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Eagle 3D PCBs with Alibre: Assembling the Model Part I

The final step in creating a 3D PCB model is to assemble all the pieces together.  Normally Alibre assemblies are created by constraining the parts.

A constraint limits how two parts can be located relative to each other.  Three constraints fully constrain a part.  For example, think of mating a through hole connector with 10 pins in one row to a PCB.  You could add three constraints like this:

  1. Use an Align constraint to align the axis of the connector’s pin 1 with the PCB hole for pin 1.  Now the connector is limited to two degrees of freedom: it can move close and farther from the PCB and it can rotate 360 degrees  around the pin-1/hole-1 axis.
  2. Use another Align constraint to align the axis of the connector’s pin 10 with the PCB hole for pin 10.  Now the connector cannot rotate: it can only move close or father from the PCB.
  3. Finally, use a Mate constraint with a zero offset to mate the bottom of the connector with the top of the PCB.  Now the connector cannot move at all; it is attached the the PCB just like you had perfectly soldered a perfect connector to a perfect PCB (pins centered in the holes, connector just touching the PCB, etc).

That sounds pretty easy, right?  Well, the reality is often different.  Extruding a PCB is straightforward.  Mating parts together is not; there are many possible ways of mating the parts together, and the best approach depends on the specific parts and PCB.  I think MCAD assemblies are complex enough that an expert could write a book just about assemblies (and I’m definitely not an expert).

I haven’t found a lot of practical information on Alibre assemblies.  My findings so far:

  • You should definitely read the Alibre Design User Guide chapter on assemblies.  It covers what’s available, including about 20 pages on constraints, but is brief and descriptive.  It does not give any examples or practical advance.
  • Based on the table of contents, the Learn 3D CAD book also only covers constraints briefly (about 20 pages), but still looks like it’s worth the price, since it has a real world example, and advice on overall design (top down vs bottom up).  I’m planning on getting the PDF version when the 2011 update is available.  Note 10/5/2011:  the Learn3DCAD website is no longer active, so this is no longer an option.
  • The Alibre forums look useful if you have a specific question, but aren’t a tutorial.  Also, if you’re on maintenance, there’s always tech support for specific questions.
  • I’m not sure how useful Alibre’s paid training materials (DVD, exercise book, online seminars) since they don’t provide detailed information on the contents.  I suspect they wouldn’t cover some of the problems I’ve had.

Next up in this series: some specific tips from my experience.

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