I suppose that depends on the meaning of the word "phenomenon"!
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| Bridges and HHG – First time questions http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=25120 |
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| Author: | dmills [ Mon Dec 21, 2009 11:07 am ] |
| Post subject: | Bridges and HHG – First time questions |
I recently had a bridge separate from a top (IRW bridge, spruce top) on one of my previous guitars. I had used LMI white to glue the bridge and felt I had well prepared and fitted glue surfaces. Obviously not as the separation occurred none the less. The bridge bottom had a uniform coating of dried glue and a fine haze of wood fibers across its length. It’s as if there was insufficient glue penetration into the spruce fibers on the guitar top. Anyone else had a similar bridge failure and identified the probable cause? Anyway, as I have a bridge re-glue in front of me I thought I would dip my toes in water and try hot hide glue for the first time. There are plenty of great posts on the preparation of HHG as well as tips and techniques but I have a few questions specific to bridge gluing in the hopes of getting it right the first time and avoiding another bridge incident. - Do most of you apply the glue to the bridge bottom by brushing it on or by running several beads across the bridge length with a squirt bottle? If you use a squirt bottle, do you then spread the glue beads evenly across the bridge bottom prior to clamping or assume the glue will spread evenly under clamping pressure? - Do you apply glue only to the bridge bottom or to both the bridge and the gluing surface on the guitar top before clamping? - Any issues with clamping pressure when using HHG – just snug it down with medium pressure and avoid over clamping assuming a good fit? I will be warming the glue surfaces on both the bridge bottom and the guitar top prior to the glue application. Thanks in advance for the help. |
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| Author: | Link Van Cleave [ Mon Dec 21, 2009 11:31 am ] |
| Post subject: | Re: Bridges and HHG – First time questions |
How warm was your shop ? (white or yellow glue doesn't work well at low temps.) Are you sure you had enough clamping pressure , was the glue old, contamination on the top ? Also don't rely on pressure to spread the glue. Thin even coats on both surfaces is ideal. More details might help to diagnose the problem. LMI white should have worked fine. Something is not right. Hide glue is fine but before you switch you need to figure out what is wrong. Link |
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| Author: | Howard Klepper [ Mon Dec 21, 2009 2:24 pm ] |
| Post subject: | Re: Bridges and HHG – First time questions |
It's a well-known phenomenon in lutherie that hide glue failures are all due to poor technique, while failures of other glues are always the fault of the glue. |
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| Author: | WaddyThomson [ Mon Dec 21, 2009 2:39 pm ] |
| Post subject: | Re: Bridges and HHG – First time questions |
I suppose that depends on the meaning of the word "phenomenon"!
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| Author: | Terence Kennedy [ Mon Dec 21, 2009 4:21 pm ] |
| Post subject: | Re: Bridges and HHG – First time questions |
A couple of things come to mind. If the wood of the top was recently cut it may not be seasoned enough even though it was OK on the humidity meter. I've had that happen with some Lutz. An unseasoned top can move quite a bit with temp or humidity and that can loosen the bridge. Baking a top that was cut in the last few years is probably a good idea. The other thing is don't just assume your top is the same radius that your braces were sanded to and radius the bottom of the bridge the same. It may be a lot different in the bridge area. I've made that mistake too. I usually cover the area with low tack drafting tape, stick some sandpaper on top of that and sand the bottom of the bridge to the exact plane of the top. Lastly, if you don't remove finish exactly to the edge of the bridge footprint create a little relief in the edge of the bridge so the finish does not hold it away from the top. For HHG I do like Todd, set the bridge on a heating blanket that is around 160. Warm the top with a hair dryer CAREFULLY and put a generous amount of glue on both surfaces so there is a lot of squeeze out. I stick some on with the bottle and spread with a brush. I put some tape over the pin holes so the glue does not run out during application. I've been using the StewMac caul as well. |
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| Author: | dmills [ Tue Dec 22, 2009 11:18 am ] |
| Post subject: | Re: Bridges and HHG – First time questions |
Todd, Link, Terrence - thanks for the suggestions. I'm going to give it a go as soon my HHG order hits my doorstep. I'm thinking the poor glue penetration into the top wood may have been a combination of a cold Illinois basement workshop and (thinking back) I may have only applied glue to the bridge bottom rather than to both gluing surfaces. I will avoid both shortfalls on this go round. |
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| Author: | dunwell [ Tue Dec 22, 2009 12:05 pm ] |
| Post subject: | Re: Bridges and HHG – First time questions |
From your description of the bridge when it pulled lose as having a thin film of glue and wood fibers, I'm taking a guess that maybe you over-clamped it and starved the glue joint. You need to get it pulled down nice and tight, but if you clamp it too hard then there is not enough glue left in the joint to make a good bond. Others have mentioned the bit about making a small shelf/relief around the edge of the bridge so that it sits over the finish, that is a good idea and makes a very clean looking final product. Also if you put the bridge in place, then mask around it, you can then apply some cheap-o Johnson's Wax or similar to the tape and the edge of the bridge. Now apply more glue than you think you need in order to get some squeeze-out. Once it is all clamped, go around and clean up the semi-dried glue. The wax will make it release cleanly. If you are using hide glue you may want to size the surfaces first. An alternative you might think about is fish glue. It works at room temperature and has a very long open time but still dries crystalline like HHG. Just some thoughts, Alan D. |
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| Author: | Howard Klepper [ Tue Dec 22, 2009 12:34 pm ] |
| Post subject: | Re: Bridges and HHG – First time questions |
1. Glue "penetration": never heard of this as an issue, and cannot see why it would matter. The glue joint is a chemical bond between molecules at the surface. 2. Starved joint from overclamping: A myth, IMNSHO. The closer the surfaces are brought together by the clamps, the better. Glue joints can get starved from grain that absorbs glue, inadequate amount of glue, or failure to size first on highly absorbent surfaces, but not from overclamping. But lots of joints fail from poor fit and underclamping, or not enough glue to wet the surfaces completely. 3. Temperature: PVA glues will not coalesce properly below 50º F. But to be on the safe side, I think it's a good idea not to glue below 60º F. Upper 50's minimum. For hide glue you will have more problems in a cold room; it's a good idea to create a local environment well into the 80's or higher. Temperature may also (IMO) affect surface energy of the wood. 4. Spreading glue on both surfaces: unnecessary. You need enough glue in the joint to thoroughly wet both surfaces. How it gets there doesn't matter. 5. Fresh surfaces are good. |
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| Author: | Laurent Brondel [ Tue Dec 22, 2009 1:27 pm ] |
| Post subject: | Re: Bridges and HHG – First time questions |
dmills wrote: Anyway, as I have a bridge re-glue in front of me I thought I would dip my toes in water and try hot hide glue for the first time. If you've never used HHG before I would recommend against gluing a bridge as your 1st experience. It's the most important joint most prone to failure. Either you have a good system down, or you're wicked fast. At 70º HHG gels in less than a minute, so heating the bridge is mandatory, as well as being relaxed and having the operation well rehearsed. It's always possible to take it off and start over again… The issue you had sounds like wood oxydisation, and/or unclean surfaces. |
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| Author: | WaddyThomson [ Tue Dec 22, 2009 1:31 pm ] |
| Post subject: | Re: Bridges and HHG – First time questions |
I'm just guessing here, but if you scribed through the finish, did you scribe too deep. Seems to me that would leave a fine mesh of wood evenly all over the bottom of the bridge. Again, just guessing. I glue before finishing, so not a problem for me. |
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| Author: | dmills [ Tue Dec 22, 2009 2:03 pm ] |
| Post subject: | Re: Bridges and HHG – First time questions |
Thanks again for the comments. Waddy, I too am now suspecting the seeds of failure may have been sown during my scribing and/or finish removal work. I’m going to pay closer attention to my technique when I rescrape the surface for the reglue. Howard – for a HHG glue joint and the associated issue of the glue gelling with a temperature drop , wouldn’t it be good insurance to wet both glue surfaces with 140 degree material? |
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| Author: | WaddyThomson [ Tue Dec 22, 2009 2:07 pm ] |
| Post subject: | Re: Bridges and HHG – First time questions |
I don't think it's necessary. Use enough glue to wet both surfaces and just put it on one. IMO, the time to wet both surfaces just extends the time till the clamps are on and tight. Heat everything, have clamps in place, put the glue on the bridge, place it and clamp it. Practice till you are moving efficiently. |
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