Putting a little back bow in a Carbon Fibre reinforced neck.

[Michael Tulloch]

I have to finish up a neck started by someone else. No truss rod, but it has a carbon Fibre rod in the centre where the truss rod would go. With a truss rod, I add a little back bow , and re-plane flat (fretboard)  and add the fretboard radius then.

How does one do this sort of thing with a neck without a truss rod, but with the CF rod? 

Does one even do this to this type of neck?

Thank you;   Michael

[Mark Pollock]

I have no help here, but a thought -

Wouldn't it make sense to have a jug where you can stress a neck with a certain amount of torque and then measure the deflection? Perhaps suspend it on its ends, and then add a weight and measure how much it bends? Then adjust the truss rod and see how much it bends now?

The same technique would also allow someone without a truss rod to estimate how much bow there will be in the neck before setting up the fretboard.

So, if it were me, and I knew nothing about how much cf will bow, I would take a guitar with a truss rod, pull the strings, and run the weight test on the neck, and then run the weight test on this new neck, and see what the defelction is. If it is similar, then set the new neck up similarly. Otherwise, you can adjust accordingly for the difference.

Shooting from the hip here.

[Michael Tulloch]

I can see how much, but it's the how, I'm askin. Only way I can see would be to introduce it to the frtboard after it's glued on and flat, to sand / plane in the back bow a wee bit...then radius the thing.

Thanks Mark, that all seems well, and good. 

[Josh Fuller]

Just a thought-What if you jigged it up in a way that allowed you to weight the headstock down in order to introduce a slight backbow?  You could then plane the neck flat under that tension, and see some relief with the weights removed. 

[Hesh Breakstone]

Perhaps consider first determining how much back bow, if any, this neck may need.  I say if any because it's possible that with the CF reinforcement this neck may not move much at all.

My approach would be to wait until the neck is on a guitar, string her up with the target strings, and see what you have.  If it comes to pass that you need more back bow under string tension consider compression fretting as is done with pre-truss-rod-Martins.  Using thicker and thinner tangs in strategic locations can induce or correct back bow as desired and needed.

One more thing I have to mention for guitar builders - truss rods are important and the fastest way to get repair folks mad at you is to leave out the truss rod....  Rant over (and was never directed at the OP) ;)

Hope this helps, Michael.

[Michael Tulloch]

Yes Hesh.....this helps. Thank you ! I use truss rods in my builds....but in this instance, I must do what this Roman (when in Rome) want's me to do, under his banner. Interesting experience so far. I gravitate toward light builds, and this light weight neck goes a long way toward that. 

m.

[Kerry Krishna]

 Just thinking on my feet here. It seems to me that the person who 'fixed' this before did the routing for the CF rod while the neck was already bowed. At least that is what I am thinking. So  bending the neck back, and plaining  the neck flat should do 100% of nothing, because the neck will spring back.

 Just supporting the neck as is and plaining it should work, but you could be loosing quite a bit of wood at the nut, which COULD end up being a pretty big problem. 

  Am I totally out to lunch here folks?

[Michael Tulloch]

This is a new neck sent to me as a roughed in blank...( headstock veneered, Heel shaped, CF rod installed, capped and flattened. I'm to take it the rest of the way. His work is all good and proper. I'm going to anticipate the pull load of the strings eventual (I'm now thinking), by introducing a 1/32 crown in the radiused fret board, then fretting. Any small further adjustments I'm figuring will be sorted out when I level the frets (few thousandths give or take.) if need be. Fingers crossed.

Michael

[Kerry Krishna]

 Compression fretting may be your only answer...