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any continuous or interrupted three-dimensional cutting process. As the heating period decreases or the cooling period increases, the maximum tool-chip interface temperature for interrupted cutting decreases. Jen et al. [5] numerically solved nonlinear transient heat conduction problems using a control volume approach to study transient cutting temperatures with temperature-dependent thermal properties. Experimentally, many techniques for measuring the milling temperature by thermocouple or radiation methods have been developed. Wang et al. [6] devised a technique for measuring the milling cutter temperature using garter spring pickups and a standard thermocouple positioned at the back of the inserts. Temperature signals for as many as eight teeth were recorded simultaneously. Mcferon et al. [7] measured an average tool-chip interface temperature using a tool-work thermocouple technique in plain peripheral milling. A mercury slip ring contact was used to make electrical contact with the rotating member. Schmidt et al.[8] measured the temperature in a workpiece while it was being milled using a thermocouple embedded in the workpiece. They experimentally determined the temperature gradient in a work- piece as it was milled. As a radiation method, Ueda et al. [9] measured a flank face temperature in high-speed end milling by a two-color pyrometer with an optical fiber. The effects of cutting speed, feed rate, and depth of cut on the temperature were investigated. Toh et al. [10] measured the chip surface temperature in up/down milling using an infrared measuring system to assess the effect of tool wear and axial depth of cut on cutting temperatures. Although many studies have examined milling temperatures and much valuable knowledge has been obtained, the temperature variation in the cutting tool throughout the cutting and concurring cycles, which is an essential factor in the thermal impact to the cutting tool, has not been measured experimentally. The thermo- couple technique is a popular measuring method but cannot measure the temperature during a noncutting period since the electrical circuit between the cutting tool and the chip opens. A conventional radiation technique cannot measure the temperatureduring the cutting period since the rake face cannot be seen from the outside during chip formation. Recently, we developed a new pyrometer system with two optical fibers and used it to measure the translucent cBN cutting tool temperature in end milling [ll]. In this study, we applied the new pyrometer system to measure the cyclical temperature variation in the cutting tool during cutting and noncutting periods in end mill- mg 2 Experimental Method 2.1 Experimental Setup. The experimental setup is illustrated schematically in Fig. 1. A single-tooth end milling cutter, a sintered carbide Kl0, is used. The workpiece made of titanium alloy Ti-6Al-4V is mounted on a piezoelectric dynamometer. The rake angle is o deg, the axial depth of cut is 2 mm, the radial depth of cut is 21 mm, the feed rate is 0.105 mm/tooth, the spindle speed is 1300 rpm and the diameter of the cutter is 52 mm. Figure 2 shows a detailed illustration of the tool insert. A small blind hole for inserting an optical fiber is drilled from the under- side of the tool insert to near the rake face by electrodischarge machining. The diameter of the hole is approximately 0.5 mm. Measurements of tool temperature are taken at several different depths in the tool insert. The distance z between the rake face and the bottom of the hole is varied from 0.1 mm t0 0.5 mm at intervals of 0.1 mm. The hole is 0.6 mm from the major cutting edge and l.3 mm from the
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tool top. A cross-sectional view of the hole is shown in Fig. 2(b). On the left, the distance is z=0.1 mm and on the right, z=0.5 mm. As shown in Fig. 1, optical fiber l runs through the inside of the machine tool spindle and is inserted into the hole in the tool insert Fig. 2 Detail of tool insert: (a) location of the hole inserting optical fiber and (b) cross-sectional view of the blind hole until the incidence face of the fiber reaches the bottom of the hole. During chip formation, the cutting tool is heated, and thermal radiation depending on the temperature is emitted as infrared rays from the bottom of the hole. The infrared rays are accepted by the inserted optical fiber l and are guided along the fiber core by total internal reflection. The other end of fiber l is inserted into the fiber couplet and connected to fiber 2. In the couplet, fiber l can be rotated upon its axis at high speed, and fiber 2 is fixed. Infrared rays emitted from the end of fiber l are accepted and transmitted by fiber 2. Fiber 2 is connected to the two-color pyrometer, and the transmitted rays are focused on the infrared detectors using a condenser. The electric currents derived from the detectors are converted into voltage and amplified. The output signals are stored in a data acquisition unit and then converted into temperature. The combination of two optical fibers makes it possible to transmit infrared rays from the rotating cutting tool to the pyrometer. 2.2 Pyrometer. The characteristics of the pyrometer are determined from the composition of all of its components. For the optical fibers, multimode step index fluoride glass fiber is used. The numerical aperture of the fiber is 0.22, and the acceptance angle is 12.7 deg. The fiber transmits light of wavelengths from 0.5 um t0 4 um, which are longer than those transmitted by a silica glass fiber. The core diameters of the two optical fibers, fibers l and 2, are about 195 um and 430 um, respectively. The fiber ends are cut by a sharp blade, after which the face condition is checked for cracks by microscopic observation. The incidence faces of the two fibers at the fiber couplet are kept approximately 100 ,u.m apart. Two photocells, an indium arsenide (InAs) cell and an indium antimonide (InSB) cell, are used as infrared detectors. The InAs cell is mounted in a sandwich configuration over the InSb cell. The InAs cell detects radiation at wavelengths from about l ru,m t0.3 um. The InSb cell, which is set below the Inst cell, detects radiation through the InAs cell at wavelengths from about 3 um to 6 um. The temperature can be obtained by taking the ratio of the output signals from each photocell. The photocells are maintained at 77 K using liquid nitrogen to increase their sensitivity. A calcium fluoride (CaF2) lens is used as a condenser. It offers constant transmission of more than 90a/o for wavelengths from about 0.5 um to 10 um. The frequency characteristics of the amplifier for a rectangular wave are shown in Fig. 3. The amplifier has a flat response from 10 Hz t0 100 kHz. Since the response time of the photocell is very high, approximately l ,u.s, the frequency characteristics of the pyrometer are determined by the amplifier. The response of the pyrometer is quick enough to use in this experiment. 2.3 Influence of Fiber Setting Error at the Coupler. In this experiment, infrared energy radiated from the tool insert is accepted by fiber l and transmitted to fiber 2 in the fiber coupler.Ideally, the center axes of both fibers coincide with the rotational center axis of the
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