Sohmer

[Robin Hufford]

Hello Owen,
     I measured the crown on the Sohmer across the top side, using a method
similar to that described by Newton, but, in this case using an assistant,
three hammers shanks that had been sanded even, and a thread.  I did this
mainly to indicated the sense of the curvature, whether positive or negative,
as this point was under discussion.  .
     The shanks are placed wherever one choses  close to the edge of the board,
and of course, maintained as squarely upright as possible.  The string is then
pulled tight across the tops of the shanks, indicated approximately a straight
line between them. A straight edge will serve equally well.  The third shank is
then placed on the board wherever and its height is judged relative to the
string.  Its varying height indicates, essentially, the curvature.
     As has been noted, however, the results of this method can be inaccurate
due to thinning of the board.  With judicious use, though,  it is possible to
make  reasonable inferences as to the curvature of the board.  First, the
board indicated a little more than an eighth of an inch in the middle and this
feathered down in both directions along the string to zero as the shanks
supporting the threads were reached, readings taken with the thread at
different places, but still crossing the grain  were approximately similar.
These suggest to me several things:  if the differences in height indicated at
the center were due to thinning then the amount removed in that process  would
have to have in excess of an eighth of an inch, at two different edges,
something unlikely enough that it seems reasonable to suppose the reading
indicates curvature imposed by crowning.  Second, the important  fact of a
curve being indicated by the sytematically varying readings of the  indicator
shank, as it is ranged along the string,  suggests again that this reading is
not due merely to thinning but rather to crown.  If the hiehgt differential
perisits and increases  as one approaches an area somewhat in the middle of the
board, declines after you pass the apex, and passes to zero at the other
support, and, in particular, one finds this pattern at a number of places,
then, again, the inference of an actual curvature seems unavoidable, at least
to me.  Otherwise, we must admire an effort of the part of the installer of the
board such as to result in a degree of  uniformity and distribution of thinning
of the board as to seem implausible, so the supposition that the height
differential indicates actual curvature, is once again, at least defensible,
and to me, seems likely.
     However,  this method and its interpretations are not perfectly  reliable,
although, in fact,  I would be hard pressed to remember a case where  a board
that indicated crown, where  the method used above had been employed employed
and correctly interpreted, that did not check out when verified by the use of
other methods such as the use of a thread or a straightedge next to the ribs in
the ordinary fashion.  I will use the method described above on the bottom side
of the board and report back.  When using it on the bottom of the board  the
white plastic foot long rulers used to measure hammer angles which Wally Brooks
hands out at his classes work well, but anything that clears the beams, and is
the same length, can be used.
     The indicidence meter I described in the postings on the soundboard thread
in the last month or so,  can be used across the top of the unloaded board to
guage the curvature, in a quick, casual method, again, with the qualifications
given above.  It is, as was noted by Ron N. actually an inclinomenter.
Regards, Robin Hufford

Hi Robin, i

> Could you elaborate on how you were able to measure the crown on your
> Sohmner?  I have a 7 foot, serial # 25512 model #11 in my shop, and am
> pondering how to measure the crown with that maple reinforcing strip in the
> way.  I don't think that this piano has any crown, but would love to hear
> your method.
> Thanks, Owen.
> ----- Original Message -----
> From: "Robin Hufford" <hufford1@airmail.net>
> To: <pianotech@ptg.org>
> Sent: Thursday, March 21, 2002 3:45 AM
> Subject: Re: Sohmer
>
> > Dale,
> >      I have a Sohmer 9B in the shop.  It has conventional crown which is
> > about 1/8 in the middle.  This is after restringing and being within 1/2
> > step of pitch.   I think these are great pianos and I like their bridge
> > agraffe system.  This particular Sohmer had a very pronounced long crack
> > running about an inch and a half or so in front of the bridge.  This
> > crack, which I have seen on several other Sohmers in virtually the same
> > configuration, is obvious stress relief from the forces produced by the
> > bridge operating and pulling on the board, that is, relief of shearing
> > stresses.  Usually, there is about an eighth of an inch or more of
> > offset in height between the two parts of the board where this occurs.
> >       In the one in the shop the ribs were dowelled to the board in this
> > area and the crack was filled with epoxy.  This crack is about the only
> > characteristic failure of  the c. 6 feet Sohmers with the agraffe system
> > I have seen, although, as I have seen perhaps, only 8 or 9 over the
> > years, this may be a generalization on shaky grounds. The factory,
> > apparently, was aware of this as they have attached on the botton side
> > of the board, under the bridge, a long reinforcing strip of maple, let
> > in and passing through the ribs. The crack occurs in front of this
> > strip.
> >      As to the agraffe termination at the bridge, I think this works
> > quite well and, is probably a better, but more expensive mousetrap.
> > Perhaps the expense is not that much more than the conventional method
> > but I would guess that it would be as the bridge requires, of course,
> > planing, notching of a somewhat different fashion,  the maple strip,
> > notching of ribs, AND an agraffe and its installation.  The ringtime of
> > this system seems distinctly superior to my ear in comparison to the
> > conventional method.
> > Regards, Robin Hufford
> >
> > Erwinspiano@AOL.COM wrote:
> >
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