CNC’s changing role in aerospace factory automation

In automated aerospace assembly, computer numerical controls (CNCs) are better able to address complex interpolated motion than programmable logic controllers (PLCs). CNC technologies are finding increased application in complex airframe alignment tooling, robot control, additive machining, fiber placement and tape laying, and 5-axis machining.

As CNC use grows, manufacturers must integrate them into product lifecycle management (PLM), manufacturing execution systems (MES), and manufacturing operations management (MOM) environments. To optimize production processes, MOM software focuses on efficiency, flexibility, and time-to-market, including:

• Integration with order management
• Advanced planning, scheduling
• Quality management
• Tracking and tracing
• Supervisory control and data acquisition (SCADA)
• Research and development (R&D) management

As CNC technologies and software management tools have grown individually – often counter-productively – challenges have emerged:

• Production planning, operational management of a variety of machines
• Optimizing performance, flexibility of PLC-, CNC-based production machines
• Extracting data for monitoring, putting the right information back into the machines, proper human machine interface (HMI)
• Managing operator skill levels on variety of controllers
• Maintaining variety of machines, spare parts

One platform, all technologies

Aircraft parts machining and assembly technologies can be standardized using today’s CNC platforms. With system openness, technology can be adapted to fit the machine tool and other production machining technologies – not only for traditional metal cutting, but for composites production. In-cavity fiber molding, composite tape laying, ceramic, and powdered metal additive processes use CNC technology with adaptive modes and high customization for motion control and data transmission.

Using CNC as a single standard for various production technologies enables:

• Uniform operation, programming of various machines
• Machine data integration on a standard communications platform; consolidated information from various brands of sensors, motion components
• Consistent, global programming, operations, maintenance training

The open architecture of advanced CNC comes from standard systems, the virtual numeric control kernel (VNCK) for virtual simulation, and simple language commands on the HMI – all engineered on the control at NC language level. PLCs can be adapted via standard engineering tools, and CNC applications can be supplemented with software tools from third-party suppliers – tool and process monitoring systems, measurement systems, tele-services, and video monitoring systems.

High performance 5-axis machining

To machine complex parts, CNC software can improve performance and precision, since machine kinematics are no longer the sole determinant when machining. The CNC can adjust interpolation of machine axes to integrate with orientation vectors to the workpiece, increasing surface finish quality, optimizing cutting speed, and increasing efficiency.

Workpieces can be programmed in Cartesian coordinates and system-specific cycles, and function macros can automatically calculate machine axis movement.

Volumetric compensation

Volumetric compensation software (VCS) implements compensation at machines with five axes, if the tool with two rotary axes can be orientated to the workpiece. Measured error compensation of the linear axes is calculated, depending on the position of the tool, to the tool center point.

For use on 3- and 5-axis portal milling machines, VCS expands CNC technology by implementing volumetric compensation of 21 geometrical machine errors on a Cartesian machine tool.

Geometrical machine errors cause tool-center-point-offset and tool-orientation error. Because of the positions of the rotary axes, their mutual offsetting, and the orientation of the tool, parts such as turn and swivel heads may exhibit systematic geometric errors. Small errors in the guide system for feed axes – positional, horizontal and vertical straightness, pitch, yaw, and roll – will arise in every machine.

In a 3-axis machine, 21 geometry errors can be ascribed to the tool holder – six error types per linear axis plus three angle errors. Deviations form volumetric error, the deviation of the real machine tool center point (TCP) position from an ideal, fault-free machine. Qualified CNC technicians can determine volumetric errors onsite using laser measuring devices. Entire measuring curves are recorded, as individual error sizes are dependent upon the position of the relevant feed axis and on the measuring location. For example, errors attributed to the X-axis turn out differently if the Y-axis and Z-axis are in different positions – even if the errors are at the very same position on the X-axis. On today’s CNC, rotary axis errors can be determined in just a few minutes so the machine’s accuracy can be constantly checked and corrected, even during production.

System integration

Digitalization of industrial processes will eventually encompass every step of product life cycle – design, production planning, engineering, execution, and a network of global services.

Multi-axis machining workflow is typically characterized by the computer-aided design (CAD), computer-aided manufacturing (CAM) CNC process chain. Enhanced CAM with specific post-processing combines realistic machine simulations driven by a VNCK to create a digital twin.

CNC units can be networked to transmit data to the cloud for monitoring and corrective action, tracking back from production output and maintenance alerts on the machine to integration with CAD files and part prototypes. Production of components and assembly of finished aircraft benefit from the controls on the machines. Core elements of a full digital enterprise system begin to emerge from the data generated by the machine controls.

Simulation

Advanced simulation software and virtual production subjects the process chain to a simulative analysis from the CAD/CAM system to the workpiece surface. Instead of repeated testing on an actual machine, programs can be optimized in computer simulation, with the exception of the numerical control program, which is simulated on a real machine.

Virtual production can be performed in three steps:

• Analysis of the data quality as provided by the part program
• Execution of the part program by the numerical control (NC) where set-point positions on the NC output are evaluated and velocity can be optimized and controlled
• Simulation based on characteristic features of position-controlled drives including machine dynamics

Virtual production reduces machining times, improves surface finish, and shortens start-up times for new workpieces.

The mechatronic twin can be run with only a CNC simulator, through Mechatronics Concept Designs (MCD), a Siemens machine design tool. Production times, collision avoidance between spindle and workpiece, tool path optimization, and machine kinematics can be virtualized and evaluated prior to test runs. This simulation can drive the creation of G-code programs on the final machine.

Motors, drives, shafts, slides, cam discs, spindles, etc. are stored with all technical data and animated in the MCD. Positions, feeds, and speeds from the central processing unit (CPU) can be transferred to the design engineering computer in the MCD.

The 3D model can be operated like a machine tool via the CNC control. In manual mode, individual axes can be triggered. In production mode, axes can be moved synchronously toward each other via electrical cam discs. Machine functions can be actualized, checked, and optimized.

Photo courtesy of Coast Composites

Communication at all levels

With Profinet, the Sinumerik 840D sl CNC integrates with the Siemens Totally Integrated Automation (TIA) environment – from field level to production level to manufacturing execution level.

The TIA portal’s engineering framework supports planning, programming, and optimization of machine and process tasks. Consistent, standardized operating enables users to program and integrate all Siemens controllers, distributed input/output (I/O), HMI, power supply and distribution, drives, network components, motion control, and motor management. CNC data seamlessly channels through this engineering tool to a centralized workstation or control room.

With smart library shared data storage, users can tap existing libraries of proven CNC code to program universal hardware and software functions, and they can develop and store libraries of their own proprietary code.

The digital factory and digital enterprise remain key targets for aerospace, and the power of the CNC on all operational machine motion control, data gathering, and communications levels continue to feed that process, evolving at light speed.

Siemens
www.siemens.com