the old jig:

But around the center, the angle is now different:

It looks like this will counteract the backbow on the bass side (left in photo) and the upbow on the treble/right side. We will know for sure tomorrow after routing.

 

[TFM94]

Routing is done!

I was trying to install a height-adjustment via screws to raise the neck blank instead of lowering the router bit (which can induce a twist error, when it is sitting much lower than the rails), but decided not to use it. There is just too little room underneath the neck blank in this jig. Maybe I will build a better jig at some point, but for this project I wanted to get started with routing and decided to use little shims under the neck blank to raise it to an appropriate height. Small router bit depth adjustments (<1-2mm) are still possible and will not induce significant error to the twist angle.

Here we go. First, I used the big tenon cutter bit again to get the rough shape right:

Note that the tool path is not straight along the blank, and that is because the router sled is following along the left rail. I think this is not an issue, as long as the sled stays at the correct angle. I could even rout this crosswise if I wanted to. Here's the router base with the guide clamped on.

The clamped guide is a temporary solution and annoying to adjust, but it works!

This is after routing with the large bit. you can see the ridges left by the bit. Then I started routing with a dome bit in increments of 2mm. This requires about 50 passes for the full width of the neck:

and done! I still need to clean it up with a scraper, but I can already tell that the centerline is roughly straight and so are the string paths.

 

[TFM94]

Hi guys, I need your advice.

Since drawing a plan on flat paper isn't representative here, I decided to draw the fretboard outline and string paths directly on the neck blank. With the angles it looks quite a bit different than expected. First of all, the string paths are perfectly flat now, I checked that with a straight edge, and more importantly, with a thread pulled across the surface.

As I mentioned above, the jig I built is designed for string spacing ratio of 11/16, meaning for 16mm string spacing at the bridge I need 11 mm at the nut, all measured center-center. I referred to my Billy-Bo 4-stringer for the 11mm nut spacing, which I find very comfortable to play. Now, with this being a 5 string, the nut becomes really wide, 4x11mm+2mm(string diameter)+2x3mm(fb edge)=52mm nut width. I generally like wider (and thicker) necks (vintage P for example), but this seems to be a lot, even for me. Note that with the twist, the neck is about 2mm narrower at the 12th than a corresponding straight neck would be.

Choosing a narrower nut width would mean building a new jig from scratch, and probably a new router sled, too.

Now I have two options:
1. Leave it as it is, go for the string spacing above and hope it will be comfortable to play.
2. Build a completely new jig from scratch with adjusted taper, and re-rout the entire blank.

With the time already spent on this jig, I am tempted to choose 1. On the other hand, if the bass is uncomfortable to play, I would always regret not having spent a few extra days to buid a better jig.
What do you all think? is this a reasonable design?

 

[BAG]

I'm way late to this thread and I see you are already in build mode but have you seen the necks that are on .strandberg guitars? They call it the EndurNeck and rather than having the fretboard twisted they have the neck profile twisted. Supposedly it makes for a very comfortable playing neck similar to the Torzal twist but without the difficulties in production.

 

[MPU]

Hi guys, I need your advice.

Since drawing a plan on flat paper isn't representative here, I decided to draw the fretboard outline and string paths directly on the neck blank. With the angles it looks quite a bit different than expected. First of all, the string paths are perfectly flat now, I checked that with a straight edge, and more importantly, with a thread pulled across the surface.

As I mentioned above, the jig I built is designed for string spacing ratio of 11/16, meaning for 16mm string spacing at the bridge I need 11 mm at the nut, all measured center-center. I referred to my Billy-Bo 4-stringer for the 11mm nut spacing, which I find very comfortable to play. Now, with this being a 5 string, the nut becomes really wide, 4x11mm+2mm(string diameter)+2x3mm(fb edge)=52mm nut width. I generally like wider (and thicker) necks (vintage P for example), but this seems to be a lot, even for me. Note that with the twist, the neck is about 2mm narrower at the 12th than a corresponding straight neck would be.

Choosing a narrower nut width would mean building a new jig from scratch, and probably a new router sled, too.

Now I have two options:
1. Leave it as it is, go for the string spacing above and hope it will be comfortable to play.
2. Build a completely new jig from scratch with adjusted taper, and re-rout the entire blank.

With the time already spent on this jig, I am tempted to choose 1. On the other hand, if the bass is uncomfortable to play, I would always regret not having spent a few extra days to buid a better jig.
What do you all think? is this a reasonable design?

Draw the narrower string spacing lines on the blank and check how much difference do they make for the straightness of the string paths. I doubt there will be much difference and if there is you can scrape or sand the neck blank to get straight string paths.

 

[TFM94]

I'm way late to this thread and I see you are already in build mode but have you seen the necks that are on .strandberg guitars? They call it the EndurNeck and rather than having the fretboard twisted they have the neck profile twisted. Supposedly it makes for a very comfortable playing neck similar to the Torzal twist but without the difficulties in production.

View attachment 4548485

View attachment 4548488

I am aware. It's a great design byt itself and has a similar ergonomic purpose. I have played an endurneck once (it was on a guitar, though), and I couldn't get used to the hard edges in the back of the neck. That's when I realized that this design could be more comfortable, because it could utilize a very similar neck profile to the basses I am already used to.

 

[TFM94]

Draw the narrower string spacing lines on the blank and check how much difference do they make for the straightness of the string paths. I doubt there will be much difference and if there is you can scrape or sand the neck blank to get straight string paths.

I just drew this up. If I assume a nut string spacing of 9mm (36mm for outer strings), then I have a relief of 1.5mm in the treble string, and the equivalent bump on the bass string.

 

[TFM94]

The more I think about it and the more I research other 5-string basses, the more I lean towards a narrower nut spacing. 9mm spacing would give me 4x9mm+2mm+2x3mm=44mm nut width, that's 8mm narrower than what it currently is. I think that the wide nut would be a bit awkward together with the narrow bridge.

At this point of the project I don't want to call it "close enough" and just use a narrower spacing with the current twist, I think it's just asking for trouble during fret levelling/setup.