what causes brushes to arc?

Worn down brushes can be the problem but other reasons need to be discounted.
If the saw started to loose power,slow down or smell prior to the the problem then the brushes are probably not the problem. More serious I'm afraid most likely armature could be field coil.
If possible pull the brushes out and look at them. They should be evenly worn and feel free to move in the brush holders. If they have a centre wire connecting brush to machine or to back plate check it is intact. A broken or damaged wire can cause sparking. If they felt stuck as you removed them then clean them off and refit. try the machine again you never know.
Before refitting the brushes look at the armature for an even colour all the way round the com bars. If a single or a couple of coms are darker or burn,t then probably armature fault.
If you need brushes then try Solent Tools on 023 8057 8057 they have carbon brushes for absolutely every make of powertool. They can even make specials for obsolete machines.
Email sales@solenttools.co.uk

all the best

Martin

Solent Tools
July 2007