# Vision-Based Path Guidance to Achieve Dies-Free Roller Hemming Process
## 1 Introduction  
Sheet metal is the most commonly used component in automobiles, for which stamping is the main forming process. Recently, due to increasing demand for lightweight vehicles, automakers have introduced aluminum alloys or high-strength steels to replace traditional steels. With the inclusion of the new materials, many innovative metal forming processes have been derived. Hot/warm forming processes proposed and investigated by [PERSON] et al. [1], [PERSON] et al. [2], and [PERSON] et al. [3] are innovative forming processes to solve the problems of insufficient ductility of aluminum alloys and spring-back of high-strength steels.  
Hemming on the other hand is a sheet metal forming and joining process used in the automotive industry to join inner and outer closure panels and components. Such panels or components include hoods, doors, trunk-lids, tailgates, etc. This hemming processing is the process of bending/folding the edge of the outer part (skin) over the reinforcing inner one, which largely represents and determines the external quality of the final automotive products. Common hemming equipment can be catalogized into classical hemming (hem die-central), table-top hemming, robotic roller hemming, etc. In recent years, in order to achieve cost reduction and increase manufacturing flexibility, the industry has been phasing out expensive technologies, such as classical hemming, and transforming into the use of more economical means of production. Due to the high production flexibility of robotic roller hemming, it has become the mainstream process for joining metal sheets in the automotive industry.  
In the past, most hemming studies focused on simulating and analyzing the properties of materials, the complex interactions between geometric shapes and process parameters,or predicting the final quality and defects of parts. The approach used in the literature mainly includes finite element (FE) simulation of the roll hemming process. For instance, [PERSON] et al. [4] utilized FE to investigate the roller hemming process on an Al-Mg alloy. [PERSON] et al. [5] used FE simulation to accurately predict the geometry of the part before hemming, and also the required forces for the hemming operation. [PERSON] et al. [6] conducted numerical analysis of the roller hemming process; they found that evolution of the load on the roller, resulting in plane strain bending of the part, varied during the course of the hemming process. [PERSON] [7] studied the influence of hemming parameters, such as bending angle, thickness, and roll diameter, on the hemming force and final quality of the products. Other researchers proposed some novel strategies for hemming. For an example, [PERSON] et al. [8] used electromagnetic forming (EMF) for an aluminum sheet hemming process. Laser heating to assist the roller hemming process was proposed by [PERSON] et al. [9; 10] who discussed the effect of increasing heat in the hemming process on aluminum sheet metal after forming. [PERSON] and [PERSON] [11] presented a systematic review of the sheet metal hemming processes.  
Due to increasing market demand and the trend fo small-volume, large-variety production, saving costs while increasing productivity has become a common research and development focus in the automotive industry. However, in present practice, even the highly flexible robotic roller hemming still requires specific dies to support the sheet metal parts. In the present robotic roller hemming, the repeated and offline manual adjustment of the robot planning route is required, which has been found to result in high die costs, time consumption, and labor costs. The goal of the research presented in this paper was to solve this issue. With the proposed method of vision-based automatic compensation path guidance that is discussed in detail in the following sections, a universal fixed platform replacing specific dies is feasible, which allows for a dies-free roller hemming process that further reduces costs and expands production flexibility.