What causes excessive arcing at brushes when running

If there is a lot of sparking, it's likely the armature is gone, especially if arcing across from one brush to the other, the motors can be bought for 15 to 20 from eBay.

The armature may be shorted out. The brushes may not conform to the arc of the copper. Did you clean the armature? Did you deepen the grooves between the individual coppers? Sometimes brushes will arc for a while until they wear into the curve. If it is not hot, and not excessive, try running for a while to see if it lessens its arcing.

It could be that your brushes (PN CB440) are worn down so much that they can't make full contact with the armature or the armature (PN 619247-1) has a short in it causing excessive arcing. This would be evident by excessive burning or pitting on the commutator where the brushes make contact or missing sections of the commutator..

One can expect a small amount of arcing from a carbon brush. If one brush is arcing more than another, or if the arcing is following around the diameter of the commutator toward the other brush, the brushes need to be replaced and commutator cleaned with a commutator stone.

armatures over heat from 3 problems 1 The brushes are worn out and there is heavy arcing on the commutator bars causing excessive heat and 2 the armature is shorting out 3 the armature is touching the field shoes from worn out bushes/ bearings ( poling out). have an appliance electrician check it out.

Problem will be either low brush lengths or burnt armature. Check for arcing when running and excessive sparking will mean motor over haul or replacement.

you have a problem with the bearings or the motor/brushes are burnt. Does this tool arc when running. If so I would check the brushes first, unplug the tool and remove them and while they are out rotate the motor slowly and look at the commutator bars for excessive discoloration or unusual wear.

Did you dress the brushes to the contour of the commutator?

You wrap the commutator with fine sandpaper and rotate under the brushes till the curve conforms.

If this does not reduce the arcing, then you may have an armature with a bad winding.

Then power is not reaching the motor ie bad connection past relay or the motor is toast, worn so badly theirs no connection through brushes. Some times just dirty or carbon build up. Low voltage is usually the culprit, running accessories like these without the engine running or with a weak battery can result in excessive heat and arcing can occur causing carbon deposits making contact impossible. Sometime slamming the door will instigate contact and it will run for a bit but the damage is done and repairs from cleaning to replacing are in order.

That could cause a problem not allowing the brushes to seat properly. Does it arc excessively? If so you might need a new armature. Also while you had it apart did you check the bearings for being rough or tight?