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MMD > Archives > March 2000 > 2000.03.07 > 17Prev  Next

Cracked Piano Plate Can Be Welded
By Craig Brougher

Larry Mayo, I believe, misunderstood some things about defective piano
plates, as I am sure others have, too.  I want to "stress" a few points
here again about what works, what might work, and what you definitely
should not try.

When I mentioned curing plates outdoors for a season (they call it
seasoning) I referred to stress lines, forgetting that most people
would not know what I was talking about, who have not dealt with the
metallurgy of grey cast iron.  Machinists know how important this is
when their new Bridgeport mills were unable to hold repeatable settings
month after month.  The machines were eventually sold for scrap.

It is the uneven stresses built into the iron by faulty casting or
casual, everyday variations that creates different amounts of tension
here and there in the casting.  If it's great enough, the casting could
eventually crack.  These lines can actually be seen when a model is
made from Plexiglas, cast in the same way, and polarized light is
diffracted through the plastic by these lines, which approximate what
you will get in iron.  This was years ago.  Today, it's done with

Were the model to be immediately subjected to stress similar to
stringing, you would see the lines of stress actually squirm around
like snakes, changing color, and concentrating themselves in certain
portions.  When the stress builds up enough, due to stringing, and
curing under load, the plate breaks.  The defective plate just has a
greater concentration of these lines in the weak areas.

Cast iron is really not perfectly stable, immediately, although today
there are better formulas that cure very quickly, and depending on the
plate design, some can be used immediately without curing at all (if
you don't consider 2 weeks a "cure").

Rusting outdoors actually removes the stresses that accumulate at the
surface of the iron, and which has the greatest moment of force against
the parts, anyway.  Of course, when you have to do that, then you have
to pickle off the rust in a bath, because cast iron rust doesn't make
a good primer for the sealer.

As to what works, I told my welder, in all my wisdom of 15 minutes of
trying to weld something, once, "Don't you have to heat the entire
plate if you are going to braze it?"  And I also told him, "I was told
that you have to use a cast iron rod for this job, and that braze
definitely will not work.  I'm afraid you are going to mess up my

His reply was even more scary (and, he had an oven wide enough to put
two or three plates into, side by side, if he had wanted to).  He just
said, "None of that is right.  You can't trust welds.  Sometimes they
hold.  More often, they don't.  Don't worry -- I'll fix it."  (What's
going to happen to my piano plate.  Now I've really got myself in a

What he did next was the worse thing I ever saw.  He placed the large
piece across two saw horses, got the approximate place that the smaller
piece went, leaned it up against the two places where it would
eventually get brazed onto the larger one, and left its bottom standing
on the ground!  Here's this third of the plate making about a 60 degree
angle with the plane of the larger piece.

He started to braze then with the most ferocious torch fire I ever saw
it done with.  And to make it even more interesting, he left out the
third chunk of stuff that fell out of the gap between the wide treble
web.  It was narrowly football shaped and about 8-9 inches long and 2
or 3 inches wide in the middle.  That was just filled in with braze as
he went along.

The piece standing on the floor started to raise.  He could change the
angle as he filled in the hole.  Up it came, and it was as if he really
didn't give it a second's thought if it was flat or not with the rest
of the plate, but when he was finished, it looked flat, and there
wasn't any hole left in the treble web.  It took him all of about 10
minutes or less, from start to finish.

My next comment was terrifying me, but he wasn't even concerned.
I told him, "How do you know that's going to be flat?  I don't see how
you could possibly get that flat enough to place back into a piano.
Everything has to be true, or the plate will just break again -- maybe
somewhere else."

He just laughed and said, "Well, if it breaks again, you just bring it
back.  But it won't."

So all the way back to the shop I was planning on fitting shims and
stuff to fill in the spaces so that my bolts would not flex the plate
and put it under tension.  I was also figuring how I was going to
restore bearing, and dozens of other imaginary problems.

When I got back and tried the plate, I could not believe it.  Nobody is
this good, I told myself.  This cannot be.  The repaired plate was
perfect.  It was absolutely flat.  It was precisely and perfectly tight
at every screw position.  I might say that I just "lucked out," but
that completely discredits the abilities and talent of this man.  He
already knew it would be perfect.  Credit where credit is due.  Let's
be fair, here.

It's the same difference between someone who has taken welding classes,
and maybe even taught welding, and someone who has been known as the
best in the business for 30 years and was welding aluminum truck
transmission housings, before they had aluminum welders and when it
was said to be impossible.  Can't be done.  He doesn't know what he's
talking about.  That sort of thing.  You know -- like some of the stuff
we read right here, from time to time!

This was a small thing to a guy like this.  He charged me $35.00.
Ho-hum.  And the proof is in the plate.

I have been reading in MMD how some welds broke again and they had to
be re-welded.  Well, what did I just say that Mr. Summers said about
that?  And 99% of the welders you hire would probably attempt to do it
with iron rod, and possibly have an 80% chance of success.  But what
about the unlucky people that land in that 20% bracket?

The problem, as I understand later, from another source, with iron rod
and grey cast iron is that it still "oxidizes" the iron you are welding
to, just not quite as badly as an ordinary rod might do it.  If there's
no really powerful stresses applied, it will hold.  But an upright
piano plate supports about 20 tons or more of compressive pressure,
some of which tries to deflect the plate upward.  An oxidized iron has
less compressive strength.  So it crumbles under such weight.  We've
already read about it here.

The one sure-fire way to totally ruin a piano plate is to decide to
"cleat it" with plates and bolts.  Every case I have heard about, where
this was attempted, the result was a totally ruined plate.  This is
what you should definitely not try.  The only time this will "work,"
is when it isn't really needed, anyway.  A cleat has no strength to
prevent bowing, only from being pulled apart.  If you will think
according to the basic mechanical principles, then you would never
try this or suggest it to begin with.

Craig Brougher

(Message sent Tue 7 Mar 2000, 14:18:05 GMT, from time zone GMT-0600.)

Key Words in Subject:  Be, Can, Cracked, Piano, Plate, Welded

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