Making a Fixture Bed for the Modela

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:

1. Cut out the outline (modela-pcb-bed-3.cdr) on the laser cutter.
2. Cut out some clamps (workholders2.3dm) on the waterjet.
3. Mount the bed and send bed.prn to the Modela to mill out the alignment stops.

Cutting the bed

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:

Color	Mode	Power	Speed	PPI	Purpose
Black	VECT	100%	2.5%	600 PPI	outline cut
Red	VECT	100%	50%	600	text etch

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.)

Making clamps

• Small flat clamp
  The workhorse clamp, small enough to work in tight situations. Note that it holds the board down (and thus flat) as well as pushing to keep it in place.
  
• Large flat clamp
  For larger boards or the large side of small boards. Again, it's really good at keeping a large side down and flat as well.
  
• Ell clamp
  Used to clamp the top of the board, usually, when there's not enough space for the small flat clamp. Note that only one side of the "L" is used for clamping; the other leg must stand off from the board in order to align with the mounting holes.

  The top surface of this clamp can be offset to hold the board down as well.
  

• Pac-man clamp
  The swiveling head on this one was Axel's idea. This clamp allows you to grab the free corner of the board so that it doesn't translate.
  
• Origin clamps
  The picture shown is of the prototype, with a slightly different design, but you can clearly see how the metal plates bracketing the origin overlap to press the board flat on these two sides.
  

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:

side-clamp.3dm side-clamp.dxf side-clamp-omax.dxf side-clamp-omax.ord	workholders2.3dm workholders2.dxf workholders2.svg workholders2.cdr
work3-omax.dxf work3-omax.ord	work4-omax.dxf work4-omax.ord

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:

Color	Mode	Power	Speed	PPI
Black	VECT	100%	8%	600 PPI

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.

Milling the bed

1. Mount the partially-finished bed in the Modela. The thumbscrews go through the small untapped holes.
2. Put a 1/8" end mill in the Modela.
3. Download and save the home.prn file. Download the PRNter utility and unzip it. Use PRNter to "print" home.prn to the "Roland MODELA MDX-20" device. This will move the Modela head over to the "home" position above the far left corner of the board.
4. Use the up/down buttons on the front to lower the mill so its just at the surface of the bed. You may want to use an allen wrench to loosen the bit and let it drop to touch the bed. (Be careful of setting the Z to the extreme end of the Z range of the device.)
5. Download and save the bed.prn file. Use PRNter to send it to the Modela. The Modela will start milling the bed. It will take quite a while.

That's it!

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.

Clamping down a PCB

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 bed.

More pictures of the fixture bed and clamps

 

Copyright © 2002 C. Scott Ananian. Verbatim copying and distribution is permitted in any medium, provided this notice is preserved.

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