On-line Thickness Measurement System for the Metal Spinning Process

Spun-part wall thickness is a key output parameter of spinning products. Thickness affects the spun part strength: Low thickness leads to cracks on spinning products. Hence, it is crucial to measure and control wall thickness. However, thickness measurement and a control system for the spinning process are still offline methods. That is, these parameters must be measured after the spinning process is completed. In this method, the cross section of the spun part is cut, and the wall thickness is measured using a measurement tool. Thus, the measurement system is not applicable as online method. Hence, this study proposed the online thickness measurement method for the spun. Here, the mandrel-less spinning machine and a line laser measurement system were developed. The line laser measurement system, including two sets of line lasers and cameras, was attached on the spinning machine. Both sets of line lasers and cameras were used to measure the thickness profile of the spun part. The first set of a line laser and camera was used to capture the surface profile in the front of the spun part, while the other set was used to capture the surface profile behind the part. Then, the digital image processing (DIP) was estimated the spun thickness by using both images. In the experiments, the spun part was formed by the variation of degrees of angle and spinning distance. In each experiment, the spun-part thickness was measured by the cross-section method and line laser measurement method. Both results were compared and discussed. The result revealed that the thickness estimated by the line laser measurement system is similar to that estimated by the cross-section measurement method. An average error of 3.67% was obtained by the line laser measurement system.

Digital image processing; Real-time thickness measurement; Sheet metal spinning

Metal spinning is a conventional sheet metal formation process that utilizes a compression force between the roller (tool) and mandrel to form a blank sheet. A workpiece is formed under a compression force and bending force. This process exhibits advantages of flexibility, cost-effectiveness, and using simple tools. Owing to these reasons, it is still used widely in the sheet metal industry.

First, the spinning process is employed for the formation of a symmetrical metal sheet. A mandrel and roller are used in the spinning process. The mandrel is used to support a workpiece during the roller pressing over the workpiece. If the shape of the product is changed, the mandrel also must be changed. Hence, traditional spinning is not a flexible formation process. Several researchers have proposed the development of a non-symmetrical spinning process and mandrel-less spinning. Awiszus and Meyer. (2005) has employed spring-controlled rollers to control the tool path in non-symmetry spinning. In 2019, Hirohiko (2019) has developed a computer control system for the spinning process by using servo motors and force sensors to control the tool path in non-symmetry spinning.

1.1.  Mandrel-less Spinning

In 1996, Kitazawa et al. (1996) have developed mandrel-less spinning to increase flexibility. A blank sheet was clamped to a blank holder without a mandrel. Next, the roller was moved to press the spun part. This method could be employed for the formation of a part without using a mandrel, but the blank sheet must be sufficiently strong for spinning. Shima et al. (1997) have developed two rollers for the spinning process, which were used for clamping and forming the part. In 2011, Music and Allwood (2011) have developed a flexible spinning method by using three sets of rollers: blending roller, support roller, and working roller. These rollers could form the part in the same manner as that performed by traditional spinning. In the above-described study, the spinning process can be employed to form an asymmetrical part without using a mandrel. In this study, an on-process thickness measurement system was developed and installed on a mandrel-less spinning machine as the measurement system needed to capture font and back images of the spun-part surface. Spinning using a mandrel cannot capture the back surface images of the workpiece because its surface is behind the mandrel. 

1.2.  Thickness Measurement Method for the Spun Part

The measurement method for the spun-part wall thickness involves the formation of a spun part and cutting of its cross section. Next, a measurement tool is used to measure the thickness of the cross section. This method has been employed by Avitzur and Yang (1960). These authors have drilled a blank sheet and plugged holes with the same material before spinning. Kalpakcioglu (1961) has employed the grid line method. Quigley and Monaghan (2000) have employed a surface etching method. They have etched a circular pattern on a blank sheet surface before spinning the blank sheet. Next, the blank sheet is used for spinning, and the spun part is cut and measured after the completion of the spinning process.

The spun-part wall thickness is a key parameter because thickness affects the strength of the part. However, all of the spun-part thickness measurement methods are offline methods, which cannot be applied for real-time monitoring. In this study, an online thickness measurement method for metal spinning is developed.

1.3.  Sheet Metal Thickness Measurement Method

A laser displacement sensor is typically used for measuring the sheet metal thickness. In this method, two sets of laser displacement sensors are used (Figure 1a). The first set of sensors is used for the measurement of the distance from the sensor to the top sheet metal surface, while the other sensor is used for the measurement of the distance from the bottom sheet metal surface. Next, the sheet metal thickness is calculated by using both distances (Ancheng et al., 2014). As laser measurement is a non-contact method, it can be applied to on-line measurement. Saurabh et al. (2019) have used a laser sensor for the on-line measurement of the wire steel diameter. Laser sensors have been used to measure the thickness of a workpiece placed between the laser receiver and transmitter. In addition, a line laser can represent the workpiece profile; thus, it can be applied to measure the workpiece geometry. Noll and Krauhausen (2008) have developed an on-line system to measure the flatness of a steel-rolled product by using a laser sensor. On the other hand, laser sensors have been applied for various on-line measurement systems, including the detection of microcracks on a specimen surface (Andreas et al., 2018) and on-line measurement of a liquid droplet size (Emir et al., 2021).

Laser sensors are an on-line measurement method, which can be applied to closed-loop control systems. Malik and Grandhi (2009) have applied laser sensors for the flatness control of closed-loop rolled steel. Researchers have used flatness results to compare the flatness setpoint and data sent to the controller. If the flatness out of spec, the controller will adjust the process parameters. James and Julian (2014) have used line lasers to measure profiles of the metal-spun product, corresponding to real-time measurement. The results from the system were used as a feedback signal. Hence, these results are utilized to calculate the spinning toolpath for the purpose of reducing the spring back of the spun product.

Figure 1 Application of a laser sensor for: (a) thickness measurement (Ancheng et al., 2014); (b) flatness measurement (Noll and Krauhausen 2008); and (c) the 3D DIC technique (Aydin et al., 2018)

Digital image correlation is another important technique for measuring stain and thickness in material testing applications. As a white-light speckle technique, DIC is a non-contact optical measurement technique that is based on the comparison of deformed and undeformed digital images. Aydin et al. (2018) have employed a 3D DIC method with the Erichsen cupping test (Figure 1c). The result show, DIC method can detect specimen cracks and measure thickness reduction during testing. However, this technique is more complicated, which is still only used in material testing applications (Quanjin et al., 2020).  

In this study, line laser lighting was utilized for the measurement of the spun-part thickness. The principle of the line laser thickness measurement system was adapted from the laser displacement sensor system. This system used two sets of line lasers and cameras for capturing the spun-part surface profile. The first set was used to capture the inner surface, while the other set was used to capture the outer surface of the part. Then, image processing software was utilized to calculate the spun-part thickness. Line laser thickness measurement systems are advantageous as they are non-contact measurement systems. Therefore, such a system can be applied for on-process thickness measurement. However, this system can be used only for mandrel-less spinning, and the environment light also must be controlled when using this system. Limitations of this system are discussed in conclusions.

        In this study, the line laser thickness measurement system is developed for the on-process measurement of the spun-part wall thickness. The advantages of this system are as follows: (1) This system is a non-contact measurement system. This method exhibits advantages of no measurement tool wear, high-speed measurement, and real-time measurement. Therefore, the thickness can be measured even during the spinning process; (2) This system is a real-time measurement system. Thus, it can be applied to a closed-loop control system. As part of future studies, the thickness results from this method will be used as feedback signal for the development of a real-time thickness control system. The real time thickness will be compared to the thickness setpoint. If the thickness is different from the setpoint, the thickness control system will adjust the spinning process parameters, such as tool path, spinning depth, and spinning speed; (3) In the other thickness measurement methods, the cross section of the workpiece must be cut, and the cross-section thickness must be measured. Hence, the workpiece is destroyed. However, there is no need to cut the workpiece using the line laser measurement system, and the workpiece can be used after measurement.

         However, the disadvantages of the system are as follows: (1) Camera resolution affects accuracy and processing time. If a high-resolution camera is used, the accuracy will increase. However, the processing time also will increase. If this system is applied to the closed-loop control system, camera resolution needs to be optimized because the closed-loop control needs a suitable accuracy and a short processing time; (2) The precaution for this method is environment light. The vision system can be disturbed easily by the environment light. Light from other sources can affect the vision system; (3) During the experiment, the position of camera and lighting must be fixed. The camera movement and lighting lead to result errors; (4) This system can be used only with the mandrel-less spinning process because the line laser and camera cannot be installed to capture the inner side of the part for spinning by the mandrel process.

    This research was facility supported by King Mongkut’s University of Technology North Bangkok and financial supported by the Thailand Research and Researcher for Industry (RRi) [grant number: PHD59I0080].

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