The electric motor in my circular saw has started to arc round the
brushes - lighting up the underneath of the saw table.
I took out the brushes but couldn't see anything wrong. There was about
10mm of carbon and when reinserted the spring was compressed so the
brush was pressing against the commutator. Is there anything else that
could be wrong and is straightforward to check out and deal with?

The saw is a Bosch GKS 85S

--
Frank Stacey

On Jul 6, 5:13*pm, Frank Stacey
wrote:
The electric motor in my circular saw has started to arc round the
brushes - lighting up the underneath of the saw table.
I took out the brushes but couldn't see anything wrong. There was about
10mm of carbon and when reinserted the spring was compressed so the
brush was pressing against the commutator. Is there anything else that
could be wrong and is straightforward to check out and deal with?

The saw is a Bosch GKS 85S


http://wiki.diyfaq.org.uk/index.php?title=Motor_repair

On 06/07/2011 17:13, Frank Stacey wrote:
The electric motor in my circular saw has started to arc round the
brushes - lighting up the underneath of the saw table.
I took out the brushes but couldn't see anything wrong. There was about
10mm of carbon and when reinserted the spring was compressed so the
brush was pressing against the commutator. Is there anything else that
could be wrong and is straightforward to check out and deal with?

The saw is a Bosch GKS 85S


Normally a symptom of one or more armature windings being open circuit,
so there is no back emf as the brushes pass from one pair of commutator
segments to the next.

Test (with power off) by using a multimeter between each opposite pair
of segments. The resistance should be sensibly constant.

If a winding is open circuit, it's normally time for a new armature, but
it's just possible that one end of the winding has become detached from
the commutator, in chich case re-soldering might be possible.


--
Kevin Poole

"Frank Stacey" wrote in message
...
The electric motor in my circular saw has started to arc round the
brushes - lighting up the underneath of the saw table.
I took out the brushes but couldn't see anything wrong. There was about
10mm of carbon and when reinserted the spring was compressed so the brush
was pressing against the commutator. Is there anything else that could be
wrong and is straightforward to check out and deal with?

The saw is a Bosch GKS 85S

--
Frank Stacey

10 mm sounds a little short to me, although I don't know this saw. Check for
spares on the web, you will sometimes find a photo

or a diagram like this
http://www.powertoolspares.com/tool/...7a000/spares/#

(might need to zoom in to get it to load properly)

On Jul 6, 7:23*pm, Kevin Poole
wrote:
On 06/07/2011 17:13, Frank Stacey wrote:

The electric motor in my circular saw has started to arc round the
brushes - lighting up the underneath of the saw table.
I took out the brushes but couldn't see anything wrong. There was about
10mm of carbon and when reinserted the spring was compressed so the
brush was pressing against the commutator. Is there anything else that
could be wrong and is straightforward to check out and deal with?

The saw is a Bosch GKS 85S

Normally a symptom of one or more armature windings being open circuit,
so there is no back emf as the brushes pass from one pair of commutator
segments to the next.

Test (with power off) by using a multimeter between each opposite pair
of segments. The resistance should be sensibly constant.

If a winding is open circuit, it's normally time for a new armature, but
it's just possible that one end of the winding has become detached from
the commutator, in chich case re-soldering might be possible.

--
Kevin Poole

The above is right. However the price of an armature is probably
something stupid. Probably not economic to repair. Which of course is
the intention.

BTW just connect your multi-meter (on lowest resistance setting)
across the brushes and rotate the armature slowly by hand. The need
will wave about gently. If it goes down abruptly at one point your
armature is US and unlikely to be fixable.

On Jul 7, 8:55*am, harry wrote:
On Jul 6, 7:23*pm, Kevin Poole
wrote:



On 06/07/2011 17:13, Frank Stacey wrote:

The electric motor in my circular saw has started to arc round the
brushes - lighting up the underneath of the saw table.
I took out the brushes but couldn't see anything wrong. There was about
10mm of carbon and when reinserted the spring was compressed so the
brush was pressing against the commutator. Is there anything else that
could be wrong and is straightforward to check out and deal with?

The saw is a Bosch GKS 85S

Normally a symptom of one or more armature windings being open circuit,
so there is no back emf as the brushes pass from one pair of commutator
segments to the next.

Test (with power off) by using a multimeter between each opposite pair
of segments. The resistance should be sensibly constant.

If a winding is open circuit, it's normally time for a new armature, but
it's just possible that one end of the winding has become detached from
the commutator, in chich case re-soldering might be possible.

--
Kevin Poole

The above is right. However the price of an armature is probably
something stupid. Probably not economic to repair. Which of course is
the intention.

BTW just connect your multi-meter (on lowest resistance setting)
across the brushes and rotate the armature slowly by hand. The need
will wave about gently. If it goes down abruptly at one point your
armature is US and unlikely to be fixable.

A Bosch may well be worth a rewind. But I'm not fully convinced by
this diagnosis.


NT

On Jul 7, 8:55*am, harry wrote:

The above is right. However the price of an armature is probably
something stupid. Probably not economic to repair. Which of course is
the intention.

£100 + VAT - which is a lot for a handheld circular saw.

On 06/07/2011 22:15, newshound wrote:


"Frank Stacey" wrote in message
...
The electric motor in my circular saw has started to arc round the
brushes - lighting up the underneath of the saw table.
I took out the brushes but couldn't see anything wrong. There was
about 10mm of carbon and when reinserted the spring was compressed so
the brush was pressing against the commutator. Is there anything else
that could be wrong and is straightforward to check out and deal with?

The saw is a Bosch GKS 85S

--
Frank Stacey

10 mm sounds a little short to me, although I don't know this saw. Check
for spares on the web, you will sometimes find a photo

or a diagram like this
http://www.powertoolspares.com/tool/...7a000/spares/#


(might need to zoom in to get it to load properly)

That's a great site. The exploded view of my machine could be very helpful.
I tried measuring armature winding resistance via the brushes. Nothing
seemed suspicious - the readings were all about 1.5ohm.
WikiDiy almost as helpful as his group and encourages me to expose the
commutator for visual inspection and possibly cleaning up.

Thanks everybody.

On 07/07/2011 08:55, harry wrote:
On Jul 6, 7:23 pm, Kevin
wrote:
On 06/07/2011 17:13, Frank Stacey wrote:

The electric motor in my circular saw has started to arc round the
brushes - lighting up the underneath of the saw table.
I took out the brushes but couldn't see anything wrong. There was about
10mm of carbon and when reinserted the spring was compressed so the
brush was pressing against the commutator. Is there anything else that
could be wrong and is straightforward to check out and deal with?

The saw is a Bosch GKS 85S

Normally a symptom of one or more armature windings being open circuit,
so there is no back emf as the brushes pass from one pair of commutator
segments to the next.

Test (with power off) by using a multimeter between each opposite pair
of segments. The resistance should be sensibly constant.

If a winding is open circuit, it's normally time for a new armature, but
it's just possible that one end of the winding has become detached from
the commutator, in chich case re-soldering might be possible.

--
Kevin Poole

The above is right. However the price of an armature is probably
something stupid. Probably not economic to repair. Which of course is
the intention.

Yup. My table saw has gone exactly the same way (Performance Power Pro).
Spares apparently not available ... until I spotted that Axminster used
to sell the identical machine (except for colour and badging). £110 for
a new one - I didn't even ask whether that was plus or inc. VAT

SteveW