of a ring of metal with screw threads tapped radially into it, in which screws (hex cap, socket hex cap, or set screws) serve as independent jaws. Spiders can serve various purposes: As auxiliary features that complement the main lathe chuck: To hold the bar or workpiece at the back end of the spindle bore and support it concentrically, so that it resists wobbling or whipping while the spindle is turning. Gun barrels and oil pipes are examples of workpieces that benefit. To hold the bar or workpiece at the tailstock end (thus serving as a steady rest) or following the tool (thus serving as a follower rest). In place of the main lathe chuck (for particular workpieces that can benefit—for example, in some gunsmithing work) Specialty jawed types (two-, six-, eight-jaw; other) For special purposes, chucks are available with six or eight jaws. These are usually of the self-centering design, and may be built to very high standards of accuracy. However, it is a misconception that such chucks necessarily offer more precision in holding solid workpieces than conventional three-jawed self-centering chucks. Indeed, hot-rolled or other imperfectly round workpieces may "teeter" insecurely between opposing jaws of scroll chucks having even numbers of jaws, in the same manner that a four-legged stool teeters on a rough floor while a three-legged stool never does. The primary purpose of six- and eight-jawed chucks is to hold thin-walled tubing with minimum deformation. By having twice as many clamping points, a six-jaw chuck induces less than half as much clamping distortion in a thin-walled workpiece, compared to a three-jawed chuck. Two-jaw chucks are available and can be used with soft jaws (typically an aluminium alloy) that can be machined to conform to a particular workpiece. It is a short conceptual leap from these to faceplates holding custom fixtures, wherein the part is located against fixed stops and held there with toggle clamps or toe clamps. Jaw construction Many chucks have removable jaws (often the top part is removable leaving the base or 'master jaw' assembled with the scroll), which allows the user to replace them with new jaws, specialised jaws, or soft jaws. Soft jaws are made of soft materials such as soft (unhardened) metal, plastic, or wood. They can be machined as needed for particular setups. The typical interface between the master jaw and the removable jaw is a matching pair of serrated surfaces,