"Jack Shaping"
by Robert Callaghan, RPT

I would like to comment on the Journal, October 2009 Q&A Roundtable article on Bobbling Hammers – An Upright Conundrum.  Many good points were made, but I would like to add one more that was not included. 

I have found the condition of mysteriously “bobbling” hammers on many Asian pianos, and few American pianos as well.  The problem usually doesn’t start until after the piano has been played a while, and then usually on hammers that are played more often, such as in the middle range.

What causes this to happen on a soft blow is the trailing end of the jack, the edge that touches the hammer butt leather last, has too sharp an angle. That edge needs to be “softened” slightly, to make it more rounded so it will “roll” off the hammer butt leather more smoothly. If you can imagine the sharp side edges of snow skis digging into the snow sideways so you can stop.  If your ski’s edges are not sharp you will keep sliding. What is happening here is the sharp angle on the trailing edge of the jack is digging into the leather as it slides off, stopping it like skis on snow.

The remedy for this is to remove the offending wippens to a workbench. With sheets of 1000 and 3000 grit sandpaper flat on the bench, using a curving motion, drag the back corner of the jack over the 1000 grit sandpaper, just enough to round it slightly.  Then repeat this with the 3000 grit sandpaper to smooth it more, and then apply new graphite to the bare wood.  Reinstall and you will find much of the resistance at let off is gone.

I think you will see this process also works well on some grands that have this problem.