Double Bass Dovetail Joint ?

[Matthew Tucker]

But on basses (especially those with sloping shoulders), where the ribs intersect the neck at a steep angle, such a wide mortise would cause the upper corners of the neck block to be thin and pointy, thus quite weak and incapable of offering much support to the joint.

So I don't doubt what you are saying, I'm just surprised that this is standard contemporary practice. It seems to me that having what a cabinetmaker would call 'cheek cuts' on either side of the tenon would make the whole joint more robust.

There is little or no lateral force on the cheeks, the force on the joint is totally leverage forwards. So with a well fitting glue joint at the bottom, sides and button, it holds very well.

Sure, there's not much sideways strength but unless the bass takes out a door jamb quite hard, it's not going to be needed. However some neck joints are better than others - Rubners and other german models can tend to have thin mortise cheeks whereas any bass with a decent decorative strip along the sides has enough robustness for most situations.

The other thing to consider is that with a well fitted hide-glue joint, it will be perfectly strong enough for normal use
but if knocked hard, the hide glue joint can "pop" like a safety valve with minimal damage to the block, and be easily reglued. Well that is the theory anyway. It works some of the time

 

[gbaker]

tstone,
Here is a German factory student bass that I worked on. You can see how simple the mortise is.

http://s657.photobucket.com/albums/uu298/ghbaker/D Bass Mortise and Stuff/

This one had failed at least twice because it had be incorrectly repaired with white glue when I got it with the neck off and it was a SOB to clean up. Also had to rebuild the cheeks of the mortise, you can see how shredded they were. The neck is a full, tapered (front to back) tendon as Jake said.

There is also an assembled shot before the trim was installed to hide the joint.

As Matthew said the sideways forces are not great unless trauma occurs. If this bass had had a Kay neck/dovetail it would have had a cracked neck at least, I think. With the simple shallow mortise the neck rolled out and the damage was cosmetic.

Note how important the button is in transferring the forward tipping force from the string tension to the back and body.

 

[tstone]

>> If you'd take the trouble to visit the second link I posted and look at some of the photos, you'll see a giant honking neck block in there which is where the 'robust' comes from.​

Yes, I have studied those pictures, and that neck block is indeed substantial. I have taken the liberty of posting a picture of it as the first thumbnail below.

But I think you would also agree that that bass has much squarer shoulders than typical contemporary basses, and also has that unusual S-curve in the ribs right before they join the heel. On a bass with more sloping shoulders (second thumbnail) I think it is easy to visualize that if the mortise is the full width of the neck, then once the overhanging lip of the top and the thickness of the ribs are taken into account, the top corners of the neck block are reduced practically to feather edges.

So with the trend towards sloping shoulders, perhaps the design in that area has become a bit unbalanced. That's really my only point, I'm not trying to start an argument.

 

[mesmithnm]

tstone,
Here is a German factory student bass that I worked on. You can see how simple the mortise is.

http://s657.photobucket.com/albums/uu298/ghbaker/D Bass Mortise and Stuff/

This one had failed at least twice because it had be incorrectly repaired with white glue when I got it with the neck off and it was a SOB to clean up. Also had to rebuild the cheeks of the mortise, you can see how shredded they were. The neck is a full, tapered (front to back) tendon as Jake said.

There is also an assembled shot before the trim was installed to hide the joint.

As Matthew said the sideways forces are not great unless trauma occurs. If this bass had had a Kay neck/dovetail it would have had a cracked neck at least, I think. With the simple shallow mortise the neck rolled out and the damage was cosmetic.

Note how important the button is in transferring the forward tipping force from the string tension to the back and body.

I'm just following this thread as a casual observer, but these pictures really finally explained to me what was going on. Thanks!

So the tension from the strings gets transferred through a moment arm (the heel) to the button/neck glue joint, where the glue joint is almost in pure shear and I presume from my experience (not with basses) that a glue joint in shear is many times stronger than a glue joint in tension. The glue joint from the bottom of the neck to the block just provides strength to keep the neck from twisting or such. I would think that if you never had to remove the strings (remove tension from the neck) you could leave this joint unglued and the friction between the heel and neck block would hold things in place just fine. But I'm just speculating as I have no firsthand knowledge.

Mark

 

[Jake deVilliers]

I'm just following this thread as a casual observer, but these pictures really finally explained to me what was going on. Thanks!

So the tension from the strings gets transferred through a moment arm (the heel) to the button/neck glue joint, where the glue joint is almost in pure shear and I presume from my experience (not with basses) that a glue joint in shear is many times stronger than a glue joint in tension. The glue joint from the bottom of the neck to the block just provides strength to keep the neck from twisting or such. I would think that if you never had to remove the strings (remove tension from the neck) you could leave this joint unglued and the friction between the heel and neck block would hold things in place just fine. But I'm just speculating as I have no firsthand knowledge.

Mark

You've pretty much got it!

 

[tstone]

>> Note how important the button is in transferring the forward tipping force from the string tension to the back and body.​

Thanks for pointing that out, gbaker. I hadn't hitherto appreciated what an important structural role the button plays, but it makes sense once you think about it. That particular button-to-heel part of the neck joint is also face grain to face grain, which of course is much stronger than face grain to end grain or -- even worse -- end grain to end grain.

 

[TroyK]

I don't spend a lot of time in the setup/repair forums because I accepted long ago, that most things are best left to a trusted luthier. But, I couldn't resist clicking on this thread and let me just say two things:

1) You folks are really smart and it's comforting to know how well you all understand these instruments.
2) I thought this thread was going to be about something completely different.

Keep up the good work.

 

[Jake deVilliers]

I don't spend a lot of time in the setup/repair forums because I accepted long ago, that most things are best left to a trusted luthier. But, I couldn't resist clicking on this thread and let me just say two things:

1) You folks are really smart and it's comforting to know how well you all understand these instruments.
2) I thought this thread was going to be about something completely different.

Keep up the good work.

LOL!!

 

[Matthew Tucker]

Tstone, look at it this way.

Imagine you saw off the neck just above the D stop and attach the strings there instead. Even on a bass with thin block sides, you have about an inch and a half of leverage pulling down due to the strings. You have around 20 square inches of hide glue joint resisting that pull. It feels because the neck extends up a foot or two that there's more leverage than there actually is.

Of course, if the bass takes a big knock at the scroll the leverage is much more.

But I can tell you, removing a well-glued neck from a bass just by wiggling and knocking is no piece of cake.

I guess I'm saying that in practice, the thinner mortise sides don't really seem to be much of a problem unless they are damaged or the neck tenon is badly fit in the first place.

In my opinion the button on a bass is much less critical for strength than on a violin where the mortise is much shallower and the button is proportionally much larger. Some basses after a neck reset have virtually NO button and provided the neck joint is good it's not a problem.