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.

1) 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. 

   2) Thermal Stabilization Technology
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.

  3) Speed
A second machine feature to consider is speed. The spindle should run at least 20,000 rpm and metal removal rates should be at least 30 ipm. For example, there is a large mold machining center with rapid traverse and cutting feedrates of 787 ipm.

  4) Accuracy
Accuracy is important in large molds if you plan to finish and rough on the same machine. The machining center you choose should be capable of similar positioning accuracies and repeatability of smaller machines.

  Use a large mold machining center with a positioning accuracy of ±0.000060 inches (±0.0015 mm) and repeatability of ± 0.000040 inches (±0.001 mm). Also keep in mind that pitch accuracy is important, and should be kept within about 5 microns.

  6) Feedback Resolution
To measure just how accurate you are machining, you must be aware of the machine’s feedback resolution. With a standard one micron feedback resolution, imperfections are common. When you get down to a 0.050 micron feedback resolution, the finish is nearly flawless. Control resolution, scale feedback and fine pitch ball screws all allow for this improved surface finish.

   7) Spindle
Another machine design consideration is the spindle. The spindle on a large mold machine should be capable of predictable roughing, semi-finishing and high-quality finishing. As a benchmark, surface finishes should be able to get to around 2 microns. Tight finishing characteristics are especially important in shut-off and parting lines, where many moldmakers are forced to hand polish to make up for the inaccuracies of their tools. Also, because large machines are much more expensive, buying multiple machines for these three tasks is impractical.

  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.

  8) Movable, Multi-Axis Head
A desirable feature for the machining of especially complex large molds is a movable, multi-axis head. A variable geometry head design allows for simultaneous three-axis machining, capable of milling deep mold cavities and cooling holes, as well as many other cut combinations in the same setup. Because the size and weight of the workpiece translates into long setup times, reducing setups with three-axis capabilities without losing accuracy can go a long way to improving the ability for a shop to machine large molds. In addition, a multi-axis head will allow for slanted-hole machining, accomplished by tilting the spindle at an optimum angle for improved access to the milling point.

 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.

  9) Chip Management
Chip management becomes a serious issue of concern with large molds. On some large mold machining centers eighteen outlet holes are provided below the table of the machine to reliably catch chips, regardless of the table position. Four built-in hinge-type chip conveyors evacuate chips at high speed toward the front of the machine. The bed and column are protected by covers to prevent any freshly cut hot chips and coolant from direct contact with the machine’s cast-iron surfaces. The areas around the column and the bed as well as underneath the bed are insulated to eliminate thermal changes in the cast-iron machine structure caused by variation in the shop temperature.
Without an effective chip management system, no large mold machine will be able to operate without break-downs or thermal issues.

   10) High Pressure Coolant Capabilities
A final feature that is important when choosing a large mold machine is high pressure coolant capabilities. For example, when drilling angled holes with a 2+3 machining approach, 1000psi coolant will allow for effective chip evacuation and more accurate cuts. Without the high pressure coolant, the tool will often block the chips from leaving the hole, causing chips to be re-cut. Having the ability to effectively machine angled holes will eliminate setup time and additional machinery needed for gun drilling, as is typically done for these types of cuts.     

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