In article , Glenn Ashmore wrote:

I would surmise that if you use a bar magnet the flux lines have to
flair out to get back around to the other pole. The chuck is a lot of
narrow horse shoe magnets side by side so the flux just makes a short
loop through the surface of what it is holding to get to the other pole.

But that is just an opinion and you know what opinions are like... :-)

Hmm... I like it. Now to figure out why the special v-blocks and parallels
work... I suppose since they have narrow slices of magnetic material
sandwiched between non-magnetic, then the flux has to continue up and do its
looping over the top of the vee block. So I suppose if this is all correct
that they would work the best when the iron/brass strips were lined up with
the strips in the chuck.

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Mike Graham | Metalworker, rustic, part-time zealot.
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http://www.metalmangler.com| Caledon, Ontario, Canada