my own Soundboard

[Erwinspiano@AOL.COM]

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In a message dated 1/18/2002 6:23:50 PM Pacific Standard Time, 
gnewell@ameritech.net writes:


> Subj:Re: my own Soundboard 
> Date:1/18/2002 6:23:50 PM Pacific Standard Time
> From:<A HREF="mailto:gnewell@ameritech.net">gnewell@ameritech.net</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 
> 
> 
> 
> Dale and Terry, 
>     I whole heartedly agree!!!! 
>   
> Dale, 
>     Can you describe your method for laying out and calculating the crown? 
> Greg Erwinspiano
> 
> 
       Greg
 I changed my mind. Its' still fri.
       It's not very scientific but I have a crown chart that has lengths in 
inches going down the left hand column of the page. Across the top is radius 
no.'s 25 ft. 30 ft. and so on out to about an 80 ft. Radius. I simply select 
the rib length and the radius I want and read to the right of the rib length 
no. under the 60 ft. radius column and voila!! precalculated crown no.s . The 
chart was compiled and shared with me by John Hartman and is a handy resource 
for quick calculations. No  number crunching is required. Not very glamorous 
ehh. oh well
     The decsion making process for choosing a crown dimension is a different 
matter. I find that many techs use very similar rib crown specs . I. e. I use 
a 60 ft. in the bottom of the panel except no 1 which gets a bit less. why 
because it get less bearing and need  a bit more flexiblity IMHO. The top 
five treble ribs get a bit more crown with the top being as tight as 25 or 30 
ft. radius rib this is my current process.
   My procedures have changed with time. I used to crown ribs less and dry 
more (4.5%). Now I dry to about 6% And crown more. 
  This yields a nice healthy hemispherical crown after installation about 10 
mm at roughly 8 to 8.5% e.mc. (45 to 50 R.H.) of about. This measured on the 
longest rib prestrung. The shortest rib appox. 1 to 1.5mm crown. Huge ehh?
  My Rib material of choice is nice tight grain sugar pine. Sometimes I've 
used spruce in the bottom 6 or seven. I've made a decision recently that 
increasing rib depth more than a mm except on the longest ribs is usually not 
necessary or          ( some   may disagree and that's O.K. ) in original 
design boards because of the increased stiffness of rib crowning itself.  
What rib crowning does as I see it is take a component of stiffness away from 
the soundboard panel and adds it the stiffness factor of the ribs without 
changing the mass of the whole system.  I.e. duplicating the original rib 
depth  providing the were reasonable as well as the soundbord thickness.
  The idea, as has been stated many times, is of course to provide for a 
longer lasting crown without the damage of over compressing the spruce while 
maintaining excellent tonal characteristics.
 There are two ways I know to cut crown into ribs. One is a router jig or 
jigs and the other is a jig made to cut the crown o a table saw. This is my 
personal weapon of choice as it has great and easy flexibility to cut a crown 
of any reasonable radius leaving an excellant machined gluing surface. Easy 
to construct and safe unless brain dead. I could post a picture if need be. 
Pictures are worth a thous. words aren't they?
   I radius the top (botom) of the ribs on a router table using a large and 
wicked radius bit. I cut the scallops on the ends, sand and save the pieces 
for glue up in the press.
 I put all the ribs on the board in a fire hose press that is setup all the 
time so I can easily set it up and press the ribs on in about 1 1/2 hrs. I 
use about 50 psi. Fabulous glue joints. 
    All the ribs are crowned, fit to the case rib notches &sanded prior to 
glue up.
     I'm Leaving out details but hey it's friday night.
    More fun than a barrel O monkeys

         Dale Erwin
   

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