Improving Tone By Reshaping Hammers

Hammers with grooves in the felt on the left. Freshly sanded hammers on the right.

Over time, the pounding of a piano's hammers against the strings will dig deeper and deeper grooves into the hammer felt. As the felt gets compacted and the area of the hammer that strikes the strings get flatter, larger, and harder, the felt provides less cushion during impact. This causes the tone of the piano will become sharper, more harsh, and more percussive. In addition, harder hammers are more likely to break strings. To restore the tone and correct felt density, the hammers need to be sanded until the grooves disappear.

Before beginning the sanding process, remove the action from the piano and gently lay it on a sturdy workbench.

Marking a guideline for the hammer's strike point.

The marked guideline.

The hammers must be carefully sanded so that they retain their original shape. The point of the hammer that strikes the string (a.k.a. the “strike point”) needs to be at the very tip of the hammer. To make identifying the strike point easier, use a straightedge and a pencil to mark a line along the center of the wooden hammer core and out to the edge of the felt. This guideline will let you easily see the optimal strike point during your sanding.

A hammer sanding block.

Make a sanding block by gluing a strip of coarse sandpaper to an inch wide piece of wood. Gently but firmly hold the hammer in one hand and use the sanding block to shave felt off of the underside of the hammer. Take your time with this. While sanding, follow the curve of the hammer by moving the sanding block in an arc over it. Always sand toward the strike point of the hammer.

Sanding the underside of the hammer.

The lopsided hammer after sanding its underside.

Once the grooves on the underside of the strike point have been sanded down, repeat the same process on the top side of the hammer. Keep sanding until the hammer is round again and the tip of the hammer is located on the guideline. While you are doing this, take the bit of extra time to make sure the hammer is round and looks good. Don't leave any flat spots.

Sanding the top side of the hammer.

Work your way through each of the hammers. Once you get to the high treble, the felt will be so thin that you shouldn't touch it with sandpaper. Just leave that section alone. Sanding wont change its tone much anyway.

The finished, well-rounded shape.

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Leveling Keys

Over time, the felt underneath the keys as well as the felt in the action becomes compressed.  Between this settling of the felt and dry rotting of paper and cardboard spacers (not to mention rodent infestations) the keys of many pianos tend to undulate up and down all the way across the length of the keyboard.  This can also be caused by damage to the keytops.  In this case, you will need to recover the keys.

The keys can be re-leveled by carefully adding or subtracting paper spacers underneath the felt punchings.  The tool of choice here is a 4 foot level (or any 4 foot straight-edge).  Make sure that it is true.  One way to do this is by laying on large piece of paper, cardboard, or wood and tracing the bottom edge of the level.  Flip the level over the line so the line is along the top edge of the level.  The line you drew should still match the edge perfectly.  If not, the level is not true and you should find something else to use.

Lay the level across the keys.  It should reach perfectly from the first key to the last.  Crouch down so that the tops of the keys are at eye level.  Generally, the first and last keys receive such little use, that they will already be at the ideal height.  Make sure the level is touching these two keys and not causing them to drop.

By repeatedly lifting and dropping one end of the level, you can find high spots in the keyboard.  Any key that drops from the weight of the level will need to either have paper spacers removed or have a thinner cloth punching installed.  These changes must be made underneath the keys at the balance rail, not at the front rail.

The level resting on the keys

The level slightly lifted at one end.

To find which keys are low, look for spaces between the tops of the keys and the bottom of the level.  Paper spacers must be added to the balance rail below these keys to bring them up to the level.  I get these spacers from either Schaff Piano Supply or Pianotek.  A general rule of thumb is that the spacers added should be about half the thickness of the gap between the key and level.  Move along the keyboard a leave an appropriately sized stack of spacers on top of each key that needs to be raised.

After you've made it through the whole keyboard, pull the level away.  Go through the keyboard one key at a time, pulling a key up, removing the cloth punching, placing the paper spacers on the pin, and replacing the cloth punching.  Reinstall the key and move onto the next one.  After you've gone through the keyboard place the level again and make a second pass.

After the key is removed

Cloth punching removed

Paper spacers added and cloth punching about to be replaced.

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