Pre-crowning ribs

[Erwinspiano@AOL.COM]

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In a message dated 3/16/2002 4:31:01 PM Pacific Standard Time, 
gharvey@netsource.co.nz writes:


> Subj:Pre-crowning ribs 
> Date:3/16/2002 4:31:01 PM Pacific Standard Time
> From:<A HREF="mailto:gharvey@netsource.co.nz">gharvey@netsource.co.nz</A>
> Reply-to:<A HREF="mailto:pianotech@ptg.org">pianotech@ptg.org</A>
> To:<A HREF="mailto:pianotech@ptg.org">pianotech@ptg.org</A>
> Sent from the Internet 
> 
>            Hi Grahm

             There has been much info on list about this subject in the 
recent months. I posted pictures and a brief description or my procedure 
using a table saw style jig just a month or so ago. Check archives under 
subject title "Dale Erwins Rib crowning jig."
  A word of warning on increasing rib dimension. Unless you have good reasons 
it is unwise IMHO to arbitrarily increase rib depth more than 1 or 2 mm. For 
example I don't find that increasing rib depth on model O Steinways except  
on the longest ribs helps  tonally. I find that to much depth here just 
creates a stingy board. also I personally don't think it's a very forgiving 
design. 
   Also increasing the rib depth as much as you've suggested is running the 
risk of a too stiff system which could end up being tonally stingy, meaning 
not a real free sound.  A 4 or 5 mm increase is a mile.
     I am relatively unfamiliar with Bechstein or European pianos in general  
but am currently installing a belly in a  6'-3" Bluethner which has all the 
Hallmarks of the Del and Ron's cutoff bar and fanned rib design. It has 14 
ribs with the tallest being 19 mm and only about two at that the rest range 
from 15 to 18.5. I'm only increasing the rib depth in the bass end by 1 to 
1.5 mm. I'm also increasing the the fan. This piano had a big tension spike 
in the tenor which I've reduced from about  average 190lbs per string to 
about 170. I believe the original rib scale was to light for the heavy 
tension which contributed with father time to the boards total collapse. Also 
realize that the board thickness is something to investigate closely and make 
some educated decisions based on that and the type of spruce your using. The 
Bleuthner is a beautiful design but  I'm also making a few other changes in 
the grain angle of the board so as I'll be able to float the board behind the 
bass bridge.I'm sure this will also be an improvement to the bass/bottom end. 
(Ron Del thanks for the ideas)
    I believe Del or Ron could contribute more in the numbers crunching dept. 
as this is something they've done a good deal of.
          Best----Dale Erwin

> 
> 
> List,
> 
> A tech friend and myself are rebuilding two identical Bechstein A's. We've
> made (glued) up the soundboard panels and are ready to thichness and sand
> them along with prepare (precrown) the ribs. We are on the search for an
> appropriate method including jigs to do this job. We have plans for one
> method, but it may not be the best way to do the job.
> 
> My questions are... What system or jig arrangement do the seasoned
> soundboard folk use for shaping (precrowning) the ribs?
> Also for this model piano are there improvements to be made in altering
> (reducing) the soundboard thickness?
> We are going to increase the rib height from the original minimal 19mm to
> somewhere near 23 to 24mm, but will be happy to incorporate advice and
> suggestions from those of you familiar with this model piano and who have
> successfully altered / improved some of the original soundboard / rib 
> specs.
> 
> Any input here would be very welcome.
> 
> Thanks,
> 
> Graeme Harvey
> New Plymouth
> New Zealand.
> 


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