Cutting rib radii

[pianoguru at cox.net]

> ..."But the shape of the curve wont' be an arc"
I've heard that argument before.  I'm not so sure that is the case, and even less sure that it is of any consequence.  

Another method for scribing a large diameter arc is to convert the radius data to "rise" over a given distance, say 6'.  Construct a bow, as in an archery type bow, where the string is music wire secured by tuning pins at each end.  The frame is an otherwise straight wooden beam, bent by the tension of the string.  Adjust the tension until you measure the correct "rise" for the desired radius.  Trace along the edge to transfer the "arc" to the work piece.  Of course, bending the work piece itself has essentially the same result.

In some manufacturing procedures, the ribs are bend, but without clamps.  A "shoot board" is constructed for use in a large industrial drum sander with conveyer.  A group of ribs are placed on the shoot board, supported at each end such that there is a gap at the center between the rib and the lower surface.  The downward pressure of the sander drum forces the rib into contact with the lower surface.  The speed of the movement of parts under the drum is critical.  At a lower speed, the rib can recover a little from the bending pressure and reduce the resultant radius.

There are similar manufacturing operations to produce a concave crown on the bottom surface of treble bridges.  The shoot board in this case has an elevated center.  It doesn't work quite as well in the absence of clamping press.  In this case, the sander cut a relatively straight line to the center, where the bridge flops to the other side, to cut a relatively straight line on the down-side of the cut.  Not only is this clearly not an arc, but the lateral curve of the bridge causes it to tilt out of parallel to the top side.  While this is little more than an approximation of the soundboard crown, is seems to be close enough for some fairly high-end manufacturers.  Others manufacturers simply leave the bottom of the bridge flat.

Frank Emerson