Homebrew PCBs

Hi Stephen,

I'll answer your questions from the Electronics 101 thread:

ST wrote:

>Yes, i have read rotating the electrode is good for edm.
>I don't fully understand electrode erosion of different materials
>tho.
>Rigging a pump would not be a problem, i'm quite used to that sort of
>thing with a watercooled PC by now..


I used a combo dielectric duct and return ring around the electrode on
a EDM machine based on an HP plotter.  The idea was to use gravity
feed of distilled water ( the dielectric ) through a copper pipe
around the elctrode which also provided the pulse return path.  The
system had a spring to keep the duct in contact with the PCB and
attached plastic rod to provide capacitive feedback for depth.  My
hope was to speed up cutting the board by leaving a groundplane
however the drag of the duct slowed things down too much.



ST wrote:

>I would like to PC control it like a normal cnc drill.

You could do that with the A3967SLb s on a driver board and control
software on a PC.  Maybe EMC?

>How do you scan one line, at fixed speed or is it variable with
>current?

The gantry moves along the x axis in 0.001" increments.  The holding
torque of the steppers and the "stiction" of the acme threads holds
the gantry in place while the EDM head moves across the PCB and the
pulser removes 1 mil "dots" where there are no traces.  At the end of
the traverse the gantry advances another 0.001" and EDM head then
"etches" another line.  As I develop the system I'll probably follow
your suggestion and match the traverse speed to the pulse rate, but
for now -- Keep It Simple Stupid is the rule.

>You said you regulate height to have the right current? What happens
>if
>the electrode is "done" and is not moved, will it enlarge the cut or
>just stop because the gap is too large?


The gap ( height ) is regulated by measuring the gap breakdown voltage
as outlined in the power supply reference.  The EDM head is moved by a
stepper and the software puts out a pulse if copper is to be removed.
  At the end of the traverse the gantry is moved another 0.001" and
the  EDM head returns while cutting the next line. 


>I understand the going line be line, a fixed width thing. But i don't
>yet understand fully how you move along the line, or depth, and how
>you "lift" the cutting over the traces. Plase clarify again.

You don't lift the EDM head, you turn on the pulser and "burn" ( or
"etch" ) the away the copper you don't want.  The height or z axis
position of the electrode is controlled by the gap voltage.


>Also, i think making ground planes would ensure faster cutting of the
>overall board?

See my comments about the HP plotter.  Since you can raster scan any
image and output lines in the resolution of your choice you can move
in increments of that resolution along the x axis without having to
worry about the pulse return path.  


ST wrote:

>I would really like making boards that way, i think.
>what do you think the resolution would be?

If I could grind a fine enough point on the electrode my stepper and
leadscrew combo could "etch" 0.0001 inch dots ;D

Doing double sided boards with SMT is a pleasure since the number of
holes to be drilled is minimized.  You avoid all the mess and hassle
of TT.  No board scubbing.  No etching ( or mist or titrating ). No
paper residue.  Fine line widths and best of all minimal hole
drilling!!!

Curt