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Ergonomic bass build "Twisted Sister"

#41
TFM94 said:
The next step is to resaw it into two thinner pieces, which I can then bend and laminate around the neck blank to form the twist
Click to expand...

This raises an interesting question that I haven't been able to find an answer for: does Torzal bend a flat fretboard around the twisted neck, or carve the shape of both the neck and fretboard first, then glue the two matching shapes together? I had guessed it would be the latter, since I would expect it would be hard to twist a 1/4" thick fretboard about 25 degrees over a couple of feet, but maybe not.
 
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#42
Jeff Siddall said:
This raises an interesting question that I haven't been able to find an answer for: does Torzal bend a flat fretboard around the twisted neck, or carve the shape of both the neck and fretboard first, then glue the two matching shapes together? I had guessed it would be the latter, since I would expect it would be hard to twist a 1/4" thick fretboard about 25 degrees over a couple of feet, but maybe not.
Click to expand...

I believe it's possible to bend wood like that if you use steam.
 
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#43
I cannot speak for Torzal and don't know their processes, but I'd assume that they CNC the fretboard out of a larger block with the matching twist. This way they don't have any tension in the neck.

It is somewhere between difficult and impossible to twist a 1/4" board that much, depending on the wood itself. In any case, it should flex back a fair amount once the neck is thicknessed and shaped.

I will laminate the fretboard from thinner pieces and I hope to minimize any tension in the neck this way. Only the top layer has to be about 3mm to ensure a uniform look after applying the radius. My plan is to use binding to cover up all the layering in the fretboard.
 
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#46
I will be using a single straight double-action truss rod. It's going down the center of the neck, and I'll rout a straight channel so that it's slightly tilted to the treble side at the heel and to the bass side at the nut. The plan is to rout the channel slightly deeper than usual and then fill the gap above the rod with a strip of wood to blend it in with the twisted surface. It's a bit of a compromise, but should work ok. I'll also be using carbon stiffening rods to reduce the need of adjustment in the first place.
 
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#47
Slidlow said:
I've not paid much attention to Torzal necks. How do they arrange the truss rod(s)?
Click to expand...

I had this question also. They don't say much (of course) but I expect they cut a straight slot in the neck after the twist. This means that, relative to the fretboard, the truss rod angle will change along the length of the neck. But that begs the next question: what do you do about the flat bar of the rod (assuming a typical DADR design) that is angled relative to the bottom of the fretboard? Maybe a equally twisted insert over the truss rod? Hmmm...

EDIT: thinking about this a bit more, I guess it might not matter much as the bulk of the force will be at the middle of the rod, so if you made the slot perpendicular to that then the rest can probably contact wherever and it won't make a big difference.
 
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#49
Jeff Siddall said:
This raises an interesting question that I haven't been able to find an answer for: does Torzal bend a flat fretboard around the twisted neck, or carve the shape of both the neck and fretboard first, then glue the two matching shapes together? I had guessed it would be the latter, since I would expect it would be hard to twist a 1/4" thick fretboard about 25 degrees over a couple of feet, but maybe not.
Click to expand...
I suspect you could saw twisted fingerboards from a plank using a bandsaw and a jig similar to the neck routing jig, but moving the workpiece relative to the tool rather than the other way around. Then just clean it up on the CNC. You wouldn’t bother if you were just making one or two, but the reduced waste might make it worthwhile for production.
 
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#50
Why? WHY? Why did I let myself believe that I could resaw 3" wenge by hand? :banghead:

I thought I could do it with my crosscut Dozuki, because it has the smallest kerf, but that was an epic fail. Switched to a rip cut Ryoba after that and it was 10 times faster, but it still turned out pretty rough. I had to flatten it with a router sled afterwards, which left me with one piece of 2.4mm and one piece of 2mm. FYI, I started with an 8mm board:rollno:.

On the bright side, these pieces are easy to bend around the twist. I will need some extra veneers to get to the right thickness. That's ok, I still have some maple veneer for that. I only wish I had at least 3mm thickness on one piece for the radius. I'll have to think about that.
 
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#51
2.4 mm should be enough for radius unless you are doing a really small radius. Even the tight 7.25" radius at the nut on my 5 string build is just over a mm thinner at the edge than the centerline. If you were doing a constant radius board it would get thinner towards the end of the board but still likely less than 2.4 mm.
 
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#52
Jeff Siddall said:
2.4 mm should be enough for radius unless you are doing a really small radius. Even the tight 7.25" radius at the nut on my 5 string build is just over a mm thinner at the edge than the centerline. If you were doing a constant radius board it would get thinner towards the end of the board but still likely less than 2.4 mm.
Click to expand...
I was planning a 12" or 16" radius. All my guitars and basses so far have 12", but 16" could be more convenient here. And yes, I'm doing a constant radius board. Compound radius with a twist is beyond the things that my router jig can do. For now.

For the required thickness h I calculate h = (2r - sqrt(4r² - w²))/2 = 1.58 mm for r = 400mm (16") and w = 71mm fretboard width at the 24th fret. 2mm binding on either side reduces the width to 67mm and the required thickness to 1.4mm, so I have about 1mm left, that's ok.
 
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#53
More geometry, more math, more fun.

I was thinking about the fretboard, and assumed I would have to glue it centered onto the neck blank, as we all usually do. Taking into consideration that all required surfaces run along the top of the fretboard/frets, I considered a 2mm fretboard offsetoffset to the right at the nut (looking from the headstock end), and 1mm to the left at the body. The reason is fretboard thickness. This is hard to explain, but here's a sketch:
fretboard.jpg


Alternatively, I could glue the fretboard on the center of the blank, but then the center string would run at an angle, and I would have to offset the neck in the jig when radiusing the fretboard.
fretboard2.jpg

Now, which way is better?:help: I lean towards the first option, because then the center string would run parallel to the sides of the neck blank, which is useful for a neck-through.

To keep things symmetrical, I will also rout the trussrod and CF rod channels depending on these offsets. I love that I can figure out something new about this design every day:hyper:but seriously, I hope I'm not missing something important:nailbiting:.

The trussrod will arrive tomorrow, I think I can rout some channels during the week.
 
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#54
I know what you are saying, but I instead of thinking about everything relative to the neck blank I would think of it reversed: Strings should be centered on the fretboard, fretboard needs to be aligned to body so strings run down the middle of the body. That only leaves the question of where the neck blank goes under the fretboard. Since that is going to be mostly carved away, I would leave that adjustment to the carving of the back of the neck. Just make sure the blank is a few mm wider than the board and it should take care of itself.
 
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#55
Jeff Siddall said:
I know what you are saying, but I instead of thinking about everything relative to the neck blank I would think of it reversed: Strings should be centered on the fretboard, fretboard needs to be aligned to body so strings run down the middle of the body. That only leaves the question of where the neck blank goes under the fretboard. Since that is going to be mostly carved away, I would leave that adjustment to the carving of the back of the neck. Just make sure the blank is a few mm wider than the board and it should take care of itself.
Click to expand...
Good point. If I understand correctly, that would correspond to the first option. I am referring to the neck blank because that's what I'm starting with. As soon as I have the fretboard in place I will arrange everything according to it, including body, bridge etc.
 
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#57
Made some progress today. The roussrod is installed, and so are the CF rods. Since there have been a few question here about trussrods in twisted necks, here's some details: I used a normal router sled to cut a straight channel with even depth, and arranged the neck blank underneath the sled to the desired angles. That was a bit tricky, because I had to tilt the blank sideways for the correct angle (so that the tilt at the nut and the FB end is the same, just opposite), and I had to shim the nut end side of the neck for even depth for the length of the slot. It's not perfect, but good enough and nobody except you guys is going to see it ;-)

TR channel with the rod inside:
20220126_123129.jpg


I glued a strip of pine on top and blended it in with the twisted surface. This way I have a good glue surface for the fretboard lamination and the TR has a square surface to push against. I'll call it a success:
20220126_141915.jpg


Next step was the CF rods. Same channel routing technique, and they came out quite nice. They're glued in with epoxy with wood strips on top.
20220126_183806.jpg

I will cut them flush tomorrow when the epoxy is cured.

Btw, the trussrod is a titanium trussrod from Tatsuta in Japan. It's great quality, and super light at only 110g (compared to the 200g of a typical steel trussrod).
 
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