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| Author: | Bobc [ Mon Jul 25, 2005 4:02 am ] |
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I was thickness sanding a couple of curly mahogany backs and a sitka top for BobJ this morning and thinking about the posts on various types of glue. I don't re-call reading anything about joint preparation. I don't use clamps or wedges or rope but just plain brown masking tape from Stew-Mac to hold the halfs together untill dry. Pulling alternately from each side of the joint as I place the tape. Then flip over and do the same on the other side. I don't have anything against any of the above methods as long as you start out with a nice tight joint. If you rely on mechanical means to close the joint you are looking at possible joint failure down the road. If the joint fits tight dry you don't need any pressure to close it. Here's a link to some pic's of my sophisticated method if your interested. http://rctonewoods.rcefaluguitars.com/temp_pics.htm |
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| Author: | crazymanmichael [ Mon Jul 25, 2005 4:33 am ] |
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i couldn't agree more. if the joint doesn't pass a candling test don't glue it. whilst you can jam it closed with clamps, it will be prone to failure. that being said, there are a variety of means to keep it closed while the glue sets, clamps, rope and wedge, tacks and steeple, etc. just don't apply too much pressure. |
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| Author: | tippie53 [ Mon Jul 25, 2005 7:24 am ] |
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I joint that needs to be forced is doomed to fail john hall |
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| Author: | John Mayes [ Tue Jul 26, 2005 12:06 am ] |
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seriously guys I thought you just liked the wrapper a little bit and the joint would seal easily.... woah wait wrong joint...flashback to high- school. Ok.. um Yeah I do my uke tops this way because my clamps are too big. Joint has to be perfect before glue :) |
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| Author: | Dave-SKG [ Tue Jul 26, 2005 12:49 am ] |
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I use the exact same method as BobC and have never had a joint fail. Never.  ( hope I didn't just jinx myself). Never say Never...but it's true!
p.s. learned that from Harry Fleishman. Harry has tons of little tidbits that just seem to work well. Anyone interested in taking one of Harry's classes... it is well worth it! Funny guy too! |
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| Author: | Alan Carruth [ Tue Jul 26, 2005 5:26 am ] |
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I'm going to register a mild disagreement: even well-fitted joints may need clamping. Glue is not itself a strong material; what's strong is the bond between the glue and the wood. There is a 'correct' thickness of glue for the joint, which will depend on the kind of glue. Too much and you're loading the weak glue, too little and there is not enough there for a proper bond. I have an article someplace on glue joints in wood aircraft construction. The Forest Products lab found back in WW II that you needed to have 125psi of clamping pressure +/- 10%, on spruce, to get the strongest joint with resorcinol glue. I will note that this stuff usually has some wood flour in it as a filler to give it better gap filling properties, so that's probably on the high side for what we do. Still, it's not zero. I've used the old 'rub joint' technique with hide glue, where you assemble the pieces and rub them together to expell the excess glue. If you line them up properly just as the glue grabs and leave them you will end up with a strong joint. I was demoing this to a friend once and we got 100% wood shear from a glued up sample that had been sitting only 15 minutes. I've used rubber bands for years to glue up top and back joint, with very few problems. It doesn't take much pressure, but I think it's good to have some, and your masking tape probably works as well as anything. |
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| Author: | Dave-SKG [ Tue Jul 26, 2005 5:37 am ] |
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The tape I use is the brown "binding" tape that both LMI and S.M. sell it's pretty tacky and has some stretch to it. Harry F. said he it tested the amount of pressure it created but I can't recall. I believe it was around 16 lbs... not sure...anyone know? p.s. I used this method on all the common types of top wood as well as everthing from Cocobolo to ziricote to braz. rose never had a joint fail. |
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| Author: | Bobc [ Tue Jul 26, 2005 5:51 am ] |
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Alan I don't think we disagree at all. My point was that an ill fitting joint that is drawn together with clamps is almost certain to fail in time not that no clamping was required. I have used this method on veneers for about 35 years and just transferred the technique to joining plates. Although I also used veneer tape over the seam. I don't think tightbond was around because I remember using a white glue possibly Elmers wood glue.  The masking tape technique produces some squeeze out. I have not had a joint open up yet. Oops I guess my first post should have stated that "you don't need much pressure" instead of "you don't need any."  |
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| Author: | Alan Carruth [ Wed Jul 27, 2005 8:07 am ] |
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Didn't mean to jump on you Bob, but ya gotta admit there's a lot of difference between 'none' and 'a little'. :) |
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| Author: | Bobc [ Wed Jul 27, 2005 8:11 am ] |
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Absolutely Alan. I didn't realize what I said untill your post caused me to review mine. Agree, agree, agree.
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| Author: | letseatpaste [ Wed Jul 27, 2005 2:11 pm ] |
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I took Harry's class, too. I use that Stewmac tape for everything. I ordered a case of it when I got back home from class. :) It's like a luthier's version of duct tape. My problem is I can't find the glue joint after the plates go together. Not a bad problem to have, unless you're trying to line something up with that glue joint. I keep forgetting to notch the ends of one of the plates before I glue them together so I have a reference for the centerline. |
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| Author: | Colin S [ Thu Jul 28, 2005 12:24 am ] |
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I use a similar method of joining plates. I think Bruce once descibed it here as the "tent" method. Place the jointed plates tightly together on a flat surface, join together with about 8 pieces of stretchy tape, I use electical insulation tape, turn over and raise centre to form a 'tent' shape, add hide glue, press down flat and add tape to second side, stretching slightly. A bit of wax paper front and back to stop it sticking to the bench and weight down. After a few minutes clean up, then let it sit, weighted, 24hrs or so. Never had one even try and fail. Colin |
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| Author: | Bobc [ Thu Jul 28, 2005 1:21 am ] |
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Colin that's exactly the way I do it except for the electricians tape. |
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| Author: | Brock Poling [ Thu Jul 28, 2005 6:31 am ] |
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[QUOTE=letseatpaste] I took Harry's class, too. I use that Stewmac tape for everything. I ordered a case of it when I got back home from class. :) It's like a luthier's version of duct tape. My problem is I can't find the glue joint after the plates go together. Not a bad problem to have, unless you're trying to line something up with that glue joint. I keep forgetting to notch the ends of one of the plates before I glue them together so I have a reference for the centerline.[/QUOTE] Just offset your plates a little bit so you can see where the plates joined -- at least you can find the centerline before you cut it to rough profile. After that... just don't lose that line. 
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| Author: | Rick Davis [ Thu Jul 28, 2005 10:21 am ] |
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I clamp, using long cam-like clamps, with more pressure for glues like Titebond and LMI and less for hide. Franklin, makers of Titebond, say that significant clamping pressure is needed for the best joint. I think you want to squeeze out all but a thousandth or so. I have no idea how much clamping pressure the tape provides but guess that it's not much. Your choice, always given a good joint to start with. I deal with finding the joint by chopping (on the bandsaw) a small triangle out of each end of both halves before gluing. Afterwards, it's a second's work to slide a pencil into the resulting wedge, push a straightedge against it, and accurately mark the center seam. I can even do it precisely without putting on the magnifier! |
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