Out of True Lathe ?

[Andy Shillito]

Now then...

I've read a bit here and there on this big old world wide web thing about comm lathes that cut out of true ? Now I have an oldish lathe by todays standards and was wondering how the hell you can tell if it cuts true. My understanding is that if the armature is spinning on it's axis, and you run a diamond bit across the armature in a controlled fashion (i.e a lathe) then how can you end up cutting the comm out of true ?? The arm is spinning, centrally as it's on the lathe, and the bit passes over it horizontally, so you must end up with a perfectly round comm ? I read somewhere that running the motor and using a small tool to touch the back of one of the brushes was an indicator as you shouldn't feel much vibration, if any. I tried this and unsuprisingly you can feel some slight vibration when the motor is running, but as it's spinning like hell I guess that's normal. Anyone got any ideas on this out of true issue ?

Andy.

[DA_cookie_monstA]

Casting my mind back to when I had a little training on lathes, and correct me if I am wrong, unless the cutting bit is set to the right hight, it wont cut efficiently or properly, and maybe get a little deflection in the tool.

Thats a guess, the only other thing is that the lathe doesn't run parallel to the armature, but then this would only make the comm conical, and not out-of-true.

[marcoski]

If the slideway is not parallel with the axis of the armature, as cookie says it will cut a cone, ie be out of true. You would need a very accurate caliper or small jaw micrometer to measure the comm at the beginning, middle and end to see if there is any taper after the cut. Ideally all three readings should be the same.