Homebrew PCBs

Andrew wrote:

>>AlanK wrote:
>>
>>  Also start thinking now about two
>>electrodes.  Could cut both sides with
>>a 2 tooth tip, but independent control
>>would be better.
>>    
>>
>
>I don't understand why independant control
>would be advantagous.  The EDM was going
>to be done raster not vector.
>
>  
>
  I didn't really read every word of the rest so may have missed 
something about raster, but raster would take a tremendous amount of 
time vs vector, and the single wire CNC setup doesn't really have a 
reason to go over areas that aren't etched, if it's following the cut 
line and in different directions it's vector regardless of being an XYZ 
table and keeping most of the paths X or Y..  Raster's more a fixed 
format scan of everywhere, not really needed for this type of thing 
normally.  Little reason to travel uncut areas with a CNC.

  And you almost always have to cut the other side of the trace, whether 
it's an isolated trace or simply the cut between this trace and the 
next.  Wasn't clear, that was independent control of the feed rate of 
both wires not different XY control, fixed or simple distance 
relationship.  You have to control seperately anyway for different 
erosion rates, simpler to just back one or the other wire off where you 
don't want to cut than control the supply too..

Really this would be simple, and do 90% of things well with an easy 
enough system:

   o

   x      o


  All three the same size, x is just the main electrode.  3 seperate 
feeds, two power supplies, the second attached to both of the secondary 
wires.  Main one cuts and does the tracing, upper cuts the other side of 
horizontal traces, right one cuts the other side of vertical traces.  
Keep most of the cuts horizontal and vertical and this will let you 
isolate both sides of a trace at once without requiring rotation, just a 
second supply and seperate feed mechanisms for the three wires.  Just 
back one of the two secondaries off or both for curves or diagonals with 
only the main wire..  Generally double the linear cutting rate of a 
single wire, with only a second power supply and 3 feeds so still not 
too complicated.  Don't care about the width change and you could do 45 
deg diagonals too, they'd just be more narrow with a fixed system.
  Just two supplies and 3 feeds on a CNC, may just have to try it out 
myself.  If it can be taken down to cutting pads and traces for SOIC 
pitch devices it'd be great, that'd do enough for most prototyping.


  The real work will be automatically controlling the feed rate for 
erosion.  With just drilling a hole, you simply look at the spark 
frequency and when it gets too low you feed, it's in the direction of 
feed.  With this you have needing XY direction feed and erosion away 
from the board that both reduce the spark rate.  No doubt figureable but 
not as easy..

>However - I agree that extra wires would
>be cool.
>
>Make it like a 24 pin dot matrix printer
>(pins offset so they dont inter-pin-spark)
>
>  
>
   Would be nice for other things even.  But with differing discharge 
rates or on or off control that will take either or both of 24 feeds and 
supplies, depending on how you want to do it.  Could probably hack a few 
motors to do it all, like a vending machine selector, but getting it all 
to work would probably still be not so easy..


Alan