For various reasons I had to solve a problem of grinding and sanding down a
bunch of flat washers. Holding them down while I attacked them with the
grinder and sander or keeping hold of them while using a belt sander
proved
to be a problem. They stuck to a board very nicely with a double sided
sticky tape, but after only a modicum of sanding the washer heated up
causing: a) the tape to let go and b) create an almighty mess on the back
of
the washer.
For the bigger (4.5") washers I finally made a jig consisting of a central
pillar and 4 rare earth magnets. This works very nicely, holding down the
washer really well (it has to be pried off at the end and when putting the
washer on the jig you have to be careful not to lose a finger).
So, all pleased with myself, I thought I would make another jig for the 2"
washers, this time with only 2 smaller magnets. Again, I congratulated
myself how cleverly I solved the problem. Until today. I was cleaning a
batch of 6 washers and had to use the angle grinder with a 120 grit sand
paper a little bit more than usual due to the patterns on the face of the
washers which had to be eliminated.
During the work progress I noticed that the magnets were gripping
progressively less and less until they barely gripped at all. The answer
presented itself when I dropped one of the washers in a bucket of water
and
it made a nice sizzling noise.
It appears that I have heated the washers and as a result also the magnets
to a temperature where the magnetic effect is lost. I dumped them into a
cold bath but sadly they have not regained their strength (now they are
rather weak fridge magnets). I did not see this with the bigger washers
due
to their bigger mass.
I knew that magnets lose their strength with heat (are you not supposed to
do a magnetic test on steel during a hardening process?) but I thought we
were talking 1000+ degrees or so.
So I guess the question is:
1) At what temperature is the loss of magnetic properties first
noticeable?
2) Can one restore the magnets to their previous strength?
--
Michael Koblic,
Campbell River, BC
|
