Belly rail crown - Why???

[Ron Overs]

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Ron N. and Del,

My thoughts concur with both your contributions, so its comforting to 
know that if I am way out in left field I'm not alone. Regarding the 
next two cut-off bars we're making, I've decided to laminate them. I 
designed the clamping system last year but didn't get around to 
building it. Wal had already cut up the RHS, so I should get it 
welded together by tonight (Sunday Sydney time).

Ron O.

>----- Original Message -----
>From: <mailto:sec@overspianos.com.au>Ron Overs
>To: <mailto:pianotech@ptg.org>Pianotech
>Sent: November 18, 2002 4:13 AM
>Subject: Re: Belly rail crown - Why???
>
>To listees concerned with sound board installation,
>
>Again I find myself questioning a long standing tradition in piano 
>making, and why it should be so done.
>
>The theory of sound board curvature (whatever that might be) tells 
>us that the shape of a sound board panel ideally should conform to 
>that of a sector of a sphere. Since it has been decided that the 
>sound board should be installed so that it experiences less stress, 
>makers historically have decided that there must be a rise in the 
>centre of the belly rail to accommodate the spherically shaped 
>'board (6mm or 1/4" would seem to be a typical figure).
>
>
>As well you should.
>
>In the above you bring up three assumptions I have questioned over 
>the years. First, that the soundboard should be installed 'less 
>stress.' Second, that there should be a rise in the center of the 
>belly to accommodate the spherically shaped soundboard. And, third, 
>that the shape of the soundboard should be spherical.
>
>The whole idea of soundboard crown and its subsequent physical 
>distortion by the effect of string loading is to distort -- stress 
>-- the thing.
>
>
>I am wondering about the validity of this practice, since the board 
>does not in reality assume a spherical shape. It would appear to be 
>crowned more or less perpendicular to the 'line' of the long bridge, 
>ie. along the line of the ribs. Furthermore, the treble corner tends 
>to curl up like a potato crisp after the ribs have been fitted, 
>which would cause the raised centre of the belly rail to place the 
>glued down belly under even greater stress than if the belly rail 
>was made straight.
>
>As you say, no soundboard panel is crowned to a true spherical 
>shape. More typically it will take on either a cylindrical shape or, 
>possibly, a slightly conical shape.
>
>With only one exception that I am aware of no manufacturer has 
>shaped a rim to accommodate a spherical soundboard shape and that 
>was Kimball in its later years. Of course by then they were using 
>laminated soundboards exclusively. Some vertical piano makers do 
>shape their soundboard liners to a more-or-less spherical shape but 
>it is generally only a fraction of the shaping that would be 
>required to really match the claimed shape of the soundboard 
>assembly.
>
>At best crowning the bellyrail takes care of only one relatively 
>short span of the soundboards edge. What about the rest?
>
>
>Just sitting at an empty case thinking about this, I can see no 
>disadvantage in setting the belly rail straight. In fact with the 
>piano I am setting up at present, the cut off bar extends so far 
>across to the treble end of belly rail that the 'board will not be 
>supported by the belly rail until the highest treble section is 
>reached. The cut off rail I am building is a continuously curved 
>member of 30 mm depth and 40 mm wide. It will be glued to the back 
>posts, and the entire non-sound board bass corner will be filled in 
>with a 19 mm thick plywood piece which will butt up to the sound 
>board panel over the cut off bar. This is intended to provide a high 
>level of lateral support for the cut off bar and sound board 
>perimeter. I can see no advantage in setting any part of the sound 
>board perimeter at a height other than that of the inner rim.
>
>It is my practice to set the rim and bellyrail, including the 
>soundboard cutoff rail, flat. Since the soundboard will be stressed 
>-- i.e., have considerably less crown -- once the strings go on I 
>prefer to have a flat, consistent mounting surface for the parameter 
>of the board. Besides, my cutoff bars tend to be so shaped that very 
>little of the soundboard is actually glued to the bellyrail.
>
>And, while we're on the subject, for the same reasons I fail to see 
>any advantage to beveling the inner rim to (supposedly) accommodate 
>the shape of the soundboard. It is claimed that this somehow aids 
>the maintenance of crown but I surely don't see how. To illustrate 
>simply clamp a piece of spruce to a bench and see how easily it is 
>bent.
>
>Del
>


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Grand Piano Manufacturers

Web: http://overspianos.com.au
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