Re: [Pcb2gcode-devel] PCB Board Expectations

[Ford P. <fo...@hi...>]

I think you found my problem(s).  Thanks to Pat for taking the time to
consider my situation.  

I was leaving the copper foil on the board and the Gerber output was
defining it.  So by leaving it on the board, suddenly pcb2gcode needed to
deal with those edges as well and started tweaking the insulation path on a
second pass.  By eliminating the copper fill, only the meaningful traces
appear in the Gerber, and consequently the GCode for LinuxCNC.  One pass.

Also, the offset attribute is defined as being 1/2 the bit size.  In a
perfect world, this is true.  My world is far from perfect.  My dremel tool
has some run out in the z-axis.  I have a new spindle on order but won't be
here for another week or so.  I disassembled the dremel and discovered they
took care in holding the shear motion (side pressure) on the cutter through
the use of ball bearings at each end of the spindle/motor shaft.  Using a
V-bit, the tool wants to climb on top of the copper, forcing me to set the
depth deeper to put down pressure on the bit.  I'm guessing this surplus
board stock has copper of roughly 0.003" or so.  I need to set it at a depth
>0.008" and a 0.010" is most reliable.  In a reliable cut, this 0.004" bit
seems to cut a 0.012"-0.015" insulation width.  So by setting the offset
attribute to 1/2 the average width (e.g. ~ 0.0075") I am able to get traces
down to 0.018" or so.  That's a fairly small trace.  I'm not sure how much
glue is left to hold the trace after soldering.

Obviously, if I want decent output, it will have to start with a decent
spindle.  I'm working on that.

Chemically tinning the circuit board is important for long term reliability
(the copper will rust).  Have others experimented with tinning?  Does it
improve routing quality or make it more complex?

Ford


-----Original Message-----
From: Patrick Birnzain [mailto:pbi...@us...] 
Sent: Monday, December 10, 2012 6:45 AM
To: pcb...@li...
Subject: Re: [Pcb2gcode-devel] PCB Board Expectations

On Sun, 9 Dec 2012 21:05:11 -0600
"Ford Peterson" <fo...@hi...> wrote:

> Good evening,
> 
>  
> 
> I'm new to this forum.  I'm trying to diagnose some problems with this
GCode
> conversion tool.
> 
>  
> 
> Using KiCAD for circuit board design and layout, I'm able to extract
Gerber
> files.  Pcb2gcode converts them for running LinuxCNC under Ubuntu 10.04
LTS,
> which is the most recent stable version for running LinuxCNC.  My router
> table seems to be able to reliably hold a repeatable trace.  I have
several
> concerns and am hoping this list can provide some reliable answers.
> 
>  
> 
> While the spindle on my CNC is light duty (a ball bearing dremel tool
> running a 0.1mm 60 degree V bit).  I have a new spindle on order and may
not
> see it for another week or so.  Adjusting the Z axis to obtain a cut
> sufficient to remove copper, the narrowest insulation I can obtain with
this
> 0.00397" V bit is about 0.030" using a micrometer.  Thinking this through,
> am I observing the run-out in the bearing at being roughly 0.026" ?
> Inspecting the spindle I simply cannot believe there is that much play.  I
> would certainly expect to visibly see 0.026" of play and I'm unable to
> observe ANY play in the spindle at rest.  While I was expecting some play,
I
> was not expecting this much.  Are my expectations unreasonable?

I'm using a small proxxon rotary tool as well (quite successfully) and
a cheap, single-toothed bit. I noticed the isolation diameter changes
somewhat if the bit is turned in the collet, sometimes requiring
adjustment for boards with ICs in smaller packages (MLF).
Maybe the collet isn't that accurate, or the center axis of the ball
bearing is somewhat misaligned to the axis of the motor? Taking into
account how ball bearings work, the bit might even end up at a very
slight angle. I can't really tell as I'm not a mechanical engineer and
as long as it works reliably there's litte incentive to investigate.

>  
> 
> Using a test circuit called TraceTest I've installed nine SM1206 resistor
> footprints and connected them in series using trace widths ranging from
> 0.018" to 0.042".   I convert the Gerber to GCode with the following
command
> line:
> 
>  
> 
> pcb2gcode --front TraceTest-Front.gbr --zsafe 0.08  --zchange 1
--mill-feed
> 6 --zwork -0.08 --offset 0.002 --mill-speed 20000
> 
>  
> 
> Even though the Gerber files were produced by specifying a 0.004" bit, a
> 0.004" insulating band, pcb2gcode insists on retracing the board a second
> time.  After a first pass, a 0.042" wide trace remains but only about
0.010"
> wide.  The second pass obliterates the traces and the pads.  I either do
not
> understand the offset attribute or my spindle is simply a POS.
> 
>  
> 
> I do not see an attribute for bit size.  Only the -offset, which I've set
at
> 0.002" (half the bit size)  It seems the second pass enlarges the
insulation
> by taking part of the trace. I experimented a little with a negative
offset
> and saw no real difference.

Do you have the remaining copper in the gerber file? The pcb2gcode
algorithm expects you to have only the actual traces in the file. It
takes your parameters (offset, extra-passes etc.) and tries to mill the
board as close to your specifications as possible in a best-effort
approach. If the remaining copper is in there as well, it will be
treated just like everything else.

I hope this helps, if not I can check your test file if you want.

-- Patrick

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