8/6/2019 Bogdan 2001

1/4

Residual stress in grindingBogdan W. Kruszynski* , Ryszard Wo jcik

Technical University of o dz , Skorupki 6/8, 90-924 o dz , Poland

Abstract

Results of investigations on residual stress in surface grinding are presented in the paper. A coefcient `` B'' combining power density andwheel/workpiece contact time was developed. Experimental set-up and software to estimate the coefcient during grinding are described inthe paper. Experiments were carried out for surface plunge grinding for several workmaterials in a wide range of grinding conditions. Theinuence of process parameters on the coefcient B as well as the relation between B and maximum residual stress were experimentallyevaluated. The usefulness of the coefcient to predict residual stress in surface grinding was proved. # 2001 Elsevier Science B.V. Allrights reserved.

Keywords: Residual stress; Grinding; Wheel/workpiece

1. Introduction

Grinding is one of the most popular methods of machininghard materials. Because it is usually one of the nal opera-tions of the technological process, properties of surface layercreated in grinding inuence directly the functional proper-ties of the workpiece such as fatigue strength, abrasive andcorrosion resistance, etc.

Creating favourable surface integrity, especially in grind-ing with aluminium oxide grinding wheels is difcult due totwo opposite tendencies. On one hand, high process para-meters are preferred in order to increase productivity.Unfortunately, such parameters usually lead to the increaseof grinding power engaged in creation of the new surface of the workpiece. On the other hand, the increase of grindingpower makes grinding temperatures grow, which may causea serious damage to the surface layer created in grinding.

Finding a compromise between high productivity andadvantageous surface layer properties is extremely difcultdue to the lack of relatively simple and universal routines,among others. Because of the importance of grinding opera-tion the investigations of this process are performed in manyresearch centres. Some general approaches are observed inthese investigations.

The rst one, strictly analytical [4,5], is based on themathematical description of physical processes involved insurface layer creation. In grinding thermal effects are usuallydescribed. On the basis of the calculations of temperaturedistribution in the workpiece, such changes in surface layer

like microhardness, residual stresses, microstructure, etc. areestimated [5]. Such an approach is very promising but at thepresent stage it is limited to theoretical investigationsbecause of complex calculations and still limited knowledgeabout material behaviour in extreme grinding conditions.

The experimental approach [1,7] aims at nding a corre-lation between grinding conditions and surface layer para-meters. This is a relatively simple method with somedisadvantages. Experimental works are usually time- andcapital-consuming which limits their application. Moreover,there is a limited possibility to extrapolate the experimentalresults on different grinding methods and grindingconditions.

There is also a third approach to the problem of control of surface layer creation, which involves a search for suchgrinding coefcients, which are strongly correlated withsurface layer properties [2,4]. There are many such coef-cients existing. The most popular are: equivalent chipthickness ( heq ) and power density ( P

0). The former is provedto be useful in grinding ceramics, the latter is often appliedwhen grinding with aluminium oxide grinding wheels isinvestigated [2].

The main disadvantage of both coefcients is that tocalculate them it is necessary to estimate the effectivegrinding depth or effective wheel/workpiece contact length.Both values are very difcult to estimate ``on-line'' grindingaccurately.

Thus, an ``easy-to-estimate'' grinding coefcient, whichwould be strongly correlated with surface integrity para-meters, is still lacking. The investigation on the correlationbetween the coefcient combining power density and the

Journal of Materials Processing Technology 109 (2001) 254257

* Corresponding author.

0924-0136/01/$ see front matter # 2001 Elsevier Science B.V. All rights reserved.P I I : S 0 9 2 4 - 0 1 3 6 ( 0 0 ) 0 0 8 0 7 - 4

8/6/2019 Bogdan 2001

2/4

wheel/workpiece contact time and residual stress in surfacegrinding is described below.

2. Grinding coefcient combining power density andcontact time

It was proved [3] that residual stresses in surface layerafter grinding are closely correlated with maximum grindingtemperature. The analysis of equations used for temperaturecalculation in grinding [6] indicates that it is not only thepower density that inuences the grinding temperature butthere is also a second important factor wheel/workma-terial contact time. In surface grinding the contact time of the particular workpiece point with heat source (grindingwheel) can be easily calculated as

t clevw

(1)

where le is an effective wheel/workpiece contact length andvw is the workspeed.

The proposed grinding coefcient B is a product of powerdensity P 0 and contact time t c:

B P 0t cP

bdle

levw

Pbdvw

(2)

where P is the total grinding power and bd the grindingwidth.

The rst advantage of this coefcient is that all quantitiesin this equation (grinding power, grinding width and work-speed) are easy to measure ``on-line'' in a grinding process.

3. Experimental set-up

Experiments were carried out for the following grindingconditions.

workmaterials: carbon steel 0.45% C, 28HRC (marked S),alloy steel 40H (0.38%C, 0.9%Cr, 0.28% Ni) 48HRC ( H ),

bearing steel H15 (equivalent to 100Cr6) 62HRC ( L );grinding wheels: 38A60J8V ( J ), 99A80M7V ( M );wheelspeed: 26 m/s (constant);grinding depth: from 0.005 to 0.06 mm;workspeed: from 0.08 to 0.5 m/s;grinding fluid: emulsion or none.

Grinding parameters in these investigations were limitedby the power of the main wheel drive, table speed regulationrange and by the appearance of unacceptable changes in thesurface layer, microcracks and burns.

To estimate coefcient B it was necessary to measuregrinding power, workspeed and grinding width. Grindingpower was measured in two different ways: by the measure-ment of power consumed by wheel main drive ( P m ) andsimultaneous measurement of tangential grinding force F tand wheelspeed vs. The grinding power can then be calcu-

lated as P c F tvs. The comparison of the results obtainedfrom both methods is shown in Fig. 1. A very good correla-tion can be seen from this gure, which proves that mea-surement of power consumption of wheel main drive isaccurate enough to estimate coefcient B in the case whenonly grinding wheel is driven by this drive. The wheelspeedwas measured by means of displacement transducer andgrinding width was taken as a width of the sample beingground.

4. Experimental results

On the basis of measured values of P, vw and bd in surfacegrinding, the coefcient B was calculated in each grindingtest. Measurements carried out during grinding allowed, rstof all, to evaluate the inuence of grinding conditions on thecoefcient B, cf. Figs. 27. The linear dependence betweeneffective grinding depth and B can be seen from Figs. 2, 4and 6. Slopes of these lines depend mainly on grindingwheel, workspeed (Figs. 2 and 6) and on grinding uid(Fig. 4). The correctness of linear approximation was provedin a statistical way values of R2 were higher than 0.9 in allcases.

Fig. 1. Comparison of measured and calculated grinding power.

Fig. 2. The inuence of grinding depth and grinding wheel grade oncoefcient B for carbon steel ( S).

B.W. Kruszyn ski, R. Wo jcik / Journal of Materials Processing Technology 109 (2001) 254257 255

8/6/2019 Bogdan 2001

3/4

The inuence of workspeed on coefcient B, Figs. 3, 5and 7, is not as uniform as those obtained for grinding depth.Much higher inuence of vw on B is observed for a lowerrange of workspeeds. It indicates that there is a limitedpossibility to inuence coefcient B by changes of the

workspeed. Very similar dependencies were obtained forthe third workmaterial investigated alloy steel ( H ).

For all experiments, in which microcracks and/or burnswere not present, residual stress distribution was measuredby means of the well-known material removal method. Fromresidual stress vs. depth below surface diagrams obtained foreach grinding test, maximal residual stresses in the surfacelayer were determined. Usually, residual stresses reach theirmaximum (tensile values) close to the surface on depths of 1020 mm.

Relations between coefcient B and maximum residualstress for investigated workmaterials are shown in Figs. 810. In these diagrams the results are summarised for eachworkmaterial regardless of other grinding conditions (grind-ing wheel properties, grinding uid, grinding parameters). Ineach case the linear dependence was assumed which wasproved in a statistical way ( R2 from 0.8529 to 0.9074).

It results from these gures that the slopes of residualstress-coefcient B lines are characteristic for the givenworkmaterial and seem to be independent of other grindingconditions. The highest slope was obtained for bearing steel(L ), Fig. 10, and the lowest one for alloy steel ( H ), Fig. 9.

Fig. 3. The inuence of workspeed and grinding wheel grade on coef-cient B for carbon steel ( S).

Fig. 4. The inuence of grinding depth and grinding uid on coefcient Bfor carbon steel ( S).

Fig. 5. The inuence of workspeed and grinding uid on coefcient B forcarbon steel ( S).

Fig. 6. The inuence of grinding depth and grinding wheel grade oncoefcient B for bearing steel ( L ).

Fig. 7. The inuence of workspeed and grinding wheel grade oncoefcient B for bearing steel ( L ).

256 B.W. Kruszyn ski, R. Wo jcik / Journal of Materials Processing Technology 109 (2001) 254257

8/6/2019 Bogdan 2001

4/4

Some additional observations recorded during investiga-tions indicate that there is a possibility to use the coefcient

B to predict and/or control such changes in surface layer likemicrocracks, burns or microstructure changes. Additionalinvestigations are necessary to conrm the usefulness of thiscoefcient in other grinding methods.

5. Conclusions

1. The grinding coefcient B combining power density andwheel/workpiece contact time was developed to predictresidual stress in surface grinding.

2. A linear correlation between coefcient B and maxi-mum residual stress was found experimentally. It wasconrmed for several workmaterials.

3. The relation between coefcient B and maximumresidual stress seems to be independent of grindingconditions.

4. Coefcient B increases linearly with the increase of grinding depth and decreases with the increase of workspeed. This decrease shows less intensity in therange of higher workspeeds.

5. The coefcient B is easy-to-estimate, even on-line, inindustrial practice.

6. The coefcient B may be useful in predicting suchsurface layer properties in grinding like microcracks,burns or microstructure changes.

References

[1] P.G. Althaus, Residual stress in internal grinding, Ind. Diamond Rev. 3(1985) 124127.

[2] E. Brinksmeier, H.K. Tonshoff, Basic parameters in grinding, Ann.CIRP 42 (1) (1993) 795799.

[3] E. Brinksmeier, S.T. Comet, W. Konig, P. Leskovar, J. Peters, H.K.Tonshoff, Residual stress-measurement and causes, Ann. CIRP 31 (2)(1982) 491510.

[4] B.W. Kruszynski, C.A. Luttervelt, An attempt to predict residualstresses in grinding of metals with the aid of the new grindingparameter, Ann. CIRP 40 (1) (1991) 335337.

[5] H.K. Tonshoff, J. Peters, I. Inasaki, T. Paul, Modelling and simulation

of grinding process, Ann. CIRP 41 (2) (1992) 677688.[6] E. Vansevenant, A subsurface integrity model in grinding, Ph.D.

Thesis, KU Lueven, 1987.[7] Y. Zheyun, H. Zhonghui, Surface integrity of grinding of bearing steel

GCr15 with CBN wheels, Ann. CIRP 38 (1) (1989) 677688.

Fig. 8. Maximum residual stress vs. coefcient B for carbon steel ( S).

Fig. 9. Maximum residual stress vs. coefcient B for alloy steel ( H ).

Fig. 10. Maximum residual stress vs. coefcient B for bearing steel (L) .

B.W. Kruszyn ski, R. Wo jcik / Journal of Materials Processing Technology 109 (2001) 254257 257