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Author Topic: Continually incorrect thread pitch  (Read 5928 times)

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Re: Continually incorrect thread pitch
« Reply #10 on: August 12, 2009, 09:46:03 AM »
Well I'm running 1250 steppers on a 12X36 lathe. I just input 86 RPM in the wizard and am now getting 20 threads per inch. I don't understand why I have to enter any rpm in the wizard or need it if the threading movement is synch'd with the spindle index pulse.

Offline RICH

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Re: Continually incorrect thread pitch
« Reply #11 on: August 12, 2009, 10:37:29 AM »
Ok, backlash should not be a culprit.

Can you confirm that the steps / unit are correct.?

Do the lathe test i mentioned.

I do recall threading where my actual spindle rpm was higher than what i told the wizard  ( say 101 / 102 instead of
100 ) and the pitch was still ok. Also i will note that , the program will modify the the feed rate if the rpm is lower
but not higher on the next pass based on what it interpreted in the prior pass.

I assume you have one of those lathes that have the rpm readout and you know it's off by  20%.
I assume that the a slotted index is being used for the basis of determing the DRO readout.
Now if you are using something downstream from the Chinese readout / electronics that is incorrect to begin with
then that's a different story as the spindle to feed per revolution just is not right from the start.

Just somne thoughts,
RICH

Offline RICH

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Re: Continually incorrect thread pitch
« Reply #12 on: August 12, 2009, 10:55:36 AM »
The wizard only checks to see that the acceleration and max velocity are within your configuration settings for your motors. And warns you if not. it doesn't change the feed rate. Look at the code ( better yet change it over so that it's generating a G32 and you will see what i am talking about.

So you slowed the speed down, the contoller then was able to compensate the feed rate based on a slower spindle speed. SO the feed per revolution was correct for the feed rate.

You need to fix what / where you are getting the index pulse from so the rpm is correct to Mach.

It is not Gcode problem, it is not a Mach / threading problem, it is bad rpm input to the program.

RICH
 
Re: Continually incorrect thread pitch
« Reply #13 on: August 12, 2009, 11:21:02 AM »
I did not slow it down. I tried 70 RPM (geared on the lathe) yesterday and it did not work. I got the plugin today which told me I was actually running 86 RPM when the gears told me I was running 70. When I plugged in 86 rpm in the wizard the only difference was the insertion of S86 into the code. The point I am trying to make is somehow the Mach3Turn engine is synchronizing the spindle index to the threading movement of the lathe, right? If this is so, then the only thing the wizard is doing is commanding the spindle speed. The Mach3Turn engine (software) is taking the gcode for threading and adjusting it for the actual spindle index, right?

So why does putting in 86 into the wizard all of a sudden change my lathe movement to 20 threads per inch? Since my spindle is not really controlled by the gcode S86, I assume the Mach3Turn engine uses this to adjust the threading.

So one question is, does Mach3Turn use the spindle index to control the movement of the lathe during threading and does it take into account any S commands?
« Last Edit: August 12, 2009, 11:40:50 AM by kdoney »

vmax549

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Re: Continually incorrect thread pitch
« Reply #14 on: August 12, 2009, 12:10:45 PM »
When you run you rcode are you in G94 mode or G95 mode?  IF the calculations were done for G94 mode then the feed will be based on the programed Sspeed not the indicated spindle speed.

Same with G95 mode IF you do not check" sync spindle" on It has to go by the programed NOT the indicated.

NOW IF your programed and you actual speed is  off SO will your thread pitch.

Just a thought (;-) TP

Offline RICH

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Re: Continually incorrect thread pitch
« Reply #15 on: August 12, 2009, 12:26:24 PM »
"So one question is, does Mach3Turn use the spindle index to control the movement of the lathe during threading?"
YES

Yes, it will adjust the Z axis movement, within some range, and try to correct for variations in the spindle speed.
So if the spindle were to slow down during the threading pass as compared to a feed rate it will try and fix it in the next pass. During the threading it will try and slow the axis, but if, out of range too far  then the thread is
beyond repair practicaly speaking.

It won't' correct for higher speeds, since that should not be possible if i remember correctly... during the threading.
Now the "trigger" so to speak , going  from G94 to G95 is based on seeing a stable rpm.  If "green light is given", the axis will ramp in speed and the controlled point should hit the material in sink and based on the gcode.

What is not clear in my memory, is, the basis / control of what exactly happens during the ramping ( lot going on!).

My lathe is simple, no BOB's, no spindle speed control so there are no complexities to deal with. Just a dumb index
saying to Mach,  this is the spindle rpm and here is a spot / trigger you can use to start threading. The true spindle rpm ( as measured measured with a independant indicator ) is close to the  DRO display over the whole rpm range.

RICH

« Last Edit: August 12, 2009, 12:39:38 PM by RICH »

Offline Hood

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Re: Continually incorrect thread pitch
« Reply #16 on: August 12, 2009, 01:40:24 PM »
Is it a licenced or demo version of Mach?
Hood
Re: Continually incorrect thread pitch
« Reply #17 on: October 01, 2009, 07:24:30 PM »
Kdoney,

If the lathe is set (geared) to 70 rpm, but it runs at 86, maybe you are running a 50Hz lathe motor at a 60 Hz frequency ...
« Last Edit: October 01, 2009, 07:48:07 PM by guiacnc »