High-Performance Surface Grinding - The Influence of Coolant on the Abrasive Process

E. Brinksmeier (2), E. Minke, University of Bremen Received on January 14,1993

The availability of newly designed grinding machines with high stiffness. high spindle power and wheel speeds. super-abrasive CBN-grinding

wheels. adaptive dressing techniques, coolant delivery. coolant filter, and wheel surface cleaning systenis enables application of the high-

performance grinding method. This grinding technique. using CBN-grinding wheels. can be characterized by high cutting speeds. relatively

high material removal rates. and good surface finish. This paper presents the consideration of chip thickness and its influence on the grinding

process, due to the grinding parameters, such as cutting speed, depth of cut and work speed. Results obtained in practical investigations

concerning the impact of the coolant drlivery. on the emerged forces and on the power demand at high peripheral wheel speeds are discussed

Keywords: Grinding, grinding fluids, cutting speed Introduction

During recent years much attention has been focussed on the

development of high-performance grinding to improve understanding of

the process and to apply this technology to industrial purposes. Today

specifically designed machines and CBN-tools are available and basic

knowledge concerning grinding and dressing processes has been attained

[ I - 51. There are, however. a number of problems connected with this

process, for the overcoming of which increased efforts must be made in

the future. The application of this method to materials which are difficult

to grind, such as high-speed steel, tungsten carbide, Ni-alloys, titanium.

ceramics and glass, will open up a wide new field. In practical research

work various questions will have to be looked into, e. g., the grinding

forces and power which emerge, the process

temperature, the induced sub-surface damage and the work quality. The

effects and possibilities of substituting the mineral oil commonly used up

to now by soluble oil or alternative fluids will also have to be

investigated. This paper presents. on the basis of theoretical

considerations, some solutions to problems arising from coolant delivery

in high-performance grinding investigations.

Fundamentals of hieh-performance erlnding

High peripheral cutting speeds in grinding offer the alternatives of improving surface finish or increasing the material removal rate and

realizing a sufficient product quality. The latter is obtained through high-

performance grinding. This method can be regarded as a combination of

creep-feed grinding and high-speed grinding in which, by a

multiplicative superposition of high work speeds and large depth of cub.

extremely high material removal rates have been made possible. Fieure I shows the development in surface grinding and that today metal

removal rates of Q A = 1000 mm3/(mm. s) and more are achievable in

high-performance grinding [6].

From the theoretical point of view the possibility of achieving high

removal rates can be defined by the thickness of a chip, which, besides

the wheel specification, depends on the grinding parameters such as

cutting speed, work speed and depth of cut.

If the material removal rate remains consmt and the cutting speed is

increased, the chip thickness will decrease and the frictional speed of the

Annals of the CIRP Vol. 42/1/1993

1000

500

,mJ mm.s

100

50

10

5

I I I

0 50 100 150 mls 200

cutting speed vc

I

Fieure 1 : Working areas in surface-grinding

grains in the work material will rise. These effects result in better

surface roughness, lower mechanical loading, higher G-ratio but

simultaneously in higher risks of thermally-induced change in the

workpiece surface zone [7]. &J shows the relationship between force

components. spec. material removal rate and cutting speed. The gear

profile (hot work tool steel) was machined in one pass in up-grindulg

mode.

tA-

LLC

9 c

2500 i

1500

1000

500

O : ' " ' . " ' I " ' " ' ' ' 1 ' ' : " " I 0 100 200 300 400 500 & 600

mm s spec material removal rate 0;

Fieure 2: Grinding force components depending on spec. material

removal rate for different cutting speeds

On the other hand higher cutting speeds can be used to increase the

material removal rate by increasing the chip thickness to an extent

comparable to that obtained at conventional grinding speeds.

367

The advantage of high material removal rates such as short grinding

times leads to higher grinding force components and required power. As

material removal rate increases. the required power generally also

increases ffi..). accompanied by higher process temperature and a rise

in thermal loading of the workpiece

50 I

~ I j [rinding wheel , ;5Z-MZOO-G ' work speed vw= 1,O mlmm coolant oil. 150 llrnin , wheel cleaning 20 I/min I material 26 NiCtMoV 14 51

I

0 20 40

spec. material removal rate 4;

60 111(113 80 mm.s

Firmre 3: Total spindle power depending on spec. material removal rate

for different cutting speeds

Nevertheless grinding results have been obtained in which high-

performance grinding has led to a decrease of thermal influences and to

compressive residual stress in the surface layer [8]. This is due to a

phenomenon well known from the heat treatment of steel, i. e.. that the

type and depth of the altered structural zone depends on the temperature

level and in particular on the exposure time [9]. Increased work speeds

in high-performance grinding technology have the effect of shortening

the exposure time of the workpiece to the heat source, resulting in

application of heat to the workpiece for a shorter period. Under adapted

grinding wheel specifications and grinding and coolant delivery

conditions, thermal damage of the workpiece can be avoided even in the

grinding of sensitive materials such as Inconel, Ti-alloys, high-speed

steel and tungsten carbide. At the same time relatively high material

removal rates can be achieved [lo, I I].

750 0 0 - f 500

I " " ,

0 50 100 150 m 200

work profile length

Firmre 4; Grinding force components versus work profile length

An example is shown in u. A Hirtli-gear profile, material hot work

tool steel, was ground with a good surface integrity at a spec. material

removal rate of Q ; = 278 mm3/(mm~ s ), showing slightly increasing

force values during the total grinding time. Even 'after a ground profile

length of 170 m the tool life was not exceeded A s a result of this

investigation. grinding with conventional wheels was replaced by the

high-performance grinding method with a considerably improved

grinding time and increased economy.

Intluence of the coolant s u p p ~

The coolant has the task of removing heat from the contact zone between

wheel and workpiece and reducing the frictional forces between the

grains and material. In addition. the coolant undertakes the removal of

the swarf from the contact zone. The removal of the swarf depends on

the abrasive grit size and the grit concentration and can be improved by

using a newly developed metal single layer (MSL) bonding system [IZ]. For high-performance grinding a coolant flow rate of roughly

Qksr= 200 llmin at a pressure of some 20 bar is required. Further

requirements are an adaptation of a nozzle shaped to the workpiece, a

cleaning of the wheel rim in order to prevent deposits, and the

installation of an additional nozzle. This latter is neccessary to quench

the spray of sparks and thus to prevent any danger of fire or explosion

in the workshop while grinding with oil. The commonly used mineral oil

requires a coolant supply of an amount of ZOO0 likes, an adapted filter

system, high-pressure pumps and a workshop smog exhauster. &J

shows a high-performance surface grinder and, on the left, coolant and

filter equipment installed.

Firmre 5: High-performance surface grinder with coolant system

However, the high coolant flow rates described are accompanied by the

disadvantage of a higher power demand and the emergence of increased

cutting forces. presents the relation between increasing cutting

speeds and the different fractions of power [ 131.

As cutting speed increases, there is only a slight increase in the no-load

power demand (PI = 2 - 4 kW). With the grinding wheel positioned in

a pre-profiled groove of 6.5 mm depth with the coolant system switched

on, the power demand increases markedly from P, = 12 kW to P, =

26 kW with an increase of cutting speed from v, = 120 to 180 m/s.

However, the total power Ptor also increases against vc at the same rate.

I t is significant that in high-performance grinding using CBN-grinding

36%

1 kW 1

Fieure 6: Power demand depending on cutting speed [13]

grinding wheel . B252GSS I , spec material rein rate depth ofcut ae= 6 5 mm I

0; = 217 mm31(mm.s)

This disadvantage in high-performance grinding is caused by the high

coolant tlow rate. F&J shows the effects of an increase in the coolant

supply on the spindle power.

4 C

The experimental results shown in this illustration were obtained during

"grinding" with a depth of cut a, = 0 mrn. If the wheel speed increases

(v, = 60 to 180 mls) there is only a slight increase in the no-load spec.

power demand when the coolant tlow rate is low (Qrsa = 30 Ilmin).

With increasing tlow rates (up to 130 limin) the no-load spec. power

increases rapidly up to a value of P, = 0.58 kW per inm wheel width.

I t can be seen that at conventional grinding speeds of vc = 60 m/s the

coolant tlow rate does not affect the required spindle power.

- - mm 1

0 3 kW Zii

0.5 depth of cut coolant flow rate

a' = 0 pm akS= 130 I/mm

0.4

jL coolant . mineral oil

G

P b Q

m

wheel speed

0.3

0.2

0

grinding wheel 81 51 G wheel diameter depth of cut work speed v,= 10 mlmin

d, = 400 mm a, = 0 pm

1

0 20 40 60 80 100 120 140 160 180 mls 220

wheel speed vc

Figure 7: Spec. power depending on wheel speed and coolant flow rate

The reason for this increase in power demand with a growing coolant

volume flow and increased cutting speed can be traced to the fact that

the coolant fluid fed in under pressure exerts a braking effect on the

grinding wheel [ I I ] and that a hydrostatic/-dynamic condition of

pressure arises in the gap between the workpiece surface and the

grinding wheel.

This effect is shown in m. When cutting speed and coolant tlow rate

increase, the measured spec. normal force increases up to a maximum

25

N mm

20

-

iL=

$ 15 m

5

wheel diameter d, = 400 mm

0 20 40 60 80 100 120 140 160 180 200 220

wheel speed vc rnls

Figure 8; Spec. normal force depending on wheel speed for different

coolant flow rates

value of F,; = 22 N/mm (Qksa = 130 I/min, vc = 180 m/s). There is however another influence, which leads to decreasing spec. normal force

values for every tlow rate after a specific wheel speed is achieved. This

effect may be caused by low coolant speeds compared with the actual

whet1 speed, so that a swirling motion of the coolant arises and the

creation of a lubricating film in the contact zone is possible only to a

very limited extent.

When grinding with low depth of cut (a; = 0.5 mm) the force-reducing

effect of higher wheel speeds is over-compensated by the hydrodynamic

effect, coupled with an increase of workpiece deformation, especially

when thin-walled parts are to be ground.

Fieure 9: Spec. normal force depending on work speed and coolant

flow rate

Increasing work speeds do not affect the spec. normal force as shown in

m. The work speed compared with the high wheel speed is too low

to have an influence on the hydrodynamic effect.

The experimental results obtained require further examination to

discover whether the coolant flow rate can be reduced in order to

369

- decrease the loss of required power

- decrease the normal force component caused by hydrodynamic

effects.

On the other hand the grinding process should be provided with a

sufficient mount of coolant in order to remove h e heat induced by the

process and to prevent thermal damage of the workpiece surface layers.

Further investigations are necessary in this area in order to arrive at a

compromise between these two contradictory demands on the grinding

process.

Conclusion

In the present paper the role of the chip thickness in high-speed grinding

processes has been discussed. If the material removal rate remains

constant, higher wheel speeds make possible a lower chip thickness and

in addition offer various advantages such as better surface finish. lower

forces and lower wheel wear. An increase of the chip thickness at higher

values tor the depth of cut tand die work speed and also an increase in

the peripheral wheel speed up to vc = 200 m/s lead to spec. material

removal rates of Q = 1000 nim3/(mm~ s) or more when CBN-grinding

wheek are used. These grinding conditions characterize the high-

performance grinding process. Grinding investigations have shown that

the total power required in high-perfonnance grinding increases with an

increasing cutting speed. While the cutting power remains nearly

constant the no-load power increases rapidly up to an extent 3 - 4 times higher than the cutting power. This result can be traced to the fact that

the coolant fluid fed into the contact zone under pressure exerts a

braking effect on the grinding wheel. and in addition that a hydro-

dynamic/-static condition of pressure arises in the gap between the

workpiece surface and the wheel rim. Practical work carried out recently

has shown that the no-load power demand and the spec. normal force

component depend on the coolant amount as well as on the wheel speed.

From the experimental resulfs obtained the question arises of whether on

the one hand the coolant flow rate can be reduced in order to decrease

normal forces and no-load power demand, and on the other a sufficient

coolant delivery can be determined in order to remove heat induced by

grinding and to prevent thermal damage in the workpiece surface.

References

[ I ] Tawakoli, T . ; Tavakkoli, S. J. , 1990. High-Efficiency Deep

Grinding (HEDG) with Shaved CBN Wheels, SME 4th Int.

Grind. Conf. MR 90-506

[3] IMeyer. H.-R.: Klocke, F.. 1990. New Developments in the

Dressing of CBN and Conventional Grinding Wheels with Rotary

Dressers. SME 4th Int. Grind. Conf. MR 90-527

[4] Giihring, K.. 1967, HocNeistungsschleifen. Eine Methode zur

Leisrungssteigerung der Schleifverfahren durch hohe Schnitt-

geschwindigkeiten. Doctoral Thesis. L'niversity of Aachen

[5] Chattopadhyay, A. K.: Chollet. L.; Hintermann. H. E. , 1990.

On Performance of Chemically Bonded Single-Layer CBN

Grinding Wheel, Annals of the CIRP. 39. 1. 313 - 316

[6] ,Minke, E.; Tawakoli, T., 1991. Hochleistungsschleifen mit

CBN-Werkzeugen. Wiss. Zeitschr. Teclm. Univ. Magdeburg,

35, 4, 61 ~ 66

[7] Brinksmeier, E.; Grabner, T., 1987, AuBenrundschleifen mit

kleinen CBN-Schleifscheiben. lndustrie Dimanten Rundschau

? I , 3 , 174 - 179

(81 Tawakoli. T., 1989. Technologie des Hochleistungsschleifens.

Doctoral Thesis. University of Brerneii

191 K6nig. W.; Fromlowitz, J . , 1986, High Performance Grinding -

Possible Methods for Increasing the Metal Removal Rate. SME

2nd Int. Grind. Conf. MR 86 - 631

[lo] Meyer, H.-R.; Klocke, F.. 1991. High-Performance Grinding

with CBN. Superabrasives '9 I , Conference Proceedings. SME

Chicago, USA

[ I l l Tawakoli, T.; Tavakkoli. S. J., 1991, High-Efficiency Deep

Grinding (HEDG) of lnconel and other Materials, SME Con-

ference Proceedings, Superabrasives '91

[I21 Barnard, J. M . , 1990, Grinding Tools at High Wheel Speeds

Using M.S.L. Wheels, SME 4th Int. Grind. Conf. MR 90-508

[I31 Werner, G. ; Tawakoli. T . , 1988, Advances in High Efficiency

Deep Grinding (HEDG), SME 3rd Int. Grind. Conf. MR 88-588

[2] Konig, W.; v. Arciszewski. A,; Ferlemann. F., 1991, Hoch-

leistungsschleifen mit hohen Geschwindigkeiten. 6. Dannstadter

Frrtigungstechnisches Symposium. Hohe Geschwindigkeit und

Prazision beim Zerspanen, TH Darmstadt

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