Nửa cơ cấu scissor linkage

The gears in this mechanism force each neighbouring pair of arms to have the same angle between them. This angle alternates between turning to the left and turning to the right, so the mechanism acts exactly like one half of an ordinary scissor linkage.

There are three files: a standard scissor linkage arm, a half scissor linkage arm, and a half scissor linkage gear. 

Copies of the standard scissor linkage arm can be combined together to make a standard scissor linkage. At each junction, connect the two arms with an 8mm long M3 bolt, fastened by an M3 lock nut. (This part is not needed if you only want to make the half scissor linkage.)

To make the half scissor linkage, you will need one half scissor linkage arm and one half scissor linkage gear for each segment of the mechanism. The arms and gears are all attached to each other using 16mm long M3 bolts, each fastened by an M3 lock nut. You can make the mechanism as long as you like, although as the number of segments goes up, so will the amount of slop in the mechanism.

It can be a little tricky to get all of the gears arranged with the correct rotation relative to each other. The easiest way to do it that I've found is to first attach all the arms together to make a floppy chain of sticks. Lay it on the table on its side, so that all of the sticks are in a straight line. Then attach each additional gear to the middle of its arm. This ensures that the mechanism can extend all the way out to a straight line.