MIMF

I've got to glue a part on to my latest instrument and the mating surfaces are not perfect and, it's pretty much impractical to make the perfect for this project. The problem is that the curved piece was made from "compwood" which twisted some and increased in curvature since the initial drying stage. With some reasonable clamping pressure, I can get it to be a pretty good fit but I know that there will still be some gaps and voids hiding in the joint. My plan is to use epoxy (system 3 T88) to make up for the imperfect joinery. I'm also going to use some mechanical fastening as well, some wedged dowels orthogonal to the joint.

I've read to not use too much pressure when glueing with epoxy but I also know there's a lot of mythology surrounding adhesives in general. I'm wondering what peoples' thoughts are on this plan and whether y'all think it might work. This is a high stress joint (the equivalent of the scarf joint on the neck of a guitar) and a failure would be spectacularly catastrophic!

Thanks

Just my opinion, but using epoxy should work fine. I'm sure more people will add their experience.

Epoxy is subject to creep, so it might be good now, but might fail later.

Can you somehow rebend the pieces?

A little bit of a wet towel on one side of the board for 10 minutes would cause it to swell and warp. It might properly fit at that time, but might also return to it's previous shape, or assume another shape when it dries.

If the wood is not too thick, you could consider just heating with a clothes iron, and trying to get it back to a better shape.

Andy,
I'm totally lost...what are we looking at? And where does the part in question fit in?

I have used epoxy for years and believe it to be the best gap filling glue (sometimes used with the micro-balloons). The main context of use was laying up laminated curved panels in furniture. I also use it when certain shapes make for difficult clamping. I think its reputation for creeping is due to a bad mix or poor quality epoxy. I use T-88. Plastic resin glue also works well.

Oh- I forgot: I go for medium clamping pressure.

Randy Roberts wrote:Andy,
I'm totally lost...what are we looking at? And where does the part in question fit in?

'

Here's a picture of the gap. The piece I'm gluing is the curved part in the middle of the shot. All the strings come from that point. It's sort of the "nut" of a bandura. Compwood has a tendency to reverse-springback i.e. if you bend it to some radius, it will increase in curvature when removed from the form rather than springback like normal. It also twisted on me a bit as well.

Judd, thanks, that's pretty much what I was hoping to hear.

Well, I did the epoxy thing and so far everything looks o.k. To help mitigate any creep, I did added 5 1/4" dowels that I slotted and wedged into position after ovaling the holes somewhat. The true test will be when I string it up.

I've glued together an entire electric solid body bass guitar with epoxy. It was made with many laminations. I did that back in 1994, and it's held up with no creep or anything since then.

I used G3 epoxy made for wood.

I still use epoxy for things like gluing on fingerboards.

For whatever reason I've always had better luck with the West systems epoxy than with the System 3 brand. Clamping pressure seems to be less critical.

Apparently not all epoxy is not created equal. I'm having a brain cramp and can't remember his name but at one of the ASIA Symposiums I heard a well known and respected mandolin builder say that he attached his necks using a plain butt joint and some sort of high strength aircraft epoxy. He was a bit evasive when asked exactly which epoxy it was. The impression I got was that it might not be available to the general public and he didn't want to say how he got it.

It's my understanding epoxy just needs moderate clamping. You need a thin layer of glue between the surfaces and too much clamping pressure can actually weaken the joint.

If you want some really strong epoxy, get West Systems and add some Microfiber 403 filler. It takes a really good epoxy to a whole 'nother level.

I usually wet the wood surfaces with straight epoxy then add the filler to the pot and spread a layer of this goop on one surface. I use this for any joint where epoxy is useful including laminating sides.

The microfiber filler basically turns the epoxy into a composite material which has increased fracture strength. I confirmed this by taking a hammer to blocks of dried epoxy. Without filler the epoxy can be broken by a moderate hammer blow. With the filler the epoxy can withstand a fairly heavy whack.

Barry, that's very interesting, I wouldn't have thought a "filled' epoxy would give you a better bond. Is the 403 a West product?
Top of the line epoxies are made by Araldite and formulated for very specific tasks. They come in tiny quantities and cost a small fortune but if you need to bond a carbon fibre tube to a titanium lug and have it withstand 50Gs at 500ºF, they make exactly what you need.

Yes 403 is a West filler. They have some others, but the 403 is the only one that includes fibers to get the added strength. The strength gain is in the glue itself, not its bond to the wood. So the benefit is mostly realized when you have thicker layers of glue. A really tight wood joint will probably not gain any in strength because most of the glue gets squeezed out.