Hello Guest it is September 22, 2019, 12:58:38 PM

Author Topic: Problems threading on the lathe  (Read 330301 times)

0 Members and 1 Guest are viewing this topic.

Offline RICH

*
  • *
  •  7,361 7,361
    • View Profile
Re: Problems threading on the lathe
« Reply #580 on: October 20, 2009, 05:13:51 PM »
ART,
115 RPM / .2P / 20 PASSES 0.002"    DATA REPORT ATTACHED

- SINGLE SCRIBED LINE / no screwy passes, CONSISTANT DECREASE IN PITCH ALONG THREAD
- @1" (-) .0038"
- @1.5" (-) .0063"
- @1.8" (-) .0072"

Now were do you want to go? I can change kernal speeds and try .2 at 402 rpm.
RICH

Offline ART

*
  • *
  •  1,698 1,698
  • Tough as soggy paper.
    • View Profile
Re: Problems threading on the lathe
« Reply #581 on: October 20, 2009, 05:52:09 PM »
Rich:

 So am I right is saying that .1 and .2 pitch look much the same as var as hgow far out they are? Be interesting
to see .3 or .4 .. just for eliimation.

Art

Offline RICH

*
  • *
  •  7,361 7,361
    • View Profile
Re: Problems threading on the lathe
« Reply #582 on: October 20, 2009, 06:28:52 PM »
ART,

Yep, looks that way.

402  RPM / .2P / 20 PASSES 0.002"   / DATA REPORT ATTACHED /   kernal speed increased to 45k
- SINGLE SCRIBED LINE / no screwy passes, CONSISTANT DECREASE IN PITCH ALONG THREAD
- @1.5" (-) .0043"

Just for you i will do a long one. The bathroom still has some plumbing left in it!
Will do two at 402 rpm, one at .1p and the other at .2p , since i can do them from the same starting point on one piece.

RICH

Offline RICH

*
  • *
  •  7,361 7,361
    • View Profile
Re: Problems threading on the lathe
« Reply #583 on: October 20, 2009, 09:57:08 PM »
ART,
Two more tests as follows all decreasing in pitch. Distances are from the start point.
        
RPM    SCRIBE       PITCH       1" DIST       2" DIST       3" DIST     3.4" DIST
-------  ---------         ----------    -------------     ------------    ------------      --------------
115    1 / SINGLE    .1           (-) .001       (-) .0032     (-) .0038"
402    2                 .2            (-).0018     (-) .0046     (-) .0078"    (-) .0089"

DATA REPORTS ATTACHED
RICH

MODIFIED: ADD PICTURES
« Last Edit: October 21, 2009, 12:14:28 AM by RICH »

Offline ART

*
  • *
  •  1,698 1,698
  • Tough as soggy paper.
    • View Profile
Re: Problems threading on the lathe
« Reply #584 on: October 21, 2009, 08:47:24 AM »
Rich:

 So it does seem to scale roughly linearly, but to pitch, not so much to RPM.. which makes sense based on what youve seen.

I need to crunch some of those numbers to see if they point to any magic number. Ill get back to you later today with results.

Art

Offline RICH

*
  • *
  •  7,361 7,361
    • View Profile
Re: Problems threading on the lathe
« Reply #585 on: October 21, 2009, 10:05:35 AM »
ART,
Sure seems that way.  I quess the only thing you can can do is pick some guideline and then test it out.
For multiple scribed lines i just use the middle as a reference unless i can identify which is which and can relate the difference. ie; different leads but within a tolerance.

What is puzzling is the screwy passes. It seems the data will identify one of them but not provide reason for the
the ones following, it kind of settles out.  If it's not the machine or the threading programing, then what is interfering........may just be a Windows thing. Discussed this with Chip and also a friend.....
sort of like that old camera commercial "it has mind of it's owne, you can try to take control, but it will still does what it wants". I will try a few simple things to see the effect ......later. For now i want to stay focused on the data
and whatever you need to make intelligent changes.

Just remember "crap in crap out"...........  :D

RICH

Offline RICH

*
  • *
  •  7,361 7,361
    • View Profile
Re: Problems threading on the lathe
« Reply #586 on: October 22, 2009, 08:10:44 AM »
ART,
How goes the number crunching?
RICH

Offline ART

*
  • *
  •  1,698 1,698
  • Tough as soggy paper.
    • View Profile
Re: Problems threading on the lathe
« Reply #587 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

Offline ART

*
  • *
  •  1,698 1,698
  • Tough as soggy paper.
    • View Profile
Re: Problems threading on the lathe
« Reply #588 on: October 22, 2009, 09:30:30 AM »
Rich:

 Just as a following up. I rechecked your error passes.

We can see that the RPM was seen to change by 1RPM from 400 to 401. The feedrate changed
as a result from 40IPM to 401.RPM. Thats the only difference and its not alwasy there so
I was discountiung it. Heres why..


40IPM at 400RPM.. is .666666 inches per second at 400RPM ( 6.666666 revsper second ) so a pitch of .1 exactly..
40.1IPM at 401RPM is .6683333 inches per second at 401RPM ( 6.683333 ) so a pitch of exacltly .1 again..


  These numbers are as near perfect as one could hope for. NOW lets assuem the RPM WAS holding at 400, and not
really getting to 401RPM.. ( Ive noticed we have a granularity of 1.0 for RPM, Im going to see why that is, we should
have fractional RPM's really, so why is RPM always .000 after the decimal. )

  But before that lest figure out what happens if the RPM was simply wrong at 401, and we were still running at 400.

The difference between 400 and 401 is the diff between 6.66666 and 6.683333 so .016673 inches per second. with
6.66666 revs per second, so the error woudl be .016673 / 6.6666 = .0025Inches per rev. ( Sounds familiar dont it.).

  SO Im suspecting now the RPM , always dropping the decimal fraction may explain the lead/advance errors your seeing.

Ill look now to see why the RPM is always an integer, if I fix that we should see a tighter fit to the pitch. None of this
explains the crazy passes.. but lets see what fractional RPM does.. it shoudl fix the .0025 error ..

Art

Offline ART

*
  • *
  •  1,698 1,698
  • Tough as soggy paper.
    • View Profile
Re: Problems threading on the lathe
« Reply #589 on: October 22, 2009, 09:43:56 AM »
Rich:

  Sorry to flood you, but crunching has shown me the error may simply be the integer nature of the rpm whcih should almost always have fractions
to it. SO heres a version that should show an RPM of 400.712536753261 or similar..

Let me know if it indeed does so, if Not, Ill have to rehook my spindle sensor ( disconnect for some shop work... ) and test further. I think we're
onto why the .0025 or .003 error. If this works as I hope, that shoudl now be much closer..

Art
( I found more integers , so I resent this message with an updated code base..)