Fun with stepper motors (Grrr!), or, CNC = Careful now! Careful!

Decided to fit stepper motor onto bull wheel of lathe to create a dividing head via a worm.
Reasoning that the stepper had a ball bearing embedded
in an aluminium casing at one end, that there was no need for a separate bearing at that end of the worm and that the worm could be mounted directly onto the stepper shaft with just one bearing at the other end.
Would I get away with it?
All seemed OK until I created the supporting frame and tightened everything up.
Then the stepper, instead of turning (by hand) with a slight purring, was cogging sharply and became difficult to turn by hand.
Diagnosis... had I damaged the motor by tightening up too much?
Dismantle everything, check the motor now moves freely, yes it does.
Slowly reassemble, checking at each stage to see if cogging has returned.
No, all OK.
Mount assemblage back onto lathe bed and tighten up. Cogging has returned. NOT POSSIBLE! because fixing bolts not near to stepper configuration.
Puzzlement reigns.
Then one of those "Condor" moments. (That must date me, wasn't the advertising campaign sometime in the early 70s?)
When the assemblage was bolted onto the lathe, the connecting wires were dangling any old how (electronic controller not yet fabricated) and were shorting together on the lathe bed.
Just like any dynamo or DC electric motor, they become difficult to turn by hand if shorted together. Similar to field telephones if too many on the circuit, the magneto becomes coggy when trying to ring the other phones.
Separate the wires, it turns freely. Re-short them together, the cogging returns.
Moral? Stick to mechanical contrivances in the future!
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On Thu, 30 Oct 2008 16:45:45 -0000, "Amateur Machinist"

Worth checking the construction of your motor (but don't be tempted to disassemble it - that can result in de-magnetising the rotor) - most (all?) of the stepper motors that I have worked with are not designed to take axial loads - they have a spring washer at the tail end that takes up any end float. If you push the shaft in the direction it will cause the spring washer to be compressed. So you need to design your bearing arrangements accordingly, so that you aren't relying on the motor to control end float.
What you are doing sounds a lot like a dividing attachment that I built a while back for my ML7 - see:
http://www.jeffree.co.uk/pages/ml7dividing.htm
The worm has a ball race at each end & as a result there is never any axial load on the motor.
Regards, Tony
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wrote:

Was aware of the spring washer as the result of one of the motors I was given being completely jammed up so I dismantled it to have a look. This woke me up to the existence of the ball race at the front of the motor and the spring washer at the back.
My solution is a thrust collar beyond the remote end bearing to hold out the axis so that the spring is not compressed. However, possibly wrongly in the light of your information, I did this, not to deal with end float in the motor per se, but because the net amount of movement if allowed to remain would result in considerable angular error when in use as a dividing head.
A strange thing happened with the mounting holes on the face of the motor. I took a large number (20) of measurements of the hole spacing, and also the spacings of the inner and outer edges of the holes, (the intention being that the remote end bearing should be exactly positioned), and found the hole spacing to be exactly 1.856". That couldn't be right, must have been some metric spacing but that didn't compute, either. Then I found the data sheet on the Internet, and it was exactly 1.856"!
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