Double Bass Gluing: where exactly?

[GretschWretch]

My University has its extended holiday intersession coming up, and one of the projects on my to-do list is to Franken-craft a three-string DB. Got my fittings, got my strings, got my body and neck/fingerboard, although there may be eighty years' difference between their ages.

The neck/body mate will be mortise and tenon, but the tenon is super narrow and will have to be built out some. A false floor also will have to be added to the top of the neck block to achieve correct alignment between the button and the butt of the neck heel; at present the heel rides low in the mortise.

My question regards gluing. I know that much of the work in holding the neck in place is done by the button and side-heel glue joins, but what about the bottom of the tenon and top of the mortise? Glue there as well? I have seen Kay basses faulted on these pages for that air gap between the base of the tenon and the mortise, but I also have read that repairmen expect that void to be there when they have to steam a neck to remove it.

Seems I am getting contradictory information and I do not know which fork in the road to take.

 

[s0707]

...
My question regards gluing. I know that much of the work in holding the neck in place is done by the button and side-heel glue joins, but what about the bottom of the tenon and top of the mortise? Glue there as well? I have seen Kay basses faulted on these pages for that air gap between the base of the tenon and the mortise, but I also have read that repairmen expect that void to be there when they have to steam a neck to remove it.

Seems I am getting contradictory information and I do not know which fork in the road to take.

I'll share the woodworking perspective: the bottom of the tenon is not a good gluing surface because it's the end-grain of the wood. The end grain absorbs a lot of the glue and starves the joint while it's setting. In woodworking, some people try glue-size (diluted glue) applied to the end grain and left to dry first to try to seal the end-grain pores, but I think the glue joint is still not strong enough. You could experiment with pieces of SPF framing lumber to see if you can get a strong joint.

 

[Steven Ayres]

I know that much of the work in holding the neck in place is done by the button and side-heel glue joints, but what about the bottom of the tenon and top of the mortise? Glue there as well? I have seen Kay basses faulted on these pages for that air gap between the base of the tenon and the mortise, but I also have read that repairmen expect that void to be there when they have to steam a neck to remove it.

You won't see that gap in a traditional joint; the dovetail is quite different engineering, taking the inline load on top of the joint where the traditional mortise takes it on the bottom. But in either case, glue on the mortise floor is essentially gratuitous from an engineering standpoint. The end-grain point that @s0707 made is correct, but going further, with or without end-grain the far more effective glue joints are vertical to the neck, along the line of tension rather than at a right-angle to it.

And yeah, the floor gap can be very handy for steaming.

 

[james condino]

Some generally accepted numbers:

-The endgrain at the end of the neck: best case scenario approx. 8% of the working strength of the joint, typically less.

-The Kay gap in the front of the neck joint is a vestigial organ leftover from them as a primarily guitar building company where it is common to leave a gap in the front of the neck joint under the 14th fret. When it is time for a neck reset: pull the 14th fret, drill two small holes for steam (one inflow and one outflow), blast the steamer for a few minutes, and the neck comes off easy-peasy. NOT such a great idea on a bass, as the tension is much higher so good contact is considered better for adhesion and less chance for a release point when impacted.

- each side of the neck joint : approx. 25%

-The same goes for the bottom of the joint, as the direction of pull is the same as the sides. When you add the extra length of the button, it is often considered. Approx. 30% of the total neck joint strength.

-Cut of wood ( quartered, off quarter, rift) runout, seasoning, glue type and wood species are other considerations.

Remember that a well fitting mortise and tenon of good quality materials is a completely different conversation than a mediocre quality joint filled with misc. "gap filling" adhesives like epoxy or such. The same with a well done two pieces of one mortise and one tenon is a completely different conversation than taking a mediocre joint and attempting to fill it with additional pieces.

Taking an old weak dovetail and then boxing it with six additional pieces of wood and the opening up the mortise is NOT a mortise and tenon; it is now a janky old weak dovetail compounded with six additional weak pieces of wood....

 

[GretschWretch]

My thanks to both Steven and James for the illumination. I will say that what I am working with is a true mortise-and-tenon construction; no dovetail. And I plan on using as few pieces of additional wood as I can (with hide glue, of course) to get good alignment and a snug fit pre-gluing. And close examination of the tenon reveals it to have already been adulterated with "gap-filling adhesives," so I have some clean-up to do before really getting down to business.

It's all good. It's one reason I venture into these (for me) uncharted waters: to learn.