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From: Ross
Remote Name: 76.101.10.109
Date: 13 Aug 2007
Time: 07:58:08 -0400
Luthier’s Weblog 8/13/07 “Inspiration is for amateurs. The rest of us just get to work.” Attributed to macro-portraitist Chuck Close. Isn’t that good. Sometimes I think that the best I can do is just keep connecting the dots. Adhesives must be considered. Enjoying a kind of renaissance is old traditional hide glue. Though inconvenient to use, with a strong odor and short working time, it has much to recommend it. It is very strong in terms of tensile strength. Shear strength is good, and it’s crystalline nature eliminates creep and facilitates repairs. I used hide glue exclusively years ago when I was doing violin-family repairs. Then, giving in to convenience and convention, I switched to Titebond when I began doing guitar work exclusively. It was, and is, a very good glue with one problem: stressed joints can creep when ambient temperatures are high. Now, this isn’t a problem at a hundred degrees, but over one-thirty it gets iffy. The luthier that has thoughtful clients who do not subject their instruments to high temps is fine. But. A dark case in a closed vehicle on a sunny day will cause conditions conducive to glue joint failure. Further, a crystalline molecular structure is a very good conveyor of vibration. Which makes for a fairly acoustically transparent joint between plate and brace. The robot luthier is planning on using hide glue for top braces and bridge gluing. He still plans to use Titebond for other, less highly stressed joints. Perhaps clamping techniques amenable to the fast set-up time of hide glue will suggest themselves for these other joints in the future. The two types of strength attributable to adhesives that most concern the builder are shear and tensile. Tensile strength is when force is directed at a right angle to the plane of the glue joint. Shear strength is when force is directed parallel to the plane of the joint. Other measurable strength parameters such as flexture and compression, are not as critical to the builder. The stresses acting on the bridge can be most easily illustrate shear and tensile forces. String pull from the anchoring point at the headstock passes over the bridge saddle and down the bridge pin holes to the anchor point for the ball ends at the bridge plate. String tension tries to straighten the string, in the process of which forward pull is exerted against the front of the bridge pin hole. This force, being parallel to the glue joint, is shear. Should the bridge plate fail or become very worn, the ball end of the string can pull upward through the soft spruce of the top to exert upward force against the bridge itself. This would be an example of tensile force. Coupled with the already existing shear force working on the glue joint, failure is imminent. Other examples of these forces can be readily imagined in the anatomy of the instrument. With the year-in, year-out pull of the strings, a careful balance of strength to responsiveness must be achieved. Ross Teigen 8:37 am