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The biggest challenges in making printed circuit boards on the Modela are a) aligning the solder and component sides of the board, and b) keeping the board flat. This page will show you how to make a fixture bed for your Modela (a prototype is pictured on the right) to solve both these problems.
The basic steps are:
The board basic board shape was drawn in Rhinoceros. You can download the Rhino-format file, modela-pcb-bed-3.3dm, or the standard DXF file, modela-pcb-bed-3.dxf. I cut the piece out of 1/4" stock using the laser cutter and this CorelDraw file: modela-pcb-bed-3.cdr. The CorelDraw file has been offset .005" using my offsetting utility. The measured tool kerf in this material for these tool settings is elliptical, .013" on the X and .008" on the Y; we're offsetting as if the kerf were circular with a radius of .005". It's close enough; this doesn't have to be super-accurate.
On the laser cutter, cut with paper on, and use these settings:
|Black||VECT||100%||2.5%||600 PPI||outline cut|
You want to use fairly thick stock to protect the Modela when
you're drilling the PCB. The limiting factor is the length of the
thumbscrews used to secure the bed to the Modela. The stock Modela
comes with thumbscrews just long enough to secure a .25" bed.
If you can find longer M4 screws, you can probably use thicker stock
to make the bed. Don't use stock thinner than about 3/16", as we're
make a 1/16" cutout in the center of the piece, and we still want
enough thread left on the other side to put our clamp screws into.
The result is shown on the right. Now you need to get a 1/4x20 NC tap
and thread all 68 holes (fun!). The small hole by the word
"Component" and the small hole below "PCB Bed" don't need to be
tapped; they're mounting holes. The Modela has M4 thumbscrews that go
through these. (Look at the CAD drawing above to find these small
mounting holes; you can't really see them in the picture to the right.)
The top surface of this clamp can be
offset to hold the board down as well.
These shapes were designed in Rhino as well; the source files are workholders2.3dm (for most of the clamp designs) and side-clamp.3dm (for the long thin pieces that form the origin clamps, pictured at right. You can download OMAX Layout files for making these shapes on the waterjet, as well:
The workholders2.cdr file is a CorelDraw 10 file for making the clamps out of 1/16" acrylic on the laser cutter. The CorelDraw file has already been offset .005" on each side to compensate for the .010" cutting kerf of the laser cutter. Leave the paper on the acrylic sheet and use these settings when cutting the parts:
The acrylic clamps will not be as rigid as if you made them out of
aluminum or steel, but on the other hand it is a lot easier to make
small parts on the laser cutter — you don't have to worry about
them shifting and sinking to the bottom of the waterjet's bed.
Note that the Modela bed is going to be specific to the particular
Modela machine which you used to perform this final milling step.
If you use this bed with a different Modela machine, it is likely
that the top/bottom/side positions will be slightly off and
double-sided boards are likely to be misaligned.
On the right is a 7M movie showing how to use the origin clamps,
a small flat clamp, and an ell clamp to mount a PCB blank on the
Copyright © 2002 C. Scott Ananian.
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