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First, I looked at the interpolated and raw signals in HalScope. As expected, one had jaggies and one didn't. In my INI file, I changed:
net spindle-position encoder.0.position => spindle.0.revs
To:
net spindle-position encoder.0.position-interpolated => spindle.0.revs

I ran the threading program at 200 RPM:
 

Jaggies gone, plus initial slope change before threading now a straight line. However, there's a break in the velocity midway on the line that was not there before. Still have the odd saw-tooth velocity while threading. (Note: I was running the program without a tool, so no actual threading took place)

I thought this might have fixed the problem. So I made another 8TPI thread at 100 and 200 RPM. Alas, they did not overlay. Same problem. In case you're wondering, I could not find another piece of 1" stock, so I used a 1/2" rod.
 

PS. Looks like I need to resharpen the cutter! Easy to see in the closeup.

 

HelloI have a plasma cutter runing on mesa 7i76e / thcad10 / linuxcnc 2.8.My Problem:As you can see on the attached picture (left cut) it starts really good but after about 100mm the THC starts to move up and down the Z Axis. The move distance of the Z Axis gets more and more up to 10mm at the end. Is there an another setting as in the Config Penel Arc - Height Per Volt? (currently set to 0.05mm)So I really don't know how to fix this and which settings could be responsible for this behaveHope that you know the reason for that and I would really appreciate a hint how to fix that.Thanks in advance for you replay.The settings:Config Panel THCDelay:1Threshold(V):1PID P GAIN(Speed):10VAD Threshold(%):90Void Sense Override(%):100PID I Gain: 0PID D Gain:0Run Panel THCAutoUse Auto Volts: (ticked)Velovity Anti Sive Enable (VAD): (ticked)Void Sense Enable: (ticked) 

The freq pin is 0 and the valid pin is false.  So it appears that there is an issue with the PWM measuring. 

PCW,

Okay, lets hope nothing is fried.  I will go try the outputs of PWM freq and valid now and report back.

I got a chance to spend some time with the oscilloscope today am getting interesting behavior out of the EDM supply.  What I am seeing is a sine wave that has an amplitude of about .5v. This entire sine wave moves up and down depending on the intensity of the EDM cut.  At no cut it is at 0 volts and at short it is at 3.3v.  In other words it seems to be behaving like an analog signal. It seems to me that will not work with the periodm func in mesa.  I have contacted the creator of the Powercore EDM to see if this is the correct behavior or if something was damaged. 

My alternative plan is to use an arduino that I have here to take the analog input and send it to the computer via USB to monitor cut duty cycle. Hopefully the python script that I am writing can accommodate communication with the arduino. 

 

EDM is very noisy and may have voltage spikes on the "ground" 
that exceed the +-25V peak limitations of the 7I96S encoder
inputs.

Since it appears the you did not connect the grounds together there is
opportunity for these spikes to damage the 7I96 inputs. It may well be OK
but the way is was connected could not work.

I think you misunderstood the PWM output. It does no appear to be
a differential signal, so must connect to PWM --> IDX+ and GND --> GND

The frequency pin should show the current PWM frequency
The valid pin is true when the PWM frequency is above the
minimum frequency (basically a fault indication)
 

PCW,

I did that, why would it damage anything? Could a high voltage be carried through the ground wire to pin 14? This is the only thing that would make sense to me since the + wire is supposed to only be 3.3v..

Can you please explain what you mean by "Is the frequency reasonable and the valid bit set?"

Best,
-Dylan

Can you post a plot that contains the encoder velocity to determine
if LinuxCNC or the hardware is to blame?

The odd thing is that the 100 RPM plot looks to be as expected
but the 200 RPM plot is obviously broken. (the Z velocity never
reaches a steady state even though it's  obviously possible with
the acceleration constraints)

Also please use the interpolated position, the non-interpolated
position combined with the low resolution encoder makes
everything worse with it's large position steps. This is the intended
use case for interpolated position.
 

This problem, and this forum thread, all started when I was getting gouging when making a 1"-8 thread. After analysis, I believe that the lathe was bogging down slightly when making later passes in the increasingly deep thread. I've confirmed that the position of the thread changes with speed. The slight decrease in speed made the cutter take a larger bite, which caused further bogging down and then an even larger bite. Eventually, the lathe reached stability and was able to speed up and quit gouging.

Today, I tried making the thread again. I kept thinking that the Monarch lathe motor is large (7HP), and should be able to power through the threading cuts. My lathe has a back gear (speed reduced, torque increased), which I have never used. I put the lathe in the back gear and tried the thread cut. The gouging stopped completely. So that problem is solved to my satisfaction.

The contributing problem, thread position varies with spindle speed, seems to be built in to the LinuxCNC code. Spindle speed should not be changed during the threading operation. I'm not convinced that replacing the Z stepper with a more powerful motor, which would allow me more acceleration, would change this behavior very much. I'm also not convinced that having more pulses per spindle revolution, either through making a new gear with more teeth or using software to synthesize more pulses, would make much difference.

Please let me know if my logic is flawed.



 

Latest Version of program. I found that the entry and exit taper part of the code didn't work. I revised and tested it, so now it's working.