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Hi all,
Some time back I mentioned on the list that we have been re-crowning
sound boards by firstly jacking crown back into the board, then
removing part of the ribs and building them up again with new Sitka
spruce to re-set the sound board crown. Dale Erwin expressed an
interest in seeing some images at the time.
Many (probably most) of the sound boards in service develop panel
weakness, or have had it since the time of construction. When a sound
board looks to be in reasonable visual shape, I have proven to my
satisfaction that it is possible to re-crown a board (and more
importantly stiffen it) to restore it to proper function.
I have re-sampled 8 images and uploaded them to one of my webspaces
(I hope you lads and lassies appreciate the 3 hours I have put into
this on this fine morning). Please find below links to the images
with accompanying text included in this post.
http://members.optusnet.com.au/ronovers/recrown1.jpg
This image shows the small jacks which I have manufactured, for using
the back beams and cross braces to jack crown back into the sound
board. The piano we are operating on here is a Kawai KG6. We have
just started to rasp back the rib height in this image. The ribs on
this piano were quite deep and generously proportioned. The sound
board panel is 10 mm thick overall. It is quite surprising that such
a seemingly robust sound board should develop short tone and
impedance problems. But this late 70's instrument, in standard form,
produced a most unpleasant and short shocky tone.
Last Tuesday I inspected another slightly younger (1979) KG6 grey
market piano at a venue one hour north of Sydney. This piano had no
compression ridges in the panel. The board was virtually flat. The
tone was simply appalling. Looks very much like this piano might get
a similar treatment to the one shown in these images.
PLEASE NOTE:
The jacks must be placed directly under a bridge to prevent the jacks
pushing a hole in the sound board panel. The bridges also act to
spread the jack-pressure to ensure that the panel moves up as a whole.
http://members.optusnet.com.au/ronovers/recrown2.jpg
Once the crown is jacked up to what we deem to be an appropriate
level of crown, plus a couple of millimeters (to allow for the
sinkage which will inevitably occur when the jack pressure is
released), we take to the ribs with a half round wood rasp to remove
the bulk of the rib height we wish to replace. Bear in mind that the
originally compression crowned ribs are now pushed back into a curve.
We remove the rib material to achieve a straight bottom on the
re-profiled rib. This means that we will be removing more rib height
from the ends and less in the middle. In the image, Wal Sullivan is
using a specially made long sanding block (with 80 grit paper glued
to it) to straighten the rib tops and get them nicely trued up for
gluing the new rib extensions to the existing ribs. Notice that we
aren't re-crowning the first two ribs in the back corner. These ribs
are subjected to much lower stress than the middle order ribs. We
also don't re-crown the highest treble rib, since its nearly
impossible to get to, and the bridge is so close to the belly rail in
the high treble that sound board collapse isn't a problem at the very
top end.
In terms of structure, the extended ribs will be significantly
superior to the uniform height original ribs. A sound board rib
should be at its stiffest right under the bridge where it is subject
to the most stress, becoming gradually weaker towards the rim. The
extended rib will be deeper and stronger in the middle since we will
have removed less height to achieve a straight surface for attaching
the extension.
http://members.optusnet.com.au/ronovers/recrown3.jpg
This image shows how we clamp the rib extension to the original rib
using a cheap-and-nasty piece of flat garden hose at 50 psi. A
radiata 4"X2" is clamped across the case to support the air hose,
with spacer blocks held in place between the back beams with masking
tape until the hose is pressurised. We use Titebond Extend for gluing
the rib extensions in place. When the pressure is applied the hose
will push the board away from the back beams so that it clears the
jacks (we need to be careful to leave the jacks in place so that the
board remains jacked up when the pressure is released from the air
hose). The rib extension for the next rib to be glued is shown in the
image. Notice that the feathering is already cut into the extension
piece and that it is already sprayed with the acid cat lacquer that
we use for sealing sound boards.
http://members.optusnet.com.au/ronovers/recrown4.jpg
With a 10 mm thick sound board panel, and the bass bridge being so
close to the inner rim, this piano in original form didn't have much
in the way of bass fullness for a 225 cm piano. In our customary
manner, we have trenched the sound board panel just adjacent to note
no. A1. The trench depth is 5 mm, half of the panel thickness.
http://members.optusnet.com.au/ronovers/recrown5.jpg
Some of the original bridge notching on this piano was pretty
ordinary. C88, as shown in the image, was notched completely in front
of the pin-line. These matters need to be rectified in conjunction
with a re-crown.
http://members.optusnet.com.au/ronovers/recrown6.jpg
This view shows the sound board after re-crowning and bridge
notching, and the application of the acid cat lacquer sealing coat.
The crown on this sound board can be clearly seen in the image.
http://members.optusnet.com.au/ronovers/recrown7.jpg
This image shows a close up of the rear of the sound board where the
panel was trenched for bass tone.
http://members.optusnet.com.au/ronovers/recrown8.jpg
This image shows an underside view of the soundboard after the
rib-crowning operation. Two of the rib featherings near the cut-off
originally were very short. We lengthened these before fitting the
rib extensions. The re-finished iron plate for this piano can be seen
in the background.
I'm just regulating the action at the moment. The piano is only
chipped up at present, but we've got a known good piano in the
workshop at the moment for its second rebuild, and after plucking the
strings of both instruments and comparing the sustain and tonal
characteristics I am very pleased with the re-crowned KG6. Looking
forward to getting a couple of tunings and some fine voicing into the
hammers. We've fitted an Overs action to this piano, and the Wurzen
felted Abel hammers sound almost balanced straight out-of-the-box.
Ron O.
--
OVERS PIANOS - SYDNEY
Grand Piano Manufacturers
_______________________
Web http://overspianos.com.au
mailto:ron@overspianos.com.au
_______________________
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