Observation and Evaluation of surfaces of grinding wheels by means of
a high speed camera
Kiyokazu KOBAYASHI
1, a *,
Gen UCHIDA
1,b,
Hwa-Soo LEE
1,c,
Takazo YAMADA
1,dand Kohichi MIURA
1,e1Nihon University, 1-8-14 Kanda-Surugadai, Chiyoda-ku, Tokyo, 101-8308, Japan a[email protected], b[email protected], c[email protected],
d[email protected], e[email protected]
Keywords: grinding, grinding wheel, high speed camera, grinding wheel surfaces, digital image
Abstract. The purpose of this study is to evaluate the state of the grinding wheel surfaces and to improve the efficiencies of the grinding operations. In order to observe the grinding wheel surfaces, a lot of approaches are carried out so far. In this study, by means of a high speed camera, surface observation of the rotating grinding wheels is trialed. And then the surface states are evaluated by digital image treatment. As the experimental results, it is known that comparing the plural images, the variation of the grinding surfaces can be evaluated quantitatively. And it is confirmed that it is possible to observe the change of the grinding wheel surface by the dressing and the grinding operations.
Introduction
In order to improve the quality of grinding operations, the surface states of grinding wheels are one of the most important issues to be evaluated. From such a viewpoint, a new evaluating method is proposed in the present paper. In this method, the surface states of grinding wheel is evaluated by using a high speed camera. That is, the surface state of grinding wheel changes continuously in dressing and/or grinding operation. So that, observing the changes of the wheel surfaces in these operations, the surface state of grinding wheel may be evaluated quantitatively. In this paper, the basic idea of this trial is introduced. And the results of the some experiments based on proposed idea are described.
Method to shoot the surfaces of grinding wheels and image processing
Method of shooting. Figure 1 shows a schematic layout for photographing the grinding wheel surface with a high-speed camera. A high speed camera FASTCAM Mini UX 100 produced by Photron Corporation is used in this experiment. The grinding wheel is WA60H6V. Table 1 and table 2 show the specifications of the high speed camera and lens used in this experiment. Since photographing with the high speed camera needs a high intensity light, the surface of the grinding wheel is illuminated with a light source from the high speed camera side. In this experiment, in order to evaluate the change of the wheel surface states quantitatively, a part of the surface is set where is not to be changed in these operations, by making a step in the surface of wheel as schematically shown in figure 1. Light High speed camera Tripot Grinding wheel
Table 1 Specifications of High speed camera
Product name FASTCAM Mini UX100
Element resolution pixel 1,280×1,024
1 pixel resolution m 10
Maximum shooting speed (full frame) fps 4,000 Maximum shooting speed (split frame) fps 800,000
Trigger mode Start mode, Center mode, End mode, Manual mode, Random mode
Lens mount F mount, C mount
Trigger input signal V +3.3～+12 Table 2 Specifications of telephoto single focal lens
Product name AF Micro-Nikkor ED 200mm f/4 D IF
Focal length mm 200
Lens configuration 8 groups 13 pieces
Angle of view 12°20’
Minimum aperture 32
Shortest shooting distance m 0.5
Maximum shooting magnification 1：1(Equal magnification)
Mount Nikon F mount
Size mm 76(Maximum diameter)×202(full length)
Mass g 1190
Image processing. Figure 2 shows a schematic diagram of a pixel image obtained in this photograph. Each pixel has color information and they are discretized with 256 gradations from 0 to 255. When grinding is performed, abrasive grains in the grinding wheel surface will crush or released, so that the photographed image after processing will be changed from the state of before processing. Therefore, by calculating the difference in gradation of the image before and after grinding, information on the grinding wheel surface state can be obtained. At the position where the abrasive grains are not crushed or released on the grinding wheel surface, the difference in gradation becomes zero. On the other hand, at the position where some changes occur, some different gradation may take place. Setting some threshold amount to classify the change of the grinding surface, the image changing may be evaluated quantitatively.
1280 pixel
1024 pi
xel
processed
Observation and evaluation of changes in grinding wheel surface state by dressing
Experimental method. In order to confirm whether the change of the surface state of grinding wheel can be evaluated or not, the surface state of grinding wheel before and after dressing are photographed and compared each other. Table 3 shows the experimental condition. In the area about one-third from the left side of the image, the grinding wheel is the area where dressing is not carried out.
Table 3 Experimental condition
Dressing depth m 20
Dressing lead mm/rev 0.61
Grinding wheel rotation speed (at dressing) min-1 476 Grinding wheel rotation speed (at photographed) min-1 72
Shooting speed fps 4000
Shutter speed sec 1/10000
Pixel size m 10
Experimental results. Figures 3 (a) and (b) show images before and after dressing. It is difficult to discriminate the difference between these two photos by the naked eyes. Therefore, setting a certain threshold, the pixel where the difference amount is larger or smaller than this threshold amount is made black and white respectively. If the image has not changed, the difference in gradation becomes zero. However, A smaller gradation difference occurs due to variations in the amount of received light by the camera, the threshold amount is set to 10. The result of this operation is shown in figure 3(c). In this figure, it is known that the left hand side area where dressing is carried out and consequently to be white. As shown in this method, using gradation difference, the surface change can be described clearly. Furthermore, counting the number of pixels where changes larger than threshold amount in vertical direction in figure 3(c) is shown in figure 4. Figure 4(a) shows a schematic diagram of the dressing locus, and the figure 4(b) shows the number of pixel where largely changes, that is, the surface state of grinding wheel is changed by dressing.
(a) Before dressing (b) After dressing
(c) Tone gap between before dressing image and after dressing image
In this experiment, the dressing lead was 0.61 mm / rev. The number of pixel where changes larger than threshyold amount repeatedly changes large and small, and it is known that their interval coincides with the dressing lead. It means that it is confirmed that the change of grinding surface state by dressing operation can be evaluated by means of the method proposed in the present paoer. Observation and evaluation of changes in grinding wheel surface by grinding of carbon steel Experimental method. After performing dressing, a carbon steel (S50C) was ground by this grinding wheel. Carrying out plunge cut 5 times with 10 micron depth of cut, the surface before and after these plunge cuts are shot by high speed camera. Table 4 shows the experimental condition. In the area about one-third from the left hand side in the image, the grinding wheel is not dressed and used for dressed and grinding.
Fig.4 Difference between before and after dressing Non dressing area Dressing area
One scale is 0.61 mm (=dressing lead) (a) Pattern diagram
Table 4 Experimental condition
Dressing depth m 20
Dressing lead mm/rev 0.61
Grinding wheel rotation speed (at dressing) min-1 476 Grinding wheel rotation speed (at photographed) min-1 72
Shooting speed fps 4000
Shutter speed sec 1/10000
Pixel size m 10
Experimental results. The difference between before and after grinding are shown in figure 5, in which the image process operation as same as the result shown in figure 4 is carried out. In figure 5, differential amounts after every 5 grindings are shown and the total changes of differential amounts are shown in figure 6. From figure 5, it is clearly understood that the surface is changed at first and second grinding. Furthermore, it is also known that the changes between first and second and between second and third grinding are projected clearly in figure 6. It may be depending on the reasons as follows. That is, the surface of grinding wheel just after dressing is unstable and so many grains and bonds are floating, and consequently crushing and releasing are easy to be occurred. And then the surface is easy to be changed in the first and second grinding. On the other hand, after third grinding, the surface of grinding wheel becomes stable rather than the first and second grinding, and then large scale changes may not be took place easily.
Fig.5 Number of changed pixel between after dressing image and each grinding process image
In addition, Figure 7 shows an image obtained from the difference of gradation after the first and second grindings and coloring a portion where the gradation is changed. In figure 6, it is known that the gradations are changed in many pixels around 4 mm, 8 mm and 11 mm in the width direction. From figure 7, it can be confirmed that the place where the surface state largily changed in the wheel width and circumferential directions. In this experiment, it can be judged that the abrasive grains were crushed or released, because it is a change on the state of grinding wheel surface immediately after dressing. Continuing grinding as it is, wheel loading can be also observed. In these case, setting an appropriate threshold amount, the change of the surface state will be evaluated, and consequently the control of the surface state of grinding wheel will be possible.
Summary
By counting the number of pixels whose absolute value of the difference is greater than a certain threshold amount, the difference in gradation from the image between before and after dressing, and before and after grinding can be evaluated quantitatively. In this paper, only the first step of the proposed image processing method is described. However applying this method, it is confirmed that the change of grinding wheel surface can be evaluated quantitatively in grinding operations.
References
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Fig.7 Tone gap between first grinding process image and second grinding process
