Keywords

1 Introduction

The work of painting and coating in the automobile industry used to be done by manpower, from prototype production and mass production to repairs. Nowadays, these tasks formerly done by manpower are replaced with automatic production. Prototypes are produced by computer graphics and mass production is done by robots. But repair work must be tailored to customers’ requirements, so manpower is essential. However, the number of engineers engaged in painting and coating is decreasing recently and the labor quality also is declining, so there is difficulty in passing on repairing techniques.

Generally, the engineers involved in painting and coating acquire the skills in colleges or vocational schools. But most schools do not allocate sufficient time for painting and coating practice. Thus, most new workers have no choice but to “watch and copy” the techniques of senior skilled engineers. However, it is not easy to acquire the optimal work skills corresponding to each customer’s request, because the required skills are different every time.

In Japan, people aged around 60 years old called “baby boomers” have reached retirement one after another, including many skilled engineers. With this social background, it is necessary to sever the decline of quality of engineers, and design a method to help unskilled engineers to acquire the necessary skills quicker and more accurately.

Currently, the ways that non-skilled coating engineers learn coating skills are from corporate or paint manufacture training, or practical guidebooks specified for coating techniques. This situation has not changed for decades. However, using the recent remarkable development of information technology, it is feasible to design a new method for learning coating techniques. We think motion analysis using 3D digital motion analysis equipment is one solution. Behavior analysis technology has been incorporated into acquisition methods of techniques in many fields. This method is effective in learning behaviors [1–10]. For instance, Murata et al. conducted a motion analysis of pitching motion in baseball and proposed a better throwing motion [2]. Marco et al. conducted an analysis of athletes in different postures on the jump phase in a downhill race and clarified the characteristic motions of the skilled skier [9]. In the succession of skills, the characteristic motion analysis of skilled technicians has been conducted in the fields of cooking, sewing, nailing, glass heat forming, and first aid chest compression procedure.

The purpose of this study is to develop the learning materials for unskilled workers to acquire necessary painting and coating techniques. As a first step, this paper intends to clarify the characteristic motions of experts. We measured the motions of the expert and non-expert during a spray gun operation by the 3D digital motion analysis equipment and analyzed the results.

2 Method

2.1 Experimental Outline

We had craftsmen who do work related to metal plate coating do coating work, and we recorded and analyzed the running of the spray gun with a three-dimensional motion capture system.

2.2 Experimental Environment

The experiment was part of an exhibition of metal plate coating that was done in Osaka Prefecture in February 2013, and was done in a period of 2 days. The guests of the exhibition were those who have to do with metal plate coating in auto repair and their family.

2.3 Participants

The participants of the experiment were the guests who came to the metal plate coating exhibition. Beforehand the participants were informed that the motions would be recorded and after the recording, immediately the results would be publicized. Of the participants of the experiment, those who were under-aged or those whose number of years of experience were not asked were omitted from the subjects of analysis. The subjects for analysis were 55 people. Out of them, those with more than 5 years of experience were counted as “experienced” and those with under 5 years of experience were counted as “inexperienced.” For the experienced people, there were 11 who had more than 5 years of experience but less than 10, there were 17 people with more than 10 years experience but less than 20, there were 4 people with more than 20 years of experience but less than 30, and one person with more than 30 years of experience, making a total of 33 years. With the inexperienced people, there were 11 people with no years of experience, 6 people with more than 1 month but less than 1 year of experience, and 4 people with more than 1 year of experience but less than 5 years, making a total of 22 people. The participants with no years of experience did not completely lack previous knowledge of coating, but were people who do work with metal plate repair and value estimations, and have a connection with auto repair.

2.4 Materials and Tools

In this research, to omit the necessary time to replace the object for coating, the coat-ing was done by having water inside the spray gun. The object for coating was a white-colored board likened to the door panel of an automobile, and board was marked off by 900 mm vertically and 900 mm horizontally with the inside of the marking as the object for coating.

2.5 Experimental Motion

The participants were instructed to coat the board using a spray gun. Specifically, the instruction was “please move the spray gun in the way you usually use it for coating.” Before the measurement the participants were told that the run-ning of the spray gun would be measured, and that the ability to “spray horizontally without moving vertically” and the ability to “move the spray gun in one steady speed” would be measured and analyzed.

2.6 Record

An infrared reflective marker was put on in one place at the top of the spray gun and was made as target position. For the gathering of the coordinates of the target position, an optical three-dimensional automatic motion analysis device MAD 3D SYSTEM (made by Motion Analysis) was used, and the sampling frequency was set at 120 Hz. The state during the recording is shown in Fig. 1. For the frame of reference, the direction of right and left was the x axis, forward and backward the y axis, and upward and downward the z axis.

Fig. 1.
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Photo of measurement

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2.7 Analysis

Usually, the running of the spray gun for coating is done by going back and forth with the whole coating object on level with the ground. At this time the first part that gets coated is the upper portion of the coating object. In Fig. 2 the tracks left on the xz surface of the target position of the spray gun is shown. In this research, for the distance and time of the running of the spray gun, all of the tracks were to be ana-lyzed.

Fig. 2.
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An example of trajectory in xz plane

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For the comparison between the experienced people and inexperienced people, an examination of the difference in average value was made. The significant standard was set at 5 %. And for the observation of the relationship between number of career experience and the results the moving of the spray gun, a cross-correlation function was done. The significant standard was set at 5 %.

3 Results

3.1 Spray Gun Handling Length

Figure 3 shows the running length of the spray gun by expert and non-expert. For the spray gun running length, the target for analysis was set for when the spraying began until the spraying stopped. The length of the expert compared with the non-expert was significantly longer. The length of the expert was 1.49 times longer than that of the non-expert.

Fig. 3.
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Spray gun handling length of expert and non-expert

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Figure 4 shows the relationship between the spray gun running length and the career experience. With the expert as the number of career experience became longer the running length tended to become shorter. However with the non-expert as the number of career experience became longer the running length of the spray gun became longer. In the non-expert, the spray gun handling length was moderately and positively correlated with the career experience (r = .530, p < .05).

Fig. 4.
figure 4

Relationship between spray gun handling length and career experience

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3.2 Spray Gun Handling Time

Figure 5 shows the running time of the spray gun by expert and non-expert. For the spray gun running time, the target for analysis was set for when the spraying began until the spraying stopped. The time of the expert compared with the non-expert was longer. The time of the expert was 1.20 times longer than that of the non-expert.

Fig. 5.
figure 5

Spray gun handling time of expert and non-expert

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Figure 6 shows the relationship between the spray gun running time and the career experience. With the expert as the number of career experience became longer the running time tended to become shorter. However with the non-expert as the number of career experience became longer the running time of the spray gun became longer. In the non-expert, the spray gun handling time was moderately and positively correlated with the career experience (r = .422, p < .05).

Fig. 6.
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Relationship between spray gun handling time and career experience

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3.3 Spray Gun Handling Speed

Figure 7 shows the running speed of the spray gun by expert and non-expert. For the spray gun running speed, the target for analysis was set for when the spraying from 1st stroke to 6th stroke. The speed of the expert compared with the non-expert was significant higher. The speed of the expert was 1.32 times higher than that of the non-expert.

Fig. 7.
figure 7

Spray gun handling speed of expert and non-expert

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Figure 8 shows the relationship between the spray gun running speed and the career experience. With the expert as the number of career experience became longer the running speed tended to become lower. However with the non-expert as the number of career experience became longer the running speed of the spray gun became higher. In both of the expert and the non-expert, the spray gun handling speed was not correlated with the career experience.

Fig. 8.
figure 8

Relationship between spray gun handling speed and career experience

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3.4 Spray Gun Swing Range

The spray gun handling is regarded as good for not shaken up and down and to parallel handling. Figure 9 shows the spray gun swing range by expert and non-expert. For the spray gun swing range, the target for analysis was set for when the spraying from 1st stroke to 6th stroke. The spray gun swing range of the expert compared with the non-expert was narrower. The spray gun swing range of the non-expert was 1.10 times larger than that of the expert. Average standard deviation of the spray gun swing range of expert and non-expert were 6.09 mm and 7.32 mm, respectively. A scattering of swing range of non-expert was larger than that of expert.

Fig. 9.
figure 9

Spray gun swing range of expert and non-expert

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Figure 10 shows the relationship between the spray gun swing range and the career experience. With the expert as the number of career experience became longer the swing range tended to become larger. However with the non-expert as the number of career experience became longer the swing range of the spray gun became narrower. In both of the expert and the non-expert, the spray gun handling swing range was not correlated with the career experience.

Fig. 10.
figure 10

Relationship between spray gun swing range and career experience

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4 Discussions

The running of the spray gun of the experts compared to the non-experts had more distance, was longer in time, and was faster. It is important to cover a long distance in order to perform the coating with evenness, but on the other hand it leads to too much usage of paint, so from the perspective of cost preservation shorter is better. The relationship between the number of years of experience of the experts becoming longer and the distance of the spray gun running becoming shorter shown in Fig. 4 suggests that as the progression of proficiency of technique takes place, so does the ability to perform the coating in the shortest distance possible with evenness.

From the perspective of cost preservation, it is good for the time of the running of the spray gun to be short. The reason is that the cost of work in auto repair is calculated based on the length of time of the work, and not the quality of the work. So the non-experts need to build enough technique to be able to do coating beautifully and in a short period of time. As it is shown in Fig. 6, as the number of years of experience became longer, the time of spray gun running became longer, and it is considered that the reason for there to be a tendency to become shorter is that when the number of years of experience is short the job itself is focused on, and once the performance level is maintained then the adjustment of the time of running the spray gun is possible. The same tendency is shown for the running speed of the spray gun.

The spray gun’s running speed that is recommended by paint makers, etc. is 2–3 m/s. In this research the target persons for the analysis all ran the spray gun faster than 2 m/s. As shown in Fig. 8, as the number of years of experience became longer, the running speed of the spray gun tended to become faster and slower. It suggests that once experience is had to allow one to maintain a certain performance level then they can maintain the best running speed. Furthermore, out of the target persons for analysis 43 people ran the spray gun over 4 m/s. It suggests that there was a separation in the education of the work and the work itself.

The space for vertical motion of the spray gun is small, and to run the spray gun parallel to the ground without shifting vertically leads to an even coating. Furthermore if the space of vertical motion of the spray gun is small, then the running distance becomes shorter, which is good from the perspective of cost preservation. However, if the time of work is prioritized, a certain amount of vertical shifting would have to be forgiven. As it is shown in Fig. 10 this is thought to be one of the rea-sons why as the number of years of experience becomes longer, the space of vertical shifting becomes less and more.

5 Conclusions

The purpose of this study is to clarify the characteristic motions of experts on spry gun handling for automobile repair. As a result, spray gun handling of expert is longer running length, longer time, higher speed, narrower spray gun swing range compare with non-expert. The reason for there to be a tendency to become shorter is that when the number of years of experience is short the job itself is focused on, and once the performance level is maintained then the adjustment of the time of running the spray gun is possible.