Moldmaking Technology Magazine November 2015 Issue

JIG BORING FOR MOLD BASE ACCURACY

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

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The mold base assembly is the foundation of the molding process. A mold base positions and anchors the mold inserts, cavities and cores, incorporates cooling channels, and enables the mold to be 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) used to guide tools in precision holemaking operations.

Manufacturers of mold bases use jig boring and jig milling machines to finish precision bores that can feature tolerances of less than five microns in roundness, perpendicularity, and straightness.

Moldmaking Technology Magazine November 2015 Issue

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Later, manufacturers began to use the jig boring machines themselves to produce precision bores. The machining tolerances for bores in mold base applications can be less than five microns in location, roundness, perpendicularity, and s t ra igh tness . 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 the worktable moves the workpiece 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 operation’s accuracy, an important consideration when machining a large mold base. A jig boring machine usually bores only the holes on a workpiece that requ i re the h ighes t leve l o f precision. While very similar to a jig boring machine, a jig milling machine adds a larger capacity tool changer and the capability to perform milling operations and other ancillary work that jig borers aren’t designed to handle.

Spindles in early jig boring and 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 won’t allow them to be processed on a vertical machine. The horizontal arrangement also can facilitate automation by permitting a part to be rotated 180˚ for machining of both sides. In addition, gravity enhances chip flow when machining horizontally. However, gravity also can make fixturing and setting up a part on a horizontal machine more difficult and time consuming, especially if the part is large.

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Mitsui Seiki PJ812 Precision Profile Center is a three-axis CNC vertical Jig Mill engineered to perform high-precision contour machining and ultra precise boring of components with critical tolerances. It is ideal for processing parts for the mold and die, optical, aerospace and medical industries.

Moldmaking Technology Magazine November 2015 Issue

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Current global competition and astronomical volume requirements for some plastic parts prompt manufacturers to build ever-larger multi-cavity molds that produce more parts in less time. The result is larger mold assemblies overall, incorporating larger mold bases. As mold assemblies 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 productive lifespan of the machine tool itself. Larger mold bases can weigh five tons or more. Handling heavy weight and maintaining tight tolerances for critical mold base features requires a substantial 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 being the principal guiding way and flat way as a load bearer. Double V-ways are used in all slide ways 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 which are correctly lubricated to allow for extremely precise motion and minimal stick/slip. Vertical Jig boring machines usually have work strokes up to 40"x 60" (1m x 1.5 m) size range and generally are engineered to carry out high-precision boring operations only. In contrast, jig mills may have larger work capacity (up to 65” x 120”)and can utilize milling cutters and other tools to perform multiple operations in one setup. Jig mills also have higher-speed spindles and ground tool steel box ways to provide greater milling capability. Some jig mills offer linear motion slide way systems that facilitate high-accuracy machining.

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).

Moldmaking Technology Magazine November 2015 Issue

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The larger worktable of a jig milling machine can make it the only choice for processing some larger, heavier 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 j ig boring machines in temperature controlled-environments.The specialized nature of jig boring and milling machines results in their being replaced infrequently; effective life cycles of twenty-five years are not unusual. The mature technological capabilities of older machines, however, usually require that they be operated by skilled personnel to produce parts of maximum qual i ty. In addit ion, machine wear over time eventually affects the accuracy of the machining process. Shops producing mold 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 bu i ld ing j ig bor ing machines since the 1930s and is continually improving the machines’ capabilities.

Over the years, advances include ceramic ball bearings for spindles, 4- and 5- axis CNC operation, 0.0001mm resolution linear scale feedback and new design and casting technologies that boost accuracy and productivity. The ability of the CNC machine to be used in a more traditional “manual” mode with hand wheels for quill feed has been improved. The mold industry is facing global competition on quality and price, and machines that offer the ability to increase output while improving accuracy are a driving force behind new investment in mold base manufacturing.

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While very similar to a jig boring machine, a jig milling machine adds a tool changer and the capability to perform milling operations and other ancillary work that jig borers aren’t designed to handle.