Re: [CAD_CAM_EDM_DRO] Re: Losing steps...

Patrick wrote:

>Jon,
>
>Thanks for the information, that's exactly the kind of objective
>statement I was looking for. I hear so much about the advantages of
>servos that I find myself wondering if stepper systems are a viable
>alternative.
>
>Next question. By swapping stepper drivers I am able to eliminate
>the possibility that the driver is the problem. How does one go
>about testing a stepper motor? It "sounds" fine, and the bearings
>are tight. Do steppers that are failing begin by failing (missing
>steps) intermittently?
>
>

It is possible that the motor in question was once overheated or given
a current pulse that has partially demagnetized the permanent magnets
in it. One test is to see if the holding torque of all your motors is
the same. If one motor is distinctly weaker than the others, on the
same drive, then it is a good bet the magnets have been damaged.
(Having a drive blow a transistor can deliver a massive current
pulse that will sometimes damage the motor.)

When the magnets are weak, steps will be missed more easily, of course.
And, don't overlook a loose shaft coupling as a source of VERY similar
errors. You can mark the shaft and coupling with magic marker, then
make a hairline scribe mark in the marker ink, and detect any small
relative movements in the parts.

On my Bridgeport conversion, the Y nut is constrained entirely by
the friction of th clamping bolts. When I have a crash, the nut can slide
up to .040", with the same sort of results.

Jon