Homebrew PCBs

Brian Chapman wrote:

> First, though, what is the thickness of the PCB you mention?

board is 1.6mm thick.

> In the CAM drawing (I import CAD .DXF drawing into CAM software for
> G-code processing), I'll draw a bounding box around the work piece (in
> this case, using the PCB's actual size) so that when this box is cut
> and removed, the PCB drops into the vacated space. For me, this has
> been a dead-on, precise fit -- absolutely no wiggle room (initially, I
> was surprised at how tight this fit actually was).

Nice

>>From this point, since the bounding box is part of the coordinate
> system used to position the cutter/drill, after touching off the Z
> axis (at zero), you're good to go.
>
> This has always worked extremely well, too, when a workpiece has to be
> repositioned, for example, to mill on the reverse side of the workpiece.

Again, nice.

> I often use mill tabel double-sided tape to fixture pieces in place.
> Johnson Plastics (http://www.johnsonplastics.com/), for instance, has
> 3/4" wide double-sided tape that is sticky, sticky, sticky. . . .
>
> Hope I accurately caught the drift of your post and this information
> is relevant.

Hi Brian,

Your ideas are valuable, and something I'll definitely think about as my
CNC evolves.

I did consider using a physical box on the CNC carriage for precise
location of the workpiece, but with my present situation I can't yet see
a time saving coming from it.

For a workpiece placement box to be effective, the board needs to be
precision cut, and the artwork on the board needs to be precisely
located relative to the board's physical edges.

Presently, I'm cutting boards by hand with a fretsaw or a powered
jigsaw, and even with my most careful cutting efforts, and my most
careful efforts in heat-transferring the artwork, I'm still prone to
translational/rotational errors that would see some some holes moving up
to a couple of millimetres - disaster.

With my primitive non-workshop situation, the interactive calibration
scheme is working best - holes are ending up within 0.04mm of their
targets, which is an acceptable margin. I can relax while
heat-transferring the artwork and cutting the boards, because the
interactive calibration ond the geometry code wipes out all the errors.

However, if/when I add board milling functionality, I'll definitely look
into putting a placement jig on the carriage, most likely just a
corner-alignment scheme, so my only worry then will be cutting the
boards with a reasonable 90deg angle.

BTW - I'm exporting the drill files from Eagle via an Eagle ULP (script)
which writes out the holes in Python syntax, ready for import to my
Python CNC host program. If anyone would like this short script I'd be
happy to post it.

-- 
Cheers
David