analytical and experimental investigation of coolant velocity in high speed grinding

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4.02.017

International Journal of Machine Tools & Manufacture 44 (2004) 1069–1076

www.elsevier.com/locate/ijmactool

Analytical and experimental investigation of coolant velocityin high speed grinding

K. Ramesh �, H. Huang, L. Yin

SIMTech Institute of Manufacturing Technology, Singapore 638075

Received 30 April 2003; received in revised form 16 February 2004; accepted 26 February 2004

Abstract

Use of water-base coolant is a pre-requisite in an high speed grinding process to avoid thermal damage and to achieve bettersurface integrity as well as higher grinding ratio. However, the presence of hazardous chemical additives in the coolant causesenvironmental problems. As a result, stringent government legislation is being practiced for the coolant use and disposal, whichconsumes 7–17% of the total machining cost. This paper reports the coolant flux minimization through controlled jet impinge-ment so as to prolong the coolant replenishment cycle. Control of coolant flux was achieved through development of a ‘‘meteredquantity coolant’’ (MQC) nozzle which supplies the required amount of coolant to the grinding zone. Also, this investigation hasshown that coolant velocity has a significant influence on the high speed grinding performance. When the coolant velocity isinadequate, coolant could not penetrate into the grinding zone. The increase in coolant velocity was realized with reduction innozzle opening area and does not use a large quantity of coolant. This is of significance to reduce environmental pollution andmachining costs through extended coolant replenishment period.# 2004 Elsevier Ltd. All rights reserved.

Keywords: High speed grinding; Coolant flux; Coolant

1. Introduction

The high speed grinding process consumes five to six

times higher grinding energy and therefore use of copi-

ous coolant to avoid thermal damage, to achieve better

surface integrity and higher tool life through friction

reduction as well as cooling is commonly seen [1].

However, the presence of chemical substances like

sulfur, phosphorous, chlorine or any other extreme

pressure additives in the coolant introduces health

hazard to the operator [2]. It is well documented that

7–17% of machining cost of a work-piece is due to

coolant-lubricant deployment [3]. The disposal of used

chemical coolants involves incineration and partially

contributes to global warming [4]. Also, use of flood

coolants does not inhibit the air boundary layer and a

protocol was made for further investigation of coolant

flow mechanism [5]. In view of this, ecological machin-ing has gained its significance through introduction of:(i) dry machining (ii) effective and prolonged usage ofcoolant through optimization and nozzle design. Thispaper reports the coolant flux minimization and therelated grinding characteristics through controlled jetimpingement.

2. Analysis

The interaction between wheel and work-piecegenerates thermal energy due to friction and plasticdeformation. The generated heat is transferred towork-piece, grinding swarf, coolant and wheel (seeFig. 1). Previously developed models quantify theactual grinding temperature rise and the cooling effectwith emphasis to the physical properties of coolant[6,7]. In this analysis, the role of coolant exit velocity indisseminating the heat generated in high speed grindingprocess was analyzed.Grinding wheel was considered to be a solid disk

with a thin abrasive layer where irregular shaped

1070 K. Ramesh et al. / International Journal of Machine Tools & Manufacture 44 (2004) 1069–1076

abrasive grains are firmly fixed. This wheel rotates at

high speed and makes in contact with a slowly moving

work-piece surface. The following assumptions are made

to establish a quantitative relation between the coolant

velocity and its heat dissemination characteristics:

. fluid motion is characterized by average velocities;

. wheel is adequately dressed to have uniform clear-ance between grits;

. all flow takes place within the wheel width and notin sideways;

. convection is the major mode of heat transfer [8].

The problem was viewed as a forced convection and

in such case convection heat transfer co-efficient was

determined based on semi-empirical relations and type

of flow. The flow type was assessed through compu-

tation of Reynold’s number as given in Eq. (1).

Re ¼ qvclcl

ð1Þ

where q is the coolant density (998.2 kg/m3), vc is thecoolant velocity (3.6–16.51 m/s), lc is the contact

length and l is the dynamic viscosity (1:002�10�3 kg=ms).Substituting the coolant properties in Eq. (1), the

Reynold’s number was computed and the value was

found to be between 16030.9 and 73719. It is well

accepted that when ‘‘Re < 5� 105’’, the coolant flowwould be predominantly laminar [9]. Therefore, the

heat transfer coefficient, h was computed using the

semi-empirical equation of theory of forced convection

as given in Eq. (2).

Nu ¼ 0:664 � Re1=2Pr1=3 ð2Þ

where Nu is the Nusselt number; Re is the Reynold’snumber; and Pr is the Prandtl number are furtherdefined as,

Nu ¼ hlck; Re ¼ qvclc

l; Pr ¼ lCp

k; ð3Þ

where k is the thermal conductivity of coolant media

(6:03� 10�4 kW=m K) and Cp is the specific heat ofcoolant (4.183 kJ/kg K).Substituting the coolant properties and combining

Eqs. (2) and (3), convection coefficient, h was simplifiedto,

h ¼ 0:759ffiffiffiffiffivclc

rð4Þ

where h is the convection coefficient (kW/m2 K) and vcis the coolant velocity (m/s).Eq. (4) suggests that in any grinding condition, cool-

ant velocity directly influences the grinding heat dis-semination characteristics and therefore an arrangementthat allows the variation of coolant velocity was plan-ned. The design includes a metered quantity nozzle inwhich the ratio of coolant entry to exit area and theflow characteristics decides the coolant velocity distri-bution. For a coolant nozzle as illustrated in Fig. 2,Bernoulli’s equation for energy conservation wasapplied to compute the coolant velocity as,

v212þ p1

q¼ v222þ p2

qð5Þ

where v2 is the coolant velocity at exit, v1 is the coolantvelocity at entry, p1 is the pressure at nozzle inlet andp2 is the pressure at nozzle exit.As the Reynold’s number computation confirms the

coolant flow as steady-state laminar Eq. (5) was rewrit-ten with application of continuity analysis (v1a1 ¼ v2a2)as,

v2 ¼2ðp1 � p2Þ

qð1� ða2=a1Þ2Þ

" #1=2ð6Þ

Fig. 1. Schematics of grinding energy transfer (i) to work-piece, Qw(ii) to swarf, Qch (iii) to grinding wheel, Qgw (iv) to coolant, Qco.

Fig. 2. Schematic of coolant nozzle indicating the flow parameters.

K. Ramesh et al. / International Journal of Machine Tools & Manufacture 44 (2004) 1069–1076 1071

This equation suggests that pressure drop and theratio of nozzle exit to inlet is of significance for admin-istrating the coolant velocity parameter.

3. Coolant supply method

In contrast to the normal coolant setup that suppliescopious amount of coolant to the grinding zone, anozzle that supplies metered amount of coolant wasdeveloped. As shown in Fig. 3, this system consists of aflow meter, a pressure gage, a manifold and changeablenozzles. Three different nozzles with cross-sectionalarea 15.14–50.26 mm2 were included in this design. Thenozzle characteristics in terms of flow, pressure andvelocity are given in Table 1. Shown in Fig. 4 is thephoto of the developed nozzle arrangement with flowmeter and pressure gage, integrated to the wheel head.

4. Experimental conditions

For each nozzle, a series of grinding tests were per-formed on SS304 at a pre-fixed grinding condition.Coolant flow rate and pressure were adjusted to varythe coolant velocity (vc) for investigating the grinding

performance. During the process both normal (Fn) and

tangential (Ft) grinding forces were measured using a

9265B Kistler dynamometer for computing the force

ratio (Ft/Fn), power-flux (Q) and grinding energy (E).

The onset of burn in grinding was quantitatively

expressed in terms of power-flux using Eq. (7) as [10],

Power-flux ðQÞ ¼ FtV

bffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiDD½1þ v=V

p ð7Þ

where Ft is the tangential force (N), V is the wheel

speed (m/s), v is the table feed rate (m/s), b is the

wheel width (mm), D is the wheel diameter (mm) and

D is the depth of cut (mm). Eq. (7) suggests that

power-flux is likely to increase upon increase of wheel

speed, tangential grinding force but reduces with

increase of contact area.The specific energy for cutting, which is that portion

of the specific grinding energy remaining after subtract-

ing the contribution due to sliding was computed from

the relationship shown in Eq. (8).

E ¼ ftV

vDð8Þ

where V is the wheel speed, v is the Table feed, D is thedepth of cut and ft is the specific grinding force in tan-

gential direction of grinding wheel.Ground samples were examined using a scanning

electron microscope at a magnification� 1000 and �50to characterize the surface integrity. Also, a 3D surface

micrograph was taken using optical interference pro-

filer (Wyko NT 3300) for examination of the coolant

velocity related surface changes. Table 2 lists the grind-

ing conditions used for the experiments.

Fig. 3. Schematic layout of grinding with coolant penetration.

Table 1

Characteristics of nozzle

Nozzle cross-sectional

area (mm2)

C

p

oolant

ressure (bar)

C

fl

oolant

ow (lpm)

C

v

oolant

elocity (m/s)

50.26 0
.2–0.8 1 1–25 3 .6–9.9
28.56 0
.2–1.6 7 –21 4 –12.3
15.14 0
.5–2.5 6 –15 6 –17


5. Results and discussions

5.1. Force ratio

Rubbing between the wheel–work interface andbetween the inter-atomic layers of the grinding zone ofthe work-piece generate heat and the Ft/Fn ratio quan-tifies the trend [11]. Fig. 5 shows the Ft/Fn trend withvs. coolant exit velocity parameter for SS304 at differ-ent nozzle cross-section and wheel speed.At 42 m/s of wheel speed, the experimented nozzles

of cross-sectional area 15.14–50.25 mm2, has reducedthe force ratio from 0.44 to 0.26, 0.44 to 0.38 and 0.52to 0.4 with increase of coolant velocity from 3.5 to 7m/s, 4 to 11 m/s and 10.5 to 16 m/s, respectively.With administration of the same grinding conditionsincluding the coolant velocity range but at higher wheelspeed of magnitude 104 m/s, the force reduction wasfound to be from 0.24 to 0.16, 0.38 to 0.28 and 0.24 to0.2. These results suggest that the higher coolant velo-city has increased the penetration depth of coolant intothe grinding zone resulting in introducing a lubricationeffect for reducing the friction between wheel andwork-piece. As the grits engage with the work-piece,rubbing causes elastic deformation before the plasticdeformation and material removal. It is expected thatduring the chip formation process depending upon the

wheel/work interaction stage such as, rubbing or plas-

tic deformation or material removal, the force ratio

would be different. Therefore, for comprehensive

understanding of the role of coolant velocity, additional

experiments were conducted to establish the power-

flux, grinding energy and surface integrity.

5.2. Power-flux

Shown in Fig. 6 is the power-flux (Q) behavior against

coolant velocity (vc) for the fixed experimental con-

ditions given in Table 2. Power-flux was computed using

Eq. (7) with measured values of tangential forces (Ft)

and the experimental grinding parameters. It is to be

noted that the power-flux value was decreased from 40

to 22 W/mm2, 38 to 20 W/mm2 and 32 to 24 W/mm2

with increase of coolant velocity from 3.5 to 8 m/s, 4 to

10.5 m/s and 11.5 to 15.5 m/s, respectively. Such reduc-

ing trend quantitatively describes the cooling perform-

ance with administration of coolant velocity parameter.

The same was also analytically proven and expressed in

terms of convection co-efficient (h) as given in Eq. (4).

Fig. 4. Photo of the metered quantity nozzle design integrated to the grinding wheel head.

Table 2

Experimental grinding conditions

Detail V
alue
Wheel speed (m/s) 3
3–104
Table feed (mm/min) 5
00
Depth of cut (mm) 0
.1
Grinding direction D
own cut
Grinding wheel S
pecification B 126N100EP
Size (mm�mm) 1
200� 10 Coolant conditions P ressure (bar) 0 .2–2.5
Flow (lpm) 7
–26
Nozzle area (mm2) 1
5.14–50.26
Work-piece S
tainless steel (SS304)
Fig. 5. Influence of coolant velocity on grinding force ratio (Ft/Fn)

at conditions: 0.833 mm3/mm s, wheel ¼ B126N100EP, work-piece ¼SS304.


The results of computed h values that increases between22 and 46 kW/m2 K were also plotted in Fig. 6. Theincreased coolant velocity incessantly introduces freshcoolant to the grinding zone enabling an increase inforce convection heat transfer, which was evident fromreducing power-flux trend.

5.3. Specific grinding energy

Fig. 7 shows the specific grinding energy consumedat different coolant velocity for SS304. At low coolantvelocity the energy was extremely large but it decreaseswith increasing coolant velocity towards a minimumvalue of approximately 10, 15, and 13 J/mm3 whenusing nozzles of area 50.26, 28.56 and 15.14 mm2,respectively. The high specific cutting energy at lowcoolant velocity cannot be reconciled with the classicchip-formation model. This would suggest that onlypart of the specific cutting energy is actually associatedwith chip formation in which case, there must be atleast one other mechanism to account for the remain-

ing energy. Another mechanism associated with abras-ive process is plowing. Plowing energy is expended bydeformation of work-piece material without removal.Hence, it can be understood that extensive primary andsecondary plowing exists at low coolant velocity due tosevere friction between wheel–work interface andbetween the work-piece inter-atomic layers. At highercoolant velocity, the specific plowing energy approa-ches zero, and the minimum specific cutting energycorresponds to the specific energy for chip formation,which is assumed to be constant. Hence, the experi-mental results obtained imply specific chip formationenergy of 10–13 J/mm3 at coolant velocity 9.5–15.5 m/s,respectively. It means that at higher coolant velocityplowing is comparatively less and cutting (chipping)action is predominant which is considered to be one ofthe benefits of high coolant velocity. Also, the surfacetopography of the ground ceramics studied under SEMindicated that more amount of plowing and side flowof material had taken place in the case of low coolantvelocity.

5.4. Surface characteristics

Past grinding research enumerates that adequateapplication of coolant enables ductile material flow,characterized by plowing grooves and streaks for alloysteels [12]. Fig. 8 compares the ground surface micro-structure produced using water-base coolant at differ-ent coolant velocity. At conditions of both low andhigh wheel speed, when the coolant velocity was at 3.5m/s the ground surface produced has less plowinggroove with surface irregularities as shown in Fig. 8(A)and (D). Fig. 8(C) and (F) shows the SS304 groundsurface texture with large amount of plowing groovesand streaks produced when the coolant velocity was at15.4–16 m/s. Also, it should be noted that when thecoolant velocity was at 3.5 m/s the ground surface haslarge material adhesion due to excessive strain andinadequate time for the heat transfer. Furthermore,large amount of shear bands in the form of white stripindicate the presence of martensite as a result of abruptcooling. In earlier findings, such phenomenon wasaddressed as adiabatic shear [13]. This result wasfurther confirmed with the study of ground surfacehardness behavior.

5.5. Micro-hardness

Depending upon the grinding related thermal sever-ity, ground surface of SS304 would experience burn orthermal damage and the surface hardness behaviorcharacterizes the same [13]. Shown in Fig. 9 is themicro-hardness behavior against coolant velocity (vc).At lower wheel speed of magnitude 42 m/s andwith increase of coolant velocity from 3.5 to 7 m/s,

Fig. 6. Behavior of power-flux (Q) against coolant velocity (vc) at

grinding conditions: (i) specific material removal rate ¼ 0:8 mm3=mm s; (ii) wheel speed ¼ 42 m=s.

Fig. 7. Effect of coolant velocity on specific grinding energy for

SS304 at 42 m/s wheel speed.


4 to 9.5 m/s and 10.5 to 15.5 m/s the ground surface

hardness was reduced from 400 to 330 Hv, 360 to 330

Hv, and from 370 to 340 Hv, respectively. Similar

micro-hardness decreasing trend was also observed at

high wheel speed (104 m/s) confirming the significant

influence of coolant velocity. Also, it should be noted

that the micro-hardness values for all the ground speci-

mens are far above that of as-received materials (200

Hv) indicating the results of grinding heat and mechan-

ical strain. The results conclude that the increase of

coolant velocity reduces the thermal severity and hence

lower micro-hardness was achieved.

5.6. Surface finish

Domination of thermal or mechanical or both on the

grinding wheel/work interface would produce coarse

surface as a result of severe interaction and abrupt cool-

ing. Fig. 10 shows the surface roughness behavior

against coolant velocity at a fixed grinding condition.

At all cases of grinding experiments, it was observed

that the surface roughness reduces with increase of cool-

ant velocity. Within the experimental range, the surface

finish was improved from 1.2 to 0.6 lm, 1.2 to 0.55 lmand 1.2 to 0.56 lm with increase of coolant velocity

from 3.5 to 8 m/s, 4 to 11 m/s and 7 to 15.5 m/s,

M images of SS304 after grinding at v¼ 500 mm=min and D ¼ 100 lm at (i) 42 m/s and coolant velocity: (a) 3.5 m/s; (b
Fig. 8. SE ) 10 m/s;
(c) 15.4 m/s, (ii) 104 m/s and coolant velocity: (d) 3.5 m/s; (e) 10 m/s; (f) 16 m/s. At wheel speed ¼ 42 m=s and coolant velocity ¼ 3:5 m=s moreamount of particle pull out (a). At wheel speed ¼ 104 m=s and coolant velocity ¼ 16 m=s improved surface finish with but large amount of plasticflow of material (f).


respectively. This is because higher coolant velocityintroduced a lubrication effect with better grindingheat dissemination characteristics.

5.7. Coolant flow rate

Within the range of grinding experiments, the cool-ant velocity has played a significant role in controllingthe grinding performance of SS304. Using nozzle ofcross-section area 15.14–50.26 mm2 and flow controlvalve, coolant velocity was varied from 3.5 to 16 m/s.Coolant velocity was increased through two differentmeans which are: (i) reducing the nozzle cross-sectionand (ii) through flow control valve. Increase of coolantvelocity within a particular nozzle increase the coolantflow rate. On the other hand, the coolant velocityincrease through reduction of nozzle cross-section areadecreases the flow rate. The coolant velocity and flow

rate range for the experimented nozzle overlaps asgiven in Table 3. To meet both targets such as environ-mental friendliness and cooling efficiency it is necessaryto minimize the coolant flow rate and to maximize thecoolant velocity for extending the coolant replenish-ment period and for enhancing the coolant penetration.Table 4 summarizes the various observations as a

function of coolant flow rate. Grinding performance interms of Ft/Fn ratio, power-flux (Q) and surface integ-rity was poor at conditions of high coolant velocitywith smaller flow rate. Also, beyond 18–20 lpm of flowrate the improvement in grinding performance wassmall indicating the existence of threshold coolant flowrate for a fixed coolant velocity. Increase of coolantvelocity from 35 to 10 m/s has brought down thethreshold value from 24–26 to 18–20 lpm. Such resultswere established for SS304, and for the experimentalgrinding conditions it was ascertained that coolant flow

Fig. 9. Influence of coolant velocity on ground surface hardness at

conditions: 0.833 mm3/mm s, wheel ¼ B126N100EP, work-piece ¼SS304.

Fig. 10. Influence of coolant velocity on ground surface roughness

at conditions: 0.833 mm3/mm s, wheel ¼ B126N100EP, work-piece ¼SS304.

Table 3

Coolant output in terms of coolant velocity and flow rate


rate (18–20 lpm) with coolant velocity (10–11 m/s) wasadequate to produce a ground surface free from sur-face damage.

6. Conclusions

Study of grinding with control of coolant velocityintroduces efficient precision grinding with ecologicalfriendliness. Experimental study suggests the existenceof threshold flow rate below which the ground surfacedamage is apparent. Coolant velocity control furtherbrings down this threshold through effective pen-etration. Also, the coolant velocity introduces lubri-cation effect at the grinding wheel–work interface thatresults in reducing force ratio from 0.56 to 0.38 andpower-flux from 38 to 22 W/mm2. Furthermore, thesurface finish was improved from 1 to 0.58 lm withdistinct plowing grooves and streaks. This is of signifi-cance in reducing machining costs and to reduceenvironmental pollution.

Acknowledgements

The authors would like to thank Mr. Sim MongChye Wilson and Ms. Teo Phaik Luan of Singapore

Institute of Manufacturing Technology (SIMTech) for

providing the measurement and experimental assistance

during the project execution. This work was supported

by the Agency for Science, Technology and Research

fund via SIMTech Project No; C01-P-067AR.

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Table 4

Summary of grinding results against the coolant flow rate

Coolant

flow (lpm)

F

r
t/Fnatio
Power-flux

(Q)

(W/mm2)

S

h

(H

urface

ardness

v)

Surface

roughness

(Ra) (lm)

9 0
.56 32 3 70 1
10 0
.54 30 3 60 0.92
12 0
.5 28.5 3 50 0.8
15 0
.43 26 3 40 0.56
18 0
.41 25 3 35 0.6
20 0
.385 22 3 30 0.58
22 0
.38 22 3 30 0.56
24 0
.38 22 3 30 0.56
26 0
.38 22 3 30 0.58

analytical and experimental investigation of coolant velocity in high speed grinding

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