On Friday 20 September 2013 13:58:51 Jon Elson did opine:
> Gene Heskett wrote:
> > The 3rd sensor, the index pulse generator establishes the start point
> > on a synchronized move, and works well as long as the spindle speed
> > is steady. Because of the acceleration delays in bringing the axis
> > chosen up to speed from its parking position, the spindle speed
> > cannot be adjusted in the middle of a G76 or G33.1 cycle without the
> > thread moving sideways just enough to muck it up. BTDT. :(
>
> Not really true.
I should have qualified that, Jon. The difference in speed vs sideways
thread movement is entirely the delay from index pulse to synch, but once
synched it will stay that way. One could reduce that effect if the G76 had
a separate speed to use in the approach, coming to a near stop until the
index is found, then tipping up the good gulf can once synch has been done
to achieve optimum cutting speed. But G76 doesn't currently do that.
Feature request?
> I demo rigid tapping on my minimill, and run the
> spindle motor
> off a servo amp, but in open-loop mode. So, the spindle slows down when
> the tap goes in the hole. I believe it is at least a 20% speed drop
> during the tapping.
> But, it doesn't bind the 4-40 tap at all. The index pulse is only used
> once at
> the beginning of the spindle-sync operation, then only the quadrature
> info is
> used to synch the axis to the spindle. The encoder counter keeps a
> continuous
> count of the spindle rotations as a signed float (derived from an
> internal signed integer).
>
This is as it should work, no argument there at all. And I would be doing
it on mine if I could figure out where to put the sensors on its plastic
gears. Open for ideas. Metal gears would be nice. :)
> I think you can get away with two sensors on a lathe, one index pulse
> and one
> sensor that produces X pulses per rev., as long as your encoder counter
> scheme
> (hardware or software) can be set to count pulses as opposed to
> quadrature.
>
> Jon
On a lathe I don't see how Jon. For a G76 probably ok because it only
tracks the active part of the cut, but for a "peck" cycle using G33.1 it
absolutely must follow the chuck in both directions. On my machine the
reversal is a .hal defined sequence, staged by issuing a stop when the dir
signal reverses (that stop is something the G33.1 doesn't issue,) stop is
then detected by a one shot watching the encoder, which then gates the
reverse into the rest of the logic starting it up in the other direction,
and even with pretty heavy suicide brakes applied to stop it, staged by the
encoders speed output, it takes around 2 to 3 seconds to do the reversal.
And 80 watts worth of braking resisters get warmed right up doing it.
The carriage drive has to track the actual rotation of the chuck, and
without that you either need a rubber tap holder, or a delivery chute from
the tap makers machine to replace the broken ones. And a lot of EDM to get
the busted ones out of the holes...
So in my case at least, the carriage follows the chuck while its slowing
down, then reverses to back the tap out of the hole just like on you
minimill. I suppose one could use the spindle is stopped info from the hal
file to reverse the carriage direction. That in my case would be a 50
pulse per rev signal with 1/4 the accuracy it has running in quadrature
where it has 200 edges per rev to track. Would that effect the quality of
the thread? I Dunno, probably not.
Cheers, Gene
--
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Jones' Motto:
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