On 12/21/24 12:22, Peter Wallace wrote:
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.
I've been told that too, but I seem to have a noticeable diff in timing
between fwd and rev, resulting in quite loose rigid tapping as the taps
scrape things loose backing out. cutting both directions it seems. With
a head that weighs at least 40 lbs on this go704 I can't believe its all
backlash. Ball screws in it are now well broke in but was under .001"
when the screw was fresh. Where else should I look? I don't have a
floating tap holder. Where can I get one that's R8 compatible?
Thanks Peter.
Peter Wallace
Mesa Electronics
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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