Agraffe reaming (seating)

[Ron Overs]

Hi all,

>I have routinely used shims for alignment.  A change in height of .007 or
>.0035 at the agraffe will not make any appreciable difference in the
>height, bearing or anything else.

Indeed.

>The agraffe can also be trimmed if you
>prefer to work that way.

Although it is considerably more difficult to do this accurately and quickly.

>. . . Sorting through the agraffes screwing them in and
>out of each hole to see which one happens to align seems like a very
>tedious task and one in which it is very likely you will not end up with
>everything working out just right anyway.

It can be reasonably efficient if you use the agraffe tool in an 
electric driver for running the agraffes down for testing, and as 
David mentioned, a shim of a few thou will have an insignificant 
influence on the string plane error, but will allow you to set the 
position of the agraffes without straining the threads and risking 
breakage later on. The fitting procedure in many factories leaves 
much to be desired in this respect.

Where do you get shims of a specific thickness? We turn them up on 
the lathe. It takes very little time to make precise-thickness shims.

With a free machining brass blank turned to the required outer 
diameter and drilled to the inner diameter, a narrow parting off tool 
can be used to turn off precise washers. But the parting tool must be 
freshly sharpened for best results.

Firstly, a test washer is parted off by taking a cut off the end of 
the blank, then advancing the compound slide (the width of the 
parting tool-face plus about 10 thou' or 0.25 mm) for parting off a 
test washer. The test washer must be turned off thicker than the 
required thin washers, since thin washers will cup quite a bit when 
they are parted off. The test washer must be flat or you won't be 
able to get an accurate reading of its thickness. The test washer is 
measured with a micrometer or caliper to determine exactly what 
length of the brass blank is lost to the parting off tool. It is then 
a simple matter to part off precise shim washers. For example, if you 
wound the compound slide forward 130 thou' and the test washer 
measured 8 thou, you will turn the compound slide forward 122 thou to 
make a 2 thou shim. When the parting off tool is re-sharpened, a test 
cut will be required again, since the cutting width of the tool will 
be reduced each time it is sharpened.

If you don't have a small lathe in your rebuilding workshop, you 
should acquire one. They are most useful. For those of you in the US, 
there should be plenty of good second hand South bends over there. 
Our late is a screw cutting lathe of 6'5"
  center height and a metre between centres (it is a Taiwan made copy 
of the Harrison tool room lathe), but the 4" Southbend machines will 
do most of what is required for piano rebuilding and they are good 
little lathes.

Using a small straight edge with precision shims, it is easy to 
achieve a beautifully height aligned and tensioned agraffe set. 
Before you pull your next few sets for repair, check out the height 
alignment. There are some pretty ordinary OEM installations out 
there, even from manufacturers who claim to be setting the standard.

Best,
Ron O.
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OVERS PIANOS - SYDNEY
Grand Piano Manufacturers

Web: http://overspianos.com.au
mailto:info@overspianos.com.au
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