Bolt on neck building problems

[Chris Richards]

Hi I hope someone may have some ideas regarding slab board type necks

I have built a few Fender style necks that have a slab board fretboard. I build them by cutting and routing the neck to an accurate plan shape then gluing the fretboard ontop, cambering the FB then fitting the frets before the neck is shaped, I find this easier since it gives a lot more stable a base to fit the frets, then finally I shape the neck.

Now here's the problem, it has happened a few times, after shaping the neck and leaving it for a few days the neck will develop a back bow sometimes upto 3mm when measured in the middle, I've had this happen a couple of times and it seems to be more pronounced when using brazilian rosewood. I make my necks using a traditional truss rod so there's no way to adjust the bend out. I use Titebond original for the FB neck joint. The only thing I can think of doing is either gluing the FB on with the neck curved or plane a curve on the maple before gluing the FB on. But I though one of you on here would know why it happens and a method for avoiding the problem.

Thanks Chris

[Mark Swanson]

Of course make sure your wood is dry and well seasoned before you use it. That means it usually has to sit around for a year in your shop before working with it.
Another thing is the Titebond. It is a water-based glue, and when you apply it, it dampens the wood because of the water in it. This causes the wood to swell, and the glue will dry in this swelled and expanded state, causing a back-bow. Using an epoxy to glue the fingerboard on will eliminate that chance, and it's a very large glue joint so most any epoxy will work fine.

[Mario Proulx]

Epoxy is the "cure" for this problem. Many will resort to a curved caul or other method to counter the backbow that the Titebond(and any other water-based glue) will induce, but epoxy solves the entire issue. Have no fear about epoxy's repairability, also, as it will release at about the same temperature as Titebond, so if a repair is ever necessary it won't be any harder to do.

[Nick Middleton]

I'm wondering how tight are you installing the frets? Maybe when shaping the neck, the reduction of wood allows the neck to give way to wedging of the frets?

In the future: I'd try fretting after shaping the neck so you can monitor the effects as you go and adjust accordingly, or install the frets on the fingerboard before glueing it on. That way you can make sure it's flat and no additional stress gets introduced.

Beyond that, you're fighting the normal movements within the wood.

[Rodger Knox]

There's several things that could cause this. First, the water in the titebond, as stated already. Second, the frets could be too tight in the slots, also stated. It could also be residual stress in the neck wood, which has not been mentioned. It doesn't usually show up on quartersawn wood, and usually the material removed in the final carving is not enough to make a significant difference.
Did you cut the neck blanks yourself, or were they slightly bowed when you got them and then you planed them flat? If you did cut them yourself, did they bow slightly as they were being cut? It's pretty unlikely residual stress is causing your problem, unless all the neck blanks were from the same piece of wood.

[David King]

I have experienced this problem for decades, long after switching to epoxy and then polyurethane glue and pre-carving my necks. I always use a curved gluing caul when gluing the fingerboard on to induce relief into my bass necks. After removing the clamps the necks come out straight and I then add more backbow to them via pre-tightening the truss rod and sand the camber/radius into the fingerboard under tension. After the truss rod is loosened there's just a bit of relief in the neck which disappears as soon as the frets are installed. Many folks will probably urge you to use a two-way truss rod to deal with this issue but I feel that it's asking too much of most of the rods I've tested over the years.
Where the backbow comes from is still a mystery to me but it's much more likely to be a problem with maple necks, much less of an issue with mahogany, wenge or walnut in my experience.

[Chris Richards]

Hi all

Thanks for your help, I'm guessing it's the Titebond I shall change and use epoxy. I buy the maple from an excellent luthier suppliers here in the UK and he mainly supplies quartersawn neck blanks but the worse bend I've experienced was on a piece of quite highly figured flamed maple so I'm guessing this was flat sawn. In the end I removed the fretboard and glued it back with the neck bent the opposite way to kind of pre stress it and that sorted out the problem but it's so disappointing to have to resort to this after finishing the neck and a few days later to consider throwing it on the scrap pile!

In my early days of building I experienced a neck curve due to the fret slots being to tight and have learned my lesson on that one. that's one of the reasons why I fit the frets whilst the neck is still a square (before shaping the back), it's so much easier to work with when everything is flat and stable on the work bench. But as you say removing wood after the frets are fitted increases the risk of the neck bending due to the increase in stress on the wood.

Do you think it would be wise to plane a fractional curve on the neck blank before gluing the fretboard on? at least then (using a single acting traditional truss rod) the range of movement would allow the truss rod to effectively move the neck from concave to convex? It's something I've thought of but never done. Mind you this will be a bit pointless when I use epoxy!

Thanks again
Chris