Piano hammers are designed to have a rounded top resulting in a precise striking surface. After a piano has a few years of use the hammer tops become flattened with string grooves cut into the hammer surface. The grooves cause a harsh, unclear tone in the piano and the flattened surface can cause strings to break. To fix this condition the hammers are reshaped.
The piano action is taken out of the piano and the hammers are filed to the proper shape. This procedure takes a few hours to do depending on how worn the hammers are. Very little hammer material is taken off the hammer top so this procedure can be done a few times in the life of the hammer. The action needs to be adjusted (regulated) somewhat after reshaping the hammers because the hammer distance from the strings changes. Usually hammer reshaping is done as part of a regulation job.
Benefits of reshaping piano hammers:
The piano has a clearer, sweeter tone
Strings are less likely to break
The proper shape of the hammers provides an optimal foundation for precise piano regulation
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