There are times I feel scared of over voicing or of wrecking the hammers because of not knowing when I’ve done too much of juicing, needling, ironing or whatever.

Is there a safe way of voicing which avoids the danger of damaging hammers beyond recall?

Here is a bit of background:

Felt sheets are either layered or not, depending on the hammer maker’s choice. Whether layered or stretched, the wool fibers tend to be positioned in the anti bass/treble direction, that is, the fibers go around the molding.  Pushing needles into the shoulders rips the fibres, hence the puffing out of a heavily needled hammer.  This means that when you push needles into the shoulder radially you lose control.  Since every set of hammers is different, one can never tell how much energy is lost when needles puncture the shoulders.  The more the shoulders are punctured – the more energy and the more control is lost.  This is neither an efficient nor a safe way of voicing.

If we remember that the tone produced by your piano's hammer depends as much upon its capacity to flex when hitting the string, as it does on the impact itself, and that the capacity to flex depends on the compression of the felt to the hammer’s pressed shape – we will have taken the first step to safe voicing.

ANSWER:

Yes, there is!

The traditional way of needling uses a tool holding three (or four) needles and pushing them radially into the same surface that contact the strings. The needles are pushed in pretty deeply going from below the strike point down to the shoulders or lower. The three needles wer, originally, meant to parallel the three strings and to be pushed in just below the strike point so as to lift the compacted felt just below where the hammer contacts the strings. The technique evolved from there.

Rules for safe, effective voicing.

• Don’t change the shape of the hammer – this does not pertain to filing.
  If the hammer requires filing, file first.
• Think of the hammer and its function as though it were a tennis ball. 
  You do not puncture the outside of the ball to make it more springy, you change the air pressure inside.
• Use a single needle, about .050" diameter (#2 crewel).  Use a pin vise like an Isaac voicing tool.  Allow the point of the needle to stick out about half an inch.
• Voice by pushing the needle home into the flat side of the hammer.
• Draw a line (on the hammer or in your mind’s eye) tracing the outer shape of the hammer and running half way between the outside of the hammer and the wood molding. Push the needle home following the line you drew. Push the needle home every 1/16" from the 9 o'clock position right around to the 3 o'clock position.
• Depending on the hardness of the particular set of hammers, you may need to stitch a second line closer to the outside of the hammer.
• Working from bass to treble; when you come to the low treble section, begin graduating down the number of needle pushes so that when you reach the last octave you push the needle home only three times: 9, 12 and 3 o'clock.
• Check your voicing often by listening to the tone.  You may find you need to add a few stitches at this or that point around the hammer –  listening carefully to the tone will be your best instructor.