# Vision-Based Path Guidance to Achieve Dies-Free Roller Hemming Process
## 4 Conclusions  
In the present industry application of robotic hemming, the sheet metal part is commonly supported by a large and bulky die to minimize positioning errors of the sheet metal part in the hemming process. Such positioning errors have been found to be the primary errors contributed by the robot arm when loading the part onto the die before the hemming process. Such errors are determined by robot arm's accuracy and can be controlled within 0.1 mm, which is certainly below the industry requirement of 0.5 mm for hemming. To increase productivity and to allow small-volume large-variety production, dies-free robot hemming is thought to be the next manufacturing method in hemming sheet metal parts. As shown in Section 3.3 in this paper, without the support die to define and minimize the positioning error, errors of up to 4.4 mm can easily be found if the hemming path is generated solely from the CAD model, which does not meet the industry requirement.  
To overcome the problem and to fill the technology gap, the work presented in this paper proposed a vision-guided positioning method to optimize the hemming path for  
Figure 21: Measured surface flatness of the hemmed edge.  
Figure 20: Photography and schematics for surface flatness measurement of the hemmed edge.  
the robot arm on the sheet metal part. Design of the quick-change sensor module and the hemming tool with an active double roller module were proposed, investigated, and validated in this research. As no clear data can be found in the dies-free robot roller hemming, a hand-eye calibration method was proposed for the robotic system to eliminate all the possible tolerances and position and orientation errors from the robot arm, the sensor module, the hemming tool, and the sheet metal part of the dies-free support. The proposed methods and system described in this paper were carefully validated through calibrated experiments with actual sheet metal parts. It was found that a single-axis error can be down to less than 0.1 mm with the proposed methods and system. The studies also indicate that the roll-in/roll-out error and surface flatness of the hemmed metal sheet are well below 0.2 mm, satisfying the industry acceptance standard of 0.5 mm. Through the investigation presented in this paper, it was concluded that a dies-free robotic hemming process can be achieved and is feasible with the proposed methods and system. The quick-change sensor module not only can provide critical information for hemming path compensation, but it can also be used to provide online measurement to ensure quality of the hemmed parts.  
The work presented in this paper has archival values for research and development of dies-free robotic hemming. From the methods and system described and the reported results, one can further investigate topics such as active hemming tools that are more flexible yet automatically controllable and verifications and limitations of the dies-free roller hemming on complex curved surface sheet metal parts.  
Conceptualization, Y.-P.H. and B.-T.J.; methodology, Y.-P.H. and B.-T.J.; software, B.-T.J.; validation, Y.-P.H., B.-T.J., and C.-H.W.; formal analysis, Y.-P.H., B.-T.J., and C.-H.W.; investigation, Y.-P.H. and B.-T.J.; resources, Y.-P.H.; data curation, B.-T.J. and C.-H.W.; writing--original draft preparation, Y.-P.H.; writing--review and editing, Y.-P.H. and B.-T.J.; visualization, Y.-P.H.; supervision, J.-Y.C.; project administration, Y.-P.H. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Industrial Technology Research Institute (ITRI) with grant # K301 AR3200.  
Not applicable.  
The authors declare no conflict of interest.