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Joining back plates http://w-ww.luthiersforum.com/forum/viewtopic.php?f=10102&t=1178	Page 1 of 1

Author:	JeremiahB. [ Mon Feb 28, 2005 5:45 am ]
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Just curious, has anyone ever used epoxy to join top/back plates on an acoustic guitar. I'm having trouble with a set of rosewood that I'm trying to join. This is the third time that I'm trying to join this particular back. The first time that the back split along the seam was understandable. I had routed out the channel for the backstrip and put too much pressure along the seam while picking it up, so I sanded both plates with sandpaper attached to a level and tried to join the plates again, this time with the backstrip in the middle. Once again the back split down the seam when i picked it up by the edge (which I won't do again). By this time, the back was so small because I had already cut out the shape of the guitar and I had to sand so much away trying to get a good joint that I had to order a custom backstrip from Bob C. to just make the back wide enough to fit the rim. I would just ditch the wood, but I already have the matching rim glued up and ready and it's a beautiful set of EI rosewood, so I would like to salvage it if I can. Both times when the back cracked, the glue seemed kind of brittle. The first time I used Titebond and the second time I used LMI white glue. I'm just wondering if at this point, I shouldn't try using epoxy. Has anyone ever used epoxy and if so, what kind? I just want to do this right, because if I screw up again, I don't think I'll be able to use it. By the way, the back is already thickness sanded to size, so I'm also going to have to try and clamp everything down with a go bar deck to get everything to line up flush while gluing which just adds to the complexity and difficulty. Any suggestion?JeremiahB.38411.5743981482

Author:	LanceK [ Mon Feb 28, 2005 5:53 am ]
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I use Hot Hide glue, no problems at all. Ive also used Titebond and LMI white. Maybe your plates are extreamly resinous? Have you tried to wipe the joining edges with acetone? you might what to try that. Also, dont give up on that back set just yet! You could always make a 3 piece back. I know twice the joining!

Author:	jfrench [ Mon Feb 28, 2005 5:56 am ]
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Titebond I should work fine. Make sure you wipe the edges before gluing to make sure there is no dust in there (should go without saying). Indian Rosewood always glues up fine for me, and often I glue with the center stripe in the join. I shy away from the LMI white glue - I've had it fail, and have heard of others having it fail (fresh glue, and good technique no less). Make sure you glue in reasonable humidity and temperature, and be careful with it until you have it thicknessed and the joint reinforcement glued. Other glues that have worked well for me are hide glue and fish glue. Best wishes, Joshua French

Author:	JeremiahB. [ Mon Feb 28, 2005 7:09 am ]
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Thanks for the replies. I think I'll give it one more with Titebond.

Author:	Brock Poling [ Mon Feb 28, 2005 7:41 am ]
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get a new bottle of titebond first...

Author:	Dickey [ Mon Feb 28, 2005 8:04 am ]
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Jeremiah, describe your technique for joining? Are you clamping it so tight it ejects all the glue. That's called a glue-starved joint. A glue starved joint is a weak joint. When I learned to tent my back with five strips of binding tape, joins are fine here at my shop. Mario and Bobcef got me started on this technique and when I find a good deal I stick with it.

Author:	jfrench [ Mon Feb 28, 2005 11:12 am ]
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Fresh glue is a very good idea. I find the best way to join backs and tops is by making the boards "wedge" shaped, so the upper bout area is not as wide as the lower bout area, then clamp down two straight piece of wood to reproduce the same wedge shape, glue the wood, weigh it down, and hammer it forward (with a block of course). This gives plenty of clamping pressure, and is about the easiest and most effective way I've ever seen to do it. I've never had one come apart, and always have a helluva time trying to find where to put the center line... (unless there's already a center stripe, obviously). Regards, Joshua French

Author:	FrankC [ Mon Feb 28, 2005 11:23 am ]
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Hey Bruce...What do you mean tent with tape?

Author:	FrankC [ Mon Feb 28, 2005 11:26 am ]
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I am new to this whole building thing but have joined 3 backs. I make the backstrip part of the joint. For example: back - glue - center strip - glue - back. Also, I glue one half of the back to the center strip. Once dry, make sure its strong, the glue the other half of the back on. No problems yet. I also glue the reinforcement strip once its thicknessed. Is this a bad approach?

Author:	Pwoolson [ Mon Feb 28, 2005 12:52 pm ]
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Frank, I've had many discussions/arguements on this topic. In my opinion, yes it's a bad practice. You are making two joints and using a strip as a structural member. If you use on of those martin zipper strips, you have no idea of the glue used in the strip. I don't use back strips by practice but I have done them in the past. I join the back and route a channel for the strip. This actually makes a stronger joing than the original joint because you are using three gluing surfaces. That's my story and I'm sticking to it. Paul

Author:	FrankC [ Mon Feb 28, 2005 1:35 pm ]
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So what you are saying is, route channel about 1/16 deep and glue the strip in? Leaving 1/16 to be sanding flush? Wouldn't the back reinforcement strip add strength as well?

Author:	Josh H [ Mon Feb 28, 2005 1:46 pm ]
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Jeremiah I use epoxy to join my Cocobolo backs and it works great. I have used titebond and CA glues for joining EIR and they have both worked just fine. I agree that a new bottle of titebond should do the trick. Josh

Author:	Dickey [ Mon Feb 28, 2005 2:05 pm ]
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The tent method is simple: make your joint until no light passes between the two back or top pieces. Lay them flat and apply five strips equidistant across the back or top seam, pulling the eight inch strips tight across the area to be joined. Then flip it over. Raise the center of the setup, that exposes the gluing surface, I coat one side. You can use titebond or hide glue. As the two panels are lowered to the worktop, excess glue is forced out of the join. I quickly wipe off the excess with a sharpened wooden stick, drying the stick off on paper towel as I go. I then apply five strips of tape to the opposite side of the panel and I'm done with either a top or back. Like I said, pretty simple. The tent method name comes from the fact it looks like a one-man pup tent when raised from the workbench.

Author:	Colin S [ Mon Feb 28, 2005 8:59 pm ]
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Another vote for the tent method, it is as foolproof as this fool has found, it holds the joint tight without the posibility of glue starvation (I always use hide glue but see no reason why titebond shouldn't work) I don't even bother with the tape on the top side, simply put some wax paper and a weighted board on top and let dry. Then if wanted rout and add backstrip, As Frank said,like the inner reinforcment, the additional glue area can only help the join. ColinColin S38412.2246412037

Author:	JeremiahB. [ Mon Feb 28, 2005 11:10 pm ]
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Thanks for all the replies. All of these methods sound great and I will probably try them, but as I said before, I am trying to join a back that is already thickness sanded and cut to shape, so my choices are limited. And I agree with Paul, I believe a back with the backstrip channel routed out and inlayed makes a stronger joint than merely sandwiching the backstrip between the plates, but since the back is already cut to shape and I had to remove more material from the center seam to get a good joint, routing out the channel is no longer an option. At this point I have to sandwich the strip just so the back will be big enough to cover the rim. I plan to use a jig similar to the one John Mayes uses and I'm going to lay it in the bottom of my go bar deck. Then I'll give everything a good bead of Titebond and clamp the strip and back plates down with go bars and then draw them together with the pipe clamps. If everything goes as planned, that should create one side thats flush and then I can scrape and sand the other side flush. If anyone has any suggestions for joining a back which has already been sanded to final thickness, I'm all ears.

Author:	Dave Rector [ Tue Mar 01, 2005 12:24 am ]
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Jeremiah, did you use pipe clamps when you joined the back previously? If so that could be tour problem. Pipe clamps are capable of exerting huge amounts of pressure on the joint. This can cause the joint to be starved of glue. The previous post about using the tent method is a good exampl of how little pressure is needed. If you still plan to use pipe clamps be very careful with how far you tighten them down. Don't eant to starve that joint and have to do it again, you are already running out of wood

Author:	Brock Poling [ Tue Mar 01, 2005 12:25 am ]
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One question I thought of..... Are you gluing right after jointing? I have found that you get better results if you glue up resinous woods right away.

Author:	JeremiahB. [ Tue Mar 01, 2005 12:39 am ]
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In response, I'm not going to wrench the clamps down, but I need to exert a little force to insure contact, because I will have the go bars exerting downward pressure. Also, I do plan to join the halves immediately after getting a good joint. I really don't remember if I did that the previous times.

Author:	Tim McKnight [ Tue Mar 01, 2005 12:57 am ]
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"Both times when the back cracked, the glue seemed kind of brittle. " Jeremiah: Brittleness is a good quality for the glue to have. You want to use a HARD glue because a soft glue has a neagative dampening effect. I would question your joint and precise fit of the mating surfaces before questioning the glue. If you are trying to sand the joining surfaces by hand than I suspect that your edges are not precisely square but may be ever so slightly rounded. Try machining your edges on a Joiner or a shooting board. Hold the mating surfaces up to a light and see if you can see any light shining through the joint. If you can then the joint will undoubtably fail regardless of the glue that is used.

Author:	Dickey [ Tue Mar 01, 2005 1:05 am ]
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Speaking of clamping, I think there is a method with hide glue called slip gluing, where practically no pressure is applied, just slide the slick joint together by hand until it catches, then weight. I used the gobar deck exclusively, using wax paper underneath, securing one side, and then laying the other side in position. By pressing the joint together, then applying the go sticks, it kept everything tightly together. The Tent method is superior until I tried adding a marquetry strip. For that I prefer the go bar deck, just less nerve-wracking. The tent method becomes a three ring circus inserting the marquetry strip in the panel. I like the marquetry glued in with the joint. When you graft the back it's strong as can be and I've never had one come apart. I rarely assemble a back without one. Recently I routed a back channel for pearl and I was really worried it was so thin it would break during handling. It didn't so I was able to flip it over and apply the center graft, no problem.

Author:	JeremiahB. [ Tue Mar 01, 2005 2:13 am ]
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You may be right Tim, but I think the first time that it cracked was pure carelessness on my part. I had routed the channel for the backstrip and just put too much pressure on the seam while it was so thin and fragile. The second may have been a bad joint, but it seemed to crack along the purfling lines of the backstrip, a backstrip which I got from Martin. I clamped the plates flat to a piece of MDF when I sanded the joint so they should be pretty square. I'm going to wipe the edges good with acetone, glue it up, and hope for the best. This time I'm also going to put in the back reinforcement strip ASAP. I figure that should add some rigidity until I get it totally braced and glued to the rim.

Author:	jfrench [ Tue Mar 01, 2005 3:12 am ]
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Two points I'd like to make... Ramirez and others have been joining their backstrips in between the two back halves since at least the 60's. There's really no harm in it. I even do several layers of veneer between the joint at once, use enough glue, and wedge tight. No problems to date. When I do something more intricate, such as an inlay that does not consist of long continuous pieces (a good example was the zipper someone mentioned, or even herringbone) I always make the join, and then chisel out a channel for it. My other point, is that I've never seen a "starved" joint. There was a discussion about this on the mimf a week or two ago as well. I think its folklore. If the surfaces mate good, then you don't need much pressure, but the pressure is not going to hurt your joint. Best wishes, Joshua

Author:	Darin Spayd [ Tue Mar 01, 2005 3:37 am ]
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Would it be safe to say that as long as there are precise mating surfaces and sufficient glue, contact is all that is necessary to join?

Author:	jfrench [ Tue Mar 01, 2005 3:45 am ]
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When i'm making rosette logs the only pressure I use to put the pieces together are my fingers, and they come out plenty solid. I'd still use clamps of some kind though Darin. The tape method sounds fine. Regards, Joshua

Author:	JeremiahB. [ Tue Mar 01, 2005 3:47 am ]
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I agree, I'm not sure I believe you can "starve" a joint of glue. I would be more apt to believe that the surfaces were not properly mated. I think that the PSI required to starve a joint is greater than what any human could exert. Having said that, I don't think that it takes much pressure at all to join a back or top if you have a tight fitting joint. I'm just using the pipe clamps for simplicity and to make sure that the horizontal force is great enough to clamp the plates together and overcome the vertical force produced by the go bars, if you can understand that.

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