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| Glue With Teeth or Not? http://www-.luthiersforum.com/forum/viewtopic.php?f=10102&t=4732 |
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| Author: | LarryH [ Wed Jan 25, 2006 1:32 pm ] |
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Seems to be the consenesus that a smooth surface will bond better than a roughened surface. Just surprised to see this on the StewMAc site. What do you think? Toothing Iron |
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| Author: | Rod True [ Wed Jan 25, 2006 1:39 pm ] |
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Interesting, anything to make a sale is my opinion. Anybody know if Martin realy does this on the bottom of there bridges, John Hall may know. |
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| Author: | David Collins [ Wed Jan 25, 2006 2:01 pm ] |
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If you're using epoxy, maybe. Anything else and it seems quite foolish. Most glues and adhesives are poor gap fillers in a structural sense, and should rely more on specific adhesion rather than mechanical adhesion. |
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| Author: | crowduck [ Wed Jan 25, 2006 2:22 pm ] |
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That tool might come in handy for removing the finish under the bridge profile prior to glueing. I've seen reference to a similar tool used by steel plate engravers that does the job. I think it is called a mezzotint roller. CrowDuck |
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| Author: | Brock Poling [ Wed Jan 25, 2006 4:34 pm ] |
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[QUOTE=Rod True] Interesting, anything to make a sale is my opinion. [/QUOTE] From everything I have seen about SM that is definitely not their way. I have personally seen Dan Erlewine use this device and talk about other uses for it. Not to say that every tool they carry is absolutely necessary... clearly everyone has their own way of doing things, but I don't think they would ever lead someone astray simply to make a sale. |
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| Author: | Rod True [ Wed Jan 25, 2006 6:21 pm ] |
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Brock your right, my comment sounds much worse than I intend. I my woodworking experience, you never what a roughed up surface for gluing, I've always read and been told that the best gluing surface is a clean knife cut edge, like right off the jointer as long as there is no tear out. Even a sanded joint is inferior as there is less surface area being glued together due to the rough edge. That's great that you've seen Dan use this device, but what for? For gluing a bridge onto a soundboard? That sure seems contradictory to everything I've ever read about this particular glue joint. Maybe for other areas of guitar repair, but I would be surprised if he used it as the final preparation for a glue joint. Here is a quote from Frank Fords web site regarding glue joints Hide glue sticks to surfaces by electrochemical attraction, or specific adhesion. Mechanical bonds, like little “fingers,” may help modern adhesives such as epoxy because of their very high cohesive strength, but with hide glue’s low cohesive strength, roughening joint surfaces will not help adhesion. It is better to be concerned about the electrochemical properties of the wood. I do believe that Stew-mac and other luthier supply houses offer essential tools for high quality building and repair for the luthier industry, but in my experience of woodworking and the wisdom of our fellow woodworkers of the past have told me that this particular product would not be the best preparation for a good quality glue joint. Here are the instructions from the stew-mac page on the toothing iron.. This sure is contradictory to everything I've read about this particular glue joint and good glue joints in general. If there is a case for this being good practice for the bridge glue joint than I appologize and I should start roughing the surfaces of my bridges when glueing, and if that's the case, a quick pass with some 80 grit would do the trick. Seems to be about the same "roughness". I also agree with your second statement that not every tool they carry is necessary. And I don't think they would lead someone astray to simply make a sale. I apologize for my blunt statement of "anything to make a sale". I do believe that they intend to offer the best products for the job. But I just don't see this as the best idea for this glue joint. |
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| Author: | Brock Poling [ Wed Jan 25, 2006 11:52 pm ] |
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Rod. I am not mortally offended or anything (that is just not my way)  , but I wanted to point out that I am sure that SM never puts a product in the catalog just because it will sell well.
And, for better or worse, I did see him rough the bottom of a bridge. Perhaps the variable in all of this is the hide glue. I will see if I can learn a little more about this and report back. |
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| Author: | Michael Dale Payne [ Thu Jan 26, 2006 1:19 am ] |
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My personal opinion is that the toothed surface may be a stronger joint. but I would think that bridge removal would be more likly to tear out pieces of the top. |
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| Author: | Mario [ Thu Jan 26, 2006 1:26 am ] |
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The toothed surface is the "old" way of doing things. It's how they were all done at one time. But we've seen these joints all fail with time. During WWII, a lot fo study was put inot wood joinery, because airplanes ised a lot of wood, not the least of were the wooden properlers that were glued up. It was found that a fresh, smoothly planed surface was indeed the strongest. Toothing has its place(epoxy, CA'd bridges, etc...), but a hide or Titebond glued bridge should be scraped smooth. |
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| Author: | Laurent Brondel [ Thu Jan 26, 2006 2:11 am ] |
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All the bridges that I reglued from Martins before the '50 have those x-scars on the bottom, not done with this tool but probably with a chisel or a knife. Obviously they all failed since they needed to be reglued… But they probably held long enough though… |
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| Author: | Mario [ Thu Jan 26, 2006 3:06 am ] |
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But they probably held long enough though… How long is long enough for something that should never come up? Sorry, but I don't see the logic in doing something to anything less than the best we can do it, especially when the scientific facts, and historic failures, are presented to us. Remember when Gibson's slogan was "only a Gibson is good enough"? But the following joke on it was 'for when good enough just isn't quite good enough...' GM and Ford thought their vehicles were "good enough" for us dumb consumers, too. Explain that line of thinking to one of the laid-off employees... |
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| Author: | Brock Poling [ Thu Jan 26, 2006 3:11 am ] |
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[QUOTE=Mario] But they probably held long enough though… How long is long enough for something that should never come up? Sorry, but I don't see the logic in doing something to anything less than the best we can do it, especially when the scientific facts, and historic failures, are presented to us. Remember when Gibson's slogan was "only a Gibson is good enough"? But the following joke on it was 'for when good enough just isn't quite good enough...' GM and Ford thought their vehicles were "good enough" for us dumb consumers, too. Explain that line of thinking to one of the laid-off employees... [/QUOTE] Wasn't it Epiphone who used the "when good enough isn't good enough" tag line in their ads? If true that is classic. I bet some marketing genius at Gibson didn't get his bonus that year. 
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| Author: | Rod True [ Thu Jan 26, 2006 3:21 am ] |
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I've always seen two levels of quality in the term "good enough". "If it's good enough for ........, it's good enough for me" or "Come on, get that thing out the door, it's good enough" So the vintage Martin joint was roughed up, and now the joints are failing. Interesting. Well, if some one is doing a replica I guess this little toothing file would work for the glue joint than.
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| Author: | Mario [ Thu Jan 26, 2006 3:25 am ] |
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Yup, I believe you're right, it seems it was Epiphone that pulled it off. |
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| Author: | Michael Dale Payne [ Thu Jan 26, 2006 3:33 am ] |
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The phrase "Good Enough" Always reminds me of the old contract engineering phrase "Good Enough for government work" I tend to think of that phrase on the lines of acceptable tolerance. "Good Enough" = with in acceptable tolerance. Isn't this the difference in our guitars and factory built guitars. A factory has little direct consumer contact so good enough is acceptable. But because we have very direct contact with very discriminating end users our acceptable tolerance is “Only Right is Good Enough” |
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| Author: | Laurent Brondel [ Thu Jan 26, 2006 3:50 am ] |
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I don't use emoticons, but I should have specified that "good enough" was a sarcastic comment… Mario, I agree with you all the way. Pas de probleme… |
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| Author: | Serge Poirier [ Thu Jan 26, 2006 5:07 am ] |
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Quoi il dit ? 
Great thread guys, learned a lot too! Thanks Serge |
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| Author: | David Collins [ Thu Jan 26, 2006 5:59 am ] |
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The idea behind scoring the surface of a glue joint is to provide greater surface area. The problem here is that while there is more surface area, there is less direct wood to wood contact, and this is what specific adhesion requires. Scoring relies more on mechanical adhesion which generally involves cohesion and gap filling strength. This may be ideal if you are epoxying a micarta bridge or superglueing a bridge to a finish, but for traditional bridge joints with hide glue (or PVA  ) I believe this would create a slightly inferior joint.
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| Author: | old man [ Thu Jan 26, 2006 7:11 am ] |
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[QUOTE=Rod True] Even a sanded joint is inferior as there is less surface area being glued together due to the rough edge. [/QUOTE] I disagree with this (sort of). A rough surface has more surface area exposed. The problem boils down to what type of glue you're using. If the glue is completely NON-gap filling, THEN there is LESS surface area being used, and a perfectly flat, smooth surface will give the best bond. With a glue that fills gaps, you have more surface area to use in a rough surface. And I agree with you, Rod, that it comes down to cohesive strength of the glue. In a rougher surface, to have maximum strength, the glue must be both very adhesive and very cohesive. With very smooth surfaces the adhesive properties is more important. It depends on how you view this, though. With smooth surfaces there is maximum "wood to wood" contact, but with rough surfaces, there is maximum "wood to glue to wood" contact. So, if the wood surfaces and the glue types don't match, there is a weakness. Sorry I got too carried away here and it probably doesn't even make sense. (You don't get old by being stupid, but you can get stupid by being old.) Ron |
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| Author: | Alan Carruth [ Thu Jan 26, 2006 7:33 am ] |
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When I was working with Carleen Hutchins she used to cite the Forest Products Lab studies of wood gluing done in WW II. They found two things: 1) Propellor laminations that were sanded to thickness tended to fail in glue-up more than ones that were planed. If you look at microphotographs of surfaces that were rough sanded to thickness they are all torn up, and have junk and dust worked into the pores. Planed surfaces can be smooth, and much cleaner. 2) Surfaces that were worked within 15 minutes of being glued held better. I got an explanation of that a couple of years ago, in a publication from the Experimental Aircraft Association, dealing with assembling fiberglass components. They pointed out that removing material breaks chemical bonds, and that it takes a while for these 'loose ends' to find something else to bond onto. If you present them with some nice, fresh glue they will bond to that, and a better glue joint is the result. Martin _used_ to tooth the undersides of bridges, but all of the more recent ones I've seen were smooth. Mario points out that he has had very good results using the 'bar' bridges, with smooth surfaces and hide glue. Martin invented the 'belly', it seems, because they didn't have good luck with bar bridges, hide glue, and toothed surfaces, and needed the extra span to keep things together. There are lots of plausible reasons for toothing, such as: "It gives the glue someplace to go". I'm sure the folks at StewMac believe this is a good thing. I believe they're wrong. |
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| Author: | LarryH [ Thu Jan 26, 2006 7:47 am ] |
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[QUOTE=David Collins] The idea behind scoring the surface of a glue joint is to provide greater surface area. The problem here is that while there is more surface area, there is less direct wood to wood contact, and this is what specific adhesion requires.[/QUOTE] Your point is well taken and this phrase must simply be mis-worded - "direct wood to wood contact". It seems to me that phrase would describe a starved joint. Agreed? So let's say we don't want wood to wood contact, but the thinnest film of glue that allows the properties of hide and PVA's to adhere the best and those properties are known as specific adhesion? Verus mechanical adhesion? Larry |
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| Author: | David Collins [ Thu Jan 26, 2006 11:40 am ] |
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Right Larry, poor phrasing on my part. |
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