On Wed, 27 Jun 2007 20:22:58 +0100, "dennis@home"
wrote:

Who is talking about chucks?

They're a controllable magnetic clamping device using permanent magnets,
big enough to resist a significant force. You're talking about quite a
different device from the sort of magnetic base used under a DTI

Its a magnetic base and need not be heavy or complex.
You can buy a magnet with 250kg of load that only weighs about 100g.

So how do you remove it, if it's not switchable?

Most importantly, a "magnet that holds 250kg" is talking about an ideal
situation against flat steel. That's not the working conditions a
magnetically clamped drill has to cope with.