Belly rail crown - Why???

[Farrell]

Sorry Ron, no answers from me, but I do have three additional questions.

Regarding planing/cutting a rise into the belly rail mid-section. Is it not even more pertinent whether there is a rise cut into the concave curve (as viewed from the outside of the case) of the case in the mid-treble area? In this area the same question would apply, but the geometry would be even more exasterbated. My understanding is that some manufacturers put a rise in this area and some don't. What are thoughts about this?

Regarding the plywood filler in the cut-off area. I wonder, would there be any benefit to using MDF or some other material that would offer the support you are looking for, but perhaps have more desirable acoustical properties. I know very little about acoustics. My thinking for posing the question is the preference for MDF in stereo speaker cabinets/installations over plywood for acoustic reasons. Thoughts?

And lastly, what do you do/think about bevel angles on rim components when rebuilding/remanufacturing. 1) Change all angles to match new soundboard (Yikes!)? 2) Leave original components (rim) as they were, but bevel new components (cut-off bar) to match soundboard? Or 3) leave original components (rim) as they were, and don't worry about putting any bevel in new rim components (install the cut-off bar on a plane with the rest of the inner-rim-top)?

Thanks for any thoughts.

Terry Farrell
  
----- Original Message ----- 
From: "Ron Overs" <sec@overspianos.com.au>
To: "Pianotech" <pianotech@ptg.org>
Sent: Monday, November 18, 2002 7:13 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).
> 
> 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.
> 
> 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.
> 
> Dale, Del, Ron N. etc. what are your thoughts on this matter.
> 
> Ron O.
> 
> 
> -- 
> _______________________
> 
> OVERS PIANOS - SYDNEY
> Grand Piano Manufacturers
> 
> Web: http://overspianos.com.au
> mailto:info@overspianos.com.au
> _______________________