Re: [CAD_CAM_EDM_DRO] Help: PCB Trace Milling Software

Jon,

Thanks for the run down. As I sort of expected, the information is not
in a readily useable format. That's why one of my ideas was to start
from a HPGL file. I think the PCB plot file I was looking at had the
pen lines at some finite granularity, i.e., something like being on .3
mm spacing (grid), related to the pen width. SO, if it routinely took
some small number of pen strokes to make up a "trace", then these could
be "assembled", and their perimeter could be described as a simple
contour (for SIMPLE PCB's, the type one might layout with express PCB).
Pads and other shapes could similarly be "built up" to yield a contour.
Once traces/pads have been reduced to simple contours, Vector CAD/CAM
could generate tool paths for me. And, given that the spacing is always
adequate, as you brought up, it could work.

http://www.expresspcb.com/

OK, this approach would have very limited use, but might be useful for
simple small stuff I'd use for express PCB anyway.

As far as I know, Vector CAD/CAM can input .DXF and PLC files (PLC
stands for Point-Line-Circle, a formatted text file with entity
definitions). So one would need a program (and an algorithm!) to reduce
the traces, and create a .DXF or PLC output file.

Thanks for your input.


Alan


Jon Elson wrote:

>
> Alan Marconett KM6VV wrote:
>
> > Hi Mike and the list,
> >
> > Yes, I was just asking about cutting PCB's with CNC. Seems that if
> > you
> > could identify each "trace" as an object, you could develop contours
> > "around" each trace, and then "cut" each contour, which would make
> > each
> > trace an island.
> >
> > So how do we get from Gerber files to CNC files? As you observe, the
> > Gerber "plots" each trace, what we want is contours AROUND the traces!
> >
> > I examined an HPGL PCB plot file (see View Companion), and studied how
> >
> > multiple pen lines were combined to draw a trace. If the pen lines
> > could be combined and handled like little objects... If they could be
> >
> > import into Vector... I could generate offsets around them. Humm
> > (brain
> > working)...
>
> The tough part is that the traces and the pads are separate items, with
> no data in the
> Gerber file to associate them, except the coordinates. So, you have to
> build a table
> of all primitives on the board, and then reorganize it as you figure out
> that this trace
> 'connects' to that pad, etc. Only when you have all this mapped out,
> and know that
> there is a 'net' which contains these traces and those pads, can you go
> on to the
> next step. Here, you have to take the connected primitives, and
> organize them
> topologically, so that you know that this trace ends ON that pad, and so
> on.
> Keep in mind that traces can just end in space (rare, but can be correct
> for a
> guard or shield trace, or text); they can cross through a pad, not
> necessarily through
> the center; they can end somewhere within a pad; they can end on or near
>
> another trace's endpoint, or just Tee into another trace. An arbitrary
> number
> of traces can connect to a single pad. Two traces can cross without
> there being
> an endpoint of either at the junction. Did I cover all possible cases?
> I'll bet
> I have missed one, here.
>
> > Any thoughts? Jon, can you shed more light on the Gerber files and
> > how
> > they do the traces?
>
> I have written a program that reads in Gerber files (which, by the way,
> are a form
> of RS-274D (alias G-code)) and generates photoplots on a raster
> photoplotter.
> Now, this is a hard exercise, but it doesn't have to "understand" what
> is being
> plotted, it just throws the requested pixels on the film, and the result
> is correct.
> The film assembles the info correctly, as it were. What I mean here is
> that the
> pads and traces connect just fine, and the drawing is correct and
> usable.
> This is despite the fact that the pads and traces are drawn in no
> specific
> order, and definitely do not need to be drawn so that one whole net is
> drawn,
> then some other whole net, etc.
>
> Unfortunately, defining the toolpath to isolate the nets requires the
> program to
> understand a lot more about the PCB information that is represented in
> the Gerber
> file, but not explicitly defined there. The connectivity information is
> not EXPLICITLY
> in the data, but, of course, it is extractable from it. The toolpath to
> isolate nets
> is not there at all, and has to be induced from the available data.
>
> Without strict design rules in the PCB CAD package to prevent it, there
> is the
> added problem of eliminating what would be called "gouges" in the
> machining
> world - ie. the isolating cut around one pad destroying some other pad
> or trace.
> If the tool is really small, ie. your design rules are something like 10
> mil traces and
> 10 mil spaces (minimum), and the tool cuts a 10 mil wide groove, then
> this may
> not be a serious problem.
>
> Anyway, as for Gerber file formats, they are standard RS-274D, but using
>
> suppressed leading zeroes and no decimal points. (This is what the
> ANCIENT
> tape-NC systems used, and is ghastly to read, by today's standards.)
> The original Gerber photoplotter used a commercial X-Y table, and a
> 'machining' head which consisted of a light source, shutter, and a wheel
>
> with 'apertures' on it. The aperture wheel functions a bit like an
> automatic
> tool changer, in principle, and they use the Dxx command word to select
> the aperture. When the shutter opens, the head projects an image of
> that
> aperture onto the film. When the machine moves, it 'draws', in light, a
>
> moving spot of that size and shape on the film.
> There are 3 special codes D01, D02 and D03, that tell the shutter what
> to
> do. D01 means "open the shutter while making the move in this block",
> D02 means "DON'T open the shutter", and D03 means open the shutter
> for just a moment, called 'flash', to draw a pad shape at the current
> location.
>
> The X and Y coordinates are pretty obvious. There are comments at
> the beginning of the file that define the number of digits before and
> after the decimal points, whether coordinates are absolute or relative,
> etc. Some systems put comments in to double check the apertures.
>
> Jon
>
>
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