Re: [CAD_CAM_EDM_DRO] stepper motor accuracy

Dan Statman wrote:

> It is a typo, they meant to say brake, not break. A mechanical brake will
> hold until its mechanical limit is exceeded, but the shaft of the stepper
> motor acts as a torsonal spring and deflects a certain rotational amount
> which causes a movement before the torque rating is exceeded. I am not sure
> any of this makes a damn bit of difference when it comes to CNC machining.

Oh, it DOES! Now, if you are machining to modest accuracies, and a full
step is .00001" on your machine, you don't care. If you are working to
high accuracy, .001" tolerance is required, and a full step equals .0005"
you may start to be concerned. If a full step is .001", then a half-step worth

of error is .0005". Add an error of .0005" on one side of a part, and an
additional error of .0005" on the other side, and you are now +/- .001"
JUST due to errors in the stepper motors' holding position, ignoring
all other sources of error. That means, you are already hopelessly
outside the .001" error you wanted to hold. obviously, such a machine
is not well suited for precision work.

Understanding all the sources of error is the first thing you need to do
when designing or evaluating a machine. After understanding the errors,
you can then judge whether the machine will perform acceptably for your
needs, or not!

Practically, this is probably a small error in most machines. Thermal
expansion of dissimilar metals, errors in leadscrews, wear, bowing
and out-of-square condition of the ways are probably larger concerns
in many smaller and low cost machines, and backlash of old Acme
leadscrews swamp all other errors.

Jon