Recent advancements in industrial PC CNC controls have opened a new technology path to improved speed, accuracy, surface finish, flexibility and convenience for moldmaking, along with greater value. Here is a review of some of the features of the next generation controls you should consider when investigating your next PC CNC control purchase.
Capability Considerations
Memory
Next generation controls are specifically designed for mold and die machining and feature dual processors separately dedicated to the interface and motion control functions of the machine. Among many basic advantages to owners, program memory in these controls is user upgradeable. A control equipped with 2MB of standard program memory can be upgraded with commercially available compact flash memory, eliminating the cost and availability issues associated with proprietary memory modules.
Processing and Execution Speed
The latest dual-processor controls can deliver a much-improved 1,500 blocks per second processing speed simultaneously across three interpolated axes, compared with conventional BPS ratings across one axis only. While block processing speed is often cited as a measure of a control’s capability, the true measure is block execution speed, which factors in servo update rates and interpolation rates.
Newer control platforms have 4 millisecond servo update and interpolation rates. As a result, data for 2-D and 3-D contouring cuts is processed quickly to allow extremely high speeds and feeds, without lag, dwell or run-on. These powerful new features work seamlessly with 3,000-block look ahead and highly accurate control path algorithms to ensure both part and path are executed exactly as programmed, without blending.
Performance-Specified Machining
Performance-specified machining is a new capability that allows machine operators to choose appropriate levels of speed and accuracy required during any phase of a part program, using a sliding scale. For example, the operator might opt for increased speed during the initial roughing passes then adjust to increased accuracy for the surface finishing passes. Performance-specified machining actually retunes the servo gains, PID loops and other parameters that would take considerable time to do manually, if the operator were capable of doing it at all.
And while older controls may require the machine to be idled or off while new servo parameters are dialed in, the next-generation controls give the operator direct access to the line of code to choose exact levels of performance desired, on the fly and without disruption. Setup of the basic servo parameters at machine startup is automatic and 100 percent accurate with software technology that identifies all hardware at startup and populates the CNC with the needed data, eliminating any potential for operator errors.
Software
Software packages designed for use with the new controls in mold/die applications include control-resident CAM systems for shop floor programming. This powerful, integrated CAM package is more intuitive, flexible and scalable than traditional conversational programming systems—empowering operators to rapidly create programs for part geometries that range from simple to complex.
PC Hardware Advantage
CNC users have long faced challenges with the most popular hardware platforms, largely because new hardware must retain some degree of interface and hardware compatibility with legacy systems, even as new capabilities and features are grafted on. The industrial PC platform has an inherent plug-and-play advantage in robustness and scalability. Built for life on the shop floor, the new controls have no moving parts, no fan or hard disc—the two most common failure modes in earlier controls.
As you’d expect on a PC, the new controls have Ethernet ports and fully functional USB 2.0 ports to enable drip-feeding a part program from an external drive. PC control platforms are customizable, and can be user-upgraded from mild to wild as machining requirements change. An owner who purchases a general- purpose control for standard machining can upgrade to a twin-CPU mold/die package with plug-in components. Specially-designed portals on the new controls make it easy to seamlessly integrate state-of-the-art third-party tools, such as a digital scanner or part probes and verification software.
Even the flash memory recommended for expansion works with the control’s software, which writes data in different areas of the card all the time to enhance life and reliability. The use of commercially available memory cards is a real cost advantage for mold shops where part programs are often quite large.
More and more, mold makers rely on high-speed cutting. Whereas EDM was once the clear choice, CNC machines have proven capable of not only meeting the necessary tolerance and finish requirements, but also streamlining processes. The right control plays a big part in accomplishing all of this.
The CNC isn’t the limiting factor in determining feed rates anymore, the mechanics of the machine are. And with mechanics like simultaneous 5-axis machining more accessible than ever, the right combination of machinery and control can be game changing for a business.
CNC control requirements for moldmaking
So, what do you need to look for in a control to take full advantage or machining capability and deliver precise, well-finished molds? First and foremost, the control should use an approach that is optimized for the milling machine and the manufacturing process. That is, the CNC control has features to either automatically smooth the axis movements or allow the operator to manually set tolerance limits.
Not only does moldmaking require precise control of both acceleration and deceleration along a programmed contour, but also compensation. For example, to properly compensate for a ball-nose end mill as the part or the tool pivots, the control should have some ability to dynamically adjust the cutter vector in X, Y and Z. Keeping the tool’s contact point constant is one of the only ways to achieve demanding finishes.
We’ve identified 6 areas that make a big difference. Let’s break it down.
Spatial plane function
Many mold cores or bases require five-sided machining without excessive coordinated movements. The CNC should have a spatial plane function that allows the machinist to set the plane that is machined on each side of the part. That way, the machinist can program each side of the part in an X-Y-Z plane without changing the CAM program and ultimately improve the tolerances between features on each side.
Contour deviation monitoring
The control directs axis movements to follow the 3D surfaces within predefined tolerance bands that consist of simple line segments. The control should be able to automatically smooth the block transitions while the tool moves continuously on the workpiece surface. An internal function that monitors the contour deviations controls that automatic smoothing. This function enables the user to define the maximum-allowed contour deviation. The default value is defined by the machine tool builder in a machine parameter of the control (typically 0.01 to 0.02 millimeter).
The tolerance also affects the traverse paths on programmed circular motions. This is particularly important if the core or cavity has cylindrical details, which a user machines with either an interpolation milling or a mill-turn function.
Linear encoder compatibility
The CNC control should be able to ensure that all machine axes follow the exact path when moving from X-plus to X-minus and then moving from X-minus to X-plus after a step over. In other words, there must be exact reproduction of adjacent paths after cutting direction reversal.
Vibration mitigation
A machine axis moving very fast and changing direction on a point or a using a higher-than-permitted feed rate for the cutting tool can generate vibration, which impacts part quality. The CNC control should be able to monitor any type of tool vibration caused by either high dynamic movements or higher-than-permitted feed rates, and then adjust feeds and speeds to avoid chatter marks on the final part.
Operator flexibility
The CNC interface should allow for optimizing the machine dynamics based on each part’s feature priorities. The control should allow the operator or programmer to verify and optimize the program on-the-fly. Consider, for example, several different components made on the same machine, each with different accuracy, surface finish and lead time requirements.
Automatic feed and speed adjustments
Milling through mold cores or cavities with variable workpiece thicknesses is another machining challenge. A solution is a CNC that can detect how much material is currently cut, so the feeds and speeds can be automatically adjusted without operator intervention. This is accomplished with sensors connected to the CNC that measure spindle load and vibration, and then identify and adjust speeds and feeds within milliseconds. This technology ensures that the machine maximizes chip removal rate, based on workpiece cutting tool engagement, and cutter and spindle life.