Multi-Robot Solutions Enable High Performance 3D Metal Cutting

Laser applications have been increasing in use as they support lightweight designs. By Dr Torsten Scheller, global director of technology, product management, and research and development, Jenoptik.

Looking back at the economic crisis in 2010, the processing machine business has completely recovered by now and is growing even further. The automotive-related sectors in the US and China are once again investing in conventional as well as laser processing systems.

By now, laser cutting is the main market for high power laser applications, especially for the cutting of 2D sheet metal parts for automotive or electronics applications, and there is also a rising need in the processing of 3D parts. Besides metal cutting of car body components, there are other processes like weakening, welding and brazing of plastic and metal parts for body-in-white, interior or exterior applications.

One of the main trends supporting the increase in laser applications is the use of light weight design in the automotive industry. This has led to the introduction of new materials like press hardening steel (PHS) or carbon/glass fibre reinforced materials which are hardly to be processed without a laser. Additionally, these materials enable the industry to cater for more complex designs of automotive parts and car design. Particularly where autonomous driving will lead us to a new approach of cars, the interior might become a second living room with special requests regarding surface quality of the parts.

Numerous Advantages In Laser Technology

Nevertheless, laser technology has to be economical. Though the technology is not cheap, the advantages in productivity, flexibility and quality have the potential to justify for higher machine investments compared to competitive technologies.

A manufacturer of lasers and material processing as well as industrial metrology, Jenoptik is one such company that provides a basic robot technology that is capable of processing 3D shaped parts. The beam in motion (BIM) system is an approach for 3D cutting and the core component of the system is a robot module named BIM.

The technology can be applied in different applications and different machine concepts to support productivity and flexibility in laser processing. Due to its light weight design, the robot system has a high level of accuracy and speed for 3D cutting tasks, with a positioning accuracy down to ±50µm.

Key applications where the performance parameters of the laser machine can be used are mainly car body components—such as hydro-formed tubes and rails as well as PHS parts—or interior and exterior of automotive parts like bumpers or instrument panels.

Highly Productive Robotised Cutting Systems

To close the gap in performance between robot systems and gantry systems, improvements with regard to path accuracies and cutting speeds had to be realised. The laser manufacturing company took up this challenge and together with their partners, developed a system approach based on robots to meet cutting standards in the automotive industry.

To reach accuracies and speeds comparable to a gantry system, it is necessary that the robot maintain its basic dynamic and accuracy. Therefore the mass which has to be moved by the robot needs to be minimised. During the development of the BIM system, this has been reached by reducing the weight of the robot using lightweight structures, as well as by reducing the load of the robot with a small cutting head. Parameters reached for cutting press hardening steel parts of about 2 mm in thickness reach up to 300 mm/s in speed with repeatability in positioning of accuracy down to ±50µm. This performance is capable of fulfilling requirements with accuracies for all OEM specifications in car body design.

To further reduce the load of the robot system, the fibre of the laser source is not coupled into the cutting head. Alternatively, the robot can be modified in a way that the laser beam can be coupled into the base of the robot. This is guided inside the robot by a sealed mirror system which is purged by air to avoid contamination.

This approach results in an almost maintenance-free system as well as highest accessibility to the work piece. Another advantage using this equidistant beam guidance system is the independence of the laser wavelength from 500 nm to 10 µm. This system is an enabling approach to process 3D components of different materials using the most suitable laser source.

Verifying The BIM System’s Performance

The BIM system approach is designed as a modular concept which enables the core component “robot module” to be combined in a multiple robot cutting cell on minimised floor space that works simultaneously on the same work piece. To verify the performance of the system, a feasibility study was done by processing a press hardening steel A-pillar. This is a standard part out of production which has a typical distribution between straight cuts and inner contours like holes or slots.

The part in Figure 1 has been processed in a production-like test run of about 1,800 pieces on a standard machine based on the 3D-laser cutting technology. Out of the test run, 26 parts were extracted and compared to 26 parts being processed in parallel on a gantry system using different fixtures, with all parts analysed after processing on a coordinate measurement system. The results were then compared regarding the variation on the hole location, hole diameter, hole circularity and edge location.

The results revealed that the performance of the processing systems is on a similar level. On the basis of cycle time, the gantry system’s performance was found to have improved by approximately five percent. This difference in performance did result in some quality issues on the 2D contours at maximum speed, and can only be improved by slowing down the movements of the robot. By doing this adaptation, the quality of the features has been in spec and confirmed the results of the processed test structures.

After realising the advantages and disadvantages of the processing systems, a focussed improvement on smaller topics was executed on the robot. By using less joints of the robot moving simultaneously, a shorter cycle time by 50 percent was achieved (depending on the contour) and there was also an improvement in the quality of the geometrical variation of the contours.

Taking into account all these improvements, the BIM processing system is capable to cut parts like press hardening steel parts up to 300mm/s, and with the accuracies of the contour down to 50 µm as well as path contours down to 200 µm. These improvements had been transferred into production trials of more than 20,000 pieces of a different A-pillar and it was shown that the improvements enable this system approach to compete with a gantry system in cycle time at comparable cutting quality.

Cutting Larger Dimensions With Multiple Heads

Within car body applications, tubes and rails represent a fixed portion of the car body. When increasing the degree of freedom in design, the shape of these parts gets more complex and by hydroforming, there are limitations regarding pre-processing of contours and end cuts of the parts.

Figure 2 shows the part with a length of over 2 m, 26 contours distributed over the complete skin surface and two complex end cuts to be processed within 45 s cycle time. Due to the large dimension of the part, the expected cycle time and the limited available floor space, a machine setup with turntable and two cutting robots had been designed. The two robots work simultaneously on the work piece to reach the expected cycle time and the machine successfully integrates into a fully automated process line.

Following the trend of lightweight materials in car body, a way to reduce CO2 emissions include the use of casted aluminium. The main obstacle for further growth of volume is component cost, compared to other technologies that are mainly driven by long cycle times of the different processes to come to a finished part.

Reducing Processing Times

One approach to reduce cycle time and to gain flexibility in tooling is to replace drilling and punching to generate the contours inside the part using laser processing. Material thicknesses of the casted parts are between 3 mm and 6 mm, making them feasible to be cut by laser, producing parts with good geometry and burr.

Compared to drilling or milling operations, processing times can be reduced by up to 50 percent which gives a clear cost reduction and generates higher productivity. Looking at the outer trim, laser cutting shows a higher degree in flexibility and better tooling cost as compared to punching as well as the ability to do undercuts.

The BIM system is capable of fulfilling the specifications in quality and speed which is necessary to run this application, and gantry systems are suitable for cutting tasks. Compared to these systems, robotised solutions are offering the possibility to work more closely and with multiple tools within one work piece.

APMEN Metal Cutting