walnut sides - how thin can they be?

[arbarnhart]

I am building two mandos in parallel - one with mahogany backs and sides and the other walnut. The walnut pieces are getting thinner than I had planned; they had some saw marks that ran a little deeper than I thought. They will probably end up in the 5 or 6 hundredth of an inch range. Is that too thin?

[labraid]

Wow, that's about 1.5mm, extremely thin. Walnut is already on the soft/pliable side. This is really borderline. You may get away with it or you may regret it. Tough call.

[Antlurz]

If it's really special wood, anyone have opinions about using it anyhow, but doing the "popsickle stick" braces inside like some guitars have?

Ron

[Chris Baird]

I usually go twice as thick as you've gone for walnut, I go with about .100".

[Michael Lewis]

Use it with a second layer laminated to it. This makes a very stable rim assembly if well and accurately glued. Many guitar makers regularly use laminated sides on expensive instruments.

[Luthier]

That is a might thin and walnut does bend very well. I shoot for between .093 and .100 for my sides.

Don

[arbarnhart]

It is decent walnut, but not out of the ordinary. I was trying for thicker, but for whatever reason the saw marks were deep in spots. I used a friend's bandsaw with a very agressive resaw blade. I think I will toss them; I would rather learn a lesson now than later.

[junksuph]

I just assembled an F body mandolin with curly sides that were thin to start with, and got thinner as the process went on. Now, on top of the scroll, the sides are paper thin in one small spot. This wood had a ton of figure in it and I could only successfully bend it in thinner strips. The mandolin has turned out beautiful and I refuse to scrap it because of one small defect that can be shaded when I finish it.

My question is...how can I strengthen this small section of thr scroll rim? I am going to sun-burst the finish. I thought about using some kind of two-part resin to coat the area, thus adding thickness/strength. The surface would be sanded to take a die in the shading process. Anybody have any suggestions? Thanks in advance

[arbarnhart]

Junksuph,

Am I right in guessing it is too late to do anything on the interior side? When you say "two part resin", you mean epoxy. I would think a little thickened epoxy spread on the inside would be best. Maybe a small paint brush could reach it through the F hole. Wait for others to weigh in; I am a relative novice.

[thistle3585]

If I understand correctly, and the side is thin on the scroll, don't you have a huge chunk of mahogany on the backside of that thin part? Am I understanding this right.

I've also wondered about making laminated sides. I've wondered if it would an easier process and be able to use material that may normally crack under the heat and pressure of convential bending.

[arbarnhart]

The chunk of wood is only on the neck side of the scroll. There is a thumb sized cavity with just the thin side on the upper part. There is discussion ad nauseum about what effect that cavity has on the sound.

I might try the laminated approach also, just to get a feel for it. If it goes well then I would probably use it. You can get some outstanding figure in real wood veneer that would save time and effort (possibly money as well).

[junksuph]

My "thin spot" is just prior to where the Mohogany block starts. Yes, it is too late to reinforce from the inside. my only choice is from the outside. I figured that either a two-part epoxy or fiberglass resin would be a good strengthener, and I could hide it with a die. I realize that a stain would not penetrate the resin, but after inspecting my first mandolin, (Kentucky 630), I know that there definately is hope for this little problem. Structurally, the instrument is strong where it really counts, and isn't being made for resale. I made it to honor my mom, and hopefully it will be an heirloom to pass on to my kids.

[thistle3585]

This might work, at least it did on my boat, drill a hole in the side then squirt some epoxy inside with a syringe. Hang or clamp the instrument in a manner that allows for it to pool in the thinnest area. Then put some tape over the hole. When dry just sand and finish. Just a thought. You might even add some filler like microlight or such to give the epoxy a little more body to it.

[arbarnhart]

It is from my boat work (I have scratch built two small boats) also that I learned what I know about epoxy. It is not too late to put epoxy in there, especially since you said it is right at the juncture. Mix some epoxy up and then suck some up into a drinking straw (in front of a mirror so you can watch the level rise and keep it out of your mouth). Put your finger over the top of the straw and turn the straw upside down. With the mandolin pointed straight up (like it was hanging on a wall) carefully feed the straw through the F hole (I would put masking tape around the F hole just in case) and into the scroll cavity. Turn the mandolin upside down, take your finger off the straw and blow into the straw. Carefully pull the straw out. Tilt the mando slightly and rock it as you do. The angles should be obvious - you are making it flow around and spread on that thin side. Secure it for drying.

Before you do that, think about it some and wait for others to post alternatives. I doubt seriously it will have any measurable effect on acoustics.

I have used epoxy for external repairs also. It is tough to hide it unless it is in a large surface area where you can contour a gentle rise to the bump/mound. In an inch and a half, there isn't enough room to play with and if there isn't a bump, there isn't enough to do much good.

[Antlurz]

I was going to suggest the exact same thing as arbarnhart, but I would, instead, use a piece of surgical tubing or tropical fish aquarium airline tubing. I'd think it would give you better control than a drinking straw when fishing it through the "F" hole. I'd think the straw would collapse before you got it bent to the right direction.I've patched hundreds of leaky motorcycle gas tanks, using the same method and fiberglass resin. It works like a dream.

If you could come up with an old medical syringe, sans the needle, you could attach the tube to it and draw it up into the syringe body if it required that much.

Ron

[arbarnhart]

I thought about suggesting a "bendy" straw, then looked at a picture of an F model and realized that the F hole points almost directly at the scroll and the distance from the top of the F hole to the scroll is only a few inches. Also, because the straw is rigid you can hold it upside down and have no dripping while positioning it and only release it when you have it in the right spot. Besides, they give them away at McDonalds...

One quick nit - "fiberglass resin" is an ambigous term because it can mean epoxy or polyester resin used in some glassing operations. Polyester resin is a poor adhesive. It's good for pouring over cloth in forms to make fiberglass parts, but the wrong choice for this application.

I had an Ossa off road bike in my teens and it ended up with a bit of epoxy on the tank as well.

[Antlurz]

Yup. A dry run or two with the straw or whatever to make sure it can be done would be a smart move before one attempted it. Acoustically, I doubt it would make any audible difference at all as long as it's all up in that corner.

Ron

[arbarnhart]

I cut another set of walnut. This time I did it on the tablesaw and gavr myself some margin for error. Of course the faces came out smooth as can be but I have to thin it anyway. I still may experiment with laminating the the thin pieces.

Edit - got the new side down to size the old fashioned way - hand planing. Something about that sound like carving turns in the snow and those walnut curlies piling up on the bench just makes me feel good...