A non-traditional thermal machining discipline.
Electrical spark discharges melt tiny bits of conductive material in a controlled environment of a liquid bath.
*Sparks jump and sparks hurt.
**This simple statement sums up a lot about EDM. The spark requires a distance from tool to workpiece (this is the jump bit).
**The spark flows thru an incredibly small resistance of a bridge of microscopic particles gathered by the magnetic lines that precede current flow. The energy that flows across this bridge is really really hot and melts some material and creates a high pressure gas bubble that implodes when the pulse ends ( this is the 'hurts' bit).
**The melt creates a tiny pool of molten material on the work surface. The implosion scatters this pool into the surrounding environment. Some material cools back onto the workpiece.
**The bath resists the sparking by inhibiting this particle buildup, and forces the spark to search for a weak spot where it is easier to form. This weak spot is usually the closest point between the tool and the workpiece. In this way, the bath is a very sophisticated distributor and causes an envelope of sparks to occur over the entire tool surface.
**BTW: this bridge spark pop cycle happens up to500000 times a second ( 500Khz )
Other languages refer to the process as Eroding and Erosion.
Deutsche = funkerodieren
Italian = elletroerosione
Swedish ? = gnisting
Mandarin = dian wah (but you'd need the chinese characters for 'electric flower')
Czech = Elektroerozijska obdelava
Polish = obróbka elektroerozyjna
Français = électro-érosion
*The process can create a square corner, something hard to do on a lathe or mill. (corner radii as small as a few microns)
*It can cut diamond and carbide and other materials that are difficult to grind or mill. ( just has to be a conductor )
*It can also produce very small and deep holes (I think the record now is near 6um dia. )
*It never touches the workpiece ( shouldn't ) so it is useful on very delicate elements ( gimbals, force sensors, flexures )
*It is 'stochastic' ( it >>usually<< does the same thing when repeated ;-)
Developed in the late 1940's by a Russian husband and wife team, the Lazarenkovs.
The use of spark discharge was known previously, but the Lazarenkovs invented the 'EDM Servo'.
The term servo in EDM is like the R/C servo concept.
But in EDM the process developes the control voltage and the servo motor moves the tool to a
position that is very close to the workpiece
(perhaps .00025" to .040" depending on the discharge energy and the resistive nature of the bath).
**If you were to remove the covers from an EDM machine and watch the motor coupling during cutting, you'd see it slowly advancing, but very quickly oscillating a tiny amount. To put it simply, it is 'nervous'. This is the control system maintaining the 'gap' even tho the gap is changing in size, form and depth.
**In non-CNC EDM, an analog of the process directly feeds the motor amplifier. It is exactly like an R/C servo. Any numerical control is merely a limit, the feed while approaching the limit is purely process controlled ( no realtime needed ). As long as only one axis is involved, this results in a very 'smooth' EDM process, it may be worth implementing when possible.
**CNC SINK EDM can use 'exact stop' mode without danger of inter-line marks ( it's almost sitting still most of the time ;-)
Tomp will try some tests with STG hardware.|
This is for sink edm, including orbiting ( a technique to refine surface finish ).
I intend a simplification where reversal is not used, but 2 safe places are remembered.
These will be constant for a single task. They are the start position, and the deeper
position where orbiting would begin. ( my term for this is the 'roughing point',
from this point all planetary excursions begin and end )
You gotta understand sink edm and i havent explained those concepts yet... but,
it simply means that 'remebering the path' is un-needed, and 'backing up' is simply
moving towards one of the 2 points/poses. Forward position increments are calculated
just like the paper describes.
The 'next place to goto' is determined by the 'trajectory planner' in EMC. These are
tiny moves that add up to a whole G01 or G02 or G03. These moves are actually the
G01 G02 G03 broken down into tiny bits that may get accomplished in the next servo cycle.
Well, if we dont use EMC's tp, then we dont get these pieces. If we dont get these pieces,
then we cant stream them to hal. We can get these pieces from Vapt, asking it to generate
all motion into short lines. These destinations, even up to 9 axis, can be hal-streamed
into a gap-width control under Hal. If there's some need, the tool can 'run-away'to one
of the 2 safe places. If the conditions get better, the tool can return to the last
hal-stream position accomplished by by the gap-controller, and proceed towards the end of
of the 'program'.