- Email: [email protected]
Lubricant testing for grinding operations
Wear, 77 (1982)
73
73 - 80
LUBRICANT TESTING FOR GRINDING OPERATIONS*
G. W. ROWE Department of Mechanical Engineering, Birmingham (Gt. Britain)
University of Birmingham, P.O. BOX 363,
(Received November 23,198l)
Summary The International Research Group on Wear has highlighted the problems of simulating real problems of tribology by laboratory tests and has emphasized the importance of considering each tribological system as a whole. The present paper is concerned with the coordination of tests of the various chemical and physical properties of grinding fluids, their physiological actions and their mechanical performance. A simplified data bank is proposed, allocating each result to one of seven categories and combining these in a computer. Various weighting factors are applied according to the practical requirements of specific grinding processes.
1, Introduction It is well known, especially by the members of the International Research Group, that simulation in tribology testing requires great care in considering the relevant parameters of the whole system. Indeed, this is the principal theme of the book written by our Chairman [l]. There has recently been much concern with the replacement of oilbased grinding fluids by water-based solutions and emulsions, The main reason for this change is that grinding produces large quantities of vapour, and the inhalation of hydrocarbon aerosols can be injurious. It is also common for grinding operators to have their hands in frequent or even continuous contact with the fluids, leading to dermatitis and other risks. It is clearly necessary in seeking replacement fluids to be able to assess their properties reliably. In the past, the selection of grinding fluids has been very largely a matter of personal discussion between the user and a representative of one or more oil companies. The main criteria are as follows: *Paper presented at the 8th Meeting of the Org~ization for Economic Cooperation and Development Internatio~l Research Group on Wear of Engineering Materials, Paris, France, May 19 - 21,198l. 0043-1648/82/0000-0000/$02.75
@ Elsevier Sequoia/Printed
in The Netherlands
74
how much of the stock material can be removed before it becomes necessary to redress the wheel; how fast the material can be removed; how closely the dimensional and property tolerances on the product can be maintained. In addition, the grinding fluid must not attack paintwork, cause corrosion or introduce any physiological or fire hazard. There is consequently a large subjective element in the selection of a new fluid.
2. Rationalization
of selection
parameters
This brief survey of the criteria usually applied suggests a primary emphasis on breakdown characteristics. For example, the “life” of the grinding wheel between redressing operations is set by some limiting feature which may be a rapid increase in force, temperature generated or surface roughness. Unfortunately, these parameters usually follow a bathtub type of curve and the instability or catastrophe point is highly specific and subject to wide random variations. As in many wear tests, it is therefore necessary to look for more controllable features that can be tested without extensive statistical trials, which would be impossibly expensive. This, of course, involves an assumption that the serviceability of the system is associated directly with the steady performance characteristics. Thus it may be assumed that a fluid which reduces the wear of a grinding wheel by 50% will allow the wheel to last much longer, although not necessarily twice as long. This is not always true; for example, very fine attritional wear will blunt the grits, producing flat areas that cause extensive rubbing and the generation of excessive heat. Nevertheless, such effects are easily detected independently, so it is possible to take an overall view of several interacting parameters and then to arrive at a realistic assessment.
3. Parameters directly related to the metal removal process The easiest parameter to measure is the torque or power required. Industrially this can be estimated from the armature current of the driving motor, but for research purposes the force FT tangential to the workpiece is measured with a dynamometer. It is directly related to the heat generated and hence to the possibility ,of metallurgical damage in the workpiece surface, which is sometimes visible as “bum” marks. The radial force FN normal to the workpiece surface is also readily measured. It is found to be very sensitive to the wheel condition and it is responsible for the elastic deflections in the machine and the wheel itself, and hence it affects dimensional accuracy. If the force is allowed to rise to too high a level, vibrations or chatter may be set up. Wheel wear is widely recognized as a critical feature of many grinding operations. It is most conveniently measured in terms of the grinding ratio
75
GR, which is the ratio of the amount of stock removed to the volume of wheel worn away. In severe grinding operations this is of direct importance in determining the life of the wheel, so a high GR.value, perhaps 100, is desirable. However, for finish grinding it may be preferable for the wheel grits to fragment and thus to provide a self-sharpening action, at the expense of greater loss of wheel material, rather than to become blunt or glazed. This is of special value in creep-feed grinding [ 21. When high surface quality is required, it may also be important to reduce or avoid wheel loading WL, which is the accumulation of metallic debris on or between the grits. The effects of wheel loading or glazing are seen directly in the surface topography ST, or in more detailed studies can be revealed by etching sections through the workpiece. Wide hardness variations in ground steel surfaces can result from the production of untempered or overtempered martensite, which also induces severe residual stresses leading to substantial reduction in fatigue life [ 31. These wider properties can be described in terms of surface integrity SI, but their determination is very time consuming t41. 4. Parameters related to the handling and the use of grinding fluids There are several physiologic~ factors P,which include dermatolo~c~ and inhalation risks as well as those due to carcinogenic compounds. Such parameters can only be assessed by clinical trials, outside normal engineering laboratory measurements, but the presence of known hazardous chemicals and particulate contamination of the atmosphere can be assessed. The stability SF of the fluid can be evaluated in terms of thermal, chemical and biological degradation [ 51, fire risk, emulsion stability and corrosivity or passivating properties. General handling qualities H include viscosity, related to pumpability, foaming, attack of organic paint, and aerosol formation. 5. Data-bank format Obviously no single test or even group of tests can fully describe the quality of a fluid accurately, Even those tests for which precise numerical results can be obtained are expressed in widely different units, so it is very difficult to indicate their relative importance. Recognizing that there is little value in determining, for example, normal force with an accuracy to 1% or better when a critical factor may actually be the paint stripping, it is proposed that all the parameters should be given quality ratings on a scale I- 7. Because the dressing of the wheel has a very important in~uenc~,on the grinding perfo~~ce it is necessary to make the me~uremen~ after
76
conditions have stabilized. Under the present test conditions, which are closely controlled and have been described elsewhere 161, we find that removal of 2000 mm3 is sufficient for this purpose. It is sometimes interesting to compare results also at an earlier stage so values are also quoted for measurements after removal of 500 mm3. Table 1 shows the raw data and Table 2 the data bank for ten fluids. TABLE 1 Input cards for the systematic assessment of grinding fluids Card Fluid
GR500
GR2000
FN500
1 2 3 4 5 6 7 8 9 10
31.0 33.0 32.0 30.0 30.0 31.0 31.0 31.0 33.0 26.0
79.0
104.0 97.0 9,Q.O 117.0 134.0 145.0 115.0 116.0 115.0 170.0
A B c D E F G H I J
86.0 85.0 51.0 52.0 54.0 53.0 53.0 56.0 39.0
ST2000
WL2000
P
S
H
96.0
0.39
91.0
0.27 0.29 0.52 0.65 0.51 0.52 0.56 0.49 1.00
5.0 5.0 5.0 4.0 5.0 4.0 4.0 4.0 4.0 5.0
2.0 2.0 2.0 2.0 6.0 6.0 5.0 5.0 4.0 7.0
4.0 4.0 4.0 5.0 4.0 5.0 4.0 4.0 5.0 2.0
5.0 4.0 2.0 6.0 6.0 2.0 5.0 6.0 6.0 6.0
FN2000
92.0
111.0 127.0 132.0 111.0 110.0 109.0 165.0
TABLE 2 Computer conversions into performance categories Fluid
GR500
CR2000
FN500
FN2000
ST2000
WL2000
P
S
H
A B C D E F G H I J
2 2 2 2 2 2 2 2 2 1
6 7 7 4 4 4 4 4 4 2
6 6 6 5 4 4 5 5 5 3
6 6 6 5 5 4 5 6 6 3
6 7 7 4 3 4 4 4 5 2
5 5 5 4 5 4 4 4 4 5
2 2 2 2 6 6 5 r 4” 7
4 4 4 5 4 5 4 4 5 2
5 4 2 6 6 2 5 6 6 6
6. Selection procedures The data shown in the format of Table 2 are more readily comprehensible than those in Table 1 where, for example, a large force was undesirable but a large grinding ratio was desirable, and the units were very different. Ail high v&es in Table 2 represent good perfo~~ce according to that particular criterion.
The table as a whole nevertheless allows for very considerable subjectivity in assessment, and of course the various values should be differently assessed for different applications. This limitation can be overcome by agreeing beforehand on the required priorities of the different features. For example, the parameters of most importance in grinding a crankshaft may be considered to be good surface finish, low wear and good fluid stability. One convenient procedure for this selection is to set minimum acceptance levels for surface topography and stability factors and then to place the short-listed fluids in an order of preference. Thus the initial pass marks may be set for the parameters A and B: A=ST+WL+FN+GR>l6 B=2FS+P+H>16 It will be seen that the fluid stability rating has been allocated double weight because of the importance of this feature in this particular application. Nevertheless, the physiological hazards cannot be ignored, so P is also included in the elimination barrier. The final selection is based on an agreed combination of factors with suitable weighting, such as C = 0.25 X GR500 + 0.5 X GRZOOO+ 0.25 X FN500 + 0.5(ST2000 + + FN2000 + WL2000 + P + S + H) The grinding ratio and force for the partly stabilized grinding performance at 500 mm3 are included as relevant but are given lower weighting than the 2000 mm3 values. Thus individual categories and the aggregate scores of each fluid can be calculated. A letter R in the print-out shows that despite a possible high overall score the fluid must be rejected on the basis of the preliminary elimination scores A or B.
7. Analysis and presentation
of results
When these data have been analysed by a computer and sorted according to the specified criteria, the results are presented in the form of a short table of not more than six fluids ranked in order of merit. There may of course be less than six acceptable fluids for any particular application because of the elimination trials. 7.1. Exampie 1 For the selection described above, on the basis of surface finish, wear and fluid stability, the results were as follows. The maximum possible score for all tests was C = 28.
78
Order
Scores
Of
Aggregate
A
3
18.7
19
20
18.2
18
19
18.0
17
20
17.2
17
18
Fluid B, oil based with sulphurized EP additives scored best in mechanical performance but was rejected because of the heaith risk
19.5R
25
14R
Fluid J, plain water, mance
14.5R
12R
17
merit
score C
1
An extreme pressure (EP) fatty “soluble oil” (fluid I), usually recommended for heavy duty 2 A high stability translucent emulsion (fluid H) used at high dilution 3 A nitrite-free “synthetic” solution (fluid E) used at high dilution 4 A stable milky emulsion (fluid G) recommended for general machining, used at moderate dilution -----_-----_--__-__--------_-----_____
was eliminated
on mechanical
perfor-
7.2. Example 2 For a different application where the machine was fully enclosed, the removal rate criterion dominated and the physiological and stability requirements could be relaxed but paint stripping was to be avoided. This eliminated one of the fluids (fluid F) containing glycol. Fluid C was rejected because of high viscosity and three aqueous fluids gave unacceptable wear. Five fluids were acceptable.
Order
of
Aggregate score C
merit 1 2 3 4 5
A mild EP “neat” oil (fluid B) containing sulphurized compounds 19.7 A mineral oil (fluid A) without additives, recommended for machining 19.0 brasses The EP fatty soluble oil (fluid I) (example 1, number 1) 18.0 The high stability translucent emulsion (fluid H) (example 1, number 2) 17.7 17.0 The stable milky emulsion (fluid G) (example 1, number 4)
7.3. Example 3 The heavy duty, higher speed machine designed in the collaborative project sponsored by the Science Research Council of Gt. Britain called for maximum removal rate consistent with acceptable surface finish and freedom from physiological hazards. Four possible fluids were identified.
7%
Order
of
Aggregate
score C
merit 1 2 3 4
The nitrite-free
synthetic (fluid E) (example 1, number 3) The high stability translucent emulsion (fluid H) (example 1, number 2) The EP fatty soluble oil (fluid I) (example 1, number 1) The stable milky emulsion (fluid G) (example 1, number 4)
18.2 17.7 17.5 17.0
8. Conclusions and further developments These combinations of the measured results are not of course the only ones that could be proposed. An important aspect of the proposed data bank is that it can “learn”. If a number of fluids are compared in this way and then subjected to extended production trials, it should be possible, as never before, to obtain quantitative comparisons between industrial performance and sets of laboratory trials. It is a very simple matter to alter the relative weightings of the various data in any of the criteria, and this is allowed for in the computer program. Many combinations can thus be examined quickly for such comparisons. It can be seen that when all factors are included, the differences in aggregate scores are not large. The computer analysis may be of most value in quickly identifying the fluids that should be rejected on the basis of performance stability or any other selected criterion. The acceptable fluids remaining can then be examined more fully in general terms or with respect to specific characteristics. The principal difficulty is in obtaining reliable industrial data unaffected by adventitious circumstances. We are at the moment engaged in a collaboration with British Petroleum Ltd. in the development of water-based grinding fluids. Some interesting features can be identified from the short lists given above. Thus it is evident that when the physiological risks were otherwise avoidable, for example by enclosing the machine, the neat oils showed advantages. A surprising result is that even the oil without additives (fluid A) performed better than the best emulsion. This supports the widely held view that viscosity itself is an important parameter. A test using plain water with a polymeric viscosity improver has since shown better performance than water alone. Nevertheless, the differences between good grinding fluids, used under controlled conditions, are not large, although individual parameters such as the grinding ratio can vary widely from one to another (in Table 1, CR = 51 - 86; fluid J was plain tap water and scored only 39). A more important aspect may be that the fluids change significantly during use for a variety of chemical, physical and biological reasons. Possibly the most important of these for the widely used oil-in-water emulsions is microbial degradation. These emulsions, especially when fortified with nitrogen or phosphors additives, provide excellent media for the growth and multiplication of a
80
range of bacteria. It is therefore important to monitor the quality and grinding performance of emulsions on a regular basis and to take steps to correct any deficiencies as they appear. A current research project is directed towards the development of a suitable set of tests, including some of those mentioned above. These will be performed at regular intervals and the results analysed in a similar way by a microprocessor. More precise indicators of stability such as pH value, droplet size, concentration and bacterial count will be incorporated. The microprocessor will then display, and perhaps in a later version control, the appropriate corrective measures. It is hoped that in this way a logical series of tests can be analysed for continuous quality control as well as initial selection of grinding fluids.
Acknowledgments Thanks are expressed to the Science Research Council for financial support, to British Petroleum Ltd., and to Dr. T. M. Porter for his valuable contribution to the work.
References 1 H. Czichos, Tribology. A Systems Approach, Elsevier, Amsterdam, 1978. 2 C. Andrews and T. D. Howes, SRC Rep., 1981 (Science Research Council) (available from Bristol University). 3 S. 0. A. el Helieby and G. W. Rowe, Influences of surface roughness and residual stress on fatigue life of ground steel components, Met. Technol., 7 (1980) 221 - 225. 4 S. 0. A. el Helieby and G. W. Rowe, Grinding cracks and microstructural changes in ground surfaces, Met., Technol., 8 (1981) 58 - 66. 5 E. C. Hill, Microbial infection of cutting fluids, Tribal. Int., 10 (1977) 49 - 54. 6 T. M. Porter and G:W. Rowe, A computerised data bank for the evaluation of grinding fluids, Jahrb. Tribal., to be published.
73
73 - 80
LUBRICANT TESTING FOR GRINDING OPERATIONS*
G. W. ROWE Department of Mechanical Engineering, Birmingham (Gt. Britain)
University of Birmingham, P.O. BOX 363,
(Received November 23,198l)
Summary The International Research Group on Wear has highlighted the problems of simulating real problems of tribology by laboratory tests and has emphasized the importance of considering each tribological system as a whole. The present paper is concerned with the coordination of tests of the various chemical and physical properties of grinding fluids, their physiological actions and their mechanical performance. A simplified data bank is proposed, allocating each result to one of seven categories and combining these in a computer. Various weighting factors are applied according to the practical requirements of specific grinding processes.
1, Introduction It is well known, especially by the members of the International Research Group, that simulation in tribology testing requires great care in considering the relevant parameters of the whole system. Indeed, this is the principal theme of the book written by our Chairman [l]. There has recently been much concern with the replacement of oilbased grinding fluids by water-based solutions and emulsions, The main reason for this change is that grinding produces large quantities of vapour, and the inhalation of hydrocarbon aerosols can be injurious. It is also common for grinding operators to have their hands in frequent or even continuous contact with the fluids, leading to dermatitis and other risks. It is clearly necessary in seeking replacement fluids to be able to assess their properties reliably. In the past, the selection of grinding fluids has been very largely a matter of personal discussion between the user and a representative of one or more oil companies. The main criteria are as follows: *Paper presented at the 8th Meeting of the Org~ization for Economic Cooperation and Development Internatio~l Research Group on Wear of Engineering Materials, Paris, France, May 19 - 21,198l. 0043-1648/82/0000-0000/$02.75
@ Elsevier Sequoia/Printed
in The Netherlands
74
how much of the stock material can be removed before it becomes necessary to redress the wheel; how fast the material can be removed; how closely the dimensional and property tolerances on the product can be maintained. In addition, the grinding fluid must not attack paintwork, cause corrosion or introduce any physiological or fire hazard. There is consequently a large subjective element in the selection of a new fluid.
2. Rationalization
of selection
parameters
This brief survey of the criteria usually applied suggests a primary emphasis on breakdown characteristics. For example, the “life” of the grinding wheel between redressing operations is set by some limiting feature which may be a rapid increase in force, temperature generated or surface roughness. Unfortunately, these parameters usually follow a bathtub type of curve and the instability or catastrophe point is highly specific and subject to wide random variations. As in many wear tests, it is therefore necessary to look for more controllable features that can be tested without extensive statistical trials, which would be impossibly expensive. This, of course, involves an assumption that the serviceability of the system is associated directly with the steady performance characteristics. Thus it may be assumed that a fluid which reduces the wear of a grinding wheel by 50% will allow the wheel to last much longer, although not necessarily twice as long. This is not always true; for example, very fine attritional wear will blunt the grits, producing flat areas that cause extensive rubbing and the generation of excessive heat. Nevertheless, such effects are easily detected independently, so it is possible to take an overall view of several interacting parameters and then to arrive at a realistic assessment.
3. Parameters directly related to the metal removal process The easiest parameter to measure is the torque or power required. Industrially this can be estimated from the armature current of the driving motor, but for research purposes the force FT tangential to the workpiece is measured with a dynamometer. It is directly related to the heat generated and hence to the possibility ,of metallurgical damage in the workpiece surface, which is sometimes visible as “bum” marks. The radial force FN normal to the workpiece surface is also readily measured. It is found to be very sensitive to the wheel condition and it is responsible for the elastic deflections in the machine and the wheel itself, and hence it affects dimensional accuracy. If the force is allowed to rise to too high a level, vibrations or chatter may be set up. Wheel wear is widely recognized as a critical feature of many grinding operations. It is most conveniently measured in terms of the grinding ratio
75
GR, which is the ratio of the amount of stock removed to the volume of wheel worn away. In severe grinding operations this is of direct importance in determining the life of the wheel, so a high GR.value, perhaps 100, is desirable. However, for finish grinding it may be preferable for the wheel grits to fragment and thus to provide a self-sharpening action, at the expense of greater loss of wheel material, rather than to become blunt or glazed. This is of special value in creep-feed grinding [ 21. When high surface quality is required, it may also be important to reduce or avoid wheel loading WL, which is the accumulation of metallic debris on or between the grits. The effects of wheel loading or glazing are seen directly in the surface topography ST, or in more detailed studies can be revealed by etching sections through the workpiece. Wide hardness variations in ground steel surfaces can result from the production of untempered or overtempered martensite, which also induces severe residual stresses leading to substantial reduction in fatigue life [ 31. These wider properties can be described in terms of surface integrity SI, but their determination is very time consuming t41. 4. Parameters related to the handling and the use of grinding fluids There are several physiologic~ factors P,which include dermatolo~c~ and inhalation risks as well as those due to carcinogenic compounds. Such parameters can only be assessed by clinical trials, outside normal engineering laboratory measurements, but the presence of known hazardous chemicals and particulate contamination of the atmosphere can be assessed. The stability SF of the fluid can be evaluated in terms of thermal, chemical and biological degradation [ 51, fire risk, emulsion stability and corrosivity or passivating properties. General handling qualities H include viscosity, related to pumpability, foaming, attack of organic paint, and aerosol formation. 5. Data-bank format Obviously no single test or even group of tests can fully describe the quality of a fluid accurately, Even those tests for which precise numerical results can be obtained are expressed in widely different units, so it is very difficult to indicate their relative importance. Recognizing that there is little value in determining, for example, normal force with an accuracy to 1% or better when a critical factor may actually be the paint stripping, it is proposed that all the parameters should be given quality ratings on a scale I- 7. Because the dressing of the wheel has a very important in~uenc~,on the grinding perfo~~ce it is necessary to make the me~uremen~ after
76
conditions have stabilized. Under the present test conditions, which are closely controlled and have been described elsewhere 161, we find that removal of 2000 mm3 is sufficient for this purpose. It is sometimes interesting to compare results also at an earlier stage so values are also quoted for measurements after removal of 500 mm3. Table 1 shows the raw data and Table 2 the data bank for ten fluids. TABLE 1 Input cards for the systematic assessment of grinding fluids Card Fluid
GR500
GR2000
FN500
1 2 3 4 5 6 7 8 9 10
31.0 33.0 32.0 30.0 30.0 31.0 31.0 31.0 33.0 26.0
79.0
104.0 97.0 9,Q.O 117.0 134.0 145.0 115.0 116.0 115.0 170.0
A B c D E F G H I J
86.0 85.0 51.0 52.0 54.0 53.0 53.0 56.0 39.0
ST2000
WL2000
P
S
H
96.0
0.39
91.0
0.27 0.29 0.52 0.65 0.51 0.52 0.56 0.49 1.00
5.0 5.0 5.0 4.0 5.0 4.0 4.0 4.0 4.0 5.0
2.0 2.0 2.0 2.0 6.0 6.0 5.0 5.0 4.0 7.0
4.0 4.0 4.0 5.0 4.0 5.0 4.0 4.0 5.0 2.0
5.0 4.0 2.0 6.0 6.0 2.0 5.0 6.0 6.0 6.0
FN2000
92.0
111.0 127.0 132.0 111.0 110.0 109.0 165.0
TABLE 2 Computer conversions into performance categories Fluid
GR500
CR2000
FN500
FN2000
ST2000
WL2000
P
S
H
A B C D E F G H I J
2 2 2 2 2 2 2 2 2 1
6 7 7 4 4 4 4 4 4 2
6 6 6 5 4 4 5 5 5 3
6 6 6 5 5 4 5 6 6 3
6 7 7 4 3 4 4 4 5 2
5 5 5 4 5 4 4 4 4 5
2 2 2 2 6 6 5 r 4” 7
4 4 4 5 4 5 4 4 5 2
5 4 2 6 6 2 5 6 6 6
6. Selection procedures The data shown in the format of Table 2 are more readily comprehensible than those in Table 1 where, for example, a large force was undesirable but a large grinding ratio was desirable, and the units were very different. Ail high v&es in Table 2 represent good perfo~~ce according to that particular criterion.
The table as a whole nevertheless allows for very considerable subjectivity in assessment, and of course the various values should be differently assessed for different applications. This limitation can be overcome by agreeing beforehand on the required priorities of the different features. For example, the parameters of most importance in grinding a crankshaft may be considered to be good surface finish, low wear and good fluid stability. One convenient procedure for this selection is to set minimum acceptance levels for surface topography and stability factors and then to place the short-listed fluids in an order of preference. Thus the initial pass marks may be set for the parameters A and B: A=ST+WL+FN+GR>l6 B=2FS+P+H>16 It will be seen that the fluid stability rating has been allocated double weight because of the importance of this feature in this particular application. Nevertheless, the physiological hazards cannot be ignored, so P is also included in the elimination barrier. The final selection is based on an agreed combination of factors with suitable weighting, such as C = 0.25 X GR500 + 0.5 X GRZOOO+ 0.25 X FN500 + 0.5(ST2000 + + FN2000 + WL2000 + P + S + H) The grinding ratio and force for the partly stabilized grinding performance at 500 mm3 are included as relevant but are given lower weighting than the 2000 mm3 values. Thus individual categories and the aggregate scores of each fluid can be calculated. A letter R in the print-out shows that despite a possible high overall score the fluid must be rejected on the basis of the preliminary elimination scores A or B.
7. Analysis and presentation
of results
When these data have been analysed by a computer and sorted according to the specified criteria, the results are presented in the form of a short table of not more than six fluids ranked in order of merit. There may of course be less than six acceptable fluids for any particular application because of the elimination trials. 7.1. Exampie 1 For the selection described above, on the basis of surface finish, wear and fluid stability, the results were as follows. The maximum possible score for all tests was C = 28.
78
Order
Scores
Of
Aggregate
A
3
18.7
19
20
18.2
18
19
18.0
17
20
17.2
17
18
Fluid B, oil based with sulphurized EP additives scored best in mechanical performance but was rejected because of the heaith risk
19.5R
25
14R
Fluid J, plain water, mance
14.5R
12R
17
merit
score C
1
An extreme pressure (EP) fatty “soluble oil” (fluid I), usually recommended for heavy duty 2 A high stability translucent emulsion (fluid H) used at high dilution 3 A nitrite-free “synthetic” solution (fluid E) used at high dilution 4 A stable milky emulsion (fluid G) recommended for general machining, used at moderate dilution -----_-----_--__-__--------_-----_____
was eliminated
on mechanical
perfor-
7.2. Example 2 For a different application where the machine was fully enclosed, the removal rate criterion dominated and the physiological and stability requirements could be relaxed but paint stripping was to be avoided. This eliminated one of the fluids (fluid F) containing glycol. Fluid C was rejected because of high viscosity and three aqueous fluids gave unacceptable wear. Five fluids were acceptable.
Order
of
Aggregate score C
merit 1 2 3 4 5
A mild EP “neat” oil (fluid B) containing sulphurized compounds 19.7 A mineral oil (fluid A) without additives, recommended for machining 19.0 brasses The EP fatty soluble oil (fluid I) (example 1, number 1) 18.0 The high stability translucent emulsion (fluid H) (example 1, number 2) 17.7 17.0 The stable milky emulsion (fluid G) (example 1, number 4)
7.3. Example 3 The heavy duty, higher speed machine designed in the collaborative project sponsored by the Science Research Council of Gt. Britain called for maximum removal rate consistent with acceptable surface finish and freedom from physiological hazards. Four possible fluids were identified.
7%
Order
of
Aggregate
score C
merit 1 2 3 4
The nitrite-free
synthetic (fluid E) (example 1, number 3) The high stability translucent emulsion (fluid H) (example 1, number 2) The EP fatty soluble oil (fluid I) (example 1, number 1) The stable milky emulsion (fluid G) (example 1, number 4)
18.2 17.7 17.5 17.0
8. Conclusions and further developments These combinations of the measured results are not of course the only ones that could be proposed. An important aspect of the proposed data bank is that it can “learn”. If a number of fluids are compared in this way and then subjected to extended production trials, it should be possible, as never before, to obtain quantitative comparisons between industrial performance and sets of laboratory trials. It is a very simple matter to alter the relative weightings of the various data in any of the criteria, and this is allowed for in the computer program. Many combinations can thus be examined quickly for such comparisons. It can be seen that when all factors are included, the differences in aggregate scores are not large. The computer analysis may be of most value in quickly identifying the fluids that should be rejected on the basis of performance stability or any other selected criterion. The acceptable fluids remaining can then be examined more fully in general terms or with respect to specific characteristics. The principal difficulty is in obtaining reliable industrial data unaffected by adventitious circumstances. We are at the moment engaged in a collaboration with British Petroleum Ltd. in the development of water-based grinding fluids. Some interesting features can be identified from the short lists given above. Thus it is evident that when the physiological risks were otherwise avoidable, for example by enclosing the machine, the neat oils showed advantages. A surprising result is that even the oil without additives (fluid A) performed better than the best emulsion. This supports the widely held view that viscosity itself is an important parameter. A test using plain water with a polymeric viscosity improver has since shown better performance than water alone. Nevertheless, the differences between good grinding fluids, used under controlled conditions, are not large, although individual parameters such as the grinding ratio can vary widely from one to another (in Table 1, CR = 51 - 86; fluid J was plain tap water and scored only 39). A more important aspect may be that the fluids change significantly during use for a variety of chemical, physical and biological reasons. Possibly the most important of these for the widely used oil-in-water emulsions is microbial degradation. These emulsions, especially when fortified with nitrogen or phosphors additives, provide excellent media for the growth and multiplication of a
80
range of bacteria. It is therefore important to monitor the quality and grinding performance of emulsions on a regular basis and to take steps to correct any deficiencies as they appear. A current research project is directed towards the development of a suitable set of tests, including some of those mentioned above. These will be performed at regular intervals and the results analysed in a similar way by a microprocessor. More precise indicators of stability such as pH value, droplet size, concentration and bacterial count will be incorporated. The microprocessor will then display, and perhaps in a later version control, the appropriate corrective measures. It is hoped that in this way a logical series of tests can be analysed for continuous quality control as well as initial selection of grinding fluids.
Acknowledgments Thanks are expressed to the Science Research Council for financial support, to British Petroleum Ltd., and to Dr. T. M. Porter for his valuable contribution to the work.
References 1 H. Czichos, Tribology. A Systems Approach, Elsevier, Amsterdam, 1978. 2 C. Andrews and T. D. Howes, SRC Rep., 1981 (Science Research Council) (available from Bristol University). 3 S. 0. A. el Helieby and G. W. Rowe, Influences of surface roughness and residual stress on fatigue life of ground steel components, Met. Technol., 7 (1980) 221 - 225. 4 S. 0. A. el Helieby and G. W. Rowe, Grinding cracks and microstructural changes in ground surfaces, Met., Technol., 8 (1981) 58 - 66. 5 E. C. Hill, Microbial infection of cutting fluids, Tribal. Int., 10 (1977) 49 - 54. 6 T. M. Porter and G:W. Rowe, A computerised data bank for the evaluation of grinding fluids, Jahrb. Tribal., to be published.