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moldmakingtechnology.com 23 between two supports, and the worktable moves the work- piece in one axis. In knee-style machines, the spindle moves vertically on a solid column, and the table moves in two axes. A moving table enables the workpiece to be precisely oriented in line with the cutting spindle, eliminating side loads and other effects of overhang. Unlike the boring process on a lathe, the part is stationary while the boring tool machines the bore to the desired size. The mass of the workpiece has no influence on the opera- tion's accuracy, an important consideration when machining a large mold base. A jig boring machine usually bores only the holes on a work- piece that require the highest level of precision. While very similar to a jig boring machine, a jig milling machine adds a larger-capacity toolchanger and the capabil- ity to perform milling operations and other ancillary work that jig borers aren't designed to handle. Spindles in early jig boring and jig milling machines were vertically oriented. Horizontal jig borers and jig mills were developed later to fulfill different end-user requirements and preferences. The horizontal configuration can accommodate components whose size or shape prevent them from being processed on a vertical machine. The horizontal arrangement also can facilitate automation by enabling a part to be rotated 180 degrees for machining on both sides. In addition, gravity enhances chip flow when machining horizontally. However, gravity also can make fixturing and setting up a part on a hori- zontal machine more difficult and time-consuming, especially if the part is large. Jig Features Current global competition and astronomical volume requirements for some plastic parts have prompted manufac- turers to build ever-larger multi-cavity molds that produce more parts in less time. The result is larger mold assemblies overall that incorporate larger mold bases. As mold assem- blies grow larger in size, the effects of deviations in mold base dimensions are magnified. The rigidity of the jig boring machine and its components affects the quality of the bores it machines and the produc- tive lifespan of the machine tool itself. Larger mold bases can weigh 5 tons or more. Handling heavy weight and maintaining tight tolerances for critical mold base features requires a sub- stantial machine tool structure composed of heavy castings. If structural strength is insufficient, weighty workpieces will push machine components out of alignment and cause premature machine tool wear as well as declining machining accuracy. The guideways of jig boring machines are engineered to maximize geometric stability. Vertical jig boring machines typically have one V-profile way and one flat way on the table axis, with the V-way as the principal guiding way and the flat way as a load bearer. Double V-ways are used in all slideways Manufacturers of mold bases use jig boring and jig milling machines to fnish precision bores that can feature tolerances of less than 5 microns in roundness, perpendicularity and straightness. While very similar to a jig boring machine, a jig milling machine adds a toolchanger and the capability to perform milling operations and other ancillary work that jig borers aren't designed to handle. of horizontal machines. V-ways are difficult to manufacture but provide precise management of machine geometry and motion. The sliding members are mated with needle roller bearings that are lubricated specifically to allow for extremely precise motion and minimal stick/slip. Vertical jig boring machines usually have work strokes rang- ing to 40"x 60" (1 × 1.5 m) in size and generally are engineered to carry out high-precision boring operations only. In contrast, jig mills may have larger work capacity (ranging to 65" x 120"), and can use milling cutters and other tools to perform mul- tiple operations in one setup. Jig mills also have higher-speed spindles and ground-tool-steel box ways to provide greater