bridge cap headache

[Michael Spalding]

Clyde,

I feel your pain.  It's very discouraging to run out of time and ideas, and
not achieve the results you had hoped for.  Happens more often than I'd
like.

Even with the kind of bridge failure you described,  the bridge pins are
usually holding the cap in place.  Or was the cap so thick that the pins
did not go through into the bridge body?  This would make it very difficult
to do a field repair, (and might also explain why the cap separated in the
first place!)

Here's my procedure for field repair of the worst repairable bridge cap
failure.  If it's not repairable this way, I don't know any way except
re-capping.   Remove the action, (if its a vertical, tip the piano,) loosen
strings and get them away from the bridge.  Push poke and shove the bridge
pins into position.  Push poke and shove the bridge cap fragments into
position.  If screws are required, do them now, while everything is clean
and dry.  If you can't get the pieces to sit where they belong, this is
when you bail out and say it can't be repaired.  Apply West System epoxy
with a stiff brush.  Use a heat gun/hair dryer to warm the bridge and epoxy
so that it penetrates all the nooks and crannies, being careful not to
"cook" it and cure it prematurely.  Continue brushing new epoxy onto the
bridge until it begins to thicken, to the point that it fills the larger
cracks and doesn't flow back out of them.  Don't worry about buildup and
brushmarks on the top of the cap or bridge pins - this stuff will flow and
level after you stop messing with it.  You can clean up with a rag and
acetone if necessary, but this often makes things worse.  Let it cure as
long as possible before un-tipping the piano - this is when I do the action
repairs, hammer filing, etc.  You can usually untip the piano after an hour
or two, but don't re-string until the next day.

Regarding string length - Changing wire size won't do any good.  Larger
wire is stronger, but needs higher tension to achieve the same pitch. 
Bottom line is, the percent of breaking strain is determined by the length,
not the diameter.  If you've only added 2mm, you probably are OK.  Most
scales are around 2" at C88, and anything less than 2 3/16 is reasonably
safe.

hope this helps

Mike


> [Original Message]
> From: Clyde Hollinger <cedel@supernet.com>
> To: <pianotech@ptg.org>
> Date: 5/8/2004 5:16:53 AM
> Subject: bridge cap headache
>
> Friends,
>
> Yesterday I worked for hours on a 1974 Baldwin spinet that had been long 
> neglected -- replacing all donut grommets and regulating lost motion, 
> replacing a torn string, tightening screws, cleaning, etc.  I had done a 
> pitchraise and tuning at a previous appointment.
>
> The bridge cap was unglued for the top four notes A7 and up and broken 
> loose as well.  Although I am not skilled at bridge repairs, I thought 
> this looked simple enough to glue in place.  Stupid me.
>
> I got the action and strings out of the way and checked the 
> positioning.  OK, I guess, although if I pushed the piece together at 
> the crack, the cap didn't line up with the bridge.  My strategy was to 
> put glue on the piece and hold it in place by putting the strings under 
> tension until it dried.  Not smart, especially if there is negative 
> bearing, which I soon learned.
>
> So I held the piece in place while drilling a pilot hole for a screw.  I 
> wasn't surprised that when I tightened the screw, it pulled the piece 
> out of proper position.  I didn't really have room to drill straight in 
> since the keybed and frame were in the way.  Grrr!  I left it that way 
> to dry, told the owners I will need to return to finish up, and went 
> home very tired and blue.  It is a repair I hope no one ever knows I 
> did.  In retrospect, I think I should have used CA glue with 
> accelerator, but I was concerned about it drying too quickly for me to 
> get it properly positioned.
>
> Okay, so you guys with lots of experience, how would you have handled 
> this?  I'm still open to learning.
>
> A couple last questions.  The bridge cap is now a little farther down 
> than it should be, maybe 2mm.  Will the added string length cause the 
> strings to tear when I bring them up to pitch?  And if they do, should I 
> try replacing them with thicker or thinner strings?  They're size 13 
> now.  It seems that a thicker string would have more strength, but I'm 
> not that good at physics.  Would a thinner string be better?  (I don't 
> stock thinner string.)  I'm eager for your responses, and I hope I don't 
> have a day like yesterday for a long time to come!
>
> Regards,
> Clyde
>
> _______________________________________________
> pianotech list info: https://www.moypiano.com/resources/#archives