EDM Noise on the LinuxCNC Mailing Lists

traditional edm gap control as developed 
by the Lazerenko's is based of 
maintaining a gap voltage.

  * non-traditional ( in the niche of EDM ) can be ignition delay based,
    current exchange
  * or a hybrid ( see JSEM journals the japanese journals or ISEM for
    the rest of the EDM world

or google 'gap control' to see other forms of cnc not based on 'be at 
predicted position at specific time'

( linuxcnc is position based on time&velocity,
   edm is position based on THE chosen process feedback
   you can choose the process variable you like to use )

to truly use a process control scheme in linuxcnc requires a huge 
re-write imo
to hack some motion that follows the process variable is not difficult,
but (imop) lies in HAL not in the linuxcnc motion module.

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At the start of a cut the machine moves at
 a positioning rate, reading the gap voltage, and possibly current, 
watching for when the signal indicates that it is close enough to the 
workpiece to stop moving.  This is the same algorithm whether the 
electrode is 5cm away or 0.002mm away.  It's all about keeping the 
erosion conditions within the specified range, regardless of how far 
away the workpiece is.  While the voltage is > high limit, the 
keep moving.
    Agie used to (and likely still does) support a rapid 
traverse mode where it moved at X-speed without monitoring the gap 
conditions.  They called it motion with crash detection turned 
off.  The only time I ever moved with crash detection turned off 
was when retracting from a cavity.  Otherwise every move was in 
erosion mode, regardless of distance.  It always works if you're 
not already stuck in a cavity.
Pete 

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Speed control is normally a direct function of a current setpoint in the 
PWM controller. EDM power supplies look at every single pulse to 
determine if the current and voltage is within a specified range. The 
quality of the cut is partly a function of the speed and the energy in 
every pulse.
So the control of the speed is integrated with the power management or 
power supply controller.

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Hi:Pete Gruendeman, long time EDM operator here. 
 I have also written EDM software which went forward, 
 backward and escaped sideways off the program path.
EDM motion is based on either:
Positioning, at whatever rate you program though it's typically rather slow;
or Erosion, with no feedrate what so ever. 
Sparks radiate from the wire or carbon/copper electrode in all directions, 
eroding the workpiece where those sparks make contact.
Motion forward in the programmed path is all about maintaining erosion gap voltage, and possibly current, 
within limits which is controlled entirely by movement of the electrode.  
Don't call it a feedrate because It's not a feedrate.  
Read the gap voltage;
If/ gap voltage > 3/4 Voc, move ahead;
If/ gap voltage > 3/4 Voc && < 2/3 Voc, stay put;
If/ gap voltage < 2/3 Voc, backup;
repeat

All of this start, stop, backup motion will result in an average rate of travel that is on the order of inches per hour.  
It's not fast.  
Though the positioning rate for forward and reverse motion can be inches per minute.
What's really needed for useful EDM control is all of the above plus backing up on or 
escaping off the programmed path at timed intervals or based on erosion conditions.  
It's not that hard to write as I have already done it in four axis moves, linear interpolation only, no wire diameter compensation.  
Code or ideas sent upon request.
Pete Gruendeman

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adaptive feed slows down till the velocity does not overshoot the 
removal rate
that aint edm, thats following the slow truck in the slow lane

reversal aint what edm is about unless things are near fatally bad.

the normal motion in edm is a jiggly motion that on-the-average moves 
towards a destination.

its jiggly because the correct position varies constantly,
the correct position is forward and backward of the present position
on a micro or miliisecond basis.

path reversal is in seconds not fractions.

path reversal is what you do when things are WRONG
path reversal is not how you move to the correct position.

path reversal is like air bags in a car,
  so path reversal is not about good motion control
  its fire prevention,
  no its the fire department.
  it's way after the control system failed.