Best way to repair router tearout

[Bob Schmidt]

First build has been going well until now. Even after practice on scrap and cutting the binding channel at 2 depths, the back managed to tear out. I have attached a picture. What is the best way to go about repairing this?

Thanks in advance
Bob Schmidt

[sunburst]

A couple of things:
1. looks like the glue joint was not the best. The back broke loose from the lining cleanly right on the joint. If the glue joint was of high quality it would have broken in the wood of the back or the binding... or perhaps it would not have broken at all.
2. Climb cut to help avoid tear-out.

With that out of the way, the best repair is to use wood from the scrap from the back, as close as possible to the area. In other words, hold the scrap up to the back like it was before cutting and use the part right next to the missing part.
Avoid cross grain joining. A "V" or a "football" would be common/usual shape that a repair person would use. Fit it very well, glue it in place and proceed with the build.

[Bob Schmidt]

Thanks John.
I am using hot hide glue which I am also new at. On the first attempt at gluing the back the glue was too thick and started to set too quickly so I removed the back, cleaned the pieces up and mixed a fresh batch. I thought the second attempt went well, but it should not surprise me that it did not. Even with using a heat gun on the two surfaces, it is a lot of area to glue and clamp in a short period of time.
I am not sure what you mean by "climb out". I had read that in that region where it is prone to do this you should move the piece with the rotation of the cutter which I was doing. Is that what you mean?
To avoid gluing cross grain, would it make sense to cut diagonally to the edge as if it were the one side of a V cut? If so how far up would I take it?

Thanks for your response
Bob Schmidt

[sunburst]

"Climb cut", once we know what it is, seems self-explanatory. Yes, it is cutting in the direction of rotation so that the cutter "climbs" into the cut. It can be difficult to control in some situations and even dangerous in some situations, but when making light, repeated cuts with small rotary tools it can be easily controlled and can yield cleaner cuts in wood when the grain is not favorable for cutting the other direction.

You sort of have a compromise situation here. The more parallel the patch is with the grain the less visible it is, but in this case, the more parallel to the grain, the larger the patch.

A "V" would look something like this:


A "football" would look something like this:


Extending farther into the back would give you glue joints more parallel to the grain but a bigger parch. Besides being less visible, joints that are more parallel to the grain can yield stronger glue joints. Straight joints ("V") might be easier to fit, curved joints ("football") might be less visible. The joint can be beveled also to increase surface area for gluing and to help control the position of the patch. Most importantly, the patch must fit well for strength and for cosmetics.

[ImTheMan_do]

Okay, I've built quite a few things. I've never built a mandolin though. So I'm not going to try to give you techniques because really I don't know. I know you say this is your first build, so I assume you want it to be perfect and look flawless, but even if something like this didn't happen the first of anything is typically not as perfect as desired. So, my - probably stupid - thing to consider would be doing something crazy like getting a completely different type or wood with different grain patterns, maybe a very light wood, and just make it unique! I'm sure you'll take one look at this and think that it's stupid, which I would too if someone told me to mess up my my beautiful hard work with a random piece of wood, but I think you might like it and look back on it fondly when you've made many mandolins and see that dumb looking patch and it reminds you of the difficulties and fun of building your first mandolin. I think that is why it would look good to do that. Eh, just something to think about. I'd love to see more pictures! It looks awesome in the little section I can see.

[sunburst]

Along the lines of what ImTheMan_do is saying, there are a lot of instruments out there in the world with "design features" that came about because of mistakes.
I've seen at least a few with some sort of inlay just inside the binding... a little pearl critter of some sort peeking out from the binding, an inlaid leaf or twig twining out from the binding... any time you see something like that, you just might be looking at the site of a mistake!

(One well know example is the electric guitar played by Carlos Santana that put Paul Reed Smith on the map. It had an abalone purfling along the center seam... can you guess why? Yep, because the Paul's jointer couldn't handle the curly maple without tear-out. Ugly glue joint? No problem; cover it with pearl!)

[Bob Schmidt]

I would not consider this a dumb suggestion, but I chose rosewood because I wanted a bit deeper tone then the production bouzouki's have. The body size will also help with that. There are other woods that I think look really nice and in time I hope to explore that, but the goal of this build was not just to get familiar with the craft, which I very much want to do, but it was also to have an instrument that sounds better then the one I currently have. It is getting close to completion now. The neck is practically done except for making the nut and finishing. What is left on the body is the binding which I am finding a bit more challanging then expected. I have already broken 2 binding strips bending them on the hot pipe. It looks like I will have to order several more spares to get them all bent. (Or get better at bending)

[Bob Schmidt]

Cover it with a pearl!! I love it. Will have to think of something appropriate.

[Dale Ludewig]

A pearl football!

[CarlM]

The problem with the glue joint may be due to oil in the rosewood rather than your gluing technique. It has to be degreased thoroughly or it will not stick. You might want to try gluing up some scrap pieces to make sure your glue is sticking. I have sometimes had to degrease it more than I felt was good.