EDM pulsae generation _is_ simple. thats true.
Some of the support circuits are not (like ionization detector)
But NB: I said 'pulse' , as in ONE pulse.
The EDM process has no use for repetitive oscilator function
( PWM is right out) .
I you use a repeating time sequence,
you will not cut as fast as you can,
you will not get a uniform crater ddpeth,
you will get longer arcs after they begin
you will get more arcs
and you will have to accomodate
by waiting langer than needed before rnext pulse.
Yes, fiexed on off will cut,
jdut slower and with more nad discharges.
HOW TO MAKE ONE SPARKK:
The pulse generation sequence is:
1) Detect when gap is ready for a pulse
Tickler or Sensing Voltage:
Apply a very weak yet high voltage difference across the gap.
(say 100V with a 250mA supply)
The potenttial will stay at 100V until...
the tool gets close enough to the workpiece,
so close that the dielectic breaks down and
a tiny weak spark occurs.
When the gap is small ecough,
the gap voltage willnose dive
markimg when the working pulse should begin.
marking when the gap gfot IONIZED
2) The genertor then makes ONE pulse:
NOTE the sprak is triggered by the porcess not a timer.
The current is connected
and ontime is decremented till 0
3) During OnTime current should be monitored.
A scope will show the current rise to avalue and stay there till OffTime negins.
The scope will ahow a very high frequency ringiing on the 'flat'
If that ringing stops mid OnTime, you got and arc!
( we wre able to shut off a bad pulse within 4us, too slow!)
Such a safety circuit is not discussed here,
other than to say itrs a good idea.
Turninf off the pulse is ghood,
but you already arced! (ask Gemini how to predict arcs)
4) Turn off the current:
Turn it off as soonas you can.
A sloping trailing edge is bad.
square is good and hard to achieve
Remove all inductance thgat uou can,
Use ultrafast diode across gap.
Sharp turn off gets better removal,
( the implosion is more explosive)
a slow turn off gets more re-cast
(material coolimg back onto wkpc)
5) Take a break: Off Time
Then a minimum delay time is waited out.
( The mfctrs say this value is usually enough
time to let the gap recuperate. They say
thius is a minumum, expect longer if you are not
in a testing environment)
6) Recuperation:
GAP restoration:
The gap _was- clean, then got chrged and dirtied.
Q: whats all thata black smoke in the oil?
A: The oil is a hydrocarbon
that gets 'cracked' into hydrogen gas ( flammabvle)
and carbon ( from oil, not from tool)
When will the gap be ready again?
The weak tickler voltage will rise to Uo
( say 100V) as soon as the gap is clear of debris.
Waiting for a clear gap
makes the same environment for the next spark.
Your circuit _could_ refire as soon as the gao clesars
or ( sort of debouncing) you could recheck a few times first.
Dont refire when Uo is not high.
That would avoid the necessary dielectic
resistance and would cause pitting or arcing.
repeat
It's not on of on of on off
It isd Wait on of Wait on off Wait on rff
HTH TomP
