Re: [CAD_CAM_EDM_DRO] Re: stepper vs. servo

Hi Ballendo,

Due to resonance etc, steppers can lose steps when you don't expect them
to. For instance I used to use a CNC router. It would run 99% of jobs
with no problems. However, occasionally it would lose steps on
particular jobs even though it had done similar jobs with no problems.
The problem jobs just happened to run at particular speed/acceleration
ranges that would make the steppers lose position. The steppers weren't
undersized in my opinion, the machine could bend a 1/2" diameter bit
into a pretzel without raising a sweat.

To reduce the likelihood of lost steps you need to run a lot of
different jobs on the machine and adjust speed/acceleration settings to
stop lost steps. This is tuning the machine.

I agree, servos aren't perfect. They have a following error that is
dependent on acceleration and load. However this is predictable. You
know that if you are taking a heavy cut at high speed you may have
following errors. If you know something is going to happen you can
compensate for it by reducing the cut depth and/or feed rate. Also the
errors will not be cumulative. You may have rounded corners etc but the
machine won't proceed to tear itself apart because the cutter is far
away from where it is supposed to be (been there, done that). With
steppers once they have lost steps your whole job is wrong.

ballendo wrote:

> Les,
>
> Well then let's just confine ourselves to the second half of my
> statement. Steppers, once tuned--and I disagree with your assertion
> that "tuning" is required--need no further attention. (steppers DO
> need proper "selection". But not tuning.)
>
> Now let's address your second assertion, in a fun way:
>
> No matter how carefully you set up SERVOS, one day you will hit a
> particular set of circumstances that will make them lose steps. It is
> most likely to happen if you are demonstrating the machine or just
> finishing a very expensive and time consuming job..."
>
> But you won't know it! Only when the part is rejected because it
> doesn't meet spec will you find that the following error ALWAYS
> PRESENT with servos, has caused the actual toolpath to be other than
> intended... Because the g320 and similar can be off by up to 128 steps
> (encoder counts, really) before a fault is issued...
>
> If you're going to defend things by using worst case scenarios, let's
> apply them across BOTH systems being considered...
>
> Ballendo
>