Acoustic Bolt on neck query

[vandenboom]

I have always used connector bolts side on through the tenon (Cumpiano method) and glued fingerboard extension to the top on previous steel string acoustics.
On guitar just finished, I followed Trevor Gore's design involving the pocket routed in the guitar top and a metal rod inserted vertically in the heel. Trevor makes the point that if you use circular rod, that the heel could split, hence why he uses square rod. Because I didn't have the square morticing tool required, I took the risk and went circular.
In order to continue this approach, what do you think about gluing 2 or 3 well placed hardwood dowels sideways through the heel tenon to 'stop' the chance of splitting?
Naturally, also interested in other ideas to deal with this which don't require outlaying a couple of hundred bucks on another tool.
Thanks. Frank

[tippie53]

I use a bolt on rarely but I use the threaded inserts. The ones I use are a brass with better side fins. There is a better contact and it is the same one Martin used for years. Don't know of a single failure.
My personal preference is dovetail.

[kalimna]

I am a complete beginner at this, currently working on my first instruments, but I am wondering why a circular rod would make the heel less stable than a square one? Surely forces are concentrated on corners, such as exist on a square cross-section, which is where failure occurs?
If I have missed something obvious, apologies.

Cheers,
Adam

[Craig Bumgarner]

I use the Cumpiano method and have not had a problem. It is important to have plenty of tenon and the cross dowel holes drilled as close to the root of the tenon as you can. My tenon's are 20mm thick by 25mm long. I use a 10mm cross dowel, leaving 15mm of wood.

I occasionally use threaded insets, most often when converting to bolt on after a neck reset. I've had one threaded insert pull out. I fixed it with epoxy and now run the inserts in with epoxy every time, no problems since.

[Trevor Gore]

I am wondering why a circular rod would make the heel less stable than a square one?

The circular rod acts as a blunt wedge and can split the heel if the bolts are torqued up too much.

Often, those using barrel bolts cross-laminate the tenon sides to help prevent splitting, but for a "vertical" bar the laminate has to be across the end grain. Frank's dowel idea will help, but I've never seen it done.

It's possible to hand chisel a round hole to square with good results if you're handy with a chisel. Also, the bolt bar hole can be drilled right through to the truss rod slot so the hole can be worked square from both ends.

I had a long talk with Colen Clenton last weekend and talked (amongst many other things) about gluing brass to wood. Not as simple as just mixing up some epoxy...

The mortise attachment seems to have disappeared from the Carbatec catalog (used to be ~$80 and is what I've always used with no problems) but McJing still lists one.

[kalimna]

Trevor - thank you for your reply to my query. I understand your explanation of a blunt wedge vs flat. However, I wonder if the lack of a wedge effect is not offset by reduced integrity of the square hole? I am not an engineer, nor have I any serious experience in luthiery, but I do like to try and understand things in my own head (a dangerous place to be, if ever there was....)
Also, is it a theoretical problem, or do the neck tenons actually split in use without reinforcement?

Cheers,
Adam

[Trevor Gore]

However, I wonder if the lack of a wedge effect is not offset by reduced integrity of the square hole?

In practice, it isn't. The square bar loads the wood in shear, parallel to the grain and has a relatively high load to failure. The round bar loads the wood in tension across the grain and has a relatively low load to failure compared to shear loading.

Also, is it a theoretical problem, or do the neck tenons actually split in use without reinforcement?

They can split if you torque the bolts up too much or shock load the neck forward at the head stock. Gerard, who developed this joint method, did the physical tests ~25 years ago. If he'd thought the round bar was up to it, he'd had left it at that.

[klooker]

I've used threaded brass inserts which seem to work fine if you can get them into the hole straight.

On my last build, I used hanger bolts that engage into a vertical dowel like Collings Guitars does. I assume this is a sound technique because I haven't heard of Collings having failure issues?

Kevin Looker

[quartpopt]

I've been thinking about this for my next build. I can't justify a mortising m/c or attachment for just this.
My thought was to upend the neck on the TS, then cut a slot the width of the square bar. Would need a simple jig for safety. I would dry fit the bar, and fill the remaining slot with a glued in piece of good quality ply. Then remove the bar leaving the square hole. Any thoughts?