Tongue Drum Glue Joint Failure

[Cliff Rohrabacher]

I made a couple of tongue drums from maple over the winter season. They were rectangular. The sides were finger jointed the tops and bases were glued to the edges of the bodies.



Logically the end pieces of the bodies are at cross grain to the tops and bottoms. Because the bottoms are solid laminated maple they are tougher than the tops as the tops have several Saw Cuts in 'em to make for the Tongues.


Then after the recipients came to enjoy the drums the tops peeled away when the glue joint at the cross grain ends failed as the wood changed dimension from humidity changes.

Apparently the glue failure issue is identical to that which causes people to not over do the gluing when bread boarding the ends of table tops.

However, with this sort of construction one does not have the luxury of simply not gluing the cross grain joints at the ends. Failure to make this joint intimate and fixed would result in vibration.

I have to tear the tops off and remount ‘em. I have had a couple of ideas:
1.) To assemble them in-situ where they will live hoping to have a stable humidity – it’s an HVAC home with an older HVAC system.
2.) machining and leaving a small gap (oh say .010”) at the end grain portions of the top where it’d normally contact the sides and simply leafing off the glue.
3.) Doing #2 above but making the gap smaller maybe .002” - .005” and filling that gap with epoxy.
4.) glue the tops on the bodies with epoxy and pray.
5.) don’t glue the cross grain at the top / end intersection but pin it with a screw in the center and plug the hole.

And if doing any of the above (except #5) mount little glue blocks about ¾” across along the inside of the end pieces (like kerfing strips one does in a guitar to increase glue area) but make them at a cross grain with the sides so that they will be inline with the tops and still


Any ideas or thoughts ??

[Andrew Williams]

I've never worked on a project like this but I am wondering if the pounding and vibration might also have played a role in causing the separation. It seems like a tongue drum would be rather small for a joint to open up just from the weather after one season.

Like a guitar body, a tongue drum is a helmholz resonator, and leaving another air hole (gap) will change the resonance and therefore change the sound. If you built them to sound a certain way when they are all sealed up (except the saw cuts) then I would try to repair it so that the original sound is back. This also precludes making a glue block, since that would change the inner volume of air, causing a change in the resonant frequency yet again.

I am sort of leaning toward the screw and epoxy idea. It is a drum after all.

[Jamie Buxton]

This idea won't work as a rebuild on the existing drums, but on new ones, you could turn the grain direction of the end panels ninety degrees. The boxes are wider than they are tall, right -- kinda like a cigar box? Turning the grain of the end panels makes the cross-grain joint shorter, so it would be less likely to fail.

[Cliff Rohrabacher]

on new ones, you could turn the grain direction of the end panels ninety degrees.

Huh~??

The boxes are wider than they are tall, right -- kinda like a cigar box?

Yes they are. Flat-ish rectangles

Turning the grain of the end panels makes the cross-grain joint shorter, so it would be less likely to fail.

You mean to take this sort of thing:
box-001.JPG

And rotate the ends as the right hand end is on the below pic??
box-002.JPG

I'm confused.

[Don Palese]

Hi Cliff,

This past spring, I made 6 drums. I used dovetail joints and fresh glue. No failures reportred ...

Don

[Jamie Buxton]

Huh~??

Yes they are. Flat-ish rectangles

You mean to take this sort of thing:
box-001.JPG

And rotate the ends as the right hand end is on the below pic??
box-002.JPG

I'm confused.

Cliff, in your first pic, the grain direction of the side is front-to-back. The top's grain runs left-right, I'm guessing. That makes the joint between the top and the side a cross-grain joint. What I'm suggesting is that you turn the side's grain direction so it is up-down. This makes the cross-grain joints the ones between the front, sides, and back. Because this cross-grain joint is shorter than in the one in your current approach, there will be less stress built up when the humidity changes, and less chance of failure.

[Cliff Rohrabacher]

Cliff, in your first pic, the grain direction of the side is front-to-back.

My bad. I expected the grain direction to be understood as running the long way on any given board.
So for the top and bottom the grain runs the same direction as the long side pieces. At the ends the grain is at 90-Deg to the tops and bases.

The top's grain runs left-right, I'm guessing.

Yah as one views the image left to right is the longer plane and is indeed the grain direction.

[/quote]That makes the joint between the top and the side a cross-grain joint. [/quote]
The cross grain joint is at the two (shorter) end pieces of the box carcase yes.

What I'm suggesting is that you turn the side's grain direction so it is up-down. This makes the cross-grain joints the ones between the front, sides, and back.

That's how I built it. The longer runs have the grain in the same direction the shorter ends are the cross grain joints. There is no way I can see that I could make this cross grain joint at the short ends into any other form of grain intersection.

I wonder if the wood movement would be attenuated If in the future I built the boxes with many sides (not merely 4 sides). Such as an octagon (or with 12 sides) with 2 long sides and . This would make for a very short of 90-Deg cross grain.

[Cliff Rohrabacher]

Hi Cliff,

This past spring, I made 6 drums. I used dovetail joints and fresh glue. No failures reportred ...

Don

Like to see pics. Where'd you place the DTs?