Re: [CAD_CAM_EDM_DRO] Re: PCB routing with CNC milling machine....

Fred Smith wrote:

>--- In CAD_CAM_EDM_DRO@yahoogroups.com, Kim Lux <lux@d...> wrote:
>
>
>>On Mon, 2003-10-27 at 10:46, Jon Elson wrote:
>>
>>
>>
>>>The Gerber file IS "gcode", alias RS-274D. It is in a very
>>>
>>>
>specialized
>
>
>>>style,
>>>with suppressed leading zeros and no decimal point.
>>>
>>>
>>OK, good to know that.
>>
>>
>>
>>> The original Gerber
>>>photoplotters used off the shelf CNC controls to move thew XY
>>>
>>>
>stage,
>
>
>>>and therefore were programmed in Gcode.
>>>
>>>
>>That makes sense.
>>
>>
>
>Jon keeps posting this nonsense and misleading people into thinking
>it is just a snap of the fingers to be able to write your own CNC
>machine programs from Gerber files.
>

I have not said anything of the sort. The original Gerber code IS G-code,
in that it is totally consistent with RS-274D, and there is a line in the
preface to the "Gerber Format Guide" published by Gerber Systems
Corporation, reading :

"Gerber Format defines a family of data formats, the most frequently
used of which are subsets of EIA Standard RS-274D."

Now, it is important to note that while these files ARE "G-code", they are
meant to create photoplots, on lithographic film, used to manufacture
PC boards by photetching processes. The creation of these plots uses
the gerber file to describe areas of light and dark on the plot, without
any information on the connectivity of the primitives on the plot.
In other words, these files describe how to make the PCB by a
SPECIFIC PROCESS. They are no longer a general description of
the PC board. That, in general, is the difference between CAD and
CAM. The CAD program has a general understanding of the item
to be produced, with important information on "how it all fits together".
By that, I mean that the electrical connectivity is understood by the
CAD program. The conversion to Gerber files loses that information,
because once the CAD program knows it is right, that information
is not needed by the manufacturing process.

This is quite similar to the manufacturing of a bracket, perhaps, by
a CNC milling machine. The CAD progam has a great deal of info
about the part, but the G-code program to cut it out of a block of
metal may not even convey whether it is going to be cutting a small part
from a block, or cutting a hole in a larger part!

> Note: Extended Gerber is not G-
>code as used for CNC machining. It is not documented in the RS-274D
>standard, but rather RS-274X, which is a whole different bird. The
>flavor used by most modern creators of Gerber files does not even
>match the RS-274X standard, and most differ considerable between ecad
>design programs. ( ecad- electronic cad design program, ie Eagle, etc)
>

RS-274X is an extension to RS-274D, and it has a a few added features.
The most important is that the photo apertures are specified in the
beginning of the file, and it has also added some features that allow
"erasing" what has already been drawn, by essentially drawing in
white in addition to black. And, unfortunately, many cad programs
generate variants of the Gerber format that they tested on only a couple
of photoplotting programs, and may not work well on all photoplotters.

>
>RS-274X is available for download from the web.
>
>There are several issues that the free and most of the expensive e-
>cad programs do not address:
>
>1) drilling and spot drilling for hole location
>

Drilling is NEVER part of the Gerber file. The drilling of the board is
specified in the NC drill file, which is a totally separate file. Most
programs
make Excellon drill files, which are ALSO RS-274D, but yet another
variant! Gerber supresses LEADING digits, Excellon supresses TRAILING
digits! Both use no decimal point, because the very early tape-NC controls
usually didn't support moving decimal points.

>2) mirror image for the bottom sides of the boards
>

Mirror image only has meaning if you are going to work from both
sides of the board, as when milling it. Commercial PCB makers do not
use milling, they use some combination of photomasks with etching
and plating processes. Two-sided boards are done both sides at the
same time.

>3) offset for tool size and detection of overlapping toolpaths if
>somebody mistakenly uses a 1/8 end mill instead of a 60 degree
>engraving point.
>
>Both DeskPCB $95 and DeskCNC $300 have these capabilities, and both
>can also save the trace outlines as DXF files, if you want to do
>additional routing or processing of the data.
>
>http://www.imsrv.com
>
>

Oh, now here comes the ad!


Any way, to sum it up, I may have said something that was misinterpreted,
but I never said that you could mill out a PC board by feeding a Gerber
file to your CNC milling machine! I certainly know that is not the case!
I have always clearly stated that you need a program to convert a Gerber
file to a toolpath that will mill out the traces, and that such a program is
not trivial.

I have been designing PCB's, professionally, since 1975, and also making
occasional prototypes myself. I have also built a laser photoplotter, which
required creating a program to read Gerber files and convert them into
a bitmap. So, I am VERY familiar wth the Gerber variant of the RS-274D
format, and the RS-274X extensions.

I strongly resent such words as

"Jon keeps posting this nonsense and misleading people"!

Jon Elson