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   * Subject: Pffffft...
   * Date: Fri, 27 May 1994 18:02:37 -0500 (CDT) [50]
   * From:

Add an axis and move it to the edge of your work area - this will be a parent axis for the group you are about to make. Add a primative disk. Add another axis and put it on the periphery of the disk (this will be the pivot point for the cylinder). Add a tube (piston), scale it down, and move and rotate it to the right (posative X) side of the disk you already added.

Now group these all together - parent axis to disk, disk to pivot axis, pivot axis to piston.

Now you have to set up the constraints by freezing parts of this group. Freeze the parent axis in X,Y,and Z world position and rotation. Freeze the disk in X and Z world rotation (you can still rotate it around Y). Freeze the piston in X,Y and Z world rotation and in Y and Z world position (the piston can still move in X).

Now, turn on constraints. Switch to object mode, and rotate the disk around Y - voila - you should get the piston moving properly. Now use states to come up with tween positions (the transformation requester can be used to get the rotation angle precise).

The group needs the parent axis because the parent object is automatically frozen, and I don't think you can rotate it. Also, you will have to create a lot of tweens, because the constraint info isn't used in animation, and the piston will take short cuts between key frames.

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