Threading under EMC

[Chris Stratton <stratton-at-mdc.net>]

Am I right in thinking that rigid threading modes are not currently
supported in EMC?  As far as I can tell, there is a cycle for tapping 
(I'm really intersted in single point threading on the lathe) but
it depends on a canonical function "SET_SPEED_FEED_SYNCH" which
doesn't do anything yet?

I'm trying to figure out what would be involved in implementing this,
and the problem I keep running up against is that everything seems to
be based on precalculating where they machine "should be", leaving it
up to the servo loop alone to fix any difference with reality.  If
this difference gets too big, a following error is declared.

Somehow, it would be necessary to slave the position of the z axis to
the spindle rotation.  One possibility would be to program a feed rate
that should be very close to what is needed, and substitute the
spindle-derived positions in somewhere instead.  Problem is EMC will
expect the motion to be done based on how long the feedrate and
acceleration parameters said it should take - perhaps there is a way
to make it wait?

Alternately, the interpolater could be altered to ignore the commanded
feedrate and simply generate points based on the actual spindle
rotation.  The problem is that EMC's PID servo loop as most actually
use it is really a P & Feedforward algorithm, and we can't know
exactly where the spindle will be several trajectory (or is it servo?)
cycles from now.  Sure, we can initially plug in numbers based on the
projected spindle rotation (it shouldn't change much) so the FF
calculations can work and then use the actual spindle position for the
P calculation - but will the loop close following errors
satisfactorily this way?

Of course there is the dodge of servoing the spindle and hoping it
will be where we tell it to be.  When not threading we could either
disable the axis and write a raw dac value, or have a flag that leaves
the servo loop enabled with a fixed velocity and no position
calculations.  Problem with this is that it requires serious
improvement to the machine, compared to simply mounting a large
encoder wheel between two bushings and boring out the assembly to fit
over the back end of the spindle.

Thanks for any ideas,

Chris


-- 
Christopher C. Stratton, stratton-at-mdc.net
Instrument Maker, Horn Player & Engineer
22 Adrian Street, Somerville, MA 02143
http://www.mdc.net/~stratton
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