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Author Topic: lathe threading pitch error  (Read 11033 times)

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Offline RICH

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Re: lathe threading pitch error
« Reply #20 on: January 20, 2013, 07:36:29 PM »
I would suggest that you set the rpm manualy. There is a difference of rpm which Mach is seeing and what your vfd is actualy providing. They are slightly out, not much, and may be fighting each other for Mach's adjustment of the feedrate. Don't change the pitch, but rather change the feedrate.The feedrate is set based on the rpm. If you use the wizard  you can put pitch out to a lot of decimal places, the wizard will show the feedrate rounded to some number, but what you input will be used. Mach can read the rpm more accuratey than you can measure it.  

Do scribing tests as that is easier and more accurate than cutting a full thread thus eliminating mechanical / cutting aspects
of threading. Thus there is no spindle slow down, but at 5HP you shouldn't have any anyway.

Do not use any diagnostics, disable them, as Hood noted.

BTW, make sure you use 3 to 5x the pitch to allow for acceleration of the Z axis, even 10x.
RICH



« Last Edit: January 20, 2013, 07:41:44 PM by RICH »

Offline fast89

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Re: lathe threading pitch error
« Reply #21 on: January 21, 2013, 09:44:03 AM »
I have not put a tach on it yet to verify rpm. I changed the screen display to give two decimal places on the rpm. It is definitely moving around but not much. The vfd says 10.4hz. My calculations say i should have 299.866 rpm but i have 309-310. At 59.8 hz i should have 1726 but i have 1780. It is a consistent percentage off. At 310rpm the vfd does not move off of 10.4hz and the incoming 0-10v from mach 3 does not budge at all either. The feedrate display is showing correctly for 310rpm and .050 pitch. I added another decimal place to those too. I am leaning towards the rpm being off from what mach 3 is saying. Maybe the c3 index pulse board is off slightly. I might try another optical sensor that goes straight to the breakout board.

Offline Hood

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Re: lathe threading pitch error
« Reply #22 on: January 21, 2013, 09:55:34 AM »
The only two things I can see it being is reported RPM is different from true RPM, but how that could be I am not sure as a single pulse per rev should be just that.
The other thing is it could be the way the servo is handling the electronic gearing. I have seen a few drives with that problem, usually the lower end ones but Tecos are a step up from them even if not what you would say top end.
I am not sure how hard it is to change the gearing in your drives, its a simple matter in the Allen Bradleys, so if its easy enough (must be easier than getting another opto ;) ) I would certainly give it a go at 1:1 to see if your result stays the same or changes.

Also have you disabled the Turn Diagnostics?
Hood
Re: lathe threading pitch error
« Reply #23 on: January 21, 2013, 10:58:55 AM »
it doesn't matter if you command 300 and it actually rotates at 310, as long as mach is reading the pulses it will respond to the pulses not the commanded speed.

Offline fast89

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Re: lathe threading pitch error
« Reply #24 on: January 21, 2013, 12:19:01 PM »
I disabled turn diagnostics and rebooted. No change. I changed the electronic gearing in the servo drive, made it 1-1. No change. The thread looked off by the same amount it has been. It just ran painfully slower at 50,000 steps per inch.lol I am leaning towards the rpm being inaccurate. My Diagnosis is that mach 3 is being fed the wrong rpm information from the c3 board. I don't know how it is possible though.
Re: lathe threading pitch error
« Reply #25 on: January 21, 2013, 01:04:16 PM »
I'm using version R3.043.022.
Not the latest version, eh?  Seems like I read there were threading problems in some versions that were corrected...somewhat.:)
Milton from Tennessee ya'll.
Re: lathe threading pitch error
« Reply #26 on: January 21, 2013, 01:10:53 PM »
i use this version without any probs

Offline fast89

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Re: lathe threading pitch error
« Reply #27 on: January 21, 2013, 01:18:59 PM »
I just tried an optek opb917 optical switch wired directly to the breakout board. Rpm's read the same as with the old one running thru the c3 board. Grrrrrr.   What version are you guys using that threads good?
Re: lathe threading pitch error
« Reply #28 on: January 21, 2013, 01:21:46 PM »
I've had issues when bumping the kernel up.
Curious to see if it straightens out if you set your kernel back to 25k and re-tune ...  just for a test.
Maybe, maybe not. Just a shot.
Russ

Offline fast89

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Re: lathe threading pitch error
« Reply #29 on: January 21, 2013, 02:41:57 PM »
tried the kernel speed at 25k, it didn't make a difference. I have figured out that if i multiply the pitch by 1.018 i get a good thread that matches up to a tap. tried it with two different threads and it works. At least i have a workaround for now.