Decapping bridges with a router

[Ron Overs]

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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