Take a paper clip. Bend it back and forth until it breaks. That's what happens. It starts with a little crack. When it cracks, the reed drops in pitch. That crack always starts in a milling mark near the rivet end, wherever force is most concentrated. There is a lot of force involved. When the reed swings one way, it is under tension force. When it swings the other way, it is under compression force. Both these forces are strongest on the outside surface of the reed. For example, let's take a reed that is swinging outside the harmonica. Let's stop it there and look what happens. The back of the reed (the one facing the inside of the harmonica) is under tension force. The side of the reed facing outside is under compression force. These two forces are opposite, so as you go deeper inside the reed - let's say from the outside - the compression force decreases. Then you get to a point where there is neither compression or tension force... kind of a force
purgatory. go deeper, you start seeing tension force, which increases until you come out the back of the reed. The milled side is the weakest, so it's going to crack there.
OK, so both these forces are strongest on the outside of the metal. A crack will appear on the surface of the reed. This crack is shallow (remember that as you go inside the reed, force decreases until you get to a point where there is no force). As you play, the crack gets longer (but not necessarily deeper) until it has gone all the way across the reed. As it does this, the pitch gets flatter. An interesting thing happens once the crack goes all the way across. The surface of the reed moves deeper. That is, the place where the force is concentrated moves deeper inside the reed as the crack gets deeper. Once that crack has moved all the way across, you have a significant defect in the structure. Now, the reed is thinner where the crack is, while the rest of the reed is much thicker. Thus, more and more force is concentrated at the location of the crack. This is why a reed will fall slowly out of tune, but then one day it's done.
Back in the 1930s, when a harmonica employed by Borrah Minnevitch's Harmonica Rascals started going out of tune, Leo Friedman would coat the reed with a very thin coat of solder, retune the reed and it would last them until they could get a replacement reed. What he was doing was filling in this crack at the first inkling the reed was going south, distributing the force over a wider area. David Payne
www.elkriverharmonicas.com
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________________________________ From: Chuck Linville<linvillec@xxxxxxxxxxxxxx> To: harp-l@xxxxxxxxxx Sent: Saturday, May 19, 2012 3:36 PM Subject: [Harp-L] ...Why do reeds go out?
Hey y'all, happy weekend. I've been getting my feet wet with harp repair and re-tuning. Been playing for a few short years, I'm a machinist by experience, and I've always been interested in opening these things up, but never made the time. Well, now I have the time (unemployment can do that) and I am just fascinated with these things! Anyway, this tuning business has really got me wondering; what exactly makes them go out of tune? I've got a box of old harps from a friend and I am going through them, mostly Special 20s. I've been able to correct some by cleaning them. Others look indistinguishable from a good reed and I can't do anything a thing to cheer them up. All the gaps look fine and the rivet is solid. The only other thing I can imagine is metal fatigue from having been vibrated a gazillion times. Am I right? Thanks in advance! -chuck (PS: If so, has anybody had success re-tempering?)