On Sat, 21 Dec 2024, gene heskett wrote:
Date: Sat, 21 Dec 2024 10:04:38 -0500
From: gene heskett <ghesk...@shentel.net>
Reply-To: "Enhanced Machine Controller (EMC)"
<emc-users@lists.sourceforge.net>
To: emc-users@lists.sourceforge.net
Subject: Re: [Emc-users] Rigid tapping.
On 12/21/24 06:39, andy pugh wrote:
On Sat, 21 Dec 2024 at 04:55, johnd <jo...@autoartisans.com> wrote:
Thanks. Is there a minimum number of slots assuming the spindle speed is
stable?I've not had issues with 60. My friend has far less.
The issue is that the spindle speed is _not_ stable, as rigid tapping
has a stop and reverse. It's catching that that is critical.
So you definitely need quadrature, to be able to tell when the spindle
changes direction.
And then you need to think about how much "electronic backlash" you
can afford at the tap.
With 60 teeth you have about 6 degrees of positional uncertainty. With
a 24 tpi / 1mm pitch tap that's about 0.0007" or 0.02mm axial error.
The tendency will be for the tap to rub on the lower thread flank as
it backs out.
If you use a floating tap holder then this won't be a concern.
I think you are just about OK at 60, but going any less I think that a
floating holder would give better threads.
But consider also the backlash in the Z-axis drive, there is probably
some, even if not very much. That will have the same effect.
There is another src of error here Andy, inherent in the way we detect
index pulses that I have not fully sorted, and both optical and hall
effect devices suffer from it, hall stuff like ats-667 outputs 2 edges,
one edge as the target screw approaches, and the reverse edge as its
target screw passes the center of the hall device. its the second edge
we want, but our choice of edge makes a simple circuit out of time by
several degrees depending on which direction the spindle is turning.
Optical means can get closer but we are still triggering on the first
edge as the screw is approaching. And that is the edge with all the
error., the second edge is the accurate one. We need a circuit that
detects both edges regardless of spindle direction, cocking its self on
the first edge, then firing on the second, and we don't have that, we
know with advance warning from a high enough definition encoder, which
direction the spindle is turning, from that we could steer the edge
detector.
But with my encoder on the rear of the motor, the effective scales,
depending on which gear its in, switch the scale from a bit over 7000
counts per rev in high gear to over 14,000 in low gear., probably
setting a record high definition for a working encoder. When I had a 64
slot optical in my early rigging, the error from the brass slotted wheel
caused the pid to hammer the gears making a racket like the bearing
balls were square, putting the encoder on the motor had the effect of
smoothing that up, allowing PGain to be multiplied by 3 to 35 or 40. W/o
any rattle from the nylon gears in that go704's head.
Now the only thing I hear is the iron in the motor squeaking when the
pico pwm-servo driving it goes into limit at 18 amps into that 90V 9.7A
motor. Making around 3hp, still on the OEM brushes after 14 years.
Question Andy, will I get a false index out of an edge detector at the
instant I switch which edge it detects with the fwd/rev from the
encoder? I've not tried that but it seems like a way around that
conundrum if I can w/o a glitch pulse at switching time. Having looked
at it with halscope, that's not fast enough to see it. I have a scope
fast enough (4 trace, 350 mhz response) but no access with a probe.
Thanks Andy.
Cheers, Gene Heskett, CET.
--
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author, 1940)
If we desire respect for the law, we must first make the law respectable.
- Louis D. Brandeis
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My understanding is that rigid tapping (G33.1) only uses
the index at the start of the cycle, so the direction of
index detection is always the same.
Peter Wallace
Mesa Electronics
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