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Woodworking Process Control using IR Camera and Pyrometer H. Madura, Z. Sikorski, H. Polakowski, M. Kastek Institute of Optoelectronics, Military University of Technology, 2 Kaliskiego St. 00-908 Warsaw, Poland, fax. (+48 22) 6668950, e-mail: hpolakow@wat.waw.pl W. Sokolowski Wood Technology Faculty, Warsaw Agricultural University, 26/30 Rakowiecka St. 02-528 Warsaw, Poland Contact

Abstract

The paper presents an application of IR camera and pyrometer for an automatic control of woodworking process and wear of tool blades during the machining process. The application makes use of relations between the wood-machining temperature and speed. These relations are dependent on the wear of tool blades. The method is simple and cheap, especially if the pyrometer is applied.

1. Introduction

Similarly to metal cutting machines, also wood cutting machines can be equipped with automatic devices for determination of tools conditions. The paper presents one of the methods used for determination of limits of woodworking parameters or admissible wear of blades of cutting tools.

Determination of the tool blade wear during a cutting process is extremely difficult. As we know, there is a close relation between the cutting process temperature and the blade's wear. Temperature measurement of the tool blade used for woodworking is an appropriate factor of the process control.

During machine cutting the balance between the temperature of tool blade and temperature on the surface of the worked material is observed. Measurement of the worked material temperature enables determination of the technical conditions (e.g. wear) of a cutting tool [1].

The paper presents the results of measurements of woodworking process parameters using continuous measurement of the wood-surface temperature with IR camera and pyrometer.

2. Measurements of temperature changes using IR camera

Figure1 presents the experimental set up. The camera is situated next to the milling machine and is focused on the working plane of a cutting tool. The worked elements were moved by means of a cylindrical, universal feed mechanism along a guide, which was made for the experiment. In the guide a rectangular hole was drilled for observation of the processed surface just behind a milling cutter.

Fig 1: Control stand: 1-milling machine, 2-worked element, 3-slot for observation of the treated surface, 4- IR camera, 5-thermovision images, 6-tool.

IR Inframetrics 760 camera was used for the measurements. The thermograms of the processed wood and tool blade (for temperature range from 17,9°C to 29,3°C with accuracy ±0.2°C) were registered on a videotape and in a computer memory.

In the analysis of thermovision images the temperature distribution was registered along a line parallel to the feed direction, in the working plane of the cutting toll (i.e., at the half-way thickness of the worked element).

Figure 2 presents temperature distribution on the worked wooden board with the use of two blades: sharp or blunt ones. The measurements were carried out for the same type of wood and at the same cutting velocity and rate of travel of the worked board. The results of experiment showed a considerable temperature difference of the worked surface between the sharp or blunt blade. Thus measurement of temperature distribution can be applied for determination of technical condition of a tool during woodworking or other wooden materials treatment [2].

Fig 2: Temperature of the surface of worked wood measured close the cutting tool: 1-blunt tool 2-sharp tool.

For temperature determination in diagnosis IR pyrometer can be used. Such pyrometer is cheaper than IR camera. Pyrometer parameters must be adequate for travel rate of the worked material and small measurement area should be only near a cutting tool. Due to high degree of dust the pyrometer should be protected from it by applying additional supply ventilation for pyrometer lens.

Furthermore, the pyrometer should be fixed in another arrangement than woodworking machine one so that it should not undergo machine vibrations.

3. Temperature measurement using pyrometer

The pyrometer with a smart measuring head and software of the RAYTEK was used for experiments. This device allows for remote temperature measurement with accuracy of 1% and resolution of 0.1°C. Diameter of measuring area is 6,4 mm (at the distance of 200 mm). Spectral range of the device is within 8-14 mm, that significantly eliminates the influence of external disturbances on the accuracy of measurement. Additionally, the pyrometer was equipped with laser diodes (670 nm) indicating precisely the position of measuring area. Autonomous air ventilation of optical elements allows for proper work of the device in a dusty industrial environment. Picture 1 shows the pyrometer on the measuring position.

pic 1: IR pyrometer on the measuring position.

Temperature measurements were carried out while milling fibreboard of medium density (MDF). The large width (thickness) of shavings causes disturbances of a cutting process because of additional material squashing by the shavings. As a result of material squashing and at the same increase in power of a friction force additional heat is produced.
The aim of experiments was to determine the influence of the shavings width on temperature of the worked surface.

Figure 3 present heating of worked surface as cutting distance increases while milling MDF with the use of tools with the same blades of various values of shavings width. Figure 4 presents the change of the medium temperature depending on the shavings width [1].

Fig 3: Relation between the value of shaving width and the temperature of worked surface.

Fig 4: Relation between the value of shaving width and the medium temperature of worked surface.

Temperature measurements of the worked surface obtained by the use of pyrometer indicate that it is possible to determine the degree of tool's wear as well as to establish permissible values of cutting process. Temperature value of the worked surface can be the basic signal for automatic control system of woodworking and at the same allows for control of the wood processing efficiency.

4. Conclusions

The experiments have shown usefulness of IR pyrometer for continuous temperature measurements of the worked surface while milling wood and wooden materials. Additionally it was concluded, that it is not sufficient to measure only the temperature of the worked surface using IR camera or pyrometer. For example, the determination of adequate shavings width for proper cutting process should be confirmed by experiment measurements of cutting forces distribution. Only then it is possible to provide exact data for practical application tools.

The experiments have also shown that as tools wear increases, the temperature of the worked surface increases as well. It is obvious, and carpenters commonly know this phenomenon. Work with blunt tool causes burns and even carbonisation of the worked elements. The application of IR pyrometers for determination of limits of cutting tools wear allows for automation of this process. Temperature of the worked surface and values of cutting forces are optimum input signals for smart systems of woodworking monitoring and in particular for determination of limits of a blades wear of cutting tool.

Further experiments are planned, the purpose of which is the determination of the influence of blades wear on temperature of the worked surface. The developed infrared pyrometer will be used to determine temperature value.

REFERENCES

1. M. Kastek, H. Madura, H. Polakowski, W. Sokolowski, "IR camera and pyrometer used for woodworking control," Quantitave Infrared Thermography 4, Lódz, Poland, September 7-10, 1998, pp. 195-199.
2. W. Sokolowski, P. Gogolewski, "Temperature of machined surface as a value for tool condition monitoring during wood products milling," 14th International Wood Machining Seminar, September 12-19, 1999. Paris, Epinal, Cluny - France. Proceedings - Volume 2, pp. 775 - 780.

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