Moldmaking Technology Magazine November 2015 Issue

JIG FOR MOLD BASE ACCURACY

By Tom Dolan, Vice President - Mitsui Seiki USA, Inc.

Manufacturers of mold bases use boring and jig machines to finish precision bores that can The mold base assembly is the foundation of the molding process. feature tolerances of less than five microns in A mold base positions and anchors the mold inserts, cavities and roundness, perpendicularity, cores, incorporates cooling channels, and enables the mold to be and straightness. opened and closed. The engineering and manufacturing quality of the mold base largely determines a mold’s performance and durability. Holemaking is a key manufacturing process, and any mold base features multiple high-tolerance bores for components such as guide pin bushings. The accuracy of those bores contributes to smooth and reliable mold operation. To maximize the quality of the critical bores, many mold base manufacturers finish them with specialized jig boring and jig milling machines. Jig boring machines were initially developed in the early 1900s to machine fixtures (jigs) 1 used to guide in precision holemaking operations. Moldmaking Technology Magazine November 2015 Issue

Later, manufacturers began to use the jig boring machines themselves to produce precision bores. The tolerances for bores in mold base applications can be less than five microns in location, roundness, perpendicularity, and straightness. To meet those requirements a jig borer must operate at even higher precision; positioning tolerances for the machines of one micron are not uncommon. The machines’ accuracy is such that some shops perform measuring operations with them in the same way they use coordinate measuring machines (CMMs). Jig boring machines are available in two basic configurations. In gantry style machines, a boring quill rides a bridge between two supports and Mitsui Seiki PJ812 Precision Profile Center is a three- the worktable moves the workpiece axis CNC vertical Jig Mill engineered to perform high- precision contour machining and ultra precise boring in one axis. In knee-style machines, of components with critical tolerances. It is ideal for the spindle moves vertically on a processing parts for the mold and die, optical, solid column and the table moves in aerospace and medical industries. 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 , the part is Spindles in early jig boring and milling stationary while the boring machines were vertically oriented. Horizontal jig machines the bore to the desired borers and jig mills were developed later to size. The mass of the workpiece has fulfill different end user requirements and no influence on the operation’s preferences. The horizontal configuration can accuracy, an important consideration accommodate components whose size or when machining a large mold base. shape won’t allow them to be processed on a A jig boring machine usually bores vertical machine. The horizontal arrangement only the holes on a workpiece that also can facilitate automation by permitting a require the highest level of part to be rotated 180˚ for machining of both precision. sides. In addition, gravity enhances chip flow While very similar to a jig boring when machining horizontally. However, gravity machine, a jig milling machine adds also can make fixturing and setting up a part on a larger capacity tool changer and a horizontal machine more difficult and time the capability to perform milling consuming, especially if the part is large. operations and other ancillary work that jig borers aren’t designed to handle. 2

Type to enter text Moldmaking Technology Magazine November 2015 Issue

The accuracy of jig boring and jig milling machines is such that some shops perform measuring operations with them in the same way they use coordinate measuring machines (CMMs).

Current global competition and flat way on the table axis , with the v-way astronomical volume requirements for being the principal guiding way and flat some plastic parts prompt manufacturers way as a load bearer. Double V-ways to build ever-larger multi-cavity molds are used in all slide ways of horizontal that produce more parts in less time. The machines. V-ways are difficult to result is larger mold assemblies overall, manufacture but provide precise incorporating larger mold bases. As mold management of machine geometry and assemblies grow larger in size, the motion. The sliding members are mated effects of deviations in mold base with needle roller bearings which are dimensions are magnified. correctly lubricated to allow for extremely The rigidity of the jig boring machine precise motion and minimal stick/slip. and its components affects the quality of Vertical Jig boring machines the bores it machines and the productive usually have work strokes up to 40"x lifespan of the itself. Larger 60" (1m x 1.5 m) size range and mold bases can weigh five tons or more. generally are engineered to carry out Handling heavy weight and maintaining high-precision boring operations only. In tight tolerances for critical mold base contrast, jig mills may have larger work features requires a substantial machine capacity (up to 65” x 120”)and can utilize tool structure composed of heavy milling cutters and other tools to perform castings. If structural strength is multiple operations in one setup. Jig insufficient, weighty workpieces will push mills also have higher-speed spindles machine components out of alignment and ground tool steel box ways to and cause premature machine tool wear provide greater milling capability. Some as well as declining machining accuracy. jig mills offer linear motion slide way The guideways of jig boring machines systems that facilitate high-accuracy are engineered to maximize geometric machining. stability. Vertical Jig boring machines 3 typically have one v-profile way and one Moldmaking Technology Magazine November 2015 Issue

The larger worktable of a jig milling While very similar to a jig boring machine, a jig milling machine adds machine can make it the only choice a tool changer and the capability to perform milling operations and for processing some larger, heavier other ancillary work that jig borers aren’t designed to handle. workpieces. A jig mill’s ability to perform operations other than boring may also eliminate the need to move a large workpiece to another machine, saving time and avoiding loss of accuracy during the transition. Thermal stability of the machine tool is a key factor in maintaining bore tolerances. When results are measured in microns, even small temperature-driven changes in machine dimensions will significantly affect machining accuracy. Machine castings are engineered to provide stable thermal characteristics, and a typical jig boring machine will have components arranged symmetrically so they will react to temperature changes evenly. For maximum accuracy and consistency, many shops operate their jig boring machines in temperature controlled- Over the years, advances include ceramic environments.The specialized nature ball bearings for spindles, 4- and 5- axis of jig boring and milling machines CNC operation, 0.0001mm resolution linear results in their being replaced scale feedback and new design and casting infrequently; effective life cycles of technologies that boost accuracy and twenty-five years are not unusual. productivity. The ability of the CNC machine The mature technological capabilities to be used in a more traditional “manual” of older machines, however, usually mode with hand wheels for quill feed has require that they be operated by been improved. The mold industry is skilled personnel to produce parts of global competition on quality and price, and maximum quality. In addition, machines that offer the ability to increase machine wear over time eventually output while improving accuracy are a affects the accuracy of the machining driving force behind new investment in mold process. Shops producing mold base manufacturing. bases should evaluate the output of their current jig boring and milling *** machines in terms of what is available in terms of present-day technology. Mitsui Seiki, for example, has been building jig boring machines since the 1930s and is continually improving the machines’ capabilities.

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