Two hard wired limit switches restrict the vertical motion by turning the vertical drive motor off when a lucite screw strikes the metallic limit switch. These limit switches are located at the base and top of the Thompson shafts preventing the spider from impacting against the top and bottom flanges. Two other hard wired limit switches are mounted under the spider restricting the angular motion to within 360 deg. In addition, software limits are set in the operating program to restrict the vertical motion from 0 in to 25 in and the angular motion from 0 to 355 .