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General Mach Discussion / Re: Problems threading on the lathe
« on: October 22, 2009, 09:15:18 AM »
Rich:
What I was concentrating on was the problem of syncronicity. Since the real world is not triggered
digitally as the cpu is, I was concerned that it may be the case of the index pulse being sensed 1 step
early or late depoending on timing and if this migth be the crazy run problem.
Since your numbers all show pretty much no aberation in the program itself, and the driver has no clue
about timing other than it knows to run at kernal frequency, all I could see is triggering.
That however, can IMO, only affect the thread run by 1 step of the pulse train, advancing it or delaying it
by one step. What is the step resolution on your machine. How many steps/inch is it set to. ( Just out of
curiosity, because I dont think your set so coarse that a single step would put you .003 off from target on those runs,
youd have to be at only 330 steps/inch.. I suspect your set to 2000 or so..
In any event, youd have to miss the trigger by 6 steps for that to occur, and that woudl make the entry look bad by that amount.
Am I right in assuming the entry alwasys seems to look good on the crazy passes, but the only thing you notice is the thread
pitch is off, that it gets longer by a set amount and gets more out of position the longer the thread?
If so, then we can only have a feedrate that is moving too fast for that pass. But given that the debug monitoring seems to show
no aberation that can explain that pass, Im left with thinking the kernal frequency must be speeding up for that pass ( or slowing if
the crazy pitch is retarded to the requested, but it seems your crazy pitches are advanced to the req. pitch. ( Am I right in assuming that
as well? ).
Si Im left with the impossible, the RPM varies only by 1RPM at most on your system, bad passes dont report any more than that, and good passes also report 1RPM variation. But 1RPM wont explain it. Weve proved that as the 400rpm tests work similarly to a 115 RPM run, but if
you call for .2 pitch, your not quite double what a .1 pitch does, but there does appear to be an error that grows as the pitch grows.
We may be too small in pitch to tell though. How high a pitch can you get on your system? Can you do .5" at 115RPM? 1Inch pitch?
The gross changes evident from .1 to 1" pitch may be more revealing, but at the moment the numbers appear to be noisy more than actually showing a direction. A .003 pitch decrease over an inch doesnt bother me too much and I can see thats just about at the correctable point,
but Im stumped as to exactly where its coming from. A 1" pitch may show us somethign we hadnt considered if you an do it.
Art
What I was concentrating on was the problem of syncronicity. Since the real world is not triggered
digitally as the cpu is, I was concerned that it may be the case of the index pulse being sensed 1 step
early or late depoending on timing and if this migth be the crazy run problem.
Since your numbers all show pretty much no aberation in the program itself, and the driver has no clue
about timing other than it knows to run at kernal frequency, all I could see is triggering.
That however, can IMO, only affect the thread run by 1 step of the pulse train, advancing it or delaying it
by one step. What is the step resolution on your machine. How many steps/inch is it set to. ( Just out of
curiosity, because I dont think your set so coarse that a single step would put you .003 off from target on those runs,
youd have to be at only 330 steps/inch.. I suspect your set to 2000 or so..
In any event, youd have to miss the trigger by 6 steps for that to occur, and that woudl make the entry look bad by that amount.
Am I right in assuming the entry alwasys seems to look good on the crazy passes, but the only thing you notice is the thread
pitch is off, that it gets longer by a set amount and gets more out of position the longer the thread?
If so, then we can only have a feedrate that is moving too fast for that pass. But given that the debug monitoring seems to show
no aberation that can explain that pass, Im left with thinking the kernal frequency must be speeding up for that pass ( or slowing if
the crazy pitch is retarded to the requested, but it seems your crazy pitches are advanced to the req. pitch. ( Am I right in assuming that
as well? ).
Si Im left with the impossible, the RPM varies only by 1RPM at most on your system, bad passes dont report any more than that, and good passes also report 1RPM variation. But 1RPM wont explain it. Weve proved that as the 400rpm tests work similarly to a 115 RPM run, but if
you call for .2 pitch, your not quite double what a .1 pitch does, but there does appear to be an error that grows as the pitch grows.
We may be too small in pitch to tell though. How high a pitch can you get on your system? Can you do .5" at 115RPM? 1Inch pitch?
The gross changes evident from .1 to 1" pitch may be more revealing, but at the moment the numbers appear to be noisy more than actually showing a direction. A .003 pitch decrease over an inch doesnt bother me too much and I can see thats just about at the correctable point,
but Im stumped as to exactly where its coming from. A 1" pitch may show us somethign we hadnt considered if you an do it.
Art
