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Journal of Materials Processing Technology 167 (2005) 110–118

Comparison of chip surface temperature between up and down miorientations in high speed rough milling of hardened steel

C.K. Toh∗

University of Birmingham, School of Engineering (Mechanical), Edgbaston Park Road, Birmingham B152TT, UK

Received 3 March 2003; received in revised form 3 May 2004; accepted 5 October 2004

bstract

High speed milling (HSM) as a manufacturing process has been in intense use in recent years. Recently, a relatively new teough milling using diameter cutters equal to or less than 10 mm has emerged such that rough milling employing high axial depAd) ofut (10 mm≤Ad ≤ 20 mm) and low pick feed without compromising cutting speed and feed rate is used. The current study investeasibility of employing high depths of cut (10–20 mm axial depth of cut) by using an infrared red technique to measure the chemperature. This is in order to assess its machinability characteristics on HSM difficult-to-machine material such as hardened steeith a view to improve the metal removal rate, by employing a cutter path strategy known as raster, a combination movement consecutive up and down milling. This paper specifically presents an investigation into the comparison of the effects of chip surface t

enerated primarily between up and down milling orientations, and the secondary effects of cutter conditions and axial depths of cut. Thexperimental results show that the chip surface temperature generated when up milling were in general lower as compared to down millingt all cutter conditions and axial depths of cut employed.2004 Elsevier B.V. All rights reserved.

tations

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eywords:High speed rough milling; Chip surface temperature; Orien

. Introduction

Cutting temperature is an important process conditiohe analysis and monitoring of the effects of a metaling process. Low cutting temperatures generated detal cutting cause pressure welding creating a built-up-

BUE) while high cutting temperatures trigger diffusion axidation processes[1]. Cutting temperature has been the

or subject of interest in the metal cutting process espeche heat generated has a major influence on tool life andace integrity[2]. A significant amount of heat is generauring metal cutting. The generation of heat arises from

ral main sources; namely the primary shear zone at the toolorkpiece interface, the secondary shear zone at the tool chip

nterface and generation of heat at the clearance face contact

∗ Present address: Singapore Institute of Manufacturing TechnologySIMTech), Machining Technology Group, 71 Nanyang Drive, Singapore38075, Singapore. Tel.: +65 67938593.E-mail address:cktoh@SIMTech.a-star.edu.sg.

o t intot wasi ed ori tin-ubtt

924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2004.10.004

; Hardened steel

.

t is reported that around 80–90% of the heat generatransferred to the chip while 10–15% of the heat goes tutter and workpiece material[3–6]. At higher cutting speedhe ratio of heat flow to the chip increases and subsequeduces the total percentage of heat flow into the workpnd cutting tool[7]. Wang and Liu[8] investigated the e

ect of tool flank wear on the heat transfer, thermal damnd cutting mechanics when hard turning AISI E52100 shey asserted that the proportion of heat that went intohip increased as flank wear increased accordingly, sugng that the heat generated at the tool chip interface andork interface were strongly dependent on the developf the flank wear. This reduced the percentage of hea

he workpiece although the overall cutting temperaturencreased. Cutting temperature measured for interruptntermittent cutting such as milling is lower than for con

ous cutting such as turning[5,9,10]. Research carried outy Stephenson and Ali[9] on intermittent cutting has shown

hat temperature depends primarily on the duration of cuttingime and non-cutting time per cut. Temperature increases as

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d in-c thee ch asfw ptotea d ac-c att ttings ct timb o thew wereh of thes -e wasn , thec to in-c sw thati twee2 mso endmt d ax-i dA d in-c toolw IE withf tion,t olsp y ofa era-t n thec get caseh e ef-f thatc l noser n. Byi turer ucedm neg-a e.

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C.K. Toh / Journal of Materials P

he cutting time per cut increases and vice versa. An impoonclusion is that the effect of cutting time has a higheract on cutting temperature generated than non-cutting

Cutting temperature increases as the cutting speereases[11–17]. More specifically, the temperature atnd of an intermittent cut increases and the time to reateady state temperature decreases with cutting speed[11]. Aact that contradicts to Salomon and Palmai’s theory[18–19]here cutting temperature rises and reaches an asymnd then falls gradually when cutting speed is increaseordingly. Matsumoto and Hsu[20] asserted that the hehat was conducted into the workpiece reduced as the cupeed increased. It was suggested that the shorter contaetween the tool and workpiece for heat to penetrate intorkpiece. It was also concluded that the temperaturesigher and heat penetration deeper as the hardnessteel increased. Minamino et al.[21] found that when hardned SKD 61 steel was milled, the cutting temperatureot significantly affected by the feed per tooth. Howeverutting temperature did rise as length cut increased duereased tool wear. Dewes et al.[4] made similar observationhen HSM hardened H13 steel. One important point was

nterface temperatures generated were measured at be47 and 385◦C. This explains why HSM is preferred in terf low cutting temperatures generated. When HSM ballilling hardened AISI H13 steel, Ning et al.[22] concluded

hat cutting temperature increased with cutting speed anal depth of cut. Abrao et al.[23] on finish turning hardeneISI H13 steel concluded that the temperature measurereased with cutting speed, feed rate, depth of cut andear. Abrao and Aspinwall[16] on turning hardened AIS52100 steel found that cutting temperatures increased

eed rate, depth of cut and tool wear, respectively. In addihey concluded that the thermal conductivity of cutting tolayed a major role where a higher thermal conductivitcutting tool induced a lower tool chip interface temp

ure. As a result, less heat would be generated to softehip. El-Wardany et al.[12] concluded that the cutting edemperature increased with feed and depth of cut whenardened AISI 1552 steel was turned at high speed. Th

ect of tool geometries was also investigated. It was foundutting edge temperature decreased with increasing tooadius since larger nose radius promotes heat conductioncreasing the tool width chamfer, cutting edge temperaose due to increased tool chip contact length that indore heat. Last but not least, there existed an optimumtive rake angle for a minimum cutting edge temperatur

A number of temperature measurement techniqueseen developed over the years for various obvious reauch as the determination of temperatures of tool chip iace, rake face, cutting tool, workpiece, chip surface, etc

ethods commonly adopted by the researchers are the

llographic method, i.e. microhardness and microstructurenalysis, embedded and tool chip thermocouples, thermal ra-iation, powders of constant melting point, thermo-sensitiveaints, etc.[23–24]. The most recent method is the use of

aeoi

ing Technology 167 (2005) 110–118 111

e

n

n infrared red (IR) method. This method is best utilisedninterrupted machining process and at the same timean be collected and analysed. However, this method iuitable for detecting the temperature at the tool chipact area due to obstructions caused by the chip and coherefore, it is largely limited to the determination of curface temperature. Wang et al.[13] used the IR methoo assess the temperature distribution within the chip cection when turning low carbon steel. Their main objecas to develop controlled localised cooling methods toble chip breaking with minimum effort. Chu and Wallba

17] on turning low carbon steel deduced that cutting spnd feed had a larger effect on cutting temperature wh

he tool nose radius was found to have little effect. Cutemperature has been found to vary within the chip duachining[25–26] when using the IR method. Young ahou[25] used the IR method to measure the chip sur

emperature in order to investigate the effect of edge euring chip formation. One important conclusion was thaaximum temperature occurred at the chip surface wa

ond the tool chip contact area. Park and Kim[26] concludedhat the IR images could reveal information about the tyf chips formed and the direction of chip flow during mach

ng. When using an IR thermo tracer, Young[27] observedhat the chip surface temperature away from the tool edgnterestingly higher than the tool chip interface temperat was suggested that the transfer of heat by conductionhe secondary shear zone along the chip thickness tohe chip surface was the root cause.

The current study investigates the feasibility of employigh depths of cut (10–20 mm axial depth of cut) by usin

nfrared red technique to measure the chip surface temture in order to assess its machinability characteristicifficult-to-machine hardened steel AISI H13, with a view

mproving the metal removal rate. Specifically, the chipace temperature was measured in order to assess thef up/down milling, tool wear and axial depth of cut on c

ing temperatures generated. The chip surface tempebtained can be used to infer the machinability impose

he high speed rough milling process due to different cath strategies used.

. Experimental work

.1. Experimental equipment, workpiece materials andooling

All cutting tests were performed on a Matsuura FX-5ical prismatic high speed machining centre. All machinests were conducted dry. In addition, high-pressure airelivered through a nozzle was directed at the cutting zon

ll machining tests conducted. A tool overhang of 40 mm wasmployed. All cutters were checked to ensure a tool runoutf less than 10�m. These were assessed prior to tests us-

ng a dial indicator with a resolution of 0.001 mm. Tool wear

1 rocessing Technology 167 (2005) 110–118

w icro-s lm

nalc Cr,1 ut thee stru-m l bulkh DMw rialss

raint illss eteroT lmw

g aT mt ncesR longw 5 mml con-n sfi edw sizeow ea ed atn onf ea-s uiredf vel-o

eitzW TM-1pTs

Fig. 1. A schematic diagram depicting the equipment set-up on emissivityd

2

em-p tos amet lockt era-t nds e wasm stem.T aluei em

acet ith af outfi lts.T tri-a eteda . Al-t flankw

TP

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NNNN0000WW√

12 C.K. Toh / Journal of Materials P

as measured using a Hilger and Watts toolmaker’s mcope magnification of 30× equipped with Mitutoyo digitaicrometer heads at 0.001 mm resolution.Hardened AISI H13 hot work tool steel with a nomi

omposition of 0.38% C, 1.00% Si, 0.35% Mn, 5.00%.30% Mo, 1.00% V and Fe balance was used throughoxperimental work. Its hardness was measured with an inatic portable hardness tester to ensure that a nominaardness of 52 HRC was achieved. A Charmilles 5-axis Eire cutting machine was used to cut the workpiece mateupplied in block sizes of 120 mm× 120 mm× 90 mm.

The cutters used in the test were Kobelco ultra-fine gungsten carbide 6-flute VC-MDRB corner radius end mpecially made for this experiment. The cutters had a diamf 10 mm, helix angle of 45◦ and a radial rake angle of−14◦.he cutters were coated with a mono-layer (Al, Ti)N fiith film thickness of about 2.5�m.Chip surface temperature was measured usin

heromovisionTM

Agema 880TM

infrared measuring systehat was loaned from the Engineering and Physical Scieesearch Council (EPSRC). It is a cryogenically cooledave analysis system that comprised a scanner with a 3

ens camera connected to a display unit that was in turnected to a personal computer, seeFig. 1. The scanner watted with a 20◦C field of lens view. The system was fittith a burst 25 Hz image acquisition system with a spotf 2 mm. It has a measurement range of−20◦C to +1500◦Cith a sensitivity of 0.07◦C at +30◦C. The size of the imagrea shown on the display unit measured was maintainot less than 2.5 mm× 2.5 mm. This ensured that radiati

rom the background would not significantly alter the mured object temperature. Data and images were acqrom a 300 MHz personal computer using specifically deped software.

Micrographs of the chips were conducted using a Letzler optical microscope and viewed through a JVC

5000PS colour video monitor and then taken using an Olym-us microscope fitted with a JVC-1280E CCD vision system.he micrographs were subsequently printed out using a Mit-ubishi CP50B video printer.

cce

able 1hase 1 test matrix chip surface temperature measurement trials

utter status/operation Cutting speed (m/min)

20

ew cutter (down milling) 400√√

ew cutter (up milling) 400√√

ew cutter (down milling) 314√√

ew cutter (up milling) 314√√

.1 mm flank wear (down milling) 314/400* √√

.1 mm flank wear (up milling) 314/400* √√

.2 mm flank wear (down milling) 314/400* √√

.2 mm flank wear (up milling) 314/400* √√orn cutter (down milling) 314/400*

√√orn cutter (up milling) 314/400*

√√

: One trial performed.* : Cutting temperatures measured only on 20 mm axial depth of cut at 400

etermination.

.2. Experimental procedure

The workpiece block was first heated to a maximum terature of 800◦C in an oven. The block was then leftoak for 30 min to attain uniform temperature. At the sime, an analogue thermocouple was inserted into the bhrough a pre-drilled hole to determine the actual tempure. The block was then allowed to cool off uniformly aimultaneously at the same instance, actual temperatureasured and calibrated against the thermal imaging syhe emissivity value was determined to be 0.73. This v

s consistent with Ng’s work[28] on the same workpiecaterial hardened AISI H13 steel.Table 1details the test matrix carried out for chip surf

emperature measurements at various depths of cut weed per tooth of 0.0667 mm/tooth. Each trial was carriedve times to improve the statistical reliability of the resuhe cutting conditions were fixed constant as in millingls. In this experimental work, the new cutter was interprs having a flank wear width land of less than 0.05 mm

ernatively, the worn cutter was construed as having aear land width exceeding 0.3 mm.

Chips were collected during trials to examine shape and

olours at various stages of tool wear. Selected chips wereold mounted in an epoxy resin and their cross section wasxamined for microstructural evaluation.

Axial depth of cutAd (mm)

15 10√√√√√√√√√ √√√√√ √√√√√√√√ √√√√√ √√√√√√√√ * √√√√√ √√√√√ √√√√√√√√ * √√√√√ √√√√√ √√√√√√√√ * √√√√√ √√√√√ √√√√√√√√ * √√√√√ √√√√√ √√√√√√√√ * √√√√√ √√√√√ √√√√√√√√ * √√√√√ √√√√√ √√√√√

m/min cutting speed.

C.K. Toh / Journal of Materials Process

F tiono radiald

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c fac-t f thet m-p ss ofa ng[ thatw t. Thei to ani ased.A thec ,r dt r andt ture.I dgea erw area[ thatr ork-p tet ribu-

ig. 2. Effect of axial depth of cut, cutter condition and milling orientan chip surface temperature at 314 m/min cutting speed and 0.5 mmepth of cut.

. Results and discussion

Fig. 2 shows the effect of axial depth of cut, cutter cition and milling orientation on the chip surface tempe

ure at a cutting speed of 314 m/min. Its analysis of variaANOVA) results on chip surface temperature are depicte

able 2. All factors in particular the individual factors suchs milling orientation, cutter condition and axial depth ofut factors were significant at 5% level. ANOVA results on

able 2NOVA results for chip surface temperature

ource Degrees offreedom (DoF)

Sequential sum ofsquares (SS)

Mean sumsquares (

1 34622 346223 1140751 380250

d 2 93309 46654× C 3 35432 11811× Ad 2 42751 21376× Ad 6 38668 6445× C × Ad 6 26937 4490

rror 96 185154 1929

otal 119 1597629

ote – M: milling orientation, C: cutter condition,Ad: axial depth of cut.

Fig. 3. Thermographic pictures depicting the cutting acti

tmfc

ing Technology 167 (2005) 110–118 113

hip surface temperature indicate that cutter conditionor was the most significant and contributed 71.04% ootal variance. In fact,Fig. 2shows that the chip surface teerature increased with increase in tool wear regardlexial depth of cut and milling orientation imposed. You

27] also made similar observation in that he concludedear phenomenon was strongly temperature dependen

ncrease in chip surface temperature is apparently duencrease in tool chip contact area as the wear land incres a result, the fraction of heat that was transferred tohip increased when tool wear deteriorated[8]. In particularesearch carried out by Leshock and Shin[14] emphasisehat cutting temperature was dependent on crater weahat flank wear had minimal influence on cutting temperan this work, crater wear occurred adjacent to the flank et all cutting conditions[29] coupled with the fact that cratear is caused by diffusion between the tool chip contact

24] as a result of a very high contact zone temperatureose up to or even exceed the heat resistibility of the wiece material[1]. ANOVA results on the other hand indica

hat the milling orientation factor has a percentage cont

ofMS)

F-test P-coefficient Percentage contributionratio % (PCR)

17.95 0.000 2.04197.16 0.000 71.0424.19 0.000 5.606.12 0.001 1.86

11.08 0.000 2.433.34 0.005 1.702.33 0.038 0.96

14.37

100

on at various high axial depths of cut when down milling.

ion ratio (PCR) value of 2.04%. Clearly, this suggests thatilling in a down or up milling orientation had minimal pro-

ound effect on the cutting temperature generated. In fact,areful observation on the graph ofFig. 2shows that the chip

114 C.K. Toh / Journal of Materials Process

F n onc deptho

s onsi re-s aticcb flankc d cor-r . Ont de-cr se inr ced axiald st ate-rssi

h t flowi ninga ownm ctiver reasei nsferi ofh to de-c . Thisc e alsoi wasm of5 toolc

thec em-p them ttomtd thep

thec andm er-ad highera encem ouldb ao eta on-duct the heat. Likewise as already mentioned and discussed,

ig. 4. Effect of cutting speed, milling orientation and cutter conditiohip surface temperature at 20 mm axial depth of cut and 0.5 mm radialf cut.

urface temperatures for up milling at all cutting conditin general were lower than down milling by 3–8%. Thisult for up milling was consistent with the dynamic and stutting forces and vibration analysis obtained[29]. A possi-le explanation is that as the crater wear adjacent to theutting edge increased, the effective rake angle increaseespondingly as tool wear increased when down millinghe other hand when up milling, the effective rake anglereased as tool wear deteriorated[30]. Lo’s [31] modellingesults showed that cutting forces decreased with increaake angle. Moreover,FX force depicts that the cutting forecreased as the tool wear increased in general at allepths of cut when down milling[29]. The opposite hold

rue for up milling because the removed workpiece m

ial would effectively push towards the rake face due to amearing effect thus increasing the effective rake angle con-equently. In addition, shear angle on the chip cross sectionncreased with increase in rake angle[30]. Ng[32] states that

Fig. 5. Changes in chip colour and

ctt

ing Technology 167 (2005) 110–118

igher shear angle promotes a greater proportion of heanto the chip rather than the workpiece. By super positioll the above analysis, it can be concluded that when dilling, an increase in crater wear increased the effe

ake angle and simultaneously the shear angle. This incn shear angle caused a larger proportion of heat to tranto the chip. In contrast when up milling, the proportioneat into the chip decreased with lower shear angle duerease in the effective rake angle as tool wear increasedould be a possible suggestion. Chip surface temperaturncreased with axial depth of cut although the increase

inimal as reflected in the ANOVA results of PCR value.60%. Such increase is apparently due to increase inhip contact area and hence higher heat generated.

Fig. 3 presents the thermographic pictures depictingutting action at various high axial depths of cut whenloying a down milling orientation. It was observed thataximum chip temperature always occurred at the bo

ip of the cutter. This was in line with Tonshoff et al.[33]ue to the highest thermal load incurred by machiningeripheral edges and the bottom workpiece surface.

Fig. 4 depicts mainly the effect of cutting speed onhip surface temperature by varying cutter conditionsilling orientations. Not surprisingly, chip surface tempture increased with cutting speed. Ng and Aspinwall[34]educed that the strain rate in the shear zone becamet higher cutting speed due to higher energy input and hore heat energy would be generated. Thus more heat we expected to transfer to the chips as reported by Hirl. [7] because of insufficient time for the workpiece to c

shape as tool wear deteriorated.

omparison between up and down milling at different cut-ing speeds suggested that the chip surface temperature forhe former was similar if not slightly lower than the latter.

rocessing Technology 167 (2005) 110–118 115

cteda e di-r ringi gthe lourp lightb rite-r on-v es tos nt ont tingt ion oft ly ass sedtstc

F steelw

C.K. Toh / Journal of Materials P

Fig. 5presents the photo macrographs of chips collet various stages of tool wear when employing a singlection raster down milling strategy. Chip collected dunitial machining were of helical type shape with its lenquivalent to the axial depth of cut imposed. The chip coroduced was violet purple and the colour changed torown/slivery white as the tool flank wear reached a cion of 0.3 mm. In addition, the shape of the chip had certed from close helical to lose helical shape. This gohow that the chip colour and its shape were dependehe cutter condition. As the tool wear deteriorated, cutemperature increased correspondingly and the proporthe heat that transferred to the chip increased accordinghown inFig. 2. This increased proportion of heat plastici

he chip and hence the tensile strength of the material chipubsequently reduced as shown inFig. 6. The reduction inensile strength would inevitably lower the hardness of thehip resulting in a ‘softer’ chip. This was characterised in a

wt

l

Fig. 7. Comparison of chip profile produced with

Fig. 8. Micrograph depicting the chip surface p

ig. 6. Effect of temperature on tensile strength of hardened AISI H13ith a workpiece hardness of 52 HRC[37].

ay that the chip was easier to break without much efforthan the chip produced by a new cutter.

Fig. 7 indicates that both the chips produced and col-ected by up and down milling orientation respectively exhibit

up and down milling orientation, respectively.

roduced using a single direction raster strategy.

116 C.K. Toh / Journal of Materials Processing Technology 167 (2005) 110–118

Fig. 9. Micrograph depicting the cross sectional area of an etched chip profile produced by a new cutter.

nal are

s hipsc thatn gth-e ont

-t siblyd face.A ignt thee gment iona ch isd alsob -e de-f sec-o turalzars

b piececT zonew tureo heatw n ofc sub-s howni

4

withrre-icalably

Fig. 10. Micrograph depicting the cross sectio

imilar shape and colour. Further comparisons of the collected under different tool wear conditions also showo major difference was highlighted. This further strenned the fact that milling orientation had minimal effect

ool wear.The chip surface shown inFig. 8 had distinct lines ex

ending across its surface. Furthermore, fragments posue to tool material or coatings were fused onto its surpart from that, its relatively smooth surface was a s

hat chatter vibrations were not dominant. Analysis oftched chip cross section revealed that saw-toothed se

al chip was formed irrespective of the milling orientatnd cutter path strategy employed, an example of whiepicted inFig. 9. Saw-toothed segmental chips haveeen observed by Dumitrescu et al.[35] when HSM hardned AISI H13 steel. Regions of high microstructural

ormation were observed to flow along the primary andndary shear zones. These regions of high microstruc

ones gradually became white as the flank wear land reachedcriterion of 0.3 mm (seeFig. 10). Boehner et al.[36] also

eported similar observation when HSM hardened AISI D2teel of 62 HRC. The formation of white layers was caused

a of an etched chip profile produced by a worn cutter.

-

y exceeding the austenisation temperature of the workhip followed by intense cooling at a very short time[30].he formation of white layers at the secondary shearas a further indication that tool chip interface temperaf the rake face became so high such that most of theas dissipated to the chip. This resulted in the formatiorater wear due to diffusion and the evidence of diffusedtrate or coating can be observed on the chip surface as sn Fig. 8.

. Conclusions

(I) The chip surface temperature measured increasedtool flank wear and axial depths of cut employed ispective of the milling orientation employed. Statistanalysis revealed that the cutter condition considerinfluenced the chip surface temperature.

(II) The experimental results showed that the chip surfacetemperature generated when up milling was in generallower compared to down milling at all cutter conditionsand axial depths of cut employed.

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C.K. Toh / Journal of Materials P

III) Statistical analysis on the chip surface temperashowed that the milling orientation contributed aPCR value of only 2.04%. This suggested that theting temperature induced when employing an updown milling orientation had no significant effectthe cutting heat generated. This further suggestedemploying down or up milling was not a contributofactor in enhancing tool wear.

Such result implies that high speed rough milling usxial depths of cut from 10 to 20 mm employing a raster

er path strategy (combination of up and down milling)iable method to reduce machining time and subsequmprove productivity without compromising tool life. Tesults on improved machining time and productivity areailed elsewhere[38].

cknowledgements

The author would like to extend his gratitude to Mr. Daspinwall for the provision of facilities and Universities U

or funding via the award of an Overseas Research Schhip. Special thanks go to EPSRC for the loan of the infred equipment. Thanks also extend to Mr. Steve Hobbslan Pearce, Mr. John Wedderburn, Mr. Richard Fashamr. Julian Price for their technical assistance and indus

upport.

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