Marchal,

I do not understand why you find the original Lehman knife edge unstable.  I
have two Lehman's with vertical knife edges.  Both are very stable.  I need
to adjust them once every few months.  The knife edge supports are small
Exacto knife blades set into the end of the stainless steel threaded boom
rod with  'Marine Tex" epoxy.

What no one ever tells you is that in order to balance the boom assembly you
must attach the suspension point on the boom at the exact point where the
whole thing is in balance when held vertically by its suspension wire.  If
the whole boom assembly rests in a horizontal position when you hold it like
a vertical pendulum, then when you attach it to the frame, to act as a
"garden gate", then it will be in perfect balance with respect to the knife
edge trying to move up or down on it's support.

My Lehman's are on Corian counter top material braced along the sides and
near the main support with sections of aluminium sliding door track.  When
my first seismometer would not centre, I found that it was because the base
flexed - that was when I added the aluminium stengtheners.

I hope this helps,

Martin


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