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Re: Source for NIB magnets



---------- Forwarded message ----------
Date: Wed, 1 Oct 1997 19:53:22 -0700 (PDT)
From: "Edward V. Phillips" <ed-at-alumni.caltech.edu>
To: tesla-at-pupman-dot-com
Subject: Re:  Source for NIB magnets

"A cheap/free source of these magnets is from junked brick-sized hard disk
drives.  The magnets are used to move the head assembly across the
surface of the disk.  The bigger the drive, the bigger the magnet, though
I'm not familiar with the older drawer-sized jobs, these might be
pre-NIB-technology.  The pole pieces must do a remarkably efficient job
at containing the field so as not to corrupt the disk data.  Just be very
careful when trying to remove the magnets from the pole pieces.  They are
glued with some very tough glue.  ANY attempt at whacking them off or
even gripping them in a vice will instantly shatter the magnets, they're
brittle like ceramic.  Better to try to slowly bend the pole pieces in a
vice or arbor press so they peel away from the magnets.  Then, keep your
fingers out from between them!

Gary Lau
Waltham, MA"

	I have stuck to a book case at work a magnet set from an
old IBM 20 megabyte disk drive which was, by todays standars,
"as big as a house".  They are so strong I can't get em unstuck
with sliding them sideways to the edge of the case.  We got two
of them stuck together about a year ago and as far as I know they
are still stuck together.  Typically they have a strength of
the order of 12000 gauss for small air gaps (that's 1.2 Teslas for
you metric fans).
	I have found that you can unglue the magnets from the
disk drive assemblies by soaking them for several days in MEK,\
or preferably methylene chrloride.  It gradually is absorbed
by the adhesive to the point where you can slide the magnets
off easily.  Since there is a very thin adhesive layer, patience
is required.  Once the magnets are removed the adhesive can
be peeled off with a razor blade, particularly if you dunk them
back in the solution.  
	A word of caution.  Many of the currently available
magnets are poled (through the thin dimension, perpendicular
to the flat faces) such that working your way along the magnet
you have a north pole (or south) at one edge and, at the very
middle, the opposite poles separated by a very small air gap.
Picture N - 
Start over.
	N - S-N - S.  Make excellent holding magnets,
but not suitable for use in such applications as magnetic
blow out of a spark gap.  Many of the bigger ones which
All Electronics sells are of that configuration.

Ed