Stepper question

When unpowered stepper motors still can be quite hard to turn. Why is this? Is there some way to prevent it? Can you buy steppers which are
easy to turn?
I ask because I would like to be able to use my little BCA mill, which is CNC-converted, as a manual mill as well as a CNC mill.
(I can turn the knobs, but it's a question of feel, which I haven't got right)
Thanks for any replies,
-- Peter Fairbrother
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Peter Fairbrother wrote:

Peter,
AFAIK all steppers will be like this. It is caused by the permanent magnets in the motors.
Servo motors will be smoother but they are quite different from steppers, require a different driver and encoder to provide feedback and as a result are not always compatible with software such as mach3 Oh yes and they cost a lot more.
Could you put an electromagnetic clutch in between the motor and feedscrew?
Bob
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wrote:

It wouldn't give you any 'feel' of the work, but you could make some manually driven pulse generators with a rotary encoder, shaft and handle. Then the leadscrews would always be driven by the stepper motors, but you could turn handles to do manual milling.
Mark Rand RTFM
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Mark Rand wrote:

The motors also have short shafts on the not-connected-to-the-ballscrew end, with knobs on them - apparently the usual type of ball-end knobs are a no-no, so they are just round knobs - which can be turned by hand.
Much simpler ... and one "click" is 0.01mm, which is handy :) but any delicate feel is lost.
-- Peter F

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The cogging effect (but not Cliff Coggin!) is caused by interaction between the permanent magnets and the salient poles. No cure, I'm afraid.

At the risk of a little more tool-making (surely the raison d'etre of most of us here?) :- ... You will no doubt be coupling your steppers by Oldham Couplers. A relatively trivial (but requiring some graft) matter to make a detachable mounting for the motor, to be replaced by a similar detachable mounting for your handwheel to fit to the Oldham couplings.
Not too dissimilar in principle to a quick-change toolpost, altho' no doubt dissimilar in configuration.
(And something that in time will also earn you the miserly fee from MEW when you publish details of it!)
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