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| Vacuum Clamp concerns http://www-.luthiersforum.com/forum/viewtopic.php?f=10102&t=1255 |
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| Author: | John Kinnaird [ Tue Mar 08, 2005 12:08 am ] |
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I am thinking about doing the vacuum clamp thing but here is my concern and I would like you guys to lay my fears to rest. I like to used hide glue to put on the bridge. Right now, I clamp the bridge on, the glue squeezes out, I let it gel and then clean up. I don't unclamp to clean up because I fear that the glue would not be strong enough at that critical junction to hold the inevitable springback I get on the ends. (what are clamps for if not to hold these things tightly to the top.) My vision of the vacuum clamp goes like this. I clamp, I let it sit till the glue gells, I unclamp to cleanup, and any little springback etc. that indicates the bridge and the top don't fit exactly right at that moment (given the vagaries of humidity fluctuation etc.) will cause a minute separation. Hide glued is not very strong when filling gaps so lines of weakness occur. I can see this happening for braces as well. Hate to be raising these kind of questions in the midst of the vacuum clamp lovefest, but I would love to join the party if I can get over this hurdle Whattdaya guys think? |
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| Author: | LanceK [ Tue Mar 08, 2005 12:24 am ] |
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John - I strive for a perfect fit between the bridge and top. I leave NO gaps under the bridge ends. There is the possibility as you say for the top to fluctuate with heat the humidity, to change shape a little, so to off set that, fit your bridge just prior to gluing it down. After its fit, I glue it up using HHG, place it, pin it, and vac clamp it - leave it under pressure for 10 minutes, remove, clean up and clamp again for 45 more minutes. Believe it or not, after just 10 minutes, that bridge is TIGHT! |
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| Author: | Sylvan [ Tue Mar 08, 2005 12:25 am ] |
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John - I will probably get chastized for these comments but it is what I found out! I have been using vacuum for a long time and I found, over time, that vacuuming the bridge on the top is not the best way. In my opinion, the concerns you express are indeed valid and verifiable, and I decided not to use vacuum for that purpose for those reasons. Vacuum is great for brace gluing and other tasks but I do not recommend it for bridge gluing! |
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| Author: | John Kinnaird [ Tue Mar 08, 2005 1:49 am ] |
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Hey I strive for a perfect fit too, but in that process I cut the base of the bridge to a 25 foot arch and since the braces are cut to a 25 foot arch, in theory they should fit. Of course the top fluctuates daily and I could wait till it hits its 25 foot arch naturally or I could just force it to do what is planned. I refuse to make that arch on the bridge deeper or flatter to fit the tops momentary shape. In other words I make the top conform to the plan and that sometimes means clamping. We are not talking about much clamping here and perhaps the hide glue can deal with the small forces required to make the top conform to the bridge. |
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| Author: | John Kinnaird [ Tue Mar 08, 2005 1:51 am ] |
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Sylvan You won't get chastized by me. You have confirmed my worst fears regarding this method. |
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| Author: | Dickey [ Tue Mar 08, 2005 5:05 am ] |
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Well I'm not a vacuum guy, but it always seemed risky to unclamp a glue joint before it is dried. I congratulate any who've successfully utilized the method. Kind of an aside, Collings uses a gobar, vacuum combination on their top bracing. The gobar sticks hold the pieces in until all braces are in place, then they remove the sticks, drop the membrane and get to sucking. They too must have the intermediate step of glue cleanup and reclamping? Looks like the glue would get all over the membrane? I used to make magnetic signs with heated plastic sheets, the vacuum sucked the plastic around the raised letters which were then painted with a roller. The Luthier's vacuum process reminds me a lot of that process. Of course, no gluing going on there. I like access to the glue as it comes out, and I clean up each brace before moving on. |
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| Author: | John Mayes [ Tue Mar 08, 2005 6:09 am ] |
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While I was employed at pantheon they used Vacuum for all the braces (except the transverse) but glued the bridges on with clamps. I currently use go bars for the braces and clamps for the bridge. I'd be weary about the un-clamping too, but hey if it works and no bridges are coming back up (it happens anyway sometimes...) then bravo! |
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| Author: | JJ Donohue [ Tue Mar 08, 2005 10:29 am ] |
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On the issue of unclamping after a few minutes for cleaning glue squeeze out...keep in mind that any water or solvents found in any glue will evaporate much more quickly once vacuum is applied. As a result, the joint will be a lot stronger a lot sooner than it ordinarily would be under normal RT&P conditions. Whether it produces a stronger joint than a gobar or clamp is still another issue. |
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