On Monday, August 1, 2011 12:54:36 PM UTC-4, steamer wrote:
DoN. Nichols wrote:

Depending on the current, the carbon brush may get quite hot.
--Welll let's call it a calculated risk..

One could double or triple up on the brushes, but it still might pay to
put an insulating barrier between the shaft and the disk (to protect the
gears and bearings from current in case of a stuck brush).

More important, though, the brush holder should be brass or bronze,
the use of aluminum (which can grow an insulating oxide VERY quickly)
is not electrically suitable to that sliding contact at the brush-to-holder
interface. There are solders that can make a good connection from
the copper plate to the aluminum, or antioxide pastes (used for copper
versus aluminum splicing, ask at any good hardware store).

Heavy brushes (like for elevator motors) use braided copper cemented into
the graphite, rather than depending on the sliding connection.

For most use, I"d consider clamping the ground via a braided cable directly
to the work; if the cable is loose enough, it'll tolerate one or two rotations of
the worktable.