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Author Topic: Problems threading on the lathe  (Read 330046 times)

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Offline ART

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Re: Problems threading on the lathe
« Reply #610 on: October 24, 2009, 01:49:54 PM »
Daniel:

   If averaging is on, then the SS is affected, if off, no effect.

Art

Offline Dan13

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Re: Problems threading on the lathe
« Reply #611 on: October 24, 2009, 01:54:59 PM »
Thanks Art. I don't use averaging ever since I've put an AC servo on my spindle and use its encoder index for speed reading - displays the speed rock solid.

Daniel

Offline RICH

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Re: Problems threading on the lathe
« Reply #612 on: October 24, 2009, 01:58:29 PM »
ART,
Well, first let me say that I am glad someone else is trying the the threading out.

The M1.5P ( 1.5x.0394=.0591" ) is close to the 20TPI i was running. Additionaly you get some

other uneven numbers in there like .xx91 for a pitch value. And we know that a small difference in

the defined pitch will change lead.  

A nice new tap is a good compare against the thread as they are ground very accurately.  I will

assume it was a ground tap. Most of mine are for H2, which simply means the diameter on it is held to

provide a tolerance of 0.001" over the basic pitch diameter.

Almost all of my threading has been done with averaging on
If averaging is on then all the rpm reported would be averaged and that is the value that would be

used for the NEXT thread pass. Correct?
If averaging was off then assume that the last reported...Or.... is it the requested used?
Unconfuse me.

I have not remarked much on what you see at the end of the thread. ie; single, double / multiple

lines.
I have used the distance in spacing to reaffirm what i thought i observed during the threading. (ie; a

later pass increases the thread) You would think that the difference between the ending of those

scribes could be used for lead error, or maybe tiiming pulse, rpm....but.... i figured if you have

some runout on the diameter of the piece or a difference of velocity as it pulls out of the thread,  

backlash as it changes direction on pullout.....just too many variables to try and make sense of

what that measurement is telling you. SO i just use it as a visual.

I need to remark that no variable speed drive or whatever is used here just a plain good old

fashioned motor.

AVERAGING
If you get a change of speed,  a practical example would be while threading and you hit a hard

spot in the metal, or, say you catch material just as the cutter hits the material at the beginning of

the thread, if that value was very influential on the next to last pass,  then a good thread
can be ruined if averaging was off . If on, the affect would not be as great. Actualy calculated
forces can easily double.

Now if your in the middle of all the passes, there is recovery, by the additional passes yet to be

done. But then, if it was spring in the material or poor setup, then who knows!

The point is, the  mechanical apsects of threading can vary the situation.
Like you, i am not sure what to make of it.

I won't even get into the acceleration and deceleration part of the cycle. I will say that whatever

someone does to test, keep the pullout at the same angle and use a common value like 2 or 3

times the pitch for free cutting before the actual thread is engaged by the cutter.


RICH
« Last Edit: October 24, 2009, 02:20:27 PM by RICH »

Offline RICH

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Re: Problems threading on the lathe
« Reply #613 on: October 24, 2009, 02:08:21 PM »
Daniel,
Can you try out the the threading without compensation and not use the SS?
What is good is that your speed is rock solid. Your an apple and i am an orange ie; my speed will vary a small amount ( like 1000 other users out there ). 
RICH

Offline ART

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Re: Problems threading on the lathe
« Reply #614 on: October 24, 2009, 03:34:44 PM »
Rich:

 When it comes to averaging.. Mach3 can keep a running average of the last 8 revolutions. At the start of any pass it either uses the averave OR the last measured time.

So in theory, hitting any hard spot has no effect on the actual planned pitch for the thread, the idea is simply to be as accurate as possible as to what the unloaded speed is, and since
every thread starts with an unloaded spindle, the 8 rev average is typically unloaded average. ( unless your turnign very slowly on the spindle.. no-one is I think less than 8 turns in the air
prior to a thread pass.

  SO the correction code is all that controls speed during the cut to take into account the slowdown caused by hitting a hard spot or an average slowdown due to harder material.
The averaging is discounted for correction, it cares only abou tthe real last index to index time, and it compares that to the locked in average or locked in last rev just prior to the
thread pass starting.


 Since your threads, as well as Johns seem to be nuts-on with no correction.... we're good till the last step in the chain..correction.


  More of a job than I thought.. be a little whlie yet..

Thx
Art

Offline RICH

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Re: Problems threading on the lathe
« Reply #615 on: October 24, 2009, 03:54:36 PM »
Thanks,
Take your time.
RICH

Offline ART

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Re: Problems threading on the lathe
« Reply #616 on: October 24, 2009, 10:02:59 PM »
Rich:

  Before I rewrite all this, I found some troubleing code in there. Can you confirm this driver does the same bad corrected thread as the last correction driver?
Turns out the driver was clippign the value used.. Im wondering if that clipping was causing the crazy passes when correction was on.

Thx
Art

Offline RICH

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Re: Problems threading on the lathe
« Reply #617 on: October 24, 2009, 11:16:06 PM »
ART,
No problem. Will try it out tomorrow.
RICH

Offline RICH

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Re: Problems threading on the lathe
« Reply #618 on: October 25, 2009, 02:02:05 PM »
ART,
1/4-20 thread 402rpm @ 40 passes.
Bad pass in Al at pass #11 in Al and #5 in CS. I only have the error report for the CS one and is attached.
At least the error correction kicked in at an early stage and I didn't need to stop the threading.  
After the CS 5th pass it had a lot of passes to recover such that the final few cleaned up the cutting.
The finished threads have a lead error of 0.050"  0.040"/ inch decrease. That's almost a full thread!
RICH

MODIFIED: Added a picture to make it more interesting.   ;)
« Last Edit: October 25, 2009, 02:48:13 PM by RICH »

Offline Dan13

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Re: Problems threading on the lathe
« Reply #619 on: October 25, 2009, 03:19:57 PM »
Daniel,
Can you try out the the threading without compensation and not use the SS?
What is good is that your speed is rock solid. Your an apple and i am an orange ie; my speed will vary a small amount ( like 1000 other users out there ). 
RICH

Rich,
It is really a hassle for me to to make my lathe run from PP. The spindle servo needs 8096 steps per revolution, so even for a 500RPM speed it would require a 75kHz kernel speed. Can't run my computer at this speed. The only choice is to reconfigure the servo drive to use electronic gearing. But then my steppers take 4000 steps per mm, again requiring a kernel of at least 45kHz to be able to get some normal feedrates.

But if it can help you, I could try it after I finish the work I'm doing now on the lathe. Another thing is I don't have any precise means of measuring my threads, so don't know how much of a help would it be.

Daniel