Modern manufacturing environments are continually evolving. To meet increasing productivity requirements, turnaround times, and cost objectives, aerospace manufacturers continue to invest in higher technology solutions such as multi-axis and multitasking machines that enable more complex processes. These processes simplify part setups, reduce handling between setups or machines, enable better (shorter) tooling in many cases, and maximize valuable floor space by consolidating multiple processes on fewer machines. Proper application of this equipment and maintaining high billable hours on these machines is essential.
To successfully realize these equipment investments, software plays a major role. CAD/CAM software has traditionally been sought out for its toolpath generation with collision avoidance and manufacturing execution and planning features, but lately there is also an increased emphasis on machine simulation solutions known as virtual machining. The virtual machining term defines a class of technology, and as with other software categories, not all solutions are the same.
Early simulation tools focused on reading neutral-language NC formats such as APT. Simulation would identify gross errors and give some confirmation to the manufacturing process, but the toolpaths and movements defined in the postprocessor would be first seen at the machine tool.
G-code simulators have grown in relevance and provide better feedback to the actual machining situation. Be aware that some of these simulators reverse post-process code in the background and then simulate all machining formats through a standard processing engine. Then they are actually simulating neutral-language code again.
G-code simulators need to factor in the language commands that are possible at the machine control, either as output by the CAM system or possibly with added codes by the end user. Otherwise, a command in the part program might not be interpreted properly by simulation software. Also, both CAM neutral-file and G-code formats often change by version. This is done to enable new toolpath strategies, new types of supported cutters, or new capabilities available in machine controls. These changes provide the need to update simulation “formatters” from time to time. A closed-loop environment including CAM and simulation increases the assurance of compatibility.
The performance of simulation software has improved continually due to greater capability of computer processors and graphics cards, and improvements to monitor displays. In addition to crisp images and faster processing, current simulation tools model the machine environment more completely including tool change, lasers and probes, and coolant blocks.
The goals of virtual machining programs are to simulate the computed toolpaths, and to add links between jobs using knowledge regarding the machine tool and its capabilities and constraints. Then the virtual machining system can optimize and add value to the computed toolpaths. Many aerospace components are produced in large lot sizes, and the tool path enhancements from virtual machining have multiplied benefits.
By its name, the goal of virtual machining is to provide the postprocessor and simulation, as well as closely model the machining process. This reduces the need for confirmation tests at the machine, thereby increasing the efficiency of machine tools and the machinists who operate them. Many use the term “Digital Twin” to describe the virtual representation of the machining process, but not all twins are alike. The standard expectation of the digital twin is geometric modeling with kinematic motion. But the system should also model the movements from home or tool change positions, and handle controller commands such as tool center point control, plane commands, canned cycles and cutter compensation.
By providing a realistic representation of the machining environment, virtual machining can also provide and add valuable information. Optimized virtual capabilities provide innovative solutions over and above simulation and identifying error conditions. These capabilities improve toolpaths that are traditionally calculated in the CAM engine. They incorporate knowledge of the part model, tooling, the toolpath calculation including parameters such as negative allowance, collision check engine, and the machine tool model with its physical constraints. Many machines used in aerospace component manufacturing have an assortment of complex geometric constraints such as asymmetric spindle housings and limited linear or rotary axis strokes.
In a virtual machining center environment, individual part programs can be linked with smooth and safe connections that enable the cutter to remain close to the workpiece. The benefit is high assurance and time savings as compared to moving the machine to a home or safety position between programming jobs. These links can produce meaningful time savings, especially in components made with fast-cutting aluminum that is often used in aerospace applications. Further, the smooth connections are also better for the machine tool dynamics compared to fast moves with hard stops and sudden direction changes.
Many toolpath programs may look good on the screen, but they do not ideally utilize the machine axes. One example is vertical toolpath orientations on a five-axis machine. Consider a circular groove or a hole pattern at a fixed radius on the part. With a vertical orientation, several solutions are possible using different C-axis positions. Typically, the X and Y axes on a machine are faster than rotary axes and may be preferred to achieve the lowest processing time. Using a common trunnion style machine design, there may be constraints at the 3 and 9 o’clock positions (looking down on the C-axis table) due to a potential interference with the trunnion motors.
Although the groove or hole pattern may be better machined by using X and Y linear axes, the machining process is improved by avoiding the trunnion motors and focusing the machining at a 6 o’clock position, rotating the part into this orientation primarily using X and C axes. Further, making a circular groove using a rotary axis may interpolate a better part feature.
Similarly, some machines cannot reach beyond the machine centerline. An optimized virtual machining process can invoke rotary motion when needed to avoid attempts to machine in such regions. Some CAM programming environments offer optimizations that require the programmer to foresee the machine limitations and compensate using a switch in the software. As with the above circular groove, an optimized virtual machining process can change a toolpath with X and Y axis motion into X and C motion. Or for some geometries, the C-axis can be pre-positioned to enable the machining to occur with X and Y motion only. Using an optimized virtual machining process provides a more efficient solution than depending on the programmer to identify all constraints.
Many machine tools have limited rotary axis ranges. This is common in gantry or fork-head style machines that are common for the manufacturing of large aerospace components. The rotary axis limitation is a constraint from the machine tool design. A programmer may recognize this limitation and avoid programming processes that continually accumulate rotary movements. But there are some processes such as swarf milling where the cutter is fully engaged with the workpiece, and it is not easy to change the programming strategy. Here, an optimized virtual machining process can anticipate the coming axis limitation and potentially pre-position a rotary table to then enable the entire cut in one motion. Or if necessary, it can invoke a retract and safe rewind routine to continue the cut within the valid machine axis range. The rewind motion is collision-checked and can be kept close to the workpiece to avoid wasted time compared to returning to a machine safe position for each rewind.
The collision check in most CAM systems during toolpath calculation is between the workpiece and the cutter, and possibly other revolved objects that are aligned with the cutter. Then the simulation can identify additional issues such as the machine head or rotary table. These systems often provide collision detection and provide warning messages, but rarely override a toolpath to create a safe result. Many milling machines, especially larger machines, may have an asymmetric shape to the machine spindle system. Or may even have just a rectangular housing around the spindle.
The mathematics used to determine the angular positions for five-axis machining usually generate two solutions. Depending on the machine axis ranges, both options may be valid. The typical process is often to select a starting position, and then subsequent positions are determined being closest to the reference angular location. The preferred solution is determined based on the available tilt axis range or may be selected for human factors, such as to have the rotary table tilt toward the machinist.
With asymmetric head geometries, one solution may be collision free while another may result in interference. Or an interfering solution may be resolved with a long tool stick-out length, albeit with a less than desirable machining outcome. The power of an optimized virtual machining solution is that the best angular option may be determined multiple times during a program. After each retract move within the programming, the best solution for the next string of instructions can be determined. This step can be performed manually in some systems, or may not be addressed at all, but on large aerospace components with many pockets and many retracts, there could be dozens of decision locations. The optimized virtual machining process determines the best solution for each of these toolpath strings automatically, and safely links the machining regions together.
Aerospace components often have requirements to machine features that are not accessible along the cutter centerline. In these cases, an angled attachment (often a right-angle attachment) is mounted in the spindle and is used to drive the machining operation (hole, slot or pocket). Most CAM systems and many CNC controls don’t adequately model these situations. Open Mind’s Virtual Machining Optimizer can process angled head solutions with many machine controls, including complete collision detection. It can also process collision avoidance in the event the head attachment housing interferes with the workpiece.
Programmers increasingly focus on software for toolpath calculations and simulations. Simulations are a great way to confirm the machining process without directly using the machine resource. Advanced simulation tools such as an optimized virtual machining process add value beyond verification of computed toolpaths and can enhance the machining process. Manufacturing of aerospace components especially benefit from these optimizations that provide savings throughout the serial production volume.
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