Thanks for your comments Frank, Not easy for me to understand all of this. > > Regarding the cutting of the radii in the ribs, forget about where the > high point will be. That is irelevent to the arc of the ribs. The only > concern in cutting the ribs is at what point will the rib be at its > thickest, once the arc is cut. *** I had been assuming that the high point and the thickest part of the rib are one and the same. Some would say that it should be > thickest under the bridge. I would say that it should be thickest at its > center. To me, it's like a roof truss. To deliberately take the lower > member of a roof truss, and make it slopped higher on one side than the > other makes no sense. It would better support the load of a heavy snow if > it connects to the other two members of the triangle at the lowest > practical point on each side. The same is true of piano ribs, having the > thickest point at the center. *** Maybe one difference is that a rib is glued to the board. I think that a good part of the spport of string downbearing on bridge and rib comes from the glue joint of the rib as there is not much rib leftover to anchor into the rim. The rib ends are tapered quite a bit. In either case I have attached a photo of a board. The tape on the old board is very close to the location of the new ribs on the new board so the bridge location imprint is accurate enough. You will need to imagine in the additional cutoff bar on the old taped up board. For the most part with exception of the treble, lo-bass the bridge is centered on the ribs. Also, the high point or thickest part of the ribs is also under the bridge to the extent possible. The bass and low tenor bridges straddle the high point or thickest part of the lowest 4 ribs. Rib #1 - lowest bass has the high point quite a bit offset from center of rib. I do not understany why would I want to support a known string bearing load with something less than the thickest part of the rib as that was the dimension used to calclate load bearing? In addition - if I am using a formula to determine crown as a result of the radius I am hopefully calculating crown at the high point - the thickest part. To place the bridge elsewhere would alter this as well. Gene > > Frank Emerson -------------- next part -------------- A non-text attachment was scrubbed... Name: new and original boards copy.jpg Type: image/jpeg Size: 61996 bytes Desc: not available URL: <http://ptg.org/pipermail/pianotech.php/attachments/20090614/8abe3a7c/attachment-0001.jpg>
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