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mboursnell
Jul-15-2012, 9:25am
I have a crack in the soundboard of my home-made mandolin. This was caused by the braces coming loose, so I had to remove the soundboard (using heat). Now I want to glue the two halves together but I have a problem in that they are both warped and when you lay it on a flat surface the edges do not meet.

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What is the best way to get these two halves flat, and what is the best glue to use?

Mike

sunburst
Jul-15-2012, 9:41am
There's no easy way. Your best bet is to make a jig of some sort to clamp the pieces flat yet allow the crack to be clamped tight. A series of little "stops" made of pieces of wood screwed to a flat board, to fit around the edges can facilitate clamping the crack closed, but aligning the edge at the split will require something else. It's hard to get into specifics because each time something like this comes up in the repair business it means a different approach and different jigs.

EDIT:
I just remembered...
You can also glue small pieces of wood to the surface on each side of the crack, in pairs so that they are opposite one another, and use them to support clamps for closing the crack. Of coarse, there still needs to be some way to keep the pieces flat and the crack aligned.

mboursnell
Jul-15-2012, 10:56am
OK, thanks. What sort of glue should I use? Titebond or cyanoacrylic? Can I use the low viscosity CD glue and allow it to seep into the crack whilst I am holding it closed?

Do I need to get the warp out first?

sunburst
Jul-15-2012, 11:11am
If you can get the warp out first, that would be best, but getting the warp out without disturbing the alignment of the crack can be a problem. If the wood is clean, the alignment is good, and the fit is tight for the whole length of the crack, hot hide glue is best. CA might work (with similar caveats), but I wouldn't trust it for a good, durable bond. Titebond fills gaps a little better than hide glue, so it could be a second choice if fit and alignment cannot be made good (as is likely).

bmac
Jul-15-2012, 2:35pm
From the photos it seems to me that the wood has shrunken to cause the crack and the warp..... (if I am reading the photos correctly), It is likely the wood wasn't seasoned before you used it. I suspect your crack is next to impossible to repair effectively. I would build a new top.

However you could cut a very thin sliver of wood to fill the crack and glue that in place taking up the space of the crack... That way it will not want to come apart... But if I am right on the shrinking of unseasoned wood there may be more shrinking in store for you. which equals more cracks. if feasable I would build a new top using well seasoned wood.

belbein
Jul-15-2012, 3:32pm
OK, thanks. What sort of glue should I use? Titebond or cyanoacrylic? Can I use the low viscosity CD glue and allow it to seep into the crack whilst I am holding it closed?

Do I need to get the warp out first?

I don't have anywhere near the experience with wooden instruments that people like BMAC and Sunburst have, but for what it's worth ... IMHO it ain't good. Cyanoacrylic isn't going to do it for you. It actually doesn't hold very well under stress. I'm not sure about the titebond, either. If it were me, and I didn't want to actually recarve something, what I'd do it figure out a way to get it flat, then on the INSIDE, do very shallow straight cuts with a mini-router vertically across the crack. Then I'd sand it flat. Then I'd use fiberglass and 2 part epoxy to cover the crack. Structurally, that'll hold. What it will do to the sound--or more cracks as it ages--I wouldn't want to guess.

mboursnell
Jul-16-2012, 2:50am
Thanks for all those comments guys. Here's the crack from inside. Maybe the other two pictures were a bit deceptive (making it look as if it had shrunk). The mandolin was made in 1976 so the wood has settled by now.
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Grommet
Jul-16-2012, 3:27am
If it is was truly a flat board prior to the crack...I'd obtain a flat piece of MDF or particle board and cover with wax paper or mylar for a work base. Make sure it's large enough to fasten three flat 1 x 1 straps across it to keep all the pieces flat. Let the cross pieces sit atop spacers of same thicness as the top. Tighten the straps a bit loosely at first so you can glue with Tirebond ! (Red Label) and then snug the crack together with clamps. Instead of clamps you might instead want to screw 4-5 small wood block stops along both outer edges as Sunburst suggested, then use small wedges between the stops and the outer edge inorder to snug up the crack after applying the Titebond. This methid would give you better conrol over alignment. Re-snug the cross pieces gently after gluing.

Scott

mboursnell
Jul-16-2012, 1:06pm
Sounds good. Thanks. I think it was flat. I think it cracked because the braces came loose underneath. I had glued them with hide glue made from boiling up solid pieces in water. I suspect the resulting mixture was not perfect and so worked itself loose over the years. Maybe the transverse braces didn't help. I think I might go for an "A frame" bracing this time.
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belbein
Jul-16-2012, 1:32pm
I guess I don't see how the braces coming loose would cause this. I mean, if you had pressure running from a line drawn from the middle of the sound hole to the butt block--with weight evely distributed all along that line--maybe then the braces giving out would allow this. But at best it seems to me that the braces are irrellevant to the problem: as my wife would say, they are "sequellae," not causes. It seems morely likely to me that whatever that force was, it pulled the braces loose.

I had my own problems with hide glue, and I'm sympathetic. But I still don't see gluing as a fix.

HoGo
Jul-16-2012, 2:20pm
Boys it's just crack. all you need is some patience and glue to get it together well. I second advice on using several pairs of flat bars with wax paper across the plate and clamp them together to hold it flat while gluing (perhaps some wetting and letting it dry while clamped between the bars before gluing). I'd try gluing the small wood blocks with one drop od gel CA to each side of crack and pulling them together with tiny c clamps. Hide glue or Titebond should be OK.

IMHO it ain't good. Cyanoacrylic isn't going to do it for you. It actually doesn't hold very well under stress. I'm not sure about the titebond, either.
I used CA to put together top of old bass. Cracked into several pieces (bass bar crack all along the top) and half dozen old cracks needed to get reglued were not worth standard attention so I used several bottles of CA and held the pieces together while the glue dried. No clamps needed. Even less mess then I expected. The bass works fine now, some 10 years later, no new cracks or signs of failing. Just one open seam between top and sides which was glued with weak HG. And the bass got some seriously rough handling and humidity "tests" in the last few years. Jam sessions, camping etc...

Marty Jacobson
Jul-16-2012, 3:17pm
CA glue is a great fixturing tool. Do not think of it as glue, think of it as double-stick tape in a bottle. The joint will fail, but if you're using it for a jig/fixture, you eventually want it to fail, right? To remove your work.
Surgeons use PMMA acrylic with similar properties as "bone cement", so that an implant will hold tight but can be removed if there is a problem with the implant.

Anyway, the main point I was going to make is that there is a "traditional" method of jointing this kind of plate. I couldn't find a good ref for it off hand, but here's a link:
http://www.woodcentral.com/woodworking/forum/archives_handtools.pl/bid/3109/md/read/id/139711/sbj/spanish-luthier-s-clamp

Maybe you could do something like this? Remember to joint the edges of the plate first on a shooting board, jointer, or something.

mboursnell
Jul-16-2012, 3:43pm
That "Spanish Luthier's Clamp" with rope looks amazing! What a clever idea. I'll look at all the suggestions and have a go with one. Thanks guys - this is the most helpful forum I've ever been on :-)

HoGo
Jul-16-2012, 4:28pm
CA is often used in restoration of valuable pernambucco bows. It will hold extremely well if properly used.
I was thinking of clamping method similar to this:
http://www.schabbon.com/12302.html?*session*id*key*=*session*id*val*
You don't have straight edges for "normal" clamping.

sunburst
Jul-16-2012, 4:54pm
A couple of things;

CA can and does form "permanent" bonds when handled correctly (as Adrian says), but fit and cleanliness are important (as with hide glue) and that is the reason I would choose against CA in this case, as I said in my first post. (I recently used it to repair some small splits in some rosewood guitar sides for bending. The CA easily withstood the heat of the bending iron, unlike any other glue I have in the shop, and the repair can be considered permanent.)

The temporarily glued blocks that Adrian linked to are the things I mentioned in my first post (in the edit, after I remembered them), and the reason for using them is the lack of straight edges on the top plate to clamp the joint tight. Also, it is worth mentioning that the strip(s) of wood you see between the blocks is important because without it, the blocks will lean toward one another when clamped and open the opposite side of the joint. Getting the strips between the blocks just the right thickness will close the crack evenly.

Marty Jacobson
Jul-16-2012, 7:04pm
Yeah, not to turn this into a CA glue thread. But CA glue is extremely strong. You can hang a car from a 5x5" joint made with CA glue.

BUT-- you tap that 5x5" joint, even not holding a car, a couple light shear loading taps with a centerpunch or deadblow hammer and it shears off completely. It is very strong, and very brittle. So it's great in situations like John and Adrian mentioned. When they are surrounded by solid material and generally not subject to impact. For a main structural joint it is a really bad idea, IMHO.

Also, most people don't know that CA glue is not in any way water proof. In fact, it dissolves pretty readily in water!

Also, really short working times are a problem for a joint requiring careful positioning.

Ok, I'll shut up about CA now, sorry for hijacking, but I thought it was relevant to the conversation.

Bill Snyder
Jul-16-2012, 10:03pm
CA is water soluble? Then why do all of us use acetone to dissolve it?

Texas
Jul-16-2012, 10:21pm
Whatever you do, remember to place a piece of wax-paper on each side when clamping with whatever method you chose. You want to make sure the glue squeeze-out does not create bigger problems.

bmac
Jul-16-2012, 10:24pm
Speaking of the failed hide glue... It is my understanding that boiling hide glue causes loss of strength and encourages failure... If you really meant you "boiled" it you probably ruined its' holding strength even before you used it.

Marty Jacobson
Jul-16-2012, 11:17pm
CA is water soluble? Then why do all of us use acetone to dissolve it?

Acetone is faster.

mboursnell
Jul-17-2012, 5:07am
As I recall the hide glue came in thin slabs like toffee. I got it from a violin maker in Birmingham (UK). I had to heat it in a metal can suspended in water in a saucepan to melt it. (I can't remember if I added water to the glue). It was in those days before the internet (1976), so I didn't have a huge amount of information. Remember those days?

sunburst
Jul-17-2012, 9:46am
...Remember those days?

Kind of hate to admit it, but yeah, I remember those days...

HoGo
Jul-18-2012, 5:16pm
As I recall the hide glue came in thin slabs like toffee. I got it from a violin maker in Birmingham (UK). I had to heat it in a metal can suspended in water in a saucepan to melt it. (I can't remember if I added water to the glue). It was in those days before the internet (1976), so I didn't have a huge amount of information. Remember those days?
I remember that very well. Internet became common few years later here. I also remember my first attempt at heating HG... My father brought me glass of granules from cabinet maker to use on my first repair attempts and he just told to heat it an a water bath. So I heated it dry. After 15 minutes no glue would melt so I removed the water bath and tried it directly on hot plate... You guess what happened... So my third attempt was with water added to glue and since then HG is always in my repertoire. It took me few years to understand how and where it works. But that was almost 20 years ago. Now you just google for hot hide glue and have all the info at hand.
Martin, where does that info on water soluble CA come from? You can clean CA off your hands with warm water and soap but I see its mostly softening my skin that makes the glue crumble off easier (typically taking some skin with it). My fingers are quite soft after that.
And talking about shear strength of CA joint I'll add that HHG is quite esily separated that way as well. I'd guess even easier than CA. In these situations would Titebond or epoxy be the winner.

Marty Jacobson
Jul-18-2012, 9:10pm
Martin, where does that info on water soluble CA come from? You can clean CA off your hands with warm water and soap but I see its mostly softening my skin that makes the glue crumble off easier (typically taking some skin with it). My fingers are quite soft after that.
And talking about shear strength of CA joint I'll add that HHG is quite esily separated that way as well. I'd guess even easier than CA. In these situations would Titebond or epoxy be the winner.

CA's are considered to have a "weak resistance" to water.. empirically, it takes so long for the water to degrade the CA that a R/C boat must be left in water for a few days before joint failure will occur. I have tested this and the glue does definitely get gummy when allowed to be saturated with moisture. Reportedly the latest generation of CA's are better than this.

However, Loctite does indicate that hot water can be used to remove CA glue as well as acetone or their proprietary debonder (which is probably acetone and MEK or something).

"Clean-up
Cured adhesive may be cut away with caution using a sharp blade, removed with Duro Super Glue Remover, acetone or with boiling water."

From this document: http://www.loctiteproducts.com/tds/SG_BOTTLE_PRO_tds.pdf

mboursnell
Jul-19-2012, 11:52am
Hi everyone. This is the conclusion of this thread. I found the suggestions by everyone really helpful, and in the end did this. I glued two blocks either side of the soundboard whilst the centre crack was raised on a piece of 6mm wood. Then when the wood was removed I could press down the crack and get a really tight join.

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Then I used Titebond, and screwed-down batons and wax paper as suggested.

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The final result looked pretty good, considering.

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The next stage is decided how to re-brace it, and indeed whether it's still useable or whether I make a new one. I'll start a new thread for that.

http://www.mandolincafe.com/forum/showthread.php?87108-Re-bracing-my-mandolin-soundboard

belbein
Jul-22-2012, 3:32pm
I think it's cute that you think that just because you finished the fix that the thread is concluded.