Decapping bridges with a router

[Fenton Murray]

Re: Decapping bridges with a routerRon,
Wow, this approach of a complete track down both sides of the bridge has occurred to me but always seemed to complicated. Thanks for the great photos and description.
Fenton
----- Original Message ----- 
  From: Ron Overs 
  To: Pianotech List 
  Cc: Bronwyn Bijl ; Miles Warner 
  Sent: Sunday, August 03, 2008 5:53 AM
  Subject: Re: Decapping bridges with a router


  Hi all,


  I've been following this thread which David started, and since the process we use doesn't seem to have been covered by anyone here is an outline, with image links, of the procedure we use when recapping a bridge without disturbing the original board (in cases where the board is deemed to be satisfactory, ie. stiff enough, for wearing out another set of hammers and wire).


  We find that most of the time, when fitting new bridge caps, we usually will want to make some adjustments to the bridge height. After removing the strings, an assessment of the current bridge heights are made before removing the plate. If the bridge height needs to be raised, we glue small blocks of an appropriate thickness to the existing bridge caps. The blocks are made precisely to the height we wish to raise the bridge at each measurement point. This will provide a reference height for manufacturing the router guides so that they conform to the revised bridge height profile. The image link below shows our most recent rebuild of a 2001 Hamburg Steinway D. Initially, I thought we were going to need to build a new board for this piano. The board's strung crown was turned quite inside-out in the high treble string sections at the rib feathering behind the bridge. But after stripping the piano the board seemed to recover its crown quite nicely, so a decision was made to keep the original board and bridge root. The original string scale of this piano had shorter speaking lengths in the high treble, when compared to model Ds we've rebuilt in the past. Our previous model D string scale re-designs wouldn't fit this piano. So I designed a new string scale similar to our previous revised model-D log scales, to enable the new string scale to fit on the original bridge root. Take a look at the first image by opening the link below.


  http://users.tpg.com.au/ronovers/CrstchrchDbd1.jpg


  You can see the router guides are finished to the desired height, ready for routing off the bridge caps. The templates for the new string scale, which must be determined before routing off the original caps, are sitting on the iron plate, just to the right of the case. For this job, we decided to retain the original board finish, since it was still in good condition. First, masking tape is laid down on the board where the runners will be attached to the board. The wax surface on the top of the masking tape is removed by wiping the tape with lacquer thinners. This ensures that the adhesive will stick to the masking tape. The runners are cut slightly higher than required, with the bottom surface cut to approximately follow the contour of the board. The accuracy of the lower contour is not critical, since we fix the runners to the tape using body filler. Once the filler hardens we plane and scrape the runners until we arrive at the desired bridge height and profile. Note the small blocks we have glued to the lower section of the bass bridge and the lower half-section of the treble bridge. This piano had insufficient downbearing in the low bass and low end of the long bridge, while there was quite a bit too much downbearing in the high treble (which was why the board's crown when strung was turned inside-out in the high treble). The down bearing can be reduced to the desired angle by packing the rear duplex strips with hardwood made to an appropriate thickness. While we could have adjusted the high treble downbearing by reducing the height of the new cap, I didn't want to do this because this piano had an original bridge height in the high treble which was only 30 mm. Taking another 1.0 - 1.5 mm off this height would make what I believe to be an already marginally low bridge height for a concert grand treble bridge even lower. So we decided a better route for the high treble in this instance would be to pack up the rear duplex strips.


  The beauty of using masking tape for board-finish protection, and using body filler to fix the runners to the tape, is that once the bridge cap has been routed off, the runners can simply be pulled off the board leaving it completely undamaged.


  http://users.tpg.com.au/ronovers/CrstchrchDbd2.jpg


  The above image link shows the same piano once the new bridge caps were fitted, and the notching, sealing and pinning completed. Notice that the new log string scale results in pin-lines which form uniform curves. Next time you get your eye under the lid of a standard D, have a look along the pin-lines, particularly those in the string section shown in the image. They don't look like this. Instruments equipped with his type of scale will make it easier to lay a tempered scale, and the tuning stability will be improved as a consequence of the piano having a smoother percentage of breaking strain deviation.


  http://users.tpg.com.au/ronovers/CrstchrchDbd3.jpg
  (this original image was provided courtesy of Reubin Bijl who took a series of images at the launch concert in early June)


  The above link shows an image of the finished piano, during its launch concert at the Music School of Christchurch. The underside of the top lid has quite a good reflection because we flatten and polish this surface to the same standard as for the outside of the case. I realise that most manufacturers don't bother too much with the under side of the lid, including the original manufacturer of this piano. But this image shows just what a difference high-polishing the underside of the lid makes. Yes I know its only cosmetic, but that's also part of the concert experience.


  Ron Overs
-- 
OVERS PIANOS - SYDNEY
     Grand Piano Manufacturers
  _______________________

  Web http://overspianos.com.au
  mailto:ron at overspianos.com.au
  _______________________
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