Scarf joint strength / failure... Add splines? is it needed?

[Rangerdave1]

So I am pretty new at building. My first build came out great, sounds wonderful, has been stable and plays well. I did a scarf joint under the fingerboard for the neck with no issue. Now on to a few new builds I am presently working on, I did the same type of joint however I was curious how strong it really was. It was plenty strong when just a blank 3" all the way across before I started carving as I stood on it and bounced up and down.

Once I carved the neck and was 95% done I thought I'd flex it and see how strong it was. Well to my surprise it broke with some force just using my hands and arms. Now there was no fingerboard on it which I believe adds a lot of strength to the joint but still.. The surfaces were very flat before I glued them, Things were clamped well. As I inspect the failed joint, the wood is what appears to have failed. Now I am worried that there is not enough strength in this joint.

Should I consider adding a spline or some sort of reinforcement in the joint?? Was I expecting just too much out of this joint given the amount of wood there once carved? Will it be plenty strong enough once the fingerboard is glued on with a trust rod inserted in to the slot?

I drew out where I think a spline or carbon rod could go on each side of the truss rod slot but is it necessary?

What are your thoughts and experiences?

[charles Tauber]

It depends on a few things. First, where is the joint? You stated under the fingerboard. I’ve seen a few people do this, but I’m not sure why or where this practice started. Traditionally, the joint attached the head beyond the fingerboard.

Second, It also probably matters whether or not the short piece is glued on top of the neck portion or beneath it. It will probably be stronger if glued beneath.

Third, it depends on how much of the joint remains after the neck and head are shaped. With the joint moved to the head, most of the 3” width of the joint remains.

Fourth, it depends how much wood, if any, is removed for the truss rod.

I’ve been using spliced heads on guitars for four decades and never had one fail, not on steel string, classical or arch top guitars. I have never found it necessary to reinforce the joint. On steel string guitars I have the truss rod adjustment through the sound hole: on arch tops, the adjustment is at the head. If your scarf joint is weak, I’d look to the design and implementation you are using.

[Bruce Sexauer]

Traditionally, a scarf joint requires a minimum 5 to 1 aspect ratio. That means that for every inch craps’s grain, there must be five inches along the grain. This translates well to the typical 15 degree angle the head would scarf to the neck at. Scarfing the neck to the head instead of the head to the neck can cause a stress riser leading to join failure. Any such joinery needs to be pretty much perfect, and clamping properly needs to be well thought out.