Well, I finally motivated myself this holiday weekend
to get my PCB EDM lash-up to cut copper, and, YES, it
does cut clean lines thru the copper without board
burning or marking. Shine a torch through the tracks
from the rear of the board and there is a faithful
line of light, (no firstborn goats or pentagrams
neccessary).
With my highest power lens and head magnifier I could
see no trace of copper whiskers on the EDM'd track.
Interestingly, the copper is blasted away entirely
below the cross section of the wire, there is no
'erosion bleeding' into the surrounding copper outside
of the wire dia.
Have only cut straight lines so far with 26 swg and
with a 0.2 mm step increment, (need to modify the
stepper gear ratio), and with multiple (continuous)
sparking between increments, (no on/off sparks or
raster yet).
One significant problem has emerged which Curt
Richards did not address, (or if he did, he did not
mention it). The wire electrode erosion is
significant, and will require an active Z axis
correction to maintain as consistent a spark gap
distance as possible between the electrode and pcb
surface. To do this I will insert a routine to 'touch
down' the electrode on the copper to zero the gap,(no
spark, closure sensed by two PIC output/inputs for
voltage present), and then retreat the electrode to a
standard clearance before any track eroding is
commenced, (Andrew, this is why I need the BMP byte by
byte transfer). This will also tend to compensate for
any board distortion.
Used tapwater as a dielectric with no problems, (the
modified Garden of EDM circuit controls the
electrolysis with no problems).
Curt controlled his spark discharge voltage by
dynamically adjusting the electrode gap on the fly. I
think that for pcb work this is not the correct
approach because of the wire erosion problem, since
one needs to KNOW the gap distance AND available
discharge energy BEFORE discharge, which is why I am
going for the touch down zeroing method, (slower but
speed is not significant at the moment). The discharge
energy will be set by presetting the voltage level
required and capacitor selection, (simple comparator
control integrated into the spark control). I think
this is the secret of successful pcb edm'ing.
This is really looking as if direct raw copper board
to finished pcb is going to be possible.
Roger
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