Adhesive Choice for a Maple Scarf Joint

A challenging problem: what adhesive to choose for a long scarf joint in a Maple drum that gets some stress from movement. January 20, 2010

I'd like to see what help there might be out there for a production application of solid hard maple snare drum shells. I am making prototypes that are bent, 1" thick, 6" wide, 12" id, 14" od, from one solid piece of maple. This is a lot of hardwood to bend, but it makes a unique product, with unique sound characteristics, and it is not currently on the market. It has a 10" scarf joint, about a 10:1 ratio.

My customer wants me to go into production on this, but I am still worrying over the adhesive. As Gene pointed out in a recent ipe thread, epoxies don't like to be clamped too tight, and this joint gets a very tough squeeze to bring it together (about 12 six inch C clamps tightened with a pipe extension). T-88 epoxy is popping about half the time. I prefer gap-filling, because the joint is so broad and will not be perfect (I can't get in there to clean it up after bending and drying).

A little elasticity seems to be warranted. This shell can exert an enormous amount of pressure on the joint if the moisture content changes, and one can assume this will happen with it being manufactured in Washington, and shipped unfinished to Southern California.

I am now testing PUs. Ashland makes the two part Isoset latex which I've tried and it seems okay. PVAs are not being considered at this time due to creep. Urea glues can't gap fill so I haven't tried them.

Note that the shell does not have a tendency to pull apart due to the bend. If the joint pops, the shell holds its shape. Joint popping just seems to be a function of slight movement in the wood. BTW, I'm gluing this at about 8% MC but it's a little hard to get a perfect read on MC for replicates because I don't want to destroy them with pin marks. I'll probably start weighing them before and after drying to confirm MC.

So here is the Holy Grail on this... I need gap filling, hard clamping, elasticity, and no creep. Is PU the closest thing, and therefore just a matter of getting the right PU? Look forward to your ideas.

Forum Responses
(Adhesives Forum)
From contributor V:
I'm not sure about some of the PUs you list, but most of the lit I have read says PUs do not have gap strength. So filling the gap here would be purely cosmetic.

However I am still trying to understand bending 1" 8% wood into that radius without steam or water of some sort. If you are using moisture and/or heat, that should be taken into consideration on the glue in drying as well.

From contributor A:
We were discussing the potential of squeezing epoxy out of a joint in the ipe thread. We are not talking about 1/32" glue lines. The problem is most often seen when a craftsman edge glues maple with the ideal protocol used for PVA (i.e. brand new jointer knives, straight as can be, with as much clamp pressure as you can muster). The PVA likes perfect joints.

Epoxy, due to its absorbent nature as well as the chemistry, prefers to have abraded that joint and the use of moderate clamping forces.

Those parts can be glued successfully with epoxy. T-88 is not your typical epoxy resin. When doing long scarf joints (10:1), you have the ability to let the epoxy soak in to those surface fibers for a few minutes. You have now created a localized wood composite, which you are using an epoxy resin to bridge to the opposing wood composite. This may in fact be the ideal joint.

Whereas with PVA and the like you are still using a surface friction adhesive. When building boats out of 1/8" veneers (often cedar), builders use the wood fibers just as you would use uni-directional glass fibers. The epoxy penetrates so far into the wood fibers that it is not a conventional glue joint by any definition.

I would stop the T-88 trials. Try the saturation protocol with more typical marine epoxy resin.

From contributor B:
Why not use a UF like Unibond 800? It's rigid, gap filling, and cure time can be accelerated with heat. Glue can be colored to match what is needed. Also, there is a company that makes flexible wood (I don't recall the name) that would easily bend into this shape. Oh wait, that's you - just saw the name.

Epoxy would probably work fine but if you let it soak in it will darken the glue line on maple and might be very noticeable. Might darken the line anyway, as joints tend to look dark with epoxy unless you add some color.

From Jeff Pitcher, forum technical advisor:
I'd agree with a UF resin. It will still allow the wood to flex but won't creep under stress. It's waterbased and much easier to use than a PUR. Granted, it won't gap fill, but an epoxy is really the only glue that will do that.

From contributor D:
I would think that you might want to consider a band that would be let into the drum body. This could be mortised in and glued with the joint of this ring opposite the joint of the drum body. You could then introduce a slightly raised element or contrasting wood, like cherry, walnut, etc. You could put it in the middle or put two of them spaced in from the top and bottom equally.

From the original questioner:
I contacted the manufacturer on Unibond 800 and they didn't recommend it because of my request for gap filling and said that only epoxy could do it. However, since I am clamping harder now, and reducing potential gaps, I'm encouraged enough by your responses to give the Unibond a try.

Yes, the gap filling of PU is cosmetic, but I'm no longer trying to get all of my Holy Grail in one adhesive, just as many as possible. Also, the shell is bent and dried before gluing, so the glue up is pretty standard for solid hardwood, except that I can't get into the joint area to prep it other than to pry it open about 1/2" to get the adhesive in.

Why trial marine epoxies other than T-88? West and MAS seem to have very similar products. By the way, I do saturate the fibers. Even so, some of those joints pop, probably from wood movement and a too-brittle adhesive.

The idea of the band is a good one. I've used it before, but this project is customer driven and needs to be just the one piece of maple.

I have tried using four 1/2" dowels to pin the joint. I just drill right through the shell four times in a square grid pattern in the scarf joint area during the glue up and insert the hardwood dowels. It seems to work well but adds ugly end grain to both sides of the otherwise perfect surface. I also recall reading in this forum somewhere that dowels don't add any strength, but don't know if this cross-grain insertion of dowels helps or not. I've stopped using the technique, though, because it doesn't look very good, and I doubt it adds anything to the joint. Comments on that point?

From contributor D:
Perhaps you could cut a spline cut in each end while the board is flat. Pry it open, wet it, glue or PVA, slip the spline in, clamp it.

From contributor L:
Given that your maple is not a figured wood, could you change the axis on your scarf joint? If the wood is holding shape, then changing the axis would let you pin the joint from the upper and lower edges where the pins would not be visible after the drum head is on.

From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
I also agree that UF is a good choice, but you can use epoxy if you make the surface unsmooth. Maybe you can make the surface slightly hollow ground. Or maybe several small (very small) grooves.

From contributor B:

You could drill for the dowels from the inside and make them blind holes. I can't see how they wouldn't add strength to a joint like this, especially if you angle them a bit like toe nailing in framing.

Depending upon your accuracy, you could also route in a dovetail slot vertically in each piece before bending, then add a dovetail shaped key during assembly. Would need to be pretty precise, though. I have used UF to fill small gaps and it works well, just depends on how big a gap you end up with. Have you tried epoxy with adhesive filler? It solves the glue line visibility issue and might add to the grip of the epoxy.

From the original questioner:
The thickness makes it quite unique. The maximum size for production steam bending this type of wood drum shell is about 3/8", then it's surfaced to about 5/16". I'm bending 1 1/8", then surfacing to 1". It's the acoustics that are interesting to my customer with the thicker shell (higher pitch), and this can't be duplicated with laminating because glue does not transmit sound like wood.

I've ordered the UF. Thanks for the recommendations on that. I've tried roughing up the surface for epoxy. I haven't done enough replicates with that, and may try again. Gene, do you think that hard clamping is going to cause too thin an epoxy line or should the rough surface take care of that?

Sounds like a few of you are in favor of increasing surface area with pegs or splines of some sort. I'll keep thinking about how to do it. This thing is tough to get on a mold, and things don't always line up perfectly after bending. 1/8" off on the scarf joint is not a big deal, but if I had to line up a groove, I probably couldn't do it repeatedly. So I won't be able to get as creative with the joinery as I might like. Great ideas though, and I'll keep them in mind.

From contributor L:
No, rather than to trying to increase the glue surface, I was trying to change the loading on the joint. If the pressure waves are opening the joint when it's approximately in the plane of the circumference, I wondered what would happen if you rotated the glue surface 90 degrees so that it's approximately in the radial plane. If you're experimenting anyway...

From the original questioner:
I'm considering trying a project with something similar. I believe you are suggesting a scarf joint that would run across the face of the shell. To get a 10:1 ratio on the scarf, I would need a 60" scarf joint, longer than the plank. The grain match would tend to give the diagonal line away too. However, I am going to try a spiral version of this product, where I take 1" x 1" squares, about 100" long, and spiral them around the mold. It will take several strips, create some more glue up issues, but gives me the option of building a taller product. It could also allow me to create a barrel-shaped product if I use a shaped mold.

From contributor L:
Wow, that's a fun project.

My impression of scarf joints is that their long ratio increases the gluing area while making any misalignment less significant. It's not clear that either is driving your problem given that the wood's static state is tight alignment and you machine down any irregularities.

Okay, talking through my hat time: My guess is that you'd not need that long of a ratio on either alignment of the scarf. So, see what happens realigning the scarf and only overlapping the joint over 90 degrees of the joint.

If you're experimenting, I'd also try avoiding the scarf entirely by using a simple splined butt joint. Spread the unglued circle to open the joint and (depending on how open you can force the joint) cut a mortise with a bandsaw or router (a cradle would be easy even for me) and insert a traditional glued in spline with the grain direction matching the shell. If the reason you're not laminating the shell is that the glue line alters the sound, then a splined joint would have the extra advantage of reducing the percentage of the shell that is disturbed by that glue line.

From contributor O:
How about a lap joint/scarf joint? This would allow you to get it together and tighten up the joint. Like contributor L said, a scarf joint across the face like 45 degrees and a lap joint for strength. You could put dowels into the joint as well, drilling from the inside out and not penetrating the face.

My all time favorite glue is Hercules by Westec sold at EB Bradley. It is a polyurethane but I have not had anything fail yet. Good luck - I am somewhat envious.