Crown without soundboards

[Ron Nossaman]

Hi Peter,

I was planning to post just this question to the list to see what other
folks were doing. I haven't installed a board for a over two years, and,
since I have a couple coming up soon, I was thinking of possibly updating my
jigs and methods if anyone had better ideas they would share. This is a
great excuse to prospect for improvements. Here is what I have done in the past.

My current rib crowning jig consists of a nominally 1" thick board (junked
upright front panel lumber) with one edge machined to a concave curve
(somewhere in the ragged vicinity of a 60' radius). It is screwed to another
board (on one side) with straight edges and has a series of screw down
clamps on the other side making a sort of stacked sandwich with straight
base, rib caul in the middle, and clamps on top. I set my rib blank in the
jig on it's side and bow it back into the curve with temporary clamps. Then
I tighten the jig clamps to hold the bow in the rib, and remove the
temporaries. I adjust the rip fence of the table saw to produce the rib
depth I want, and rip the top off the rib (rib is laying on it's side in the
jig) with the saw. When the clamps are loosened, the rib springs back
straight and has a nice square saw cut gluing surface with a uniform crown.
The jig is ugly as home made sin, looks like a committee project, and is a
bit cumbersome, but it's worked pretty well for me.        
When the assembled soundboard re hydrates, the crown increases somewhat (I
can't say I ever troubled to find out to what degree), but settles in all
right under string load.

I hope the crude description makes sense. Now let's fish for some
suggestions on how I OUGHT to be doing it.  


To the general populous:
There is one thing that confuses me concerning what I've been reading and
hearing about rib crowning. I hear more and more talk about putting the high
point of the crown along the bridge line. If the rib is machined to a
section of a circle, this is not possible. The high point will always be in
the center of the rib. You can put the center of load other than on the
center of the rib by tapering the rib, thinning the panel, or both, but the
high point doesn't move. Are the people who talk about doing this mistaken
in their belief that they are doing something they are, indeed, not doing?
Another obvious possibility is that they are machining a parabolic curve
into the tops of the ribs instead of a radius and indexing the ribs into the
jig to get the tightest part of the curve where the bridge will fall. It
also wouldn't surprise me a bit to find that the optimum curve (generally)
is different for the treble than for the mid section, and different still
for the low tenor. The 60' radius all the way through always sounded a
little simplistic to me, but I don't have the qualifications to argue
intelligently to the contrary. Not that such a consideration as that has
stopped me in the past, you understand, but a guy has to aspire to SOME
standards.   


Well, I've shown you mine. I'd love to hear all the gory details and
suggestions anyone else out there has on rib crowning jigs, methods, curve
forms, etc. Peter and I are listening attentively.


At 05:18 PM 1/2/98 +0100, you wrote:
>
>'t MUZIEKINSTRUMENTENATELIER
>PETER KESTENS
>BELGIUM
>KESTENS.P@DEBCOM.BE
>
>Ron, 
>
>You mentinonned:
>
><A better way is to machine the top of the ribs to a crown, dry the panel a
>lot less severely than the first method, and glue it to the ribs.>
>
>Can you please tell me how much crown you give to the ribs and how you do
this delicate job?  I always had in mind ribs had to be crowned in order to
give crown to a soundboard but I can't figure it out how one can give with a
saw or a router this amount exactly to the ribs.
>
>Thanks a lot.  
>
>Peter
>
>P.S.: happy new year.
>

May you learn quickly that the year is NO LONGER 1997 when you submit your
January invoices. %-) Happy new year yourself.
 
 Ron Nossaman