Author Topic: Spindle Index Pulse Problem  (Read 13843 times)

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Offline RICH

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Re: Spindle Index Pulse Problem
« Reply #30 on: April 08, 2012, 07:17:15 AM »
Hood,
Ah ...as noted in the first posting....
I never did any testing with the SS and haven't used mine in a long time. So additional electronics are involved.

If we just forget about the actual threading then there seems to be a timing issue. ie.The G95 informs Mach that the index was seen x amount of times and
based on an average of the rpm the axis is then instructed to move / accelerate over some linear distance. So the index trigger point,
acceleration value, current rpm of the spindle in a time frame, and actual movement of the axis needs to be consistant to arrive at an exact location.
If one were to use  a thin disc ( btw ....the bigger the disc diameter the better one would see any timing issues ) you can get a flavor of magnitude since the cut marks would be off by a greater amount.

If we assume that there is no backlash, the starting point is exactly the same location, and the system can respond  to the requested motion,
but, and i say but , the instructions are not the same  as the PP, then there could be something wrong in MACH. But before one guesses at that they need to verify that the system is all correct and that is hard to pin point a possible problem.

He is using a max velocity of 190 with accel around 4. So even though his kernel frequency is high that dosen't mean he can use that high velocity in a repeatable
accurate way. The max values are used during the move from standstill to the start of the threading and not the 65ipm he notes.
Maybe try a reduction in spindle rpm and velocity to say 1/2 the currrent velocity value and do some testing to "scribe" marks on the edge of a disc.
I will note to use the wizard since it will check that the spindle rpm and current tuning settings are adequate for the threading to be done.

Not sure what else to say at the moment.....

RICH



Offline Hood

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Re: Spindle Index Pulse Problem
« Reply #31 on: April 08, 2012, 08:02:16 AM »
I would say you may be right on reducing the RPM especially for testing.
With the SS the threading is done in it rather than Mach and all reports are that it is very accurate and works extremely well even on low powered spindles. One added benefit of the SS is that it can see tiny index pulse widths, I use the Index pulse on my motors encoders and it works great.

Hood

Offline RICH

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Re: Spindle Index Pulse Problem
« Reply #32 on: April 08, 2012, 12:21:29 PM »
Here is why testing at a lower velocity is worth trying.
If one assumes the acceleration, axis movement, and triggering are correct then any difference in the point of first contact would be the difference on the circumference of two marks due to variation of the spindle rpm.

For a 1" diameter disc:
800 rpm =13.3 rev/sec = 1.3 rev in 0.1sec
circumferential distance a point would travel in .1 sec =4.188"
( time of .1 sec somewhat close to the accel for 13tpi @61.4 ipm)
since 3/800=.00375 (.375%) possible variance of the rpm  then the contact point can be +- 0.16" off

Dropping the rpm to say 100 rpm the contact point could be off by +- 0.002"

So slower spindle rpm reduces the possible error of where the cutter is  in relation to the point of just starting the thread / scribing mark.

One should see a reduction in inaccuracy ...i would think

You may also be able to just do an axis move G1 Z.5 F.076923 and get a entry point and v groove on the disc. Then  pick up the thread and now do a
G32 move to see what the difference would be, but must say that you'll need to very meticulous on doing it. In the Threading on the Lathe Manual
on how to pick up a thread.

Just some thoughts on trying to find out what's wrong,

RICH




Offline GytarrMann19

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Re: Spindle Index Pulse Problem
« Reply #33 on: April 09, 2012, 05:52:38 PM »
Thanks for the replies guys. I will attempt to test some of these ideas as soon as I get some time. Note: Disabling turn diagnostics did make a big difference. I was able to produce a thread good enough to turn a nut onto, so the starting points were much closer, but still off somewhat. I ended up with a widened root, and very sharp crest.

Question for you on kernel speed v. feedrate- how does kernel speed limit my feed/rapid rate? The current max velocity is set to 190, as around 220-230 I trip the drive for following error. I assume this is because the servo lacks enough torque to accelerate any faster and keep up with the pulse train.

Same thing happens when accel gets to around 6.

I have my drives tuned such that I get around 6-8 steps max following error, and they are set to trip at 12 steps.

Thanks again,

Kevin

Offline Hood

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Re: Spindle Index Pulse Problem
« Reply #34 on: April 09, 2012, 05:58:24 PM »
Kernel doesnt affect you at all as you are not using the parallel port.
Hood

Offline GytarrMann19

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Re: Spindle Index Pulse Problem
« Reply #35 on: April 09, 2012, 06:27:09 PM »
Does it matter at all then what I have it set to?

Offline Hood

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Re: Spindle Index Pulse Problem
« Reply #36 on: April 10, 2012, 01:41:45 AM »
No, kernel has no effect on the SS. The equivelent for the SS is set for each axis on the SS config page. Like the parallel ports kernel it is best to not use more than you require so set to the nearest above what each axis requires. in other words Vel times Steps per unit divided by 60 then divide by 1000 to get KHz.

Hood

Offline GytarrMann19

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Re: Spindle Index Pulse Problem
« Reply #37 on: November 24, 2012, 06:37:15 PM »
Still not having much luck threading. One thing I notice, is that my spindle speed fluctuates quite a bit. Up and down as much as 30-40rpm between passes sometimes. In a recent test, I got 3 good passes before things got out of whack...and I think it's because the spindle speed stayed about the same.

What results do others notice they are getting as far as spindle speed variation? I'm using a 3 phase induction motor driven by a VFD. No speed feedback to the VFD...just open loop.

Thoughts?

Kevin

Offline GytarrMann19

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Re: Spindle Index Pulse Problem
« Reply #38 on: November 24, 2012, 09:44:40 PM »
Did a little more testing. Checked the output from my digispeed DC-05 board...voltage stayed pretty constant during speed fluctuations, so I assume the problem is in my VFD.


Next, I notice that my speed fluctuates slightly less at higher RPMs. Which also means the % fluctuation is quite a bit less due to the higher RPM.  Tried doing some thread testing at 2500rpm and I get much cleaner results than at 500.

I assume the index pulse must be pretty clean if I get decent results at that high speed. Which kinda reinforces to me that my trouble is the speed fluctuation. How does Mach use the index pulse during threading cycles? What kind of speed variation can it compensate for? Does it compensate during the cut or only as it is starting each pass?

Keep in mind I'm only scribing a piece of stock with layout dye on it, not taking actual cuts.

The only problem with threading that fast is that my servos can't decelerate fast enough, so the lead shortens up towards the end of the cut. But I leave plenty of room for accel before staring into the work, and most of the thread comes out with a correct, even lead.

Help!

Kevin

Offline Hood

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Re: Spindle Index Pulse Problem
« Reply #39 on: November 25, 2012, 03:17:29 AM »
Dont know what to suggest, I am wondering why your spindle is slowing down if you are just scribing, it is either massively underpowered that even a scribe is too deep or there are other issues.
Is there any way you can just configure the VFD to run at a set Hz so that there is no speed control from Mach and then do a thread test?
Oh and with the SS it is not Mach that does the actual threading, it is the SS.
Hood