Advanced Materials Research Vols. 753-755 (2013) pp 2033-2036 Online available since 2013/Aug/30 at www.scientific.net © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.753-755.2033
Process optimization of 3D optical fiber laser cutting robot for press hardening of UHSS Huiqiang Liu, Yisheng Zhang * and Peixing Liu State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, China *
[email protected],
Keywords: press hardening; 3D laser cutting; photogrammetry; path optimization.
Abstract. The hot forming parts of high strength steel are difficult to cut with the presses and the traditional high-energy beam due to its high tensile strength which can be up to 1500Mpa and its complex structure. And the 3D laser cutting is an effective way to solve the problem. However some problems in the laser process affect the efficiency and quality seriously, including interference between the robot arms and the cutting head with the untrimmed parts, difficulty in determining the entry point and over burning etc. In this research three measures are taken to cut the anti-collision beam, including using photogrammetric technology to get its uncutted 3D model for offline programming to avoid interference, selecting appropriate entry point to trim without interrupt, amending normals and adjusting the process parameters at corners to prevent over burning. The results show that the efficiency and quality are improved and a creative new offline programming method is put forward. Introduction To improve the fuel consumption of cars, the reduction in the weight of cars is intensively required in the automobile industry. The application of UHSS sheets to automobile parts is effective in reducing the weight of cars [1,2]. Since the UHSS sheets have large strength[3], not only forming but also cutting becomes a problem [4]. The laser cutting process is a very flexible and rapid manufacturing technology with various advantages including attractive processing speed, high productivity, low running cost, ability to manufacture parts with complex shapes, excellent cut quality, non-contact operation, and ease of automation [5]. In the experiment, the 3D laser cutting on the anti-collision beam is studied. Experimental Equipment and process parameters The six-axis robot used in the experiment is cell mounted Stäubli RX160L, which has a maximum load of 28kg, a repeatable accuracy of 0.05mm and a working radius of 2010mm. An auto-tracking system, which manages the focal distance, is attached to the robot laser cutting system. A CW Nd: YAG laser with the maximum power of 400W is utilized. The wavelength and the spot size of the laser are 1070-1080nm and 0.1mm respectively. The focal distance is set to be 127 mm. Oxygen gas with a purity of 99.99% is employed as an assist gas. The thickness of the anti-collision beam is 1.5mm. By taking the three factors and five levels orthogonal experiments on the quenching plates, reasonable cutting process window has been determined. Offline programming With the help of Mastercam and RobotMaster, the offline programming is processed as the workflow shown in the Fig.1.
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Fig.1 Workflow of offline programming Cutting for trial After the calibration of the robot, we place the uncutted beam on the fixing bracket which is made by ourselves with hard steel plate. Then cutting for trial is started as programmed. During the trial, we come across several problems. First, the robot has interference with the uncutted beam. Second, the robot cannot finish the connected trajectory namely the edge without a stop. And at last over burning exists at some sharp corners. Path optimization Avoiding interference Using photogrammetry equipment named PowerScan- Ⅰ with a measurement accuracy of 0.06mm, the uncutted 3D model is obtained. In order to avoid the interference between the robot and the cutting head with the part, the cutted and uncutted models are imported together. The cutted (No.1) is used to generate the cutting path and the uncutted (No.2) to avoid interference. The combination model is shown in Fig.2.
Fig.2: The combination model
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Then a creative offline programming method is put forward as shown in Fig.3.
Fig.3: The creative offline programming When the method is adopted, the cutted model is replaced by the combination model and the collision points will be found in advance. Fig.4 shows the superiority to the old method. And then we can adjust the robot to avoid interference.
No collision
(a)
Collision (b)
Fig.4: (a)Collision cannot be found with the old method (b)Collision is found with the new method Selecting appropriate entry point The purpose of the entry point is to let the robot finish the connected trajectory at a time without a stop to save time, mainly for the edge. In the Fig.5, it shows the posture of the robot when cutting the edge. The Y-axis replaces the rotational angle around Z axis of the cutting head and the X-axis replaces the points on the trajectory. The yellow portion means at least one joint reaches the joint limit and the blue part means the robot is out of reach. As shown in the Fig.7, if we put two posture graphs together, it is easy to see if we can finish the cutting at a time and where to start to realize it. The Fig.5 shows how the entry point is selected in this experiment.
Fig.5: Selecting appropriate entry point Preventing over burning In the experiment oxygen is used as the assist gas, the oxidation reaction of the iron makes a lot of heat during the cutting. On the other hand, the number of points is increased at the corner, so that the laser head will stay longer time and the over burning happens. In order to prevent it, measures are taken as shown below: 1. By reducing the points to a reasonable number to reduce the staying time at corner, and amending the other normals’ angles to make the trajectory smoothly.
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2. By reducing the laser power and the gas pressure to reduce the heat input on the premise of cutting through entirely. Results and discussion Through the optimization, the anti-collision beam is cutted. Then comparison with the 3D model is made with the help of photographic measurement, the results are shown in Fig.6.
（a）
（b）
Fig.6： ：Cutted part (a)and Photographic results(b) From the deviation distribution in the results chart, it can be seen that the deviation of more than 90% measurement points are less than plus or minus 0.6mm. Conclusion By introducing the photogrammetry technology into the laser cutting to get the uncutted 3D model, a creative method for offline programming is put forward. The quality and efficiency are improved by avoiding the interference in advance, selecting appropriate entry point, adjusting the normals and the process parameters. References [1]
Kleiner M, Geiger M, Klaus A. Manufacturing of Lightweight Components by Metal Forming. Annals of the CIRP52(2):521–542 (2003).
[2]
Neugebauer R, Altan T, Geiger M, Kleiner M, Sterzing A Sheet Metal Forming at Elevated Temperatures. Annals of the CIRP55(2):793–816(2006).
[3]
Wang Chao, Zhang YiSheng, Tian XiaoWei, Zhu Bin, Li Jian. Thermal contact conductance estimation and experimental validation in hot stamping process. SCIENCE CHINA-TECHNOLOGICAL, Vol.55(7): 1852-1857(2012).
[4] Mori K, Akita K, Abe Y. Springback Behaviour in Bending of Ultra-High-Strength Steel Sheets Using CNC Servo Press. International Journal of Machine Tools and Manufacture. 47(2):321–325(2007). [5] Wollermann-Windgasse R, Schinzel C. Laser technology in manufacturing-state of the art at the beginning of the 21st century [C]. LANE 2001. Erlangen,87−102(2001).
Materials Processing and Manufacturing III 10.4028/www.scientific.net/AMR.753-755
Process Optimization of 3D Optical Fiber Laser Cutting Robot for Press Hardening of UHSS 10.4028/www.scientific.net/AMR.753-755.2033
