The motors are attached to the uprights. When the motors actuate the arms, the whole machine rocks. In an attempt to keep the machine as steady as possible, I have crafted uprights out of MDF.

First I tried using two 2x4 pieces of construction lumber glued together. I drilled holes and threaded the all-thread rod directly into the wood. I was unable to get the threaded rod tight enough, and there was a lot of rocking.

Using a dial indicator, I checked the amount of rocking. It rocks 5 thousands of an inch. This .gif has been slowed by 8x.

Rabbets were cut in the sections of the uprights before they were glued together. Holes were drilled in the base, and threaded rod was used to connect the uprights to the base.

The threaded rod is tightened to 80foot-lbs. At that tension, it makes a middle D when plucked, and exerts a lot of force on the ends of the uprights. These steel plates keep the threaded rod from digging into the MDF.