equipped with effective in-process monitoring systems for the machining environment. ... surface of a tool tip at a distance of 0.8mm from the cutting edge.
In-process microsensor for ultraprecision machining H. Yoshioka, H. Hashizume and H. Shinno Abstract: Future ultraprecision manufacturing systems will require intelligent machine tools equipped with effective in-process monitoring systems for the machining environment. To meet these requirements, a sensor is urgently required that combines multifunctionality, reliability, sensitivity and compactness. It is particularly difficult to monitor the machining status during ultraprecision machining, because the energy emissions and cutting forces are very small in comparison to conventional machining processes. A thermometry-type in-process microsensor is proposed to solve this problem. The proposed microsensor is made using a microfabrication process that is normally used in semiconductor production. The sensor size is 520 250 mm and the line width of the sensor device is 5 mm. The developed microsensor is mounted directly onto the surface of a tool tip at a distance of 0.8 mm from the cutting edge. The performance of the developed microsensor is investigated through a series of ultraprecision cutting experiments.
1
Introduction
The demand for high accuracy and high productivity in manufacturing continues to increase. The ultraprecision cutting of hard brittle materials and low emission cutting requires the use of a closed ultraprecision machining environment since this creates the best cutting environment and allows isolation from external disturbances [1]. However, closed machining environments limit the access of operators, and hence it is important to develop an effective in-process monitoring system for the machining environment. In-process monitoring systems generally contain sensors, and allied data transfer, signal processing and status recognition functions. A high performance in-process sensor for ultraprecision machining, is difficult to produce because the cutting force, power and acoustic emissions are very small in comparison with conventional machining processes. The effectiveness and usefulness of currently available sensors [2–4] such as force, power and acoustic emission sensors [5, 6] find limited application in ultraprecision machining because of their sensitivities. However, the cutting temperature is high enough to measure. Therefore, it is possible to precisely gauge the status of an ultraprecision machining environment, by monitoring the thermal behaviour near the cutting point using techniques such as the tool-work thermoelectric effect [7], thermocouples embedded in a tool tip [8], infra-red rays [9], a combination of thermocouples and infra-red rays [10] and heat flux sensors [11]. In this study, we investigate a thermometry-type inprocess microsensor and its use in monitoring thermal behaviour near a cutting point. The use of two or more of these microsensors allows us to detect the temperature distribution of the tool and estimate the cutting temperature. The developed microsensor is a thin film platinum r IEE, 2004 IEE Proceedings online no. 20040375 doi:10.1049/ip-smt:20040375 Paper first received 9th May 2003 and in revised form 3rd December 2003 The authors are with the Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan IEE Proc.-Sci. Meas. Technol., Vol. 151, No. 2, March 2004
resistance thermometry-type sensor and it is mounted directly on the rake face of a single crystal diamond tool tip. The developed sensor is validated by its use in an ultraprecision machining environment. 2
Design of the micro-sensor
The most effective way to monitor ultraprecision machining, i.e. tool, workpiece, coolant and chip status, is to monitor the thermal behaviour near the cutting point. However, in-process measurement of the temperature using a conventional sensor has not been applied to monitor an actual ultraprecision machining environment, because of numerous problems including a low sensitivity, a long response time, a low reliability, etc. During a cutting process heat is generated at the cutting point, and it is effective to place microsized sensors near the cutting point. We have therefore mounted a platinum resistance thermometry-type microsensor directly on the rake face of a diamond tool tip. The microsized sensor, if installed very close to the cutting point, can detect the thermal behaviour with a quick response and a high resolution because of its small heat capacity and short heat transfer delay. Sensors mounted on cutting tools have been previously reported in [12, 13], but these are normal sized seasors and thus the potential advantages of microsensors have not been investigated. Figure 1 shows the developed microsensor and its positioning on the rake face of a single crystal diamond tool tip with a nose radius of 0.2 mm and a rake angle of 01. The microsensor consists of two gauge elements and three electrodes. The front gauge element (A) is mounted at a distance of 0.8 mm from the cutting edge, and the back gauge element (B) is mounted at a distance of 100 mm from gauge A. The use of two gauge elements on a tool surface, allows not only the surface temperature but also its distribution to be measured without disturbing the objective machining field. The gauge elements are made from a thin platinum film which has a high resistivity and a high temperature coefficient of resistance. The thickness of the platinum film is 250 nm and the line width of the gauges is 5 mm. The developed microsensor was designed as a simple microscopic structure, and thus it can be produced both 121
A
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Relationship between the resistance and temperature
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simply and cheaply using a microfabrication process used in semiconductor production. Figure 2 shows the microfabrication process of the developed microsensor: (I) resist coating, (II) lithography with a patterned mask and ultraviolet rays; (III) development of resist pattern; (IV) platinum deposition using sputtering; (V) lift-off of the platinum; (VI) wire bonding and overcoating with acrylic resin. Figure 3 shows a micrograph of the developed microsensor. As can be seen in this Figure, the gauge elements were mounted directly onto the rake face of the diamond tool tip using sputtering, and the thermal contact resistance between the gauges and diamond could consequently be minimised. A calibration experiment was performed to evaluate the basic characteristics of the developed microsensor. Figure 4 shows the relationship between resistance and
resist (1) resist coat
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UV (2) UV lithography
(5) lift-off of platinum
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temperature for each gauge element of the developed microsensor. As clearly shown in this Figure, the linear and parallel relationships between both gauge elements can be confirmed. The temperature coefficients of the resistances of gauges A and B are both equal to 0.11 %/K, which is smaller than the temperature coefficient of bulk platinum of 0.392 %/K. This low value of the temperature coefficient is caused by the existence of impurities, oxidised platinum and lattice defects in the thin film gauges caused by the sputtering process used to deposit the platinum. 3
Performance evaluation
3.1
Cutting experiments
Cutting experiments have been performed using an ultraprecision diamond turning machine [1] to evaluate the effectiveness of the developed microsensor for in-process status monitoring during ultraprecision diamond turning. Figure 5 shows the actual configuration of the machining environment. The workpiece is a Al-Mg alloy disk which is mounted onto the front face of the aerostatic spindle with a vacuum chuck. The inner and outer diameters of the disk are 30 and 90 mm, respectively. White kerosene mist is supplied to the cutting point to act as a coolant. As shown in Fig. 5, face turnings from inside to outside were carried out at constant spindle rotational speed, and therefore the cutting speed increases linearly through the cutting. The cutting conditions are shown in Table 1.
Fabricating procedures of the developed microsensor aerostatic spindle vacuum chuck
200 µm workpiece (disk substrate)
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cooling fluid nozzle cleaner
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Fig. 3 122
Micrograph of the developed microsensor
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Configuration of the cutting environment IEE Proc.-Sci. Meas. Technol., Vol. 151, No. 2, March 2004
Table 1: Cutting conditions for the experiments Single crystal diamond R ¼ 0.2 mm, rake angle ¼ 01
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Al-Mg alloy disk (f 90 mm)
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white kerosene mist
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010 mm
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15 mm
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Fig. 7 Typical temperature patterns during ultraprecision cutting to a depth of cut of 10 mm with a feed rate of 15 mm/rev, a rotation speed of 3000 rpm and a cutting speed of 282.7848.2 m/min
gauge element A amplifier (x250)
0.3 depth of cut = 0 µm
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Configuration of the measurement system
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Changes in the resistance of each gauge element are detected as changes in the output voltage of a bridge circuit, as shown in Fig. 6. The bridge excitation voltage is 2.5 V. The output voltages of the bridge circuits are sent to a PC through an amplifier, a low pass filter and an A/D converter. The gain of the amplifier is 250, and the cut-off frequency of the filter and sampling frequency of the A/D converter are 1 kHz and 2 kHz, respectively. The recorded voltages are converted into temperatures using the relationships between resistance and temperature as shown in Fig. 4.
0 −0.1 −0.2
3.2 Typical output pattern of the microsensor Figure 7 shows the cutting conditions and typical temperature output pattern of each gauge element in the ultraprecision face turning. The gauge elements are very close to each other, and thus the temperatures at A (TA) and B (TB) are almost identical. As shown in Fig. 7, the sensor outputs change quickly at the beginning and ending of the cutting, and increase linearly during the steady-state cutting in accordance with an increase in the cutting speed. The developed sensor is located near the cutting edge and has a very small heat capacity, which makes it possible to monitor small thermal changes with a quick response. Thus, the sensor outputs clearly contain useful information on thermal changes during the ultraprecision cutting. Figure 8 shows the difference between the outputs of the gauge elements (DT ¼ TATB). When the depth of cut is zero, TA is smaller than TB. The fluctuation of the IEE Proc.-Sci. Meas. Technol., Vol. 151, No. 2, March 2004
cutting
−0.3 −0.4 −10
0
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20 30 time, s
40
50
60
Fig. 8 Typical pattern of temperature difference for a feed rate of 15 mm/rev, a rotational speed of 3000 rpm and a cutting speed of 282.7848 m/min
temperature difference corresponds with the state of the mist supply, i.e. a mist of white kerosene has a cooling effect on the cutting edge. In the cases where the depth of cut is 5 mm and 10 mm, the temperature difference changes quickly at the start and end of cutting. These step changes are quicker than the temperature changes, because a temperature difference means heat flux to pass between two points. Furthermore, the amount of heat generated depends on both the depth of cut and the 123
cutting speed with the step height being proportional to the depth of cut. Hence, the temperature shown by the sensor contains useful information about the cutting status. Also, the difference in temperature between the integrated gauge elements denotes the amount of heat passing through the tool tip at the cutting point with a quick response.
sensor. The quick response is due to both the sensor structure and the high thermal conductivity of diamond. 4
Conclusions
In order to realise in-process monitoring for ultraprecision machining it is necessary to develop a sensor which has properties such as multifunctionality, reliability, sensitivity, a quick response and compactness. We have proposed such a sensor that is a platinum thermometry-type, sensor which was mounted directly onto the rake face of a single crystal diamond tool tip by means of microfabrication process. The sensor was integrated with two gauge elements, and consequently the temperature and its distribution on the tool tip surface can be monitored. The performance of the sensor was evaluated via a senses of experiments. From these experiments it was shown that the sensor has a linear relationship between the resistance and temperature. It can also detect temperature changes near the cutting point during ultraprecision cutting with a quick response and high resolution. It should be noted that the differential output of the sensor, which means the amount of heat passing through the tool tip at the cutting point, has a quicker response than the temperature outputs.
3.3 Evaluation of sensitivity and response time In order to clarify the sensitivity and response time of the developed sensor, face turning experiments were performed with the Al-Mg alloy disk having two holes drilled through it (Fig. 9a). The diameter of the holes, is 2 mm. When a cutting tool passes over one of the holes, the cutting is interrupted for about 0.2 ms. As can be clearly seen in Figs. 9b and 9c, both the actual temperatures and the temperature differences decrease at the holes. Figures 9d and 9e are expanded views of portions of Figs. 9b and 9c, it is clear that the sensors have detected pulses that synchronise with the spindle rotation. This pulse means an interruption of the heat inflow from the cutting edge at a hole, rapid status changes can be detect by monitoring the changes in the temperature difference measured by the
24.5 12.5
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Fig. 9 Sensor responses in cutting the workpiece with holes to a depth of cut of 10 mm with a feed rate of 15 mm/rev, a rotational speed of 3000 rpm and a cutting speed of 282.7848.2 m/min a Disk with holes b Temperature rise as a function of time c Temperature difference as a function of time d Expanded details of a section of Fig. 9b e Expanded details of a section of Fig. 9c 124
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