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 Post subject: Vacuum camping bridges
PostPosted: Sat Oct 13, 2018 12:11 pm 
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I recently tried vacuum clamping a bridge on as opposed to my usual mechanical clamp method. Easy to do and a fun process. I vacuum clamped the bridge with Titebond, removed the vacuum after 10 minutes and cleaned up the squeeze out which was considerable, and put the clamp back on for another 20 minutes. After a week or so tuned up, the bridge came cleanly off. The sole and top show very little glue residue. Is there such a thing as too much vacuum pressure? On a top my pump does not reach the shut off value of 24# that it does when using my vacuum bag, but it still reaches about 20#. I'm good enough at fitting bridges to tops that I think I can rule out the fit of the joint as a problem. Thanks, Bob

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PostPosted: Sat Oct 13, 2018 1:08 pm 
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Bob, I have successfully glued bridges at 20" of Hg. Having lots of squeeze out is obviously a good sign. Could there of been a contaminated surface?

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PostPosted: Sat Oct 13, 2018 4:45 pm 
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I'm gluing at 24Hg using TB with no issues. The only failure I've had in the last 35 was due to the surface being contaminated with buffing compound. I scraped the surface and re-glued with no issues. I use a couple of small brass nails through-drilled in the saddle slot to locate the bridge. The nails have a domed head so there little chance of damaging the vacuum clamp membrane.

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PostPosted: Sat Oct 13, 2018 5:22 pm 
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I also glue at about 24in of Hg with both Hot hide glue and Titebond—no issues. I'd have to guess you had surface contamination issues.


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PostPosted: Sat Oct 13, 2018 9:26 pm 
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Bob, a couple of more thoughts. What's the reading on your gauge without the clamping membrane sealed against the top, if the gauge is at the pump it's could give the illusion of a good seal just due to pressure loss in the line. Also were the pin holes drilled through top yet?

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PostPosted: Sun Oct 14, 2018 1:04 am 
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Thanks for the replies. I'm going to have to go with the surface contamination theory. Lots of clamping pressure at the jig, so can't be a problem there. I usually route off the finish under the bridge, but since this was an unusual project with a very thin finish I just scraped it. Probably was not good enough. The instrument is just a fast fun project done for our annual uke show here on Hawaii Island and I used it to try a number of quick fixes that I don't use on anything I normally make for sale. The uke is here http://www.pegasusguitars.com/recycled- ... ulele.html . Just for fun. I hope to have a chance to do a similar guitar project at some point.

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PostPosted: Sun Oct 14, 2018 6:22 am 
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It sounds as though contamination is the most likely culprit here, but given the very low pressure generated by vacuum clamping - less than 15 psi at sea level as a best case - is it possible that the glue gelled before the joint was fully clamped and the glue line had a chance to thin? While a well made joint needs very little clamping pressure once the glue line is thinned (e.g., rubbed joints in hide), the glue line must still be thinned before the glue gels or starts to cure for a full-strength joint.

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PostPosted: Sun Oct 14, 2018 7:49 am 
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Off topic, but where do you find redwood on Hawaii Island?

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PostPosted: Sun Oct 14, 2018 12:30 pm 
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Pat- Redwood does indeed grow at a few high elevation places on Hawaii, but it is not instrument wood. Grows too fast. Lots of branches and knots. However, tons of old growth redwood was used in construction here. Many of us live on catchment water, so there are plenty of old vertical grain water tanks around. Also, because of it's resistance to rot and insects, a lot of it was used for framing and outdoor construction. The wood for this uke came from a seaside outdoor house deck that was being renovated. Nice 14 foot long true 2" x 10"-12" planks. It is all gorgeous old growth, but full of nail holes and not vertical grain. Much of it I use for raised planting beds. but I thought it would be fun to use for this special project. In years gone by I also got lots of wood from a secret bay known as eBay. Sellers used to sell nice chunks of curly redwood at very reasonable prices. Hit and miss, but I have some beautiful wood from that source. Those days are gone. Now the sellers resaw it into what they call instrument wood and generally the prices are too high to risk buying. Times change! Happy building.--Bob

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These users thanked the author Pegasusguitars for the post: Pmaj7 (Sun Oct 14, 2018 6:20 pm)
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PostPosted: Mon Oct 15, 2018 12:09 pm 
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Good info. Thanks, Bob.

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