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Author Topic: PoKeys57CNC and Single Index Pulse Threading  (Read 4389 times)

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Offline Osker

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PoKeys57CNC and Single Index Pulse Threading
« on: July 02, 2018, 06:14:26 PM »
Has anyone successfully configured MACH4 and a PoKeys57CNC to do threading with a single index pulse per revolution?  I have found videos demonstrating threading with a PoKeys57CNC and Mach4.  However, an encoder is being used versus a single index pulse. 

The PoKeys57CNC manual indicates that it will do threading with either an index pulse or an encoder.  My index pulse generator is fairly typical.  A slotted IR sensor, an encoder disk with a single hole, and a microprocess for doing a little signal conditioning.  I have enabled threading; checked the box for using ultra-fast encoders, and checked the box for lathe(index).  The RPM DRO in Mach4 displays the correct RPM, so I think it reasonable to conclude that index pulse is being seen.  I have looked at the Index Pulse signal with an oscilloscope and it is a reasonably clean asymmetrical square wave.  The signal high is +4V, the low is essentially zero.  At 300 RPM the high pulse is about 8ms.     

I copied the threading example code from the Mach4 Lathe manual.  When the code is loaded the displayed tool path appears correct and there are no error messages.  After clicking Cycle Start, the X & Z axis move to their respective start positions.  When the G76 code line is processed, nothing happens.  It is as if either Mach4 or the PoKeys57CNC is waiting to synchronize on the index pulse.

I am running the most current versions of Mach4 and the PoKeys plugins.  Other than the threading issue, the lathe works as one would expect.  Any suggestions for what to try will be appreciated.

Dan
Re: PoKeys57CNC and Single Index Pulse Threading
« Reply #1 on: August 07, 2018, 03:00:23 AM »
Following, although from the looks of it, I don't suspect an answer is coming.
- Robert Beaubien
- Drone Plastics
-
“Dear Algebra, Please stop asking us to find your X.  She's never coming back and don't ask Y.”
Re: PoKeys57CNC and Single Index Pulse Threading
« Reply #2 on: August 07, 2018, 11:23:16 AM »
Hi,
threading requires realtime control to synchronize the thread and is consequently a controller function. Mach can't do it, it can provide
the Gcode in the interpreter and even provide the P(osition)V(elocity)T(over time) data for the controller but the controller has to do the
synchronization.

May I suggest placing this question with PoKeys, either directly by email or in the PoKeys section of the  forum. The guy from PoKeys could
answer this.

Craig
'I enjoy sex at 73.....I live at 71 so its not too far to walk.'

Offline Osker

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Re: PoKeys57CNC and Single Index Pulse Threading
« Reply #3 on: August 07, 2018, 01:30:36 PM »
Craig,

I should have posted an update for others who may be having a similar problem.  The problem was that Mach4 requires the spindle speed to be specified using G97 S__ M3.  Where S specifies the spindle RPM.  That was not the case in Mach3 and a G97 command is not used in the sample code provided in the Mach4 documentation.  I had copied the Mach4 code, expecting it to work as written.  Did not happen.  I found the problem when the PoKeys support tech sent me the code they use to test their threading algorithm.  Once I added the G97 command to my code, the lathe synchronized as expected.  Just another "Gotcha".

Regarding spindle speeds when threading, I have a Sherline lathe and drive the spindle using an high-torque NEMA 34 stepper motor.  The stepper is controlled by a PIC microcontroller, which handles among other things producing the step and direction signals . The PIC has a numerically controlled oscillator peripheral that is used to generate the step frequencies.  As one would expect from using a stepper, spindle speed is constant and is unaffected by tool loading.  Well, almost unaffected.  The stepper can be stalled at normal turning speeds.

The following link is to a small video demonstrating turning stainless steel with the stepper conversion.  https://www.youtube.com/watch?v=jWd7hNQuNxc

The following link is to a small video demonstrating threading with the stepper conversion.    https://www.youtube.com/watch?v=i20c5PwtPBM&t=20s


Dan
Re: PoKeys57CNC and Single Index Pulse Threading
« Reply #4 on: August 07, 2018, 07:12:12 PM »
Interesting videos for me.  I have a Nema 34 stepper driven lathe that I have not gotten operating yet but soon will.  What size is holding torque is your stepper and what voltage power supply is driving it?  I have a 1200 in/oz stepper with a 68vdc power supply using Leadshine digital step drive.  I only want to do occasional turning and it looks like yours works quite well.

Offline Osker

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Re: PoKeys57CNC and Single Index Pulse Threading
« Reply #5 on: August 08, 2018, 12:06:58 PM »
Gary,

I am driving the spindle with an 1805 oz/in stepper motor using an MA860H stepper drive.  The MA860H is rated for 18-80V AC or 24-110V DC.  DC power supplies in the 100+V range get pricey, so I am using an AC step up/down converter.  Not an advertisement but I am using a Rock Stone Power 1000 Watt step up/down converter, available from Amazon for around $60.  On the back of the unit one can select from various AC input levels.  I select 200V, which is the lowest step down voltage input, and plug the unit into an 110V AC outlet.  My AC is normally around 120V.  120/200 yields a stepdown factor of 0.6.  0.6 times 120V yields an output of 72V.  Still within the acceptable AC range of the MA860H.  For clarity, the MA860H plugs into the 110V output, not the 220V.   I have been using this setup for two or three years and have had no problems.   It is my understanding that stepper drivers which run directly off of 110V AC have become affordable.  Eliminating the cost of step up/down could make them even more attractive.

Dan    

 
Re: PoKeys57CNC and Single Index Pulse Threading
« Reply #6 on: August 08, 2018, 01:01:17 PM »
Dan,
Thanks for the info. I am a little disappointed your stepper and drives are already a lot bigger than mine. But CNC on small machines is more about not standing there for hours turning a hand crank, and instead working on other stuff while it chips away on its own.

Offline Osker

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Re: PoKeys57CNC and Single Index Pulse Threading
« Reply #7 on: August 08, 2018, 07:24:45 PM »
Gary,

Don't assume that you will have a problem using the equipment you have.  For threading I used a 420 oz/in stepper without problems.  The key is to balance spindle RPM and federate so that tool load is within the steppers useable torque curve.  It takes a little trial and error to find that sweet spot, but once there you should be fine.  As you eluted to, production time is not critical for most of us doing hobby CNC work.  A larger stepper should allow higher feed rates, and higher RPM, but that is not the issue for many of us.  

Threading was the driver for me.  With the Sherline motor spindle RPM would change as tool loading would change. That resulted in inconsistent threads.  My initial solution was to use a 420 oz/in stepper for threading and the Sherline motor the rest of the time.  420 oz/in was chosen simply because I had it.  Eventually, I tired of swapping back and forth and decided to use a stepper full time.  I went with the 1805 oz/in stepper to get a broader torque curve, not simply more torque.  


Dan    
Re: PoKeys57CNC and Single Index Pulse Threading
« Reply #8 on: August 08, 2018, 10:23:46 PM »
Dan,
Thanks for the clarification. I have a pretty sweet setup with a 3 axis bed type design where the table is a 205 lb cast iron surface plate.  All THK 20mm ballscrews and heavy preload high speed rails for an 18” cube work envelope.  I have a Harbor Freight 7x12 lathe stripped of everything but the headstock, chuck, and tailstock. The tool post is an 0xo Tormach wedge style mounted alongside the mill spindle. Lathe spindle drive is 30mm wide 8mm HTD belt to provide stiffness for 4th axis work and the stepper drives with a 4 to 1 reduction ratio for 4th axis work.
I also have a Do-More BRX PLC to stepper control the lathe spindle and use Mach 3 for 4 axis work.
Re: PoKeys57CNC and Single Index Pulse Threading
« Reply #9 on: August 10, 2018, 12:30:46 PM »
Since you have the stepper already on the spindle you might want to try using it as a rotary indexer(C Axis), and create a screen to do 'threading' that way.

12 tpi triplle start sqaure thread example.
g1 w-0.5000 h6703.4483 f42000.000