[pianotech] Kimball Petite Grand

[Ron Nossaman]

On 2/23/2013 12:22 PM, Jim Ialeggio wrote:
> Do we have any suppliers making what could be used as root stock in
> bridge appropriate lengths?  Pinblock stock is too short.

Someone may, I don't. It'll be awfully wasteful of material in any case. 
Economy of materials and performance are the reasons I use salvaged 
maple flooring for bridge roots, and shop made laminated veneer caps.


> The reason I noticed this was the unison stability and lack of false
> beats. This stability, not the tone of the instrument, but the
> stability, frankly beats many high value instruments I service hands down.

Yep, that's the performance part, and the cap alone will do that. If you 
want a horizontally laminated bridge, it's easy and not that time 
consuming to lay one up from multiple maple strips, with low angle cross 
plies. I haven't used one in a piano because the vertically laminated 
bridge is so easy to make, but I made one for my class use, and don't 
see any reason it wouldn't work like gangbusters, however gangbusters 
work. Techs will look at it and tell you how much better it would sound 
if you made the bridge "right", naturally, but that will be some time 
after they get past asking where the trichords and critical tuned front 
duplexes went


> My interest is a purely mechanical one. I see the multiple horizontal
> gluelines themselves grabbing the pin in a way that is less
> hygroscopically reactive.

I don't see that as much of a factor, personally. Getting a couple of 
glue joints close to the top surface will add enough support to be a 
functional tonal improvement.

>Much like the epoxy/veneer cap, only the
> epoxy/veneer purposely keeps those gluelines up near the cap surface,
> giving the pin foot more scope to float.

Yes. We don't really much care what the pin foot does, as long as it 
isn't bound up and we can't control it if we did care. The important 
part is as much support as possible as close to the cap surface as possible.


> But even so, the UF gluelines
> of pinblocks do the same thing. They effectivey keep the pin from
> walking out of the block with moisture cycles, which is a highly
> valuable trait for a bridge root/cap as well, in my view.

Sorry, no. Neither is in any danger of walking out, whatever the glue 
line configuration. People envision fence posts walking out of the 
ground with frost heave and yes, this certainly happens. The reason is 
that when the frost lifts the post, dirt from the exposed side of the 
hole falls into the space and keeps the post from going back down in the 
thaw cycle. This eventually jacks the post out. If bridges and pinblocks 
had holes with dirt (or similar) sides back filling the gap, the same 
thing would happen to them. Since they don't, the pins go right back 
where they were each cycle. This is why bottoming the pins in bridges is 
pointless because they don't stay there. In the dry season, the bridge 
shrinks in height and the bottomed pin is pushed up. Come the wet cycle, 
and the added friction (you mentioned) of the cap glue lines hold the 
pin mostly stationary there as the bridge grows taller and the bottom of 
the hole recedes from the bottom of the pin.

I made some models years back and cycled them through big humidity 
swings for a month or so, measuring overall height, cap thickness, and 
pin height above the cap surface, and did the math. It was very instructive.
Ron N