Piano Hammer Filing
If hammers are heavily worn to the point where they are more flat on the striking surface than egg shaped, replacement with a new set of Piano Hammer Heads is recommended. If, on the other hand, the wear is of a more minimal nature, with string cuts that are not too deep, filing the hammers is an option to consider.
String cuts occur over time in the striking surface as the steel of the strings bite into the felt. The condition of the hammer heads is crucial and is important for making and providing a good tone in the piano.
When hammers are produced, felt is stretched over the wooden molding of the hammers. The fibers on the outer surface of the hammers are at tension. As fibers are broken, a layer of dead felt forms on the outer surface of the hammers. The tone of the piano suffers, and as the cuts deepen, the hammers no longer impart a clean blow to strings but wrap around them instead. Skillful filing of the hammers removes this wear layer, giving the hammers a clean profile that produces a crisper tone and that is more easily voiced.
You may want to start by pencil marking the crown of the hammer all the way across. You want to start filing from the bottom all the way up to the crown. As you start filing you will see the tufts of felt layers that are coming off. Use quick strokes to gently coax the felt layer over the crown. Now you want to go to the back of the hammers and make your way down the line in an attempt to bring some consistency. The last step you will go across the back and finish up by shaping, remove the loose felt and check the shape of the hammerhead. To make sure you have the correct shape you will see a little bit of a point on the crown of the hammerhead kind of like an egg shape and minimal removal of felt from the crown of the hammers preserving height. Height is very important for the geometry of your action.
When the entire set of hammers has been shaped, and the action is cleaned, check to make sure each hammer is square to the string line by peering up over the tops of the hammers (if you are using an action cradle, simply rotate the top of the hammer line toward you) to visually compare (with one eye) the front of each hammer to the reference line provided by the damper spring rail.
When the action is replaced in the piano, use your sanding stick exclusively to touch up any hammers that are not striking on all the strings. Once the action is back in the piano, adjustments to the regulation of the action will be necessary to put things back to normal. First of all, the hammer blow distance would need to be adjusted by inserting spacers to lift the hammer rail. This, of course, will result in more lost motion which should be taken up. Finally, the let-off distance will need adjusting, as it would now be further back.
Article courtesy Gemm Piano Supply
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