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| How to understand Large Mold Machining |
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Dealing with the sheer size and weight of a large mold can be a big challenge. This often leads to excessive labor, the need for special equipment, multiple setups and potential inaccuracies. If the proper machining center is chosen for the manufacture of large molds, many of these problems can be reduced or eliminated, easing setup, change-over and other potential accuracy issues. The biggest expense often associated with the manufacture of large molds is machinery cost. Machines capable of producing big molds are expensive, so having multiple machines, one for roughing and another for finishing, for example, is a large barrier to entry for this market. When the proper machine is chosen, it is capable of roughing and finishing, even in the same setup and without losing accuracy. To counter common accuracy and setup concerns in large machines, we must look at some of the necessary design characteristics of a large mold machining center. Cast Iron Construction and Spindle Capable of Dealing with Heat The first thing any machine designed to mill large parts needs is a massive, cast iron construction and a spindle capable of dealing with heat. Cast iron is still the most stable material for machine construction, adding rigidity and heat-sink characteristics to the machine. As far as the spindle is concerned, it must have internal technology built to cool the spindle from the bearings out, ensuring that the spindle is not going to burn itself out or lose accuracy with thermal expansion. These items are important because large molds spend much longer in the cut than smaller ones, adding to the concerns of stress on the mold and heat. Without a well-built foundation, no machine tool will produce quality large molds due to these two factors. Vibration must be limited and heat must be dealt with. Most large molds will be machined for days, meaning outside conditions will change as the part is machined, and those changes must be taken into account to achieve the necessary accuracies. In a machine not designed for the cutting conditions of a large mold, a 10oC environment temperature change results in a six-degree machine column temperature change, causing a spindle-angle plate parallelism change of 0.070mm. On the other hand, with a machine utilizing thermal stabilization technology, the same environment change results in a three-degree column temperature change, or a 0.030mm spindle-angle plate parallelism change. Therefore, machine design must also be conscious of environmental thermal effects, shielding outside air influences from affecting the part. If the machine you choose is thermally stable, it will also reduce blend-line problems; and, therefore reduce the need to hand polish the part. Speed Accuracy Feedback Resolution Spindle In addition, the spindle must be designed to maximize tool life over extended machining periods by providing minimal vibration and growth. For example, when running a large fascia mold on a large mold machining center, you are able to obtain more than 30 hours of life with a 16mm CBN insert finish tool running at 314 ipm and achieving a surface finish of 0.336 micron Ra, 3.02 micron Rmax. Tooling costs add up quickly when machining a large mold, and having a machine designed to maximize tool life can save thousands of dollars per mold in tooling costs. Movable, Multi-Axis Head A multi-axis head also allows for the use of shorter tools. A shorter tool is always a more rigid, accurate tool, and prevents the spindle and the tool from colliding with the table. Finally, a multi-axis head allows for improved surface finish by using the radius edge instead of the tip of the tool. Chip Management Without an effective chip management system, no large mold machine will be able to operate without break-downs or thermal issues. High Pressure Coolant Capabilities Circle, Diamond and Square • Toolpathing was kept to a simplified circle routine (G02 and G03) common in all machine controls. This allows the machine to perform to its core of accuracy without any influences from programming software. This also showcases the machine ability to cut accurately using multiple feedrates. • Typically machines were limited to one feedrate that would cut to accuracy. • Backlash and compensation were all set to maximize one feedrate, typically the feedrate that would be used the most. As one can imagine if another feedrate was utilized inaccuracy would quickly appear. Angled Drilling Holes Sample Purpose of this detail is to the showcase the 2+3 stiffness as it relates to drilling large holes. • A standard, off the shelf high speed steel drill was used to produce the hole in the material. Typically customers will not want to spend the difference in dollars when it comes to buying large carbide drills. Carbide drills in this size are expensive. • High-speed drills don’t perform to the level of carbide but are common in many shops. This head was able to produce a hole using standard approaches with large drills. • Typically a part of this size would be roughed, semi-finished and finished as much as possible in a three-axis machine. A multi-axis machine would be used to do areas that tooling length could be reduced by tipping the tool. The reason for not doing all the work in the multi-axis machine is because of rigidity of construction and the inability of this machine to rough. Simply, some multi-axis machines are capable of finishing only. • The determining factors for removing the detail and placing it on a multi axis machine will vary from part to part. However one reason seems to be constant in all cases. The geometry gets to smaller diameters where the combination of higher spindle speeds and shorter tooling need to be used. The detail would be removed from one machine and placed onto a multi-axis machine to finish difficult areas. This would account for only about 5 to 10 percent of the total machining time.
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