> Date: Sat, 6 Jul 2013 16:28:00 -0500
> From: el...@pico-systems.com
> To: emc-developers@lists.sourceforge.net
> Subject: Re: [Emc-developers] "Open" Development
>
> Chris Morley wrote:
> > I am not an expert, just interested. I don't follow your reasoning.
> > Jerk limiting is about having the TP ask for movement that is possible
> > for the machine to actually produce.
> > infinite jerk is impossible for a machine to produce movement for.
> > While we can ignore it in relatively slow and small machines, I can not
> > see why you would want to turn it off in some cases.
> >
> G33 (lathe threading) assumes the spindle is mostly maintaining constant
> velocity. G33.1 (rigid tapping) assumes the spindle reverses fairly quickly
> at a certain point. The Z axis must follow the spindle quite closely, or
> it will break small taps and muck up the thread on larger ones.
> > It seems if you have the TP request infinite jerk, then you are must realize
> > that your are asking the machine to NOT follow the TP command for a small
> > instant.
> > I don't see how G33.1 is any different then other machine movements.
> >
> Other than spindle synched moves, ALL axes are under TP command, and
> that should always keep them synched so they are all at the correct
> coordinated position. With a spindle-synched move, the tool must
> follow the spindle, which often is NOT a servo axis, and is just
> generally obeying a velocity command. When the G33.1 gets
> to the point of reversing the spindle, it can reverse fairly quickly,
> depending on the particular machine setup, and the Z BETTER
> keep up with however fast it reverses! Having any interpolation,
> jerk limiting, etc. between the spindle encoder and the Z axis would
> apply strain to the tap, and be very undesirable.
>
> Jon
>
Ok I am starting to see what your thinking.
But I assume one would set the jerk limiting to a reasonable setting very close
to
what the machine is maximally capable. Surely if you have a very 'soft' setting,
things don't work very well, just as setting the accerleration 'soft' will
break taps too.
You seem to be assuming that if the TP command says move with infinite jerk
that the machine
will actually move that way.
What I am suggesting is that jerk limiting allows the machine to more closely
follow the TP
command. With tighter control comes - lower following error - comes more
accurate movement.
I bet there is a following error spike right when you do reversal or maybe not
badly because the
spindle does not react with infinite jerk either.
So if your tap reversal changes direction faster then the machine can react, it
doesn't matter
if you have jerk limiting or not - there will be error.
I bet we'd b surprised how much error rigid tapping can tolerate. machine and
part flex is forgiving.
And that is what I am saying, without jerk limiting, we are asking the machine
to move faster
then it actually can, so we get a small following error spike, most pronounced
at reversals.
I remember the graphs I looked at were when the mill cut circles - there were
reversal spikes at each of the four quadrants.
Chris M
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