> Date: Sun, 7 Jul 2013 08:19:30 -0400
> From: m...@mattshaver.com
> To: emc-developers@lists.sourceforge.net
> Subject: Re: [Emc-developers] "Open" Development
>
> On Sat, 6 Jul 2013 20:23:12 +0000
> Chris Morley <chrisinnana...@hotmail.com> wrote:
>
> > I guess it really comes down to at what performance machine does jerk
> > limitation show real benefits ?
>
> I would guess that almost all machines _we_ deal with would benefit
> from jerk limiting. A machine that might be able to get away without
> limiting jerk would be something where the moving parts had very little
> mass, like maybe a tiny plotter or a tiny gimbal.
>
> > And I still don't see why one would want to turn off jerk limiting
> > for some machine movements.
>
> Here's what I think Jon is talking about: When in a rigid tapping
> cycle, the Z axis is "slaved" to the rotational position of the
> spindle. The spindle will have its own characteristics of motion, but
> we don't always have tight servo control over them. For example, at the
> bottom of a tapped hole, the spindle must reverse, but it may come to a
> stop with a jerk due to high friction (you've probably experienced a
> tapping operation where the tap wants to move in small jerks rather
> than smoothly cutting). At this point the Z axis should probably be
> controlled with no limit on jerk (and maybe no acceleration limit
> either) so as to maintain the spindle-to-Z-axis position relationship
> which is critical since they are now _mechanically_ interlocked through
> the tap and it's threads in the workpiece.
>
I see what you guys are saying and I agree it would work fine as long as you
don't have a higher performance spindle then axis.
I just think it's the wrong thing to do.
Is it common to set speed, acceleration and jerk limiting leaving lots left on
the table?
My understanding is the limits are set to safe and near maximal settings.
I don't understand why one would set limits this way then ask the machine to
ignore them sometimes.
For instance if your machine could move maximally at 50 inches a minute, why
would
you allow the TP to ask it to move 52 while G33.1? Same premise as jerk
limiting.
If your machine really can run 52 then why not set the limits to 52?
I also wonder if by turning jerk limiting on and off, if this would not screw
up PID tuning.
I know when i raised my maximal speed i had to re-tune the PID, though speed of
course
has a much higher effect then jerk limiting would.
So I guess I'm saying if you are using a spindle that out performs your axis,
You should
slow down the motor not ignore the limits of the axis (assuming the limits are
already maximal)
Chris M
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