<!doctype html public "-//W3C//DTD W3 HTML//EN"> <html><head><style type="text/css"></style><title>Recrowning - was Soundboard Crown measurement</title></head><body> <div>Hi all,</div> <div><br></div> <div>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.</div> <div><br></div> <div>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.</div> <div><br></div> <div>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.</div> <div><br></div> <div><font face="Lucida Grande" size="-3" color="#000000">http://members.optusnet.com.au/ronovers/recrown1.jpg</font ></div> <div><br></div> <div>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.</div> <div><br></div> <div>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.</div> <div><br></div> <div><font color="#FF0000"><b>PLEASE NOTE:</b></font></div> <div>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.</div> <div><br></div> <div><font face="Lucida Grande" size="-3" color="#000000">http://members.optusnet.com.au/ronovers/recrown2.jpg</font ></div> <div><br></div> <div>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.</div> <div><br></div> <div>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.</div> <div><br></div> <div><br></div> <div><font face="Lucida Grande" size="-3" color="#000000">http://members.optusnet.com.au/ronovers/recrown3.jpg</font ></div> <div><br></div> <div>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.</div> <div><br></div> <div><font face="Lucida Grande" size="-3" color="#000000">http://members.optusnet.com.au/ronovers/recrown4.jpg</font ></div> <div><br></div> <div>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.</div> <div><br></div> <div><font face="Lucida Grande" size="-3" color="#000000">http://members.optusnet.com.au/ronovers/recrown5.jpg</font ></div> <div><br></div> <div>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.</div> <div><br></div> <div><font face="Lucida Grande" size="-3" color="#000000">http://members.optusnet.com.au/ronovers/recrown6.jpg</font ></div> <div><br></div> <div>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.</div> <div><br></div> <div><font face="Lucida Grande" size="-3" color="#000000">http://members.optusnet.com.au/ronovers/recrown7.jpg</font ></div> <div><br></div> <div>This image shows a close up of the rear of the sound board where the panel was trenched for bass tone.</div> <div><br></div> <div><font face="Lucida Grande" size="-3" color="#000000">http://members.optusnet.com.au/ronovers/recrown8.jpg</font ></div> <div><br></div> <div>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.</div> <div><br></div> <div>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.</div> <div><br></div> <div>Ron O.</div> <x-sigsep><pre>-- </pre></x-sigsep> <div><font face="Times New Roman" size="+1" color="#0000FF"><b>OVERS PIANOS - SYDNEY</b></font></div> <div><font color="#000000"><i>   </i></font><font color="#0000FF"><i>Grand Piano Manufacturers</i></font><font color="#000000"><br> _______________________</font></div> <div><font size="-2" color="#000000"><br> </font><font color="#000000">Web http://overspianos.com.au</font></div> <div><font color="#000000">mailto:ron@overspianos.com.au<br> _______________________</font></div> </body> </html>