On Nov 12, 2008, at 3:37 PM, Ron Nossaman wrote: > If your board has any real crown, and you're putting any real > bearing on it, this will happen. My last post (which for some reason was labeled as SPAM?? 59% - I must have cooties or something) was somewhat tongue in cheek. On a more serious note, my remark about bridge pins hitting struts had a couple of scenarios behind it. The first was a local rebuilder who believed that "more is better," so for every piano he rebuilt he increased both crown and downbearing. He started by "re-crowning" the board: took a sabre saw and cut a kerf in the soundboard about 6 inches either side of the bridge. The kerf extended about halfway through the ribs. He jacked it up from underneath with wedges, inserted hard wood wedges in the rib slots, and shimmed the board. "Very successful process": run a string under his boards, and you see a gap of as much as 1/4". To increase downbearing, he lowered the plate 1/8" or so. As a result, he often had the problem of bridge pins touching struts, and colleagues tell me he sometimes would have to "jack the board down" (wedges or jacks pressing down on the board) to be able to restring, and that didn't always work. Sometimes he'd have to remove wood from the bridge or file a strut. The second scenario is manufacturing. Crown varies by moisture content, especially for compression crowned boards. The manufacturer (or rebuilder perhaps) takes a lot of care with moisture content in gluing on ribs and installing the board. But then attention seems to wander. The rim with belly wanders around having things done to it, and eventually gets to the guy who "sets the downbearing" by setting the plate and planing bridge tops (vertical pinned plates are different, but not entirely). Is the factory humidity controlled? Not generally. Are the soundboards checked for EMC when downbearing is set? Not generally (Here vertical pinned plates become the same as other models). The point being that often, in many manufacturing or rebuilding processes, nobody knows where the soundboard is when downbearing is set. Is it dry and flat? Moist and super-crowned? Beats me, says the factory worker. I just do my job. So it really becomes anyone's guess whether bearing was established in a moist or dry period. We get to the stringing department. Sometimes they have a piano where the bridgepins are hitting the struts. Hmmm, that's funny. Well, we'll just develop a procedure for that. And they do. IOW, one hand doesn't know what the other is doing, one department doesn't communicate with the other, nobody really thinks through whether the manufacturing process actually reproduces the design. To my mind, bridge pins hitting struts should wake someone up to ask a question or two. That's the point I was trying to make. Now it may well be that a particular design philosophy would find that scenario acceptable, and that's fine by me. Regards, Fred Sturm University of New Mexico fssturm at unm.edu
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