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International Journal of Machine Tools & Manufacture 47 (2007) 2071–2076

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An experimental technique for the measurement of temperatureon CBN tool face in end milling

Masahiko Satoa,�, Takashi Uedab, Hisataka Tanakaa

aDepartment of Mechanical Engineering, Tottori University, Koyamacho-minami 4-101, Tottori 680-8552, JapanbDepartment of Mechanical Systems Engineering, Kanazawa University, Kakumatyo, Kanazawa 920-1192, Japan

Received 28 February 2007; received in revised form 18 May 2007; accepted 25 May 2007

Available online 7 June 2007

Abstract

An infrared radiation pyrometer with two optical fibers connected by a fiber coupler was developed and applied to the measurement of

tool–chip interface temperature in end milling with a binderless CBN tool. The infrared rays radiated from the tool–chip interface and

transmitted through the binderless CBN are accepted by the optical fiber inserted in the tool and are then sent to the pyrometer. A

combination of the two fibers and the fiber coupler makes it possible to transmit the accepted rays to the pyrometer, which is set up

outside of the machine tool. This method is very practical in end milling for measuring the temperature history at tool–chip interface

during chip formation. The maximum tool–chip interface temperature in up milling of a 0.55% carbon steel is 480 1C when the cutting

speed is 2.2m/s and 560 1C at 4.4m/s, and in the down milling, 500 1C at 2.2m/s and 600 1C at 4.4m/s.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Temperature measurement; End milling; CBN tool

1. Introduction

End milling is widely used as a machining process in themanufacture of mechanical components that requires highgeometric accuracy and smooth surfaces. In the machiningof high-speed steel, stainless steel, high-temperature alloys,and so on, cubic boron nitride (CBN) is often used as amilling tool tip material because of its physical andchemical stability at high temperatures, and its low affinityfor ferrous metals which is difficult for the diamond toolsto make effective cutting.

When a work material is machined, most of the powerconsumed is converted into heat. The heat generatedincreases the temperature of the cutting tool and theworkpiece and it causes various thermal damages such astool wear, thermal expansion, and the degradation ofdimensional tolerance. Tool wear especially causes dete-rioration of surface integrity, increase of cutting forces, and

e front matter r 2007 Elsevier Ltd. All rights reserved.

achtools.2007.05.006

ing author. Tel.: +81857 31 5195; fax: +81 857 31 5195.

ess: sato@mech.tottori-u.ac.jp (M. Sato).

occurrence of chatter vibration. In order to avoid theseundesirable problems and choose appropriate machiningconditions, it is necessary to acquire accurate informationabout milling temperatures.McFeron and Chao [1] developed a procedure and

necessary equations for the computation of the average,transient tool–chip interface temperature in plain periph-eral milling, and measured the average tool–chip interfacetemperature using a tool–work thermocoupling technique.Since they used a mercury bath to make electrical contact,the peripheral speed was limited to a relatively low speed.Schmidt [2] determined the maximum temperature occur-ring in or near the surface of a workpiece while it was beingmilled. He took temperatures at several different depths inthe workpiece using thermocouple embedded in the work-piece. Stephenson and Ali [3] theoretically and experimen-tally investigated the tool temperatures in interruptedcutting. Theoretically, the temperature in a semi-infiniterectangular corner heated by a time-varying heat flux withvarious spatial distributions was used to investigate thegeneral nature of tool temperature distribution. They

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Fig. 1. Pyrometer’s schematic illustration.

M. Sato et al. / International Journal of Machine Tools & Manufacture 47 (2007) 2071–20762072

compared the results of this analysis with the cuttingtemperature measured by infrared and tool–chip thermo-couple technique. An infrared video image system was usedin the end turning of a slotted tube to simulate interruptedcutting. Lazoglu and Altintas [4] presented a numericalmodel based on the finite difference method to predict tooland chip temperature fields in continuous machining andtime-varying milling process. They showed that the resultsof the simulation are in satisfactory agreement withexperimental temperature measurements reported in theliterature. Ueda et al. [5] measured the temperature ofthe flank face of a cutting tool in high speed milling us-ing a two-color pyrometer with an optical fiber. Thefiber was inserted into a fine hole drilled in the work-piece and accepted infrared energy that was radiatedfrom the flank face of the cutting tool when it passed abovethe hole.

Although these previous investigations into millingtemperatures provided valuable information, very littlework has been done on the measurement of tool facetemperatures during chip formation in end milling with aCBN tool. This is because the contact area between the tooland the chip is very small and the chip is producedintermittently in a very short time. The thermocoupletechnique is the most promising method but cannot beapplied to CBN tool because CBN is an electrical insulator.Also, a conventional thermal radiation pyrometer cannotbe applied to measure the rake face temperature, since therake face cannot be seen from the outside during chipformation. Generally, CBN tool is produced by sintering aCBN powder mixed with a binder material such as TiC andTiN. Recently, a new technology has been developedmaking it possible to sinter the CBN tool without bindermaterials. The binderless CBN tools have many advan-tages, such as high thermal conductivity, high resistance tothermal shock and oxidation, and good transmittance forinfrared rays. The translucent characteristic of binderlessCBN tools makes it possible to measure the tool–chipinterface temperature in end milling. In tribologicalapplications, using translucent heads or disks, the tem-peratures of microscopic areas in a sliding contact weremeasured [6,7]. In precision turning, Ueda et al. [8]measured the temperature on the rake face of a single-crystal diamond tool using infrared pyrometer.

In the present study, we develop an infrared radiationpyrometer with two optical fibers connected by a fibercoupler and measure the tool–chip interface tempera-ture in end milling with a binderless CBN tool while achip is producing. The infrared rays radiated from thetool–chip interface and transmitted through the binder-less CBN are accepted by one optical fiber, which isinserted into the tool and fixed in the main spindle ofmachine tool. Then the infrared energy is sent to the otherfiber, which is fixed at the column of machine tool and ledto the two-color pyrometer. The fiber coupler makes itpossible to connect two optical fibers without touchingeach other.

2. Pyrometer

2.1. Fundamental structure

Fig. 1 shows a schematic illustration of an infraredradiation pyrometer with optical fibers. Two optical fibers,fiber-A and fiber-B, are used and a fiber coupler is used toconnect fiber-A with fiber-B without touching each other.Fiber-A can be rotated upon an axis at high speed. On theother hand, fiber-B is fixed in the coupler and is in astationary stage. The infrared rays radiated from the objectare accepted and transmitted by fiber-A. The infrared raysemitted from the other end face of fiber-A are accepted andtransmitted by fiber-B and focused on the infrareddetectors using a condenser. The output signals of thedetectors are amplified and then recorded. This systemmakes it possible to transmit the rays from the rotatingshaft of the machine tool to the pyrometer on the outsideof the machine tool.

2.2. Components

The pyrometer is characterized by the composition of allcomponents. The use of an optical fiber facilitates thedetection of energy radiated from a very small object, evenif the target is in an intricate part of the object. We used amultimode step index fluoride glass optical fiber in thisstudy. The fiber has a fluoride core and a fluoride claddingwith a lower refractive index. The diameters of core andcladding of fibers-A are 195 and 215 mm, respectively, andthose of fiber-B are 430 and 477 mm, respectively. Thenumerical aperture of both fibers is 0.22. The fluoride glassfiber transmits light with wavelength from 0.5 to 4 mm. Inother words, the wavelength of the light transmission usingthe fluoride fiber is longer than that using a quartz fiber.The end faces of the optical fiber are created using a sharpblade and the faces are observed by a microscope. We thenchecked whether the output voltage of the pyrometer withthe optical fiber reaches a standard value under constantillumination.

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Fig. 2. Frequency characteristics of amplifier.

Fig. 3. Deviation and eccentricity between fibers.

M. Sato et al. / International Journal of Machine Tools & Manufacture 47 (2007) 2071–2076 2073

Two-color detector, which is used in this experiment, iscomposed of two kinds of photocells: an indium arsenic(InAs) cell and an indium antimonide (InSb) cell. The InAscell is mounted in a sandwich configuration over the InSbcell. The InAs cell detects the radiation at wavelength fromabout 1 to 3 mm. The InSb cell detects the radiation atwavelength from about 3 to 6 mm, which transmits throughthe InAs cell. Temperature can be measured by taking theratio of the signals from each photocell. The photocells aremaintained at 77K using liquid nitrogen in order todecrease thermal noise and achieve high sensitivity.Calcium fluoride (CaF2) lens is used as a condenser. Itoffers a constant transmission of more than 90% inwavelength from about 0.5 to 10 mm.

The electric current signals, which are derived from thesephotocells when they are exposed to the infrared radiation,were converted into voltage signals and then amplified. Fig.2 shows the frequency characteristics of the amplifier for arectangular wave, which clearly indicates that the amplifierhas a flat response from 10Hz to 100 kHz. Since theresponse time of the photocells is approximately 1 ms, thefrequency characteristics of the pyrometer are the same asthe amplifier.

2.3. Influence of fiber deviation and eccentricity at the

coupler

In this study, as Fig. 1 shows, the infrared energyradiated from the object is accepted by fiber-A, which isrotating upon its center axis and transmitted to fiber-Busing the fiber coupler, and led to the pyrometer. Fig. 3shows the fiber deviation ‘‘a’’ between the rotational centeraxis and fiber-B and the eccentricity ‘‘e’’ between the fiber-A and the rotational center axis in the fiber coupler. Fig.3(a) is the ideal case when both center axes of fiber-A andfiber-B coincide with the rotational center axis and theradiation energy transmitted from fiber-A to fiber-Breaches maximum. However, in practice, there is a slightdifference in position among these three axes, so that threecases of the position of axis can be considered as shown inFig. 3(b)–(d). In Fig. 3(b), the center axis of fiber-Acoincides with the rotational center axis but there is a slightdeviation between the two fiber axes. In Fig. 3(c), the center

axis of fiber-B coincides with the rotational center axis butthere is a little eccentricity between the center axis of fiber-A and the rotational center axis. In Fig. 3(d), these threeaxes do not coincide with each other, so that there is a fiberdeviation and an eccentricity, simultaneously. The devia-tion and the eccentricity cause the cyclical variation in thetransmission energy between these two optical fibers.Two-color pyrometer compensates for the measurement

errors ascribed to the cyclical variation. Since the variationof incident infrared radiation energy to each photocellcaused by deviation and eccentricity are exactly the same,their effects are cancelled in the process of taking theoutput ratio of InAs- and InSb-pyrometers. This meansthat the two-color pyrometer is independent of deviationand eccentricity between the fibers.

3. Experimental arrangement and conditions

The experimental arrangement is illustrated schemati-cally in Fig. 4, and the experimental conditions aresummarized in Table 1. We used a single point end millingcutter. The tool insert was made by bonding the binderlessCBN of 0.65mm thickness to a cemented carbide substrate.Fig. 5 shows the shape of the tool insert and the details ofthe hole position in the insert. A small hole was drilled intothe tool insert from underneath until the bottom of the holereached the surface of CBN tip. The hole is 1.7mm fromthe tool top and has a tilt of 151 in order to target on thetool–chip contact area in the chip formation. Fig. 6 showsthe spectral transmittance of the 0.65-mm-thick binderlessCBN used in this study. In the milling experiment, theoptical fiber was inserted into the hole until the incidentface of the fiber reached the bottom surface of the CBN.The diameter of the fiber’s target area on the rake face isabout 0.5mm. The target area is shown in Fig. 5.During the chip formation, the infrared rays, radiated

from the tool–chip interface and transmitted through the

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Fig. 4. Experimental arrangement.

Table 1

Cutting conditions

Tool tip material Binderless CBN

Workpiece 0.55% carbon steel

Cutting speed (m/s) 2.2–4.4

Feed rate (mm/s) 2.7–5.3

Feed per tooth (mm) 0.2

Axial depth of cut (mm) 2

Radial depth of cut (mm) 3

Radial rake angle (deg) �3

Axial rake angle (deg) 0

Diameter of cutter (mm) 52

Cutting fluid None

Fig. 5. Details of tool insert.

Fig. 6. Spectral transmittance of 0.65-mm-thick binderless CBN.

Fig. 7. Calibration set-up.

M. Sato et al. / International Journal of Machine Tools & Manufacture 47 (2007) 2071–20762074

CBN, were accepted and transmitted by the insertedoptical fiber-A. Fiber-A runs through the inside of themachine tool spindle and connects to fiber-B using the fiber

coupler without touching. The coupler makes it possible totransmit the infrared rays accepted by fiber-A to fiber-B.The infrared energy transmitted to fiber-B is led to thephotocells and converted into electric signals. The signalsare stored in a data acquisition board and analyzed using apersonal computer.The workpiece used is 0.55% carbon steel and is

mounted on the piezoelectric dynamometer. Cutting testsare performed in both up milling and down milling underthe conditions in Table 1.

4. Experimental results

4.1. Calibration

We calibrated the pyrometer by sighting on a radiatingtarget that has a known temperature, as shown in Fig. 7. Amechanical chopper is used to modulate the radiation fromthe target. For the target, we used the same work materialas used in the experiment. A thermocouple is embedded inthe target to monitor its temperature. Fig. 8 shows theresulting calibration curve for carbon steel. Using thiscurve, the output ratio of the photocells is converted intothe temperature.

4.2. Tool temperatures

Fig. 9 shows the cutting forces and cutting temperaturein up milling. In Fig. 9(a), Fx is a principal cutting forceand Fy is a normal cutting force, as shown in Fig. 4. Fig.9(b) shows the tool–chip interface temperature historyduring chip formation, which is obtained by taking the

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Fig. 8. Calibration curve.

Fig. 11. The relation between cutting speed and maximum tool tip

temperature measured during cutting.

Fig. 9. Output waves in up milling: V ¼ 3.3m/s.

Fig. 10. Output waves in down milling: V ¼ 3.3m/s.

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ratio of the output signals of the InAs- and InSb-pyrometers in up milling. The cutting length is about12.9mm, and the maximum depth of cut is about0.093mm. The cutting force increases with the cuttingoperation as shown in the figure. In Fig. 9(b), thetemperature on the rake face increases very quickly at thebeginning of the cutting and reaches a constant tempera-ture, and the temperature of 500 1C is constantly main-tained during the cutting. From the characteristics ofradiation pyrometer, the temperature measured is approx-imate to the maximum temperature on the target area [8].

Fig. 10(a) and (b) shows the cutting forces and thetool–chip interface temperature in down milling, respec-tively. During chip formation, the cutting force decreasesas the cutting proceeds. The temperature increases quicklyat the beginning of cutting, and then the temperature tendsto decrease as the cutting proceeds.

Fig. 11 shows the variation in the maximum cuttingtemperatures at the tool–chip interface, which are mea-

sured during chip formation. The tool temperatureincreases with the increase of cutting speed. In up milling,the temperature is 480 1C at cutting speed of 2.2m/s and560 1C at 4.4m/s, and in down cutting, 500 1C at 2.2m/sand 600 1C at 4.4m/s. It is found that the maximum tooltemperature in down milling is about 50 1C higher thanthat in up milling and the temperature difference at rakeface is about 90 1C when the cutting speed is doubled.Ueda et al. [9] measured the temperature of a cutting

edge of CBN tool flank in turning of 0.45% carbon steel byusing a two-color pyrometer. A CBN sintered with TiNbinder was used and the volumetric percent of CBN wasapproximately 60%. In the test, cutting speed, depth of cutand feed were 2.5–3.8m/s, 0.8mm and 0.15mm/tooth,respectively. The cross-sectional area of the uncut chip was0.12mm2. The experimental tests were performed bycontinuous cutting and the measured temperatures of the

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cutting edge for these cutting conditions were found to be640–720 1C.

In our end milling test, the maximum cross-sectionalarea of the uncut chip is 0.186mm2 and the measuredtemperature is 480 1C when cutting speed is 2.2m/s and560 1C at 4.4m/s. The temperatures in the end milling testare lower than those in the turnig test in nearly the samecutting conditions. In end milling process, since the tool issubjected to cyclic heating and cooling as it passes in andout of the workpiece [3], the temperature is lower than thatin continuous cutting. In addition, the use of binderlessCBN inserts, which have high heat conductivity than CBNwith binder contents, reduces temperatures, because theheat is more readily conducted away through the tool.

5. Conclusions

The infrared radiation pyrometer with two optical fibersconnected by the fiber coupler was developed and thebinderless CBN tool–chip interface temperature in endmilling was measured using the pyrometer. The infraredrays radiated from the tool–chip interface and transmittedthrough the binderless CBN were accepted by an opticalfiber inserted in the tool insert. These rays were then led tothe InAs- and InSb-pyrometer connected by the fibercoupler that transmits the rays outside the machine tool.The two-color pyrometer compensates for measuringerrors ascribed to deviation and eccentricity between fibersat the fiber coupler. This method is very practical formeasuring the tool–chip interface temperature in endmilling. The tool–chip interface temperature in up millingof the 0.55% carbon steel is 480 1C at cutting speed 2.2m/sand 560 1C at 4.4m/s and in down milling 500 1C at 2.2m/sand 600 1C at 4.4m/s. It is found that the maximum tool

temperature in down milling is about 50 1C higher thanthat in up milling and the temperature difference at rakeface is about 90 1C when the cutting speed is doubled.

Acknowledgment

The authors would like to thank Sumitomo ElectricHardmetal Corp. for providing the binderless CBN toolinsert.

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