Re: Threading under EMC

[Ray <rehenry-at-up.net>]

Chris

I'm not going to be much help with the details of this but I've got three
lathes out in the garage that will need it soon.  I have had some
experience with machines where a servo is not properly responding to rigid
tapping or threading and am of the conclusion that both position and
velocity of Z need to be coupled directly to changes in the position of the
spindle for this to work satisfactorily.

I set up a repo Mazak lathe recently that had a program in it for hard
tapping a 1/4-28 in aluminum.  They ran with spindle speed set to 4000
rpm.  We ran a few trials, being skeptical and the spindle speed just
reached 4k when the decel took over.  In this case, there was no constant Z
speed.

I agree that rigid threading may require us to increase the precision of
our axis tuning but I believe that 

On Sun, 27 May 2001, Chris wrote:
> 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?

Yea I've read something about tapping but with no speed feedback from the
spindle this is worse than buying a lottery ticket.

<s> 
> 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?
<s>

My thinking is that this is the way both tapping and threading ought to
be done.  

Thanks for your interest in this step forward with the EMC.  Matt and
several of us at NAMES were working on priorities for advance and this was
near the top  Let me know if there is anything that I can do to help.  I'd
be happy to set up the feedback and break a few taps with my Grizzly to test
this code out.

Ray