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Author Topic: Problems threading on the lathe  (Read 331691 times)

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Offline RICH

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Re: Problems threading on the lathe
« Reply #550 on: October 15, 2009, 09:07:02 PM »
ART,
Here is a velocity report.
RICH

Offline ART

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Re: Problems threading on the lathe
« Reply #551 on: October 15, 2009, 09:16:33 PM »
yeah..not muvh difference.. a .197rpm difference overall and only in 2 spots..

Pretty tight.

Art

Offline RICH

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Re: Problems threading on the lathe
« Reply #552 on: October 15, 2009, 09:27:28 PM »
ART and Hood,
I realy don't know.  Can certainly use Hoods as "standard" and like you said it's been around a long time, has proven good results are possible.
I have only been at the threading for say a 1 1/2 years. What stupid thing ( the rat where it is ) is the problem?
I am sure we will laugh about it someday, but in the mean time "search and destroy"!  


RICH

Offline ART

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Re: Problems threading on the lathe
« Reply #553 on: October 15, 2009, 09:39:35 PM »
Rich:

  Im starting to think its no so much of a rat , rather a problem of numbers. The random runout is looking liek an intermittant missing index pulse to me. That woudl explain it best, but we'll know if we can catch the random
bad pitch problem, if the feedrate velocity near doubles, it will indicate the spindle went to double RPM for one instant. And that just happened to be when the trajectory was planned.

  Normally, if you miss a random index pulse, youd never notice, the DRO's arent fast enough to actually show it to you, the scope would if it happened to happen during scope run, ( the advanced planners scope can grab the index pulse repeatedly for up to 4000 pulses , if any of them are double timed.. thats a missing pulse.). Anyway, that random missed pulse would double the pitch.. if averaging was off, if averaging is on and we had 100RPM for 7 of the averages.. then a 50 was seen( missing pulse..), then the average would be (7*100)+50)/8 or 93.76..so the
"random" weird advance would be advanced by 6.2% on that pass.

   That seems earily close to what that random runout looked like.. at 115rpm, thats 107.8 it would see for that pass
or 6.2%..  holds across rpm's due to the averaging.. Gotta think about that..

Art

Offline RICH

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Re: Problems threading on the lathe
« Reply #554 on: October 16, 2009, 03:31:06 PM »
ART,
Just some more testing.
These three tests were run with VER .028 / posted version 54 - reply #539 with PP.

Testing was to look for a missing index pulse and relate when it happens and is it repeatable.
Full threading of the piece 1/4 - 20unc @ 404 RPM


TEST #1 - 40 PASSES - Pass # 23, 33 and 40 were bad! In fact #40 broke the piece.
                I din't have the end supported so thought that maybe a screwy lead happened and
               caught the end of it. So did another pass, very rigidly supported, and wondered if the
               lead would change at about the same pass.

TEST#2 - 40 PASSES - Pass # 21, 24, 28 and 32 were bad and stopped it at pass 33.
               By bad, i mean a visual change in the lead and thus a heavy cut.
               
TEST#3 - 18 PASSES -  I figured that since all the other passes got screwy at above 21 passes
              time / repeatitions, then i will change the threading code to fewer passes.
               18 PASSES @ .010" first pass and remaining at .005", no spring passes.
              Pass #9,10,12,14 were bad and the 18th pass cleaned up the thread and there was
             a .010" lead  increase / inch.

Attached are the velocity reports for the three tests. Spindle slow down is not a problem as the
lathe will just break or bend something and dosn't slow down. Maybe you can make something
more of the velocity tests by looking at the bad passes i noted for each test. It was interesting to
see a possible pattern happenng atapprox 22, 32, and 40 pass and the roughly 1/2 of that for pass
when the number of pass was changed.

I have attached a pic of the thread with some notes on it. It shows the change in lead at what i think
werethe passes. At 100x you can actualy see the small stepping of the cutting on the backside
flank per pass.So for the first 20 passes there is a rather even tappering.

If the lahe was a punny one like my Sherline, the RPMs would have dropped and you realy couldn't
get what i am finding.

RICH 

Offline ART

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Re: Problems threading on the lathe
« Reply #555 on: October 16, 2009, 06:25:12 PM »
Rich:

  Now thats interesting data!!!. In the report, you can ignore all the ones with vel:40.5 , thats just the program loading up.. Your passes are obvious
when it shows the Pass number due to your comments in the code.

  SO we get..
Fri - 01:53:03 ---Decrement = 0.005 pass 14
Fri - 01:53:05 --- Threading pass:  Vel: 0.3367
Fri - 01:53:05 --- Threading pass:  Vel: 0.3367
Fri - 01:53:06 --- Threading pass:  Vel: 0.3375
Fri - 01:53:10 --- Threading pass:  Vel: 0.3358

  Thatys one of your "bad" passes. Some of the other bad passes show only the variation of .3367 to .3375.

So lets crunch that a bit..
As feedrates go, that per second, so the bad run could be considered..


Fri - 01:53:03 ---Decrement = 0.005 pass 14
Fri - 01:53:05 --- Threading pass:  Vel: 0.3367  thats feedrate 20.202
Fri - 01:53:05 --- Threading pass:  Vel: 0.3367        feedrate 20.202
Fri - 01:53:06 --- Threading pass:  Vel: 0.3375        feedrate 20.25
Fri - 01:53:10 --- Threading pass:  Vel: 0.3358        feedrate 20.148

  Now thats the highest variation I see in the reports. So we need to divide by the
spindle speed to get the pitch you requested. So 20.202 / 404 = .050000 ( middle case )
 and 20.25 / 404 = .050123 ( higest pitch called) , and finally 20.148 / 404 = .04987 ( lowest pitch called for.)


  So your numbers seem to indicate that when we consider the spindle speed, and the feedrate calcuated from it,
the pitch varies from a low of .04987 to a high of .050123 or a variance of .000253 in terms of pitch.

  Since a 1/4 20 thread is .05000 pitch , we have a pretty tight planning being done according to these numbers.

Seems to be less than 0.5% variation.. all threads , even the bad ones should have been good according to these numbers.
At most you should have seen less than .000253 difference from thread to thread. Tells me that the RPM is NOT the problem,
nor is the feedrate calculated for the pass.

 I need to think about this for a bit to see what to test next. This shows an almost perfect reading of RPM, as well as
perfect control of feedrate to match the read RPM. That variance must have meant the feedrate was solid ... hmm..lets calc that
while we're crunching. You planned for .050000 as a feedrate, it varies only due to rpm readings, so we can multiply it out
to see what rpm you were reading on each of the three pass speeds we are getting. Code assumed 404RPM, and what we got was..

perfect reading of 404 would give you .33666666 as a velocity. ( 20.2 / 404 = .05 pitch @ 20.2 / 60 = .336666666....)

.3358 * 60 = 20.148 = 405.04RPM
.3367 * 60 = 20.202 = 404.04RPM
.3375 * 60 = 20.205 = 403.0 RPM

      So at most 2RPM was floating. How accurate that was can be measured as a function of the end pitch results, which seem to
be very close, thus the measured RPM must be very close indeed.


   So all in all, Id have to say what we're seeing is just and proper. Youve disproved any problem caused by RPM measurement,
or trajectory speed calculations. This pretty much verifies all of Mach3's code with the exception of the driver itself in terms of
created the crazy result moves that have occured with the very weird high advancing leads on some passes.

  This may explain why the SS seems to be found to be very tight, it has no PP driver, and results seem good, so it
all makes great sense when you think about it. While its true the SS and Mach3 both may have a sligth advance or a slight
decrease in pitch from nominal, thatr I think may be the result of the planners conception of time, asnd probably something
that can be corrected.

  SO its my feeling we're left with "What is it that causes the crazy moves intermittantly?" This is what Id like
to focus on I think. And its where we have to go to see what it may be thats causing the trouble. And this means its
going to get complex from here on. :-) ( As if its been easy so far..)


  If we can eliminate the crazy leads, then we'll only be left with making the pitch itself as close as we can get it.


  We only have the reports of what the RPM was at the start of each thread.. this doesnt tell us if indeed the RPM slowed
signifigantly DURING any thread. We have the corrrection turned off at this point, so if the spindle slowed during any pass,
the pitch would give us a "crazy" move indication as the pitch would be way off. It would lead signifigantly for that pass.
Although it may start OK, as it slows the thread pitch would advance as the feedrate would not at the moment slow down to
match the spindle. 

   Im thinking it best to perhaps monitor the spindle rpm during the thread. This woudl create too much debug traffic,
so perhaps I can code up somethign that records the upper and lower limits of RPM seen during each pass. that may help
us better identify the culprit of the crazy passes. Im betting the warmer the spindle the less often this bug hits us,
as I think it may be related to the VFD's correction mechanisms as it slows down or speeds up the spindle to match its own
window of tolerance on the spindle rpm. (But I could be wrong..).

  Ill dwell on this for a day or two and coime up with something to feed us more data, so far youve disproved almost
any application side reason for varying pitches, as everything is pretty much as close as I think anythign could be.
  Comment on anythign you think I may be missing or screwing up here.. :)


Thx

art
 

Offline ART

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Re: Problems threading on the lathe
« Reply #556 on: October 16, 2009, 06:33:42 PM »
Rich :

 One other comment on this.. You said you stopped the thread test on pass #33 on one test,, I think this is it..

ri - 00:48:49 ---Decrement = 0.002 pass 33
Fri - 00:48:50 --- Threading pass:  Vel: 0.3367
Fri - 00:48:51 --- Threading pass:  Vel: 0.3367
Fri - 00:48:51 --- Threading pass:  Vel: 0.3367
Fri - 00:48:55 --- Threading pass:  Vel: 0.2217
Fri - 00:48:55 ---Decrement = 0.002 pass 34
Fri - 00:48:57 --- Threading pass:  Vel: 0.1033
Fri - 00:48:59 --- Threading pass:  Vel: 0.0308
Fri - 00:48:59 --- Threading pass:  Vel: 0.0100

  Look like the G32 motions to approach the start of thread were OK, but the start of pass 33 shows .2217.. which is a spindle slowing way down, followingmotion shows it stopping.. Am I right to assume you killed the spindle poer at that point ? Looks that way.. :) , as if the spindle was slowing as you attempted to terminate the run.

Art

Offline RICH

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Re: Problems threading on the lathe
« Reply #557 on: October 16, 2009, 07:38:32 PM »
ART,
When i say i killed the program, what i have done is flip a switch which disables the drives and stops all switching activity. The drive will continue recieving inputs from the PP but nothing will be sent to the steppers.  That switch also kills the power for the Hall trigger. Then i stop the program.  So Mach will continue running the program and show that the rpm drops off.  That's why you see the velocity taper off in the data. The switch was flipped sometime after the thread pass but before it started the next threading cycle.

RICH

Offline ART

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Re: Problems threading on the lathe
« Reply #558 on: October 16, 2009, 07:59:12 PM »
Rich:

  Great. Just wanted to know I was reading the data properly, that makes perfect sense.

Art

Offline ART

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Re: Problems threading on the lathe
« Reply #559 on: October 16, 2009, 08:27:53 PM »
Rich:

  In case you do any further checks, hers a version that report only on actual passes, and shows req. pitch and actual pitch as well as feedrate for each threading motion.

Art