--s-p-o-n-i-x-- brought next idea :
I have a B&D electric drill and the carbon brushes are sparking
excessively. (I'd expect a *little* sparking but this is a lot) Apart
from that the drill runs fine.

Inspecting the brushes there is about 1/4 the original length left and
the nds are rough, so they need replacement.

However, will replacing the brushes/cleaning the commutator cure the
sparking?

If not, what else could be causing the sparking?

The sparking could also be due to shorting between the commutator
segments, shorted turns in the windings, or simply poor spring
pressure. Once it starts it will only get progressively worse. Assuming
there are no other problems with the motor, such as shorted turns in
the winding or a badly damaged commutator....

As someone else suggested, it would be worth while cleaning up the
commutator first by spinning it in another drill and using very fine
glass paper to clean it up and polish it at the same time checking that
none of them are loose. This will take care of light damage.

Each segment is supposed to be insulated from the next and the gap
between them can fill up with conductive carbon from the brushes. It
would also be wise to undercut them after polishing up the commutator.
The usual method is to carefully grind down the thickness of an hacksaw
blade to fit the gap, then draw the blade down the gap working away
from the winding end towards the end of the shaft, being careful not to
scratch the copper faces or loosen any segments by twisting the blade.
Then just give it a final polish to remove any slight burrs.

If the brushes are not rounded at the end to suit the commutator, wind
a single layer of glass paper round the commutator, assemble, fit the
brushes loosely in their holders and turn the shaft to bed them in
against the paper.

--

Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.org