Re: [CAD_CAM_EDM_DRO] Re: setting up the CNC mill

FIK wrote:

>
> Well I once thought it was the cooling off of everything and once it got
> back running it sort of warned back to right size. Nope it would ton all
> these rest of time .002"undersize. So I just manually move the tool post
> with the manual move button i the Z axis. I don't touch the X axis, somehow
> a mystery to me but I can make the motors under current again, not adjust
> the undersize issue .002" but just get into power mode by moving the
> carriage +Z a few thou, back to 0, hit the play button(start, sorry I get
> mixed up when I am rocking and rolling if this is the boom box or the
> heavily beefed up CNC5, more later if anyone wants to know how to make a
> standard CNC5 better without a full conversion) and sure enough it will be
> on the money and I finish the run. Any ideas from anyone why this occurs?? I
> thought as I said it was a cooling off thing, I even tried having a lamp I
> put on the work/ways/toolpost til I got back but it was not it. I spent four
> hours testing it again and again and it just "creeps" when I go currentless.

You don't mention what type of stepper drivers are in the machine, but I'll
bet they take step/direction signals from the computer. I suspect that the
stepper driver not only shuts off the current to the motors, but also RESETS
the phase it is on when the current is turned back on again. Since 2-phase
stepper motors have 4 possible phase patterns (in full step mode) it could
cause an error of no, 1 2 or 3 steps, depending on which phase it was in
when powered off. Actually, it would turn out to be an error or -2, -1, no,
1 or 2 steps from the last correct position. If the error was -2 or +2 steps,
it would be somewhat likely to jump either way, but mechanical forces left
over from the last move might bias it to always jump the same direction.

If you hear the machine jump when the power is turned back on, then this
is the case. If the stepper was being restored to the same phase as when
it was powered off, the steppers might click slightly, but shouldn't move
a detectable amount.

I'm afraid, if my hypothesis is correct, that you should not power the
steppers off when you have homed the machine, and want to go back
to the same position.

Jon