surface and coatings technology volume 148 issue 2-3 2001 [doi 10.1016_s0257-8972(01)01349-4] shiva...

8/17/2019 Surface and Coatings Technology Volume 148 Issue 2-3 2001 [Doi 10.1016_s0257-8972(01)01349-4] Shiva Kalidas…

1/12

Surface and Coatings Technology 148 (2001) 117–128

0257-8972/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved.PII: S 0 257- 8972 Ž0 1 . 0 1 3 4 9 - 4

Experimental investigation of the effect of drill coatings on hole qualityunder dry and wet drilling conditions

Shiva Kalidas, Richard E. DeVor, Shiv G. Kapoor*

University of Illinois at Urbana-Champaign, Department of Mechanical and Industrial Engineering, 140 Mechanical Engineering Building,

MC-244, 1206 West Green Street, Urbana, IL 61801-2906, USA

Received 5 November 2000; accepted in revised form 12 June 2001

Abstract

The performance of three different coatings in dry and wet drilling conditions are investigated to identify and understand theireffect on the surface texture of the hole produced, the maximum temperature rise in the workpiece, chip clogging at the drillflutes and the size and profile of the hole produced. A statistical analysis of these responses demonstrates that refractory coatedHSS drills (TiAlNyTiN and TiAlN) perform best in terms of the dimensional accuracy of the hole. The MoS -coated carbide2drills produced holes larger than the ones obtained with refractory coated HSS drills. Drill coatings did not have a significanteffect on the measured workpiece temperatures nor on the surface texture of the hole produced. The use of coatings led to fluteclogging in wet conditions only, flute clogging was not observed in the absence of cutting fluids. The experiments conducted alsodemonstrate, within the experimental space investigated, increasing feed rates induce lower workpiece temperatures in the absenceof cutting fluids. 2001 Elsevier Science B.V. All rights reserved.

Keywords: Drill coatings; Hole quality; Dry; Wet

1. Introduction

The Green Manufacturing paradigm was initiated bymanufacturing companies to meet the challenges of rising recycling costs and to provide healthier and saferworking environments. The elimination of cutting fluidsfrom the production floor is at the core of realizing thisideal w1x. Since the early 1990s, numerous efforts have

been undertaken to overcome the challenges posed bydry machining w2,3x. A large majority of this researchhas been concentrated in developing custom tools andcoatings to meet the unfavorable machining conditionsthat a tool has to withstand in the absence of cuttingfluids. There has been little attention paid to the effectof these coatings, in the absence of cutting fluids, onthe quality of the part produced.

* Corresponding author. Tel.: q1-217-333-3432; fax: q1-217-244-9956.

E-mail address: [email protected] (S.G. Kapoor).

In dry drilling the tool has to withstand extremeenvironments which include high temperatures, highfrictional forces and large mechanical loads. Thisrequires the tool to posses high hot hardness, highrefractivity and low coefficients of friction w2,3x. Therehave been numerous publications on various substrate–coating systems that posses these properties andimproves tool life w4–9x. In large part these advances

can be attributed to the progress made in coatingtechnologies, especially the advent of efficient andflexible physical vapor deposition processes (PVD) likemagnetron sputtering w6x, which has led to numerouspossibilities in terms of coatings. The demands of lowthermal conductivities and low coefficients of frictionw2,3x on the tool in dry drilling operations has led tothe advent of novel multi-layer coatings. A good clas-sification of the various types of multi-layer coatingsand their capabilities is presented by Kustas et al. w6x.

These studies have helped considerably in providingviable solutions for the realization of dry drilling. The


2/12

118 S. Kalidas et al. / Surface and Coatings Technology 148 (2001) 117–128

Table 1The relevant properties of the coatings used in the experimentation

Characteristics TiAlN TiAlN w22x MoS w22x2yTiN w22x

Coating process PVD PVD PVDLayer structure Multi Mono plus Al O layer2 3 Mono layer

Thickness (mm

) 0.0–5.0 1.5–5.0

-

0.5Micro hardness (HV0.5) 3500 3300 20–50Thermal conductivity 0.05 0.05 Not available

(kWymK)Friction coefficient vs. steel 0.25 0.3 0.05–0.15

performance of the coatings in many of these studieshas been characterized solely on the improvements intool life. Their performance in terms of improvementsin product quality has not been explored. The importanceof these coatings in terms of maintaining or improvingthe performance of the process in terms of finishedproduct quality grows in the context of dry machining.

The temperatures induced in the part, surface finish of the part and dimensional accuracy are key metrics thatwould factor in the decision to implement these coatingsin a production setting.

In this study, the effect of three types of coatings onthe quality of the hole produced in drilling of as castaluminum 356 alloy in dry and wet conditions isinvestigated. Drilling experiments are conducted over arange of spindle speeds and feed rates in dry and wetconditions with HSS uncoated drills, TiAlNyTiN (multi-layer) coated HSS drills, TiAlN-coated HSS drills andMOS -coated carbide drills. Induced workpiece temper-

atures, hole size and surface texture of the hole arequantified. The measured torque is inspected for theoccurrence of flute clogging. The effect of drill coatingsand process conditions (presenceyabsence of cuttingfluid) on these responses is determined using statisticalanalysis techniques and discussed.

2. Experimentation and quantification of responses

Three candidate coatings are compared based on theireffect on induced workpiece temperatures, hole surfacetexture and dimensional accuracy in wet and dry drillingof as-cast aluminum 356 alloy. Three types of coated

drill, TiAlNyTiN multi-layer coated HSS drills (FIR-EX, series no. 205), TiAlN-coated HSS drills (seriesno. 205) and molybdenum disulfide-coated carbide drills(MOVIC series no. 732) from Guhring , are compared ¨

to the HSS drills (series no. 205) with no coating. Therelevant physical properties of these coatings are pre-sented in Table 1.

The TiAlNyTiN w10x multi-layer coated drills consistof alternating layers of TiN and TiAlN designed towithstand high machining temperatures and offer highertool life. This coating is designed for universal applica-tion across a wide range of workpiece materials. It

combines the characteristics of its components andprovides higher thermal resistance, wear resistance,shock resistance and hot hardness than monolayer TiNcoatings. TiAlN coatings are designed for the machiningof abrasive materials such as high silicon content alu-minum alloys (like as-cast aluminum 356 alloys). It isalso recommended for high thermal stress conditions

that occur in applications such as dry drilling. Theperformance of the TiAlN coatings have been shown tobe a function of the aluminum content in these coatingsw7,11x, higher aluminum contents further enhance itsproperties.

The need for lowering friction, to facilitate chipmovement in the drilling process has led researchers todevise coatings that act as solid lubricants at the drillmargins w6,8–10,12x. In this study one such coating(molybdenum disulfide, MoS ) is utilized in the com-2parison. Lubricating coatings such as MoS have been2shown to discourage the propensity of chip clogging

due to the low coefficients of friction prevalent in thesecoatings. The addition of these coatings to refractivecoatings like TiAlN offer the dual advantage of hightool life and good chip evacuation characteristics.

2.1. Experimental setup

Experiments were carried out on a cylindrical, work-piece 25.4 mm (1 inch) in diameter and 43.18-mm (1.7inch) long. Four tapped holes were machined in theworkpiece to facilitate the accurate placement of ther-mocouples for temperature measurement. The workpiecegeometry and thermocouple locations are presented in

Fig. 1a. The experiments were conducted on Mori SeikiTV30 CNC light milling, drilling and tapping center.Four Type E fast response co-axial thermocouples(Medtherm TCS-031-E0.12-O-TGS10-B2SR-AC) alongwith 5B signal conditioners (National Instruments) wereused in measuring the temperatures. A Kistler four axisdynamometer (KISTLER 9273) was used to collect thetorque and thrust data. All the signals were collectedusing LabView software. All the experiments wereconducted on workpieces with pilot holes (2.38-mmdiameter), 10-mm deep. As-cast aluminum 356 alloywas used in the experimentation. The drills used were


3/12

119S. Kalidas et al. / Surface and Coatings Technology 148 (2001) 117–128

Table 2Experimental design matrix

Test Speed Feedrate Cutting(rev.ymin) (mmyrev.) fluid

1 1600 0.083 Present2 2200 0.083 Present

3 1600 0.162 Present4 2200 0.162 Present5 1600 0.083 Absent6 2200 0.083 Absent7 1600 0.162 Absent8 2200 0.162 Absent

Fig. 1. Workpiece geometry and thermocouple locations used in the experiments (a) and the experimental setup used to obtain the responses (b).

9.13 mm (23y64 inch) in diameter. Fig. 1b shows theworkpiece held in the fixture which is mounted onto thedynamometer fixed to the machine tool table.

A 2 factorial design of experiments was conducted3

with each coated drill, with spindle speed, feed rate andpresenceyabsence of cutting fluid as design variables.Microsol 265 with 10 parts of water was used as thecutting fluid during drilling experiments in the wetcondition. The design matrix used in the experiments ispresented in Table 2. Three responses, drilling torque,thrust and workpiece temperatures were measured duringdrilling. The workpiece temperatures were measured atfour distinct locations to aid in the thermal modeling of the workpiece w13x, in this study the maximum measuredtemperature rise among these four will be utilized in thecomparison of the coatings. The measured responses fortests conducted at 1600 rev.ymin and 0.162 mmyrev.feed rate in the presence and absence of cutting fluidswith TiAlN-coated HSS drills is presented in Fig. 2.

The increase in workpiece temperatures with theabsence of cutting fluids is apparent from Fig. 2a,b. Aninspection of the measured torque and thrust signalsdemonstrates the increase in steady state torque andthrust with the removal of cutting fluids. Chip cloggingin the presence of cutting fluids is also observed fromthe steady increase in torque and thrust observed in Fig.2a approximately 6–7 s into the process. The occurrenceof chip clogging when the various coatings are employedand the process conditions at which they occur will bediscussed when each coating is compared.

The drilled holes were measured for dimensional

accuracy using a Brown & Sharpe XCEL 765 CMM.The drilled workpiece along with fixture were trans-ported to the CMM and 40-radial measurements along28 planes traversing the entire length of the hole werecollected. The surface roughness of the drilled holes

were measured with a SurfTronic 3q surface texturemeasurement machine using a traverse length of 25 mm.The surface texture data, collected using ST3PLUSsoftware, was used in calculating the roughness using aGaussian filter with 10 cutoffs. The measurements wererepeated at four angular positions 908 apart for each

test.

2.2. Quantification of responses and experimental results

Four responses obtained from the experiments areused in comparing the three candidate coatings. Theseinclude: (a) maximum temperature rise in the work-piece; (b) average hole radius; (c) variation in holeradius along the depth; and (d) surface roughness of thehole ( R ). The methods used to quantify these responsesafrom the experimental data and the rationale behindtheir selection is detailed here.

The maximum temperature rise (

as shown in Fig. 3)

measured among the four thermocouples (which occursat TC4 shown in Fig. 3) was used in the comparisonof the drills. The scheme used to quantify the responsesis represented pictorially in Fig. 3. The maximum


4/12


Fig. 2. Forces and temperatures measured in drilling aluminum 356 alloy with TiAlN-coated HSS drills at 1600 rev.ymin and 0.162 mmyrev. with(a) cutting fluid present and (b) cutting fluid absent.

Fig. 3. The measured temperature rise when drilling at 2200 rev.yminspindle speed and feed rate of 0.162 mmyrev. with MoS shown to2demonstrate the method used to collect the maximum measured tem-perature rise.

Table 3Maximum measured temperature rise (in 8C) across the various exper-imental conditions for each type of drill used

Test No coatings TiAlNyTiN TiAlN MoS2

1 9.72 13.13 12.85 9.042 20.81 17.35 16.69 17.583 19.65 18.41 18.34 14.764 24.61 20.68 24.48 25.015 51.17 40.32 41.95 49.216 47.66 48.64 41.32 57.737 45.40 50.84 34.64 32.238 38.46 28.93 36.64 44.38

measured temperature rise across the different processconditions and when the four types of drills wereemployed is summarized in Table 3.

The TiAlN and multi-layer coatings investigated inthis study are employed to isolate the high temperaturespresent at the chip tool interface w14x forcing more heatto be carried away by the chips and the workpiece. Thehigher workpiece temperatures could induce debilitating

thermal effects like tensile residual stresses in the partand increase hole diameter (relative to the ideal). Inthin-walled parts, induced stresses could be a majorcause for dimensional inaccuracies w15x and hole war-page. In quantifying the maximum workpiece tempera-ture rise and comparing them with non-coated drills, theeffect of coatings in controlling these effects could beunderstood.

The quality of a drilled hole is determined by asynthesis of the errors due to the dynamics of theprocess and errors due to thermal regime in the drillyworkpiece. The mechanisms that induce these errors


5/12


Fig. 4. Measured average hole radius when drilled with TiAlN-coateddrills at a spindle speed of 1600 rev.ymin and 0.083 mmyrev. feedrate in the presence and absence of cutting fluids.

Table 4Average hole radius ( in mm) across the various experimental condi-tions for each type of drill used

Test No TiAlNy TiAlN MoS2coatings TiN

1 4.6187 4.5834 4.5660 4.5751

2 4.6306 4.5663 4.5672 4.59043 4.6179 4.5764 4.5701 4.58754 4.6107 4.5695 4.5637 4.59025 4.6504 4.5964 4.5992 4.60996 4.6364 4.5825 4.5869 4.58387 4.6271 4.5535 4.5596 4.58778 4.6229 4.5654 4.5602 4.5840

Table 5Measured variation (in mm) in hole radius across the various exper-imental conditions for each type of drill used

Test No coatings TiAlNyTiN TiAlN MoS2

1 y0.0107 y0.0230 y0.0094 y0.00172 0.0192 0.0047 y0.0195 y0.00803 y0.0141 0.0248 y0.0010 y0.00804 0.0049 0.0050 0.0047 0.00455 0.0502 0.0314 0.0440 0.05736 0.0328 0.0304 0.0389 0.03187 0.0401 0.0764 0.0240 0.00108 0.0278 0.0296 0.0180 0.0272

include: wandering w16x or whirling w17x of the drillpoint at drill entry; dynamic deflections of the drill dueto unbalanced forces w18x; errors due to process faults(viz. runout) w18x; errors due to cutting at the drillmargins w19x; errors due to thermal expansion of thedrill w20x; and errors due to thermal expansion in theworkpiece w20x. The rigidity of the drill, which deter-mines, in a large part, the errors induced due to thedynamic mechanism, would be unaffected by the

presenceyabsence of coatings. Thus, in comparing theperformance of coatings, the average hole size andvariation in the average hole size with depth is utilizedto understand how critical drill coatings are in holdingthe hole size close to the ideal. The average radius ateach plane (Fig. 4) was used in determining the averagehole size errors across different process conditions anddrill coatings. The average measured radius at each testconditions and coating is presented in Table 4.

The characteristic shape of the holes obtained in theabsence and presence of cutting fluids (Fig. 4) aredifferent. To understand this variation and quantify its

magnitude, the difference in the average radius obtainednear the top of the hole to that at the bottom wasquantified for all the test conditions (Table 5). Theaverage radius at the first plane was ignored in thisquantification as it is affected by dynamic effects likedrill wandering and whirling w16,18x. A negative valuein Table 5 indicates that the average hole size at thebottom of the hole is larger than that observed at thetop of the hole, while a positive value indicates that theaverage hole size at the top of the hole is larger than atthe bottom of the hole (tapering of the hole). I t isobserved from Table 5 that all the holes drilled in the

absence of cutting fluids produce positive variations inhole size.

The measured surface roughness (two representativeprofiles presented in Fig. 5) obtained from the ST3PLUSsoftware using a Gaussian filter with 10 cutoffs was

analyzed to determine the effect of coating on thesurface texture of the hole produced. The average surfaceroughness obtained from the four angular positions wasused in comparing the performance of the drills. Themeasure R across all the test conditions is listed inaTable 6. The surface roughness of the holes producedby the drilling operation have been traditionally over-looked due to the fact that other finishing operationslike boring, reaming and honing are used to obtain thedesired surface roughness w21x. The cost incurred inthese finishing processes could be reduced if the bettersurface qualities are obtained from the drilling process.In this regard, the effect of coatings in decreasing(improving) the surface roughness would have a largeimmediate impact in wet drilling conditions. The quan-tification of the surface roughness in dry conditionswould also serve in future efforts for the selection of process conditions and coatings that would producesurface roughness comparable to those obtained withwet drilling.

Flute clogging causes an abrupt monotonic increasein torque and thrust (as in Fig. 2) which often leads todrill breakage in deep-hole drilling applications. In theexperiments conducted in this study, flute clogging was


6/12


Fig. 5. Measured surface roughness profile of the hole drilled withTiAlN-coated drills at a spindle speed of 1600 rev.ymin and 0.162mmyrev. feed rate in (a) the presence of cutting fluid and (b) itsabsence.

Table 6Measured R (in mm) across the various experimental conditions foraeach type of drill used

Test No TiAlNy TiAlN MoS2coatings AlN

1 2.64 2.85 0.78 0.95

2 3.48 1.03 1.44 3.063 3.87 2.29 1.73 2.094 3.40 2.21 3.23 4.915 8.98 8.75 9.68 8.726 6.68 7.48 8.16 7.257 8.65 6.47 8.46 9.348 7.22 5.56 7.83 7.52

Table 7The presenceyabsence of flute clogging at each test condition

Test No TiAlNy TiAlN MoS2coatings AlN

1 1 2 2 12 1 2 2 23 1 2 2 24 1 2 2 25 1 1 1 16 1 1 1 17 1 1 1 18 1 1 1 1

observed at certain test conditions (identified by thenear exponential increase in torque as in Fig. 2). Theoccurrence of flute clogging is listed in Table 7, where1 denotes the absence of flute clogging and 2 itspresence. It is evident from Table 7 that flute cloggingoccurs when coatings are employed in wet conditions.When uncoated drills are employed no flute clogging isobserved, under either wet or dry conditions.

3. Performance comparison

In comparing and evaluating the performance of the

drill coatings in dry drilling, the aforementionedresponses will be compared to the responses obtainedfrom the HSS drills with no coating in the wet and dryconditions as the baseline. The performance comparisonof the three coatings with respect to the non-coated HSSdrill were obtained by constructing three 2 designs and4

performing an effect analysis. The significant effectswere obtained using a 99% confidence interval withthird and higher order interactions used as an estimateof the error. The significant effects borne out from thismethod for each response and comparison are detailedbelow.

3.1. Multi-layer coatings

The summary of the effect magnitudes obtained inthe comparison of multi-layer coated HSS drills withuncoated HSS drills is given in Table 8. It is observed

from Table 8 that in the case of the maximum measuredtemperature rise, hole radius variation and the surfaceroughness of the hole, the positive effect of cutting fluid(increase in the response with the absence of cuttingfluids) is the only significant effect observed. The multi-layer coating does not seem to affect the inducedworkpiece temperature or the surface roughness of thehole. The strong effect of the absence of cutting fluidsin increasing the temperatures in the workpiece isexpected, since a prominent agent of temperature controlin the workpiece is lost with removal of cutting fluids.The increase in hole radius variation in the absence of

cutting fluid indicates the prevalence of hole tapering(larger hole size at the top portions of the hole incomparison to those observed at the bottom of the hole)with the removal of cutting fluids. This variation is dueto the accrual of incremental errors that occur along thedrill margins. These incremental errors have been shownto arise due to the thermo-elastic deformations in theworkpiece caused by the removal of cutting fluids w13x.The increase in the surface roughness of the hole withthe removal of the cutting fluids suggests increasedinteraction between the chips at the margins of the drilland the hole walls.


7/12


Table 8Magnitude of the effects observed in the comparison of uncoated HSS drills with multi-layer coated HSS drills

Effect Max. temp. Avg. hole Hole radius Surfacerise radius variation roughness ( R )a

Mean 30.99 4.6005 0.0206 5.10Speed y0.19 y0.0049 y0.0026 y0.93

Feed y0.23 y0.0152 0.0074 y0.28Cutt. fld. 25.88 0.0076 0.0385 4.75Coating y2.40 y0.0527 0.0036 y1.04SpeedyFeed y5.22 0.0033 y0.0124 0.21SpeedyCutt. fld. y5.82 y0.0001 y0.0168 y0.55SpeedyCoat. y1.59 y0.0016 y0.0074 y0.09FeedyCutt. fld. y5.81 y0.0090 y0.0002 y0.72FeedyCoat. 0.08 y0.0008 0.0156 y0.62Cutt. fld.ycoat. y1.09 y0.0071 0.0006 0.22Conf. int. 10.72 0.0199 0.0211 1.58

Fig. 6. Measured average radius profile when drilling at 1600 rev.ymin and 0.083 mmyrev. in the presence and absence of cutting fluidswith multi-layer coated and uncoated HSS drills.

The multi-layer coating has a significant effect onimproving hole quality. The mean measured radius of

4.6005 mm is reduced by approximately 26 mm withthe employment of the multi-layer coatings over uncoat-ed drills. These coated HSS drills produces holes thatare closer (considering the fact the drill radius is 4.5641mm) to the ideal radius. The mechanism for thisdecrease does not seem to be due to its effect onworkpiece temperatures in light of its insignificant effecton temperatures. The coatings ability to thermally shieldthe HSS drill substrate w14x is the likely driving mech-anism behind this observed effect.

The removal of cutting fluids does not have a signif-icant effect on the measured average radius of the hole

while it does on the variation in the average hole radius.The plot of the average radius profile along the depthof the hole (Fig. 6) aids in understanding this behavior.The difference in average hole size is larger betweenthe holes produced with coated and uncoated drills incomparison to those observed between the presenceyabsence of cutting fluids. The multi-layer coated HSSdrills produces holes that closer to the ideal radius (drillradius of 4.5641 mm) than those obtained with theuncoated drills throughout the depth of the hole.

Chip clogging, defined as the adherence of the chipsto the flutes, was observed in all the drilling tests

performed in the presence of cutting fluids with themulti-layer coatings (Table 7). In the worst case,extreme welding to the drill flutes was observed. Thecharacteristic increase in forces that is observed whenchip clogging occurs is represented in Fig. 7 along withthe profile of the hole obtained at this condition. Thisobserved trend of chip clogging in the presence of cutting fluids could be due to the high thermal gradientsat the drill-chips and drill-hole wall interfaces. The chipsproduced at, the cutting lips are hotter than the drillbody and the hole walls. As the chip travels along thedrill flutes, this high gradient could encourage the

adhesion of the hot chips to the drill flutes promotingchip clogging.

3.2. TiAlN coatings

A summary of the effects observed in the comparisonof TiAlN-coated drills with the uncoated HSS drills ispresented in Table 9.

Similar to the comparison of uncoated HSS drillswith multi-layer coated drills, cutting fluid has a signif-icant positive effect on the maximum measured temper-ature rise, variation in the radius of the hole and thesurface roughness of the hole.

With regard to the measured maximum temperature

rise, an interaction effect of cutting fluids with feed rateis also found to be significant. A two-way diagramexplaining this interaction is presented in Fig. 8. It isevident from Fig. 8 that in the absence of cutting fluids


8/12


Fig. 7. The increase in forces observed when drilling wet at 1600 rev.ymin and 0.162 mmyrev. with multi-layer coatings along with the measuredtemperature rise (a) and measured average radius profile (b).

Table 9Magnitude of the effects observed in the comparison of uncoated HSS drills with TiAlN coated drills

Effect Max. temp. Avg. hole Hole radius Surface

rise radius variation roughness ( R )a

Mean 30.27 4.5992 0.0156 5.39Speed 2.12 y0.0038 0.0005 y0.42Feed 0.01 y0.0154 y0.0051 0.32Cutt. Fld. 23.76 0.0122 0.0377 5.63Coat. y3.82 y0.0552 y0.0063 y0.45SpeedyFeed y0.58 y0.0005 y0.0107 0.16SpeedyCutt. fld. y4.39 y0.0037 0.0011 y1.05SpeedyCoat. 0.72 y0.004 y0.0107 0.42FeedyCutt. fld. I6.75 y0.0104 y0.0043 y0.65FeedyCoat. 0.32 y0.0010 0.0031 y0.02Cutt. fld.ycoat. y3.21 y0.0025 y0.0002 1.10Conf. int. 5.32 0.0168 0.0252 1.13

higher feed rate offers the benefit of inducing lowertemperatures in the workpiece than at lower feed rates.This trend is opposite to that observed in the presenceof cutting fluids. With increasing feed rates the totalenergy involved in the machining process increases,which should increase the temperatures in the workpieceas is the case in the presence of cutting fluids. Theopposite trend observed in the absence of cutting fluids

could arise due to the decreased time for heat diffusioncoupled with the steep gradients present at the workpieceboundaries. In the absence of cutting fluids the temper-atures at the boundaries in contact with the drill arehigher, therefore, larger heat fluxes are required to effectincreases in the existing steep gradients. This factcoupled with the shorter times for diffusion leads todecreases in temperature rise with increasing feed rate.

The TiAlN coating has a significant effect on averagehole radius, decreasing the hole size similar to the oneobserved with multi-layer coatings. Once again thecoated drills produce holes that are closer to the idealhole radius. The character of these coatings in providinga thermal shield to the drill while discouraging moreheat to be absorbed by the workpiece is similar to thoseobserved with multi-layer coatings. The comparison of

the average hole radius profiles (

Fig. 9)

shows trendssimilar to those observed with the multi-layer coatingcomparison (Fig. 6).

The combination of wet machining and the use of refractory coatings produces holes very close to the idealradius. However, this combination of wet condition andrefractory coatings also promotes flute clogging (Table7). The occurrence of chip yflute clogging when TiAlN


9/12


Fig. 8. Two-way diagram of the significant two-factor interactionbetween feed rate and cutting fluid use in the context of maximumtemperature rise in the workpiece. Fig. 9. Measured average radius profile when drilling wet at 2200

rev.ymin and 0.162 mmyrev. in the presence and absence of cutting

fluids with TiAlN coated and uncoated HSS drills.

Fig. 10. The increase in forces observed when drilling at 1600 rev.ymin and 0.083 mmyrev. with TiAlN coatings along with the measuredtemperature rise (a) and measured average radius profile (b).

coatings are employed along with average radius profileis presented in Fig. 10. The characteristics of the forceand temperature increase is similar to that observed withthe multi-layer coatings (Fig. 7). Flute clogging isinitiated at a penetration depth of 26 mm while in thecase of the multi-layer coatings it occurs at a depth of 24 mm.

3.3. MoS coatings2

The summary of the effect analysis for the comparisonof the MoS coated carbide drills with uncoated HSS2

drills is presented in Table 10. As with the comparisonof the TiAlN-coated drills, in the case of MoS -coated2drill comparisons, the positive effect of the absence

cutting fluids on the measured temperature rise as wellas the two-factor interaction of feed rate with cuttingfluid use (Fig. 11) are significant. The mechanism forthis interaction is similar to that explained with regardsto the TiAlN-coated drill comparison.

As with the multi-layer coatings and the TiAlNcoatings, the MoS -coated drills have a significant effect2on the average hole radius. On the average, the employ-ment of the MoS -coated drill reduces the hole size to24.5886 mm which is larger than the ideal radius by

approximately 20 mm. It is evident that the decrease inaverage radius in the case of MoS -coated drills is not2as pronounced as the other two. A plot of the average


10/12


Table 10Magnitude of the effects observed in the comparison of uncoated HSS drills with MoS coated drills2

Effect Max. temp. Avg. hole Hole radius Surfacerise radius variation roughness ( R )a

Mean 31.71 4.6077 0.0163 5.55Speed 5.63 y0.0032 0.0024 y0.21

Feed y2.30 y0.0084 y0.0100 0.66Cutt. fld. 28.13 0.0101 0.0344 4.99Coat. y0.94 I0.0383 y0.0049 y0.14SpeedyFeed y0.53 0.0001 0.0072 y0.01SpeedyCutt. fld. y3.08 y0.0088 y0.0096 I1.54SpeedyCoat. 4.23 0.0002 y0.0024 0.62FeedyCutt. Fld. I9.02 y0.0063 y0.0090 y0.38FeedyCoat. y1.99 0.0060 y0.0018 0.32Cutt. fld.ycoat. 1.16 y0.0046 y0.0035 0.46Conf. int. 8.03 0.0166 0.0326 1.25

Fig. 11. Two-way diagram of the significant two-factor interactionbetween feed rate and cutting fluid use.

Fig. 12. Measured average radius when drilling at 2200 rev.ymin and0.162 mmyrev. in the presence and absence of cutting fluids withMoS coated carbide drills and uncoated HSS drills.2

hole radius profiles (Fig. 12) observed with the uncoatedHSS drills and coated carbide drills also highlights thisfact. This is due to the fact that the MoS coatings are2not meant to act as thermal barriers, the decreaseobserved could be mainly due to the lower thermalexpansion coefficients of the carbide substrate (3.9–2.5mmy8F) in comparison to the HSS substrate (9.4–6.8mmy8F). The presence of tapered holes (larger at thetop) in the absence of cutting fluids in the case of TiAlN-coated and multi-layer coated HSS drills is alsoobserved with the MoS -coated carbide drills. The2increased thermal deformations in the workpiece causethe observed variation in hole size with depth in theabsence of cutting fluids w13x. Thermal deformationalong the hole walls promote cutting at the drill margins

which is reflected in the tapered profile observed.With regards to the measured surface roughness, two

significant factors are observed. The positive effect of the absence of cutting fluids observed in the previoustwo comparisons is found to be significant in thiscomparison too, that is, the absence of cutting fluids

increase surface roughness. In the use of MoS -coated2carbide drills only, the interaction of spindle speed withcutting fluid use is marginally significant and is ignored.

Chip clogging also occurred when the MoS coating2was employed in the wet conditions at test conditions2 , 3 a n d 4 (Table 7). The characteristic force andtemperature signals experimentally observed when chipclogging occurs is presented in Fig. 13. The effect of flute clogging on the shape of the hole produced ishighlighted in tests conducted with MoS -coated carbide2drills. A distinct increase in hole size is observed in thehole profile when chip clogging occurs (Fig. 13b). Thisincrease is effected at an axial depth (16 mm in thecase of the profile shown in Fig. 12) close to thepenetration depth at which flute clogging starts (28 mm

in Fig. 13). Such a magnified effect of chip cloggingon the hole profile is not observed when refractorycoatings are employed (Fig. 7b and Fig. 10b). This


11/12


Fig. 13. The increase in forces observed when drilling wet at 1600 rev.ymin and 0.083 mmyrev. with MoS coatings along with the measured2temperature rise (a) and measured average radius profile (b).

could be attributed to the superior thermal shield offeredby the refractory coatings preventing the drill substratefrom expanding.

The surface roughness of the hole and the maximumtemperature rise in the workpiece are unaffected by theuse of coatings. The coatings affect the average size of the hole obtained, providing holes closer to the idealradius. Flute clogging was also observed when therefractory and MoS coatings are employed in wet2conditions. This was not observed with uncoated HSSdrills in either dry or wet conditions. The absence of cutting fluids has an overwhelming effect in increasingthe induced workpiece temperatures and surface textureof the hole. Hole radius variation is affected largely bythe presenceyabsence of cutting fluids. In the absenceof cutting fluids the holes produced are larger at the topportion of the hole than at the bottom of the hole. Thischaracteristic variation is due to the increased thermo-elastic deformation in the workpiece along the holewalls w13x.

4. Summary and conclusions

The performance of drills with three different coatingsis compared to that of HSS drills with no coating.Surface roughness of the hole produced the maximumtemperature rise in the workpiece, the average holeradius and variation in hole radius along its depth wereemployed as the key metrics in the comparison. Theinvestigation demonstrated the overriding effect of theabsence of cutting fluids in increasing the surfaceroughness and temperatures in the workpiece. The com-parison of coatings demonstrated that presence of coat-ings has no significant effect on the induced workpiece

temperatures nor on the surface texture of the holeproduced. The presence of coatings was found to havea significant effect in determining the final hole sizewith TiAlN, multi-layer coated HSS drills and MoS -2coated carbide drills producing holes closer to the idealhole radius than the uncoated HSS drills. The extent of decrease in hole size is not as pronounced with themolybdenum disulfide coated carbide drills. The analysisdemonstrated the following trends:

● HSS drills with refractory coatings

(TiAlN and multi-layer) tend to produce holes closer to the ideal radius

than the uncoated drills due to the superior thermalbarrier provided by these coatings, discouraging drillsubstrate expansion.

● In comparison to the refractory coated (TiAlN andmulti-layer) HSS drills, the MoS -coated carbide2drills produced holes that were somewhat larger. TheMoS coating does not possess the refractory prop-2erties of the multi-layer and TiAlN coatings, thus itdoes prevent thermal expansion of the carbidesubstrate.

● ChipyFlute clogging, defined as the adherence of the

chips to the flutes, occurred with coated HSS drillsand MoS -coated carbide drills. All instances of chip2clogging occurred in the presence of cutting fluids.By comparison, HSS drills with no coating did notproduce flute clogging under wet conditions. Whilethis combination of wet conditions and coatingsprovides holes closer to the ideal size, the occurrenceof flute clogging at these conditions is a cause of concern as it could lead to drill breakage.

● The use of coatings did not seem to affect the surfaceroughness of the hole produced nor the temperaturesin the workpiece.


12/12


● In the absence of cutting fluids the holes are tapered,with the holes getting larger toward the top of thehole. The observed shape is due to cutting at the drillmargins as a result of the increased thermal defor-mations in the workpiece along the hole walls w13x.

Acknowledgements

The authors are grateful for the support of the Nation-al Science Foundation and IndustryyUniversity Co-operative Research Center for Machine Tool SystemsResearch at University of Illinois at Urbana-Champaign.

References

w1x G. Byrne, E. Scholta., Ann. CIRP 42 (1993) 471.w2x F. Klocke, G. Eisenblatter., Ann. CIRP 46 (1997) 1.w3x H.K. Tonshoff, W. Spintig, W. Konig, A. Neises., Ann. CIRP¨ ¨

43 (1994) 551.

w4x T. Cselle, A. Barimani., Surf. Coat. Technol. 76y77 (1995) 712.w5x N.M. Renevier, N. Lobiondo, V.C. Fox, D.G. Teer, J. Hamp-

shire., Surf. Coat. Technol. 123 (2000) 84.w6x F.M. Kustas, L.L. Fehrehnbacher, R. Komanduri, Ann. CIRP 46

(1997) 39(1).w7x K. Tonshoff, A. Mohlfeld, T. Leyendecker et al., Surf. Coat.¨

Technol. 94y95 (1997) 603.

w8x V.C. Fox, N. Renevier, D.G. Teer, J. Hampshire, V. Rigato.,Surf. Coat. Technol. 116y119 (1999) 492.

w9x V. Derflinger, H. Brandle, H. Zimmermann., Surf. Coat. Technol.¨

113 (1999) 286.w10x T. Cselle. In: 2nd International Conference on High Speed

Machinery, pp. 137–146. Institut fur Produuktionstechnik und¨

Spanende Werkzeugmaschinen, March 1999.w11x I.J. Smith, W.D. Muntz, L.A. Donohue, I. Petrov, J.E. Greene.,

Surf. Eng. 14 (1998) 37.w12x M. Lahres, O. Doerfel, R. Neumuller., Surf. Coat. Technol. 120y¨

121 (1999) 687.w13x S. Kalidas. Modeling and prediction of the thermal effects on

hole quality in dry and wet drilling conditions. Master’s thesis,University of Illinois at Urbana-Champaign, 2001.

w14x W. Grzesik., Intl. J. Mach. Tools Manuf. 39 (1999) 355.w15x Y. Zheng, H. Li, W.W. Olson, J.W. Sutherland., J. Manuf. Sci.

Eng. 22 (2000) 377.w16x S.J. Lee, K.F. Eman, S.M. Wu., J. Eng. Ind. 109 (1987) 297.w17x H. Fuhi, E. Marui, S. Ema., J. Eng. Ind. 108 (1986) 157.w18x V. Surendran. A study of hole quality in drilling. Master’s thesis,

University of Illinois at Urbana-Champaign, 1998.w19x M.N. Jalisi, C. Lin, and K.F Ehmann. In: Manufacturing

Processes and Materials Challenges in Microelectronic Packag-ing, pp. 25–33, ASME, 1991.

w20x K. Watanabe, K. Yokoyuma, R. Ichimaya., Bull. Jpn. Soc. Prec.Energy 11 (1977) 71.

w21x D.A. Stephenson, J.S. Agapiou., Metal Cutting Theory andPractice, Marcel Dekker, Inc, 1996.

w22x Guhring, Inc. High Performance HSS, HSCO and CarbideCutting Tools, Standards Sourcebook, 1998.

surface and coatings technology volume 148 issue 2-3 2001 [doi 10.1016_s0257-8972(01)01349-4] shiva...

Documents