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Statistical analyses of the effects of ball burnishing parameters on surface hardness
Wear, 129
(1989)
STATISTICAL PARAMETERS N. H. LOH* Nanyang Nanyang
235
- 243
235
ANALYSES OF THE EFFECTS ON SURFACE ‘HARDNESS
OF BALL BURNISHING
and S. C. TAM
Technological Institute, School of Mechanical Avenue, Singapore 2263 (Singapore)
and Production
Engineering,
S. MIYAZAWA Mechanical (Received
Engineering May 12, 1988;
Laboratory, revised
Namiki
August
1-2, Tsukuba
19, 1988;
City (Japan)
accepted
September
21,1988)
Summary Burnishing is used increasingly as a finishing operation which gives additional advantages such as increased hardness, fatigue strength and wear resistance. Experimental work based on 34 factorial design was carried out on a vertical machining centre to establish the effects of ball burnishing parameters on the surface hardness of flat AISI 1045 specimens. Statistical analyses of the results (at 1% level of significance) show that the following parameters have significant effects on the surface hardness: depth of penetration, feed, lubricant and ball material. An interaction effect between the ball material and the linear feed was also evident. A 33% - 55% increase in hardness can be obtained.
1. Introduction The finishing of metals with a hardened surface layer has attracted the interest of researchers, e.g. those in the optomechanical industry [l]. The functional performance of a component, such as fatigue strength, load bearing capacity and wear, depends on its surface characteristics such as hardness, induced residual stresses and topography. Czichos and Habig [2] studied the tribological behaviour of medium carbon steel. They found that different wear values, wear patterns and wear mechanisms result depending on various factors: material parameters (e.g. hardness), operating variables (e.g. kinematics), geometry of the tribological system, and environmental and interfacial conditions. Studies (e.g. by Kudryavtsev [3]) have established the beneficial effects of residual compressive stresses and work hardening in improving the fatigue strength of various materials. *Author
to whom
0043-1648/89/$3.50
correspondence
should
be addressed.
@ Elsevier
Sequoia/Printed
in The Netherlands
236
Burnishing, a plastic deformation process, is commonly used to achieve good surface finish. Unlike machining processes, burnishing was also found to give several additional desirable surface characteristics, i.e. higher fatigue strength [ 41, increased surface hardness due to work hardening ]4 - 61 and higher wear resistance [4, 63. In order to utilize the increased hardness effect of the burnishing process effectively, the parameters affecting surface hardness have to be established. In this paper, a statistical systematic study of the effects of ball burnishing parameters on the final surface hardness of flat AISI 1045 specimens is reported. For the experiments to be performed efficiently, statistical techniques such as 34 factorial design were used.
2. Statistical
techniques
Factorial design is more efficient than the conventional one-factorat-a-time experiments commonly employed by researchers. It also enables the study of both the main and the interaction effects between factors. Also, if a parameter (e.g. surface hardness) needs to be maximized with respect to a combination of factors, the factorial design will give a combination near the m~imum whereas the one-factor-at-a-time procedure will not. In the 34 factorial design used, four factors (ball material, lubricant, feed and depth of penetration) were studied and they were designated alphabetically A - D. Each qualitative and quantitative factor has three TABLE 1 Experimental burnishing factors and levels Fat tor
Level
A Ball material
A0 Tungsten carbide WC (hardness: 1300 HV) Ai Ball bearing steel SUJZ (hardness: 800 HV) AZ Silicon nitride SisN4 (hardness: 2100 HV)
B Lubricant
Be Brown greasea B1 White greasea Bz Kerosene
C Feed
D Depth of penetration
80 pm Cl lOO@m C2 120pm CO
81.tm Do Dl 13pm D2 17 pm
aBrown grease (Shell ALVANIA EPl) - extreme pressure lithium soap grease, Pb-S extreme pressure additives. White grease (Grade PQ40AA-2 from American Oil and Supply Co.) - aluminium complex soap grease.
237
types of materials and three values respectively and they are commonly known as “three levels”. The three levels are designated by the digits 0, 1 and 2. The four factors and their respective levels are shown in Table 1. A more detailed treatment of factorial design can be found in ref. 7. Computation of the experimental data was carried out using Yates’ algorithm. The main effect of each factor and the effects of interactions between factors were determined using the analysis of variance (ANOVA) technique and the F-test. It was assumed that third and higher order interactions were physically impossible and could be used as an estimate of experimental error. In 34 factorial design, the sum of the squares for any main quantitative effect may be partitioned into linear and quadratic components, each with a single degree of freedom. These two components are denoted by the subscripts L and Q respectively. For example, CL denotes the main effect of the linear component of factor C, &Do denotes the interaction effect between the linear component of factor C and the quadratic component of factor D.
3. Experimental work The experimental work was conducted on a vertical machining centre with automatic tool changer. A numerically controlled program was written for the consecutive pre-machining and burnishing operations. The schematic set-up of the ball burnishing operation is shown in Fig. 1. The ball burnishing tool shown in Fig. 2 is the main element in the burnishing process. The ball is located in position and held rigidly by means of a rod and screw. The burnishing tool was held stationary and rigidly by means of a special locking mechanism attached to the machining centre. The machining centre has automatic tool length measuring and linear scale feedback systems so as to ensure accurate tool settings and workpiece movements. A Kistler piezoelectric dynamometer (see Fig. 1) was used to measure the normal and tangential forces. The forces were recorded by a pen recorder and the corresponding average forces calculated. The depth of penetration and feed terminologies are illustrated in Fig. 3 which is seen from view A of Fig. 1. The depth of penetration is the distance of the ball tip below the pre-machined surface. The feed is the horizontal distance between two successive ball centres. Both the depth of penetration and the feed were preset in the numerically controlled program. To ensure a uniform pre-machined surface for all the experimental work, the following constant pre-machining specifications were used: single carbide milling cutter of nose radius 0.8 mm, table feed rate of 150 mm mini, cutting speed of 408 m min-’ and 0.2 mm depth of cut. The burnishing operations were conducted at a constant speed of 300 mm min-’ using balls of 9.52 mm diameter.
L&king contact
Workpiece
.
locator
Dynamometer
Machine
w
bed
Machine bed movement
Fig. 1. Schematic set-up of ball burnishing operation.
-_( Burnishing
1
I_
Feed
1 ball
\ Ball
holder
sc;ew
Fig. 2. Ball burnishing tool.
View
A
Fig. 3. Schematic illustration of terminologies.
The hardnesses of the pre-machined and burnished surfaces were measured using Vickers’ hardness equipment. A pyramid diamond indenter with 136” apex angle and indentation load of 1 kgf was used. The average hardness of the pre-machined surface was 196 HV.
239
4. Results and discussion A total of 81 experimental results comprising all possible burnishing conditions are shown in Table 2. Statistical discussion on the effects of the various factors on surface hardness is based on a 1% level of significance. The ANOVA table (Table 3) shows only the s~nificant main effects and interactions, TABLE
2
Experimental
results Average
surface
hardness
Ao
(HV)
AI
A2
30
BI
B2
Bo
B1
32
Bo
31
32
DI D2
266 274 280
273 280 288
260 275 283
261 270 216
270 280 282
260 274 277
268 292 280
272 281 296
268 278 281
CI
Do Dl DZ
274 281 287
284 291 297
269 281 291
265 277 283
281 286 291
266 280 282
275 282 286
277 290 297
273 278 286
Cz
Do DI D2
279 294 297
295 300 303
272 286 296
271 282 290
284 290 296
270 281 289
274 284 294
281 294 299
281 280 285
Co
Do
TABLE
3
ANOVA Source
table 0 f uariution
Sum of squares
Degree of freedom
Mean squares
Calculated F ratio
Fo.01
Main effect A B CL DL DQ Two-factor
2 2 1 1 1
213.98 702.42 1688.96 3313.50 90.38
18.79 61.67 148.28 290.91 7.94
5.10 5.10 7.22 7.22 7.22
210.70
2
105.35
9.25
5.10
546.85
48
11.39
interaction
A% Estimated
427.95 1404.84 1688.96 3313.50 90.38
error
From the ANOVA table, it can be seen that the surface hardness of the burnished specimen is dependent on the type of ball material used.
240
The interaction effect of ball material and linear feed (A&) is also significant. Of all the main effects of the four factors on surface hardness, the ball material effect is of least significance. From the experimental results in Table 2, the tungsten carbide ball (designated by A,) gives the highest hardness value of 303 HV which is about 55% higher than the pre-machined surface hardness of 196 HV. Under the same burnishing conditions, different ball materials give different surface hardness values (see Figs. 4(a) - 4(c)) .
1::: /Is 2
2 (li250--j,, 9 Depth
of
penetration
(em)
13 Depth
of
(a)
penetration
17 (pm)
(b)
5310
2
m
250
A"
, 9
I 13 Depth
of
penetration (C)
17 (WI%)
Depth
of
penetration
(pm)
(d)
Fig. 4. Relationships between depth of penetration and surface hardness. (a) WC ball and kerosene. (b) SUJ2 ball and kerosene. (c) Si3N4 ball and kerosene. (d) WC ball and brown grease. Feed: n, 80 pm; 0, 100 pm; A, 120 pm.
4.2. Effect of lubricant From the ANOVA table, only the main effect of the lubricant type on the surface hardness is significant. A comparison of Figs. 4(a) and 4(d) shows that the effect of lubricant is dependent on the depth of penetration. For the same burnishing conditions, burnishing with brown grease gives higher hardness values than kerosene for a 9 pm depth of penetration and, at the greatest depth of penetration of 17 pm, the reverse effect seems more significant.
241
4.3. Effect of depth of penetration The ANOVA table shows that the linear effect of depth of penetration on surface hardness is the most significant factor. The general relationship between depth of penetration and hardness is shown in Fig. 4. For a particular burnishing condition, as the depth of penetration increases, the normal and tangential forces increase too, as shown in Fig. 5. The increase in forces causes an increase in work hardening and thus an increase in hardness. __ 8007 z
13
17
<
Depth of penetration (pm) Depth of penetration (pm) (b) (a) Fig. 5. Relationships between depth of penetration and burnishing forces (normal and tangential) for WC ball and brown grease. Feed: n, 80 pm; 0, 100 pm; A, 120 pm.
4.4. Effect of feed Prom the ANOVA table, the linear effect of feed is the next highly significant factor. Figure 6 shows some results of the relationship between hardness and feed. Belov [8] found that the work-hardening effect on the burnished surface is greater at lower feed and decreases with increase in feed. This is so, since at lower feed the number of times a ball deforms over the same spot is greater than at higher feed. Thus at lower feed the plastic deformation is more intensive, causing a greater increase in surface hardness. However, the reverse phenomenon seemed to occur here as shown in Fig. 6. It was observed that, with an increase in feed from 80 pm to 120 pm, the surface hardness increases. This trend was also found by Pande and Pate1 [9] in their work on vibratory ball burnishing on a lathe machine, in the feed rate range of 0.05 - 0.11 mm rev-‘. The reverse phenomenon could be explained by observing the relationships between the forces and feed as shown in Fig. 7. For the same burnishing conditions, as the feed increases, the normal and tangential forces increase too, causing the surface hardness to increase. In this particular case, the increase in hardness due to the increase in force is greater than the reduction in hardness due to higher feed. It is also obvious from Table 3 that the effect of depth of penetration on surface hardness is much more significant than the effect of feed on hardness.
242
270: E
3
* 250-A" Feed
, 80
(@m)
Feed
100 (pm)
(b)
(a)
G '
1”
250L8p
v) 250~$1 120 Feed
80
(pm)
Feed
100 (pin)
Cd)
(C)
Fig. 6. Relationships between feed and surface hardness. (a) WC ball and kerosene. (b) SUJ2 ball and kerosene. (c) SisN4 ball and kerosene. (d) WC ball and brown grease. Depth of penetration: w, 9 pm; 0, 13 pm; A, 17 pm.
800-
A 100 (a)
8o
Feed
(pm)
120
(b)
So
Feed
100 (pm)
120
Fig. 7. Relationships between feed and burnishing forces (normal and tangential) for WC ball and brown grease. Depth of penetration: w, 9 pm; 0, 13 pm; A, 17 pm.
5. Conclusions The use of factorial design enables the following conclusions, at a 1% significance level, to be drawn.
243
(1) The main effects on surface hardness of ball material, lubricant, feed and depth of penetration were significant. (2) An interaction effect between the ball material and the linear feed was also evident. The experimental work shows that an increase in surface hardness of 260 HV to 303 HV, an improvement of about 33% - 55%, can be obtained by the ball burnishing process.
Acknowledgments The authors would like to thank the Nanyang Technological Institute, the Ministry of International Trade and Industry (MIT1 of Japan) and the Mechanical Engineering Laboratory (Tsukuba City, Japan) for supporting this research programme on ball burnishing.
References 1 I. Ya. Vakhrameyev and P. G. Balyura, Metal finishing with a ball tool, Sov. J. Opt. Technol., 41 (1974) 168 - 170. 2 H. Czichos and K. Habig, Wear of medium carbon steel - a systematic study on influence of materials and operating parameters. In Y. Tamai (ed.), Proc. JSLE Znt. Tribology Conf, Tohyo, July 8 - 10, 1985, Elsevier, Amsterdam, 1985, pp. 879 884, 3 I. V. Kudryavtsev, Strengthening machine parts by surface plastic deformation, Russ. Eng. J., 50 (1970) 8 - 12. 4 D. D. Papshev and Yu G. Golubev, Effectiveness of surface work-hardening of titanium alloy components, Russ. Eng. J., 52 (1972) 48 - 51. 5 D. D. Papshev, Effectiveness of method for finishing-strengthening treatment, Sou. Eng. Res., 3 (1983) 34 - 47. 6 P. G. AIekseev, The wear resistance of burnished flat surfaces, Mach. Tool. U.S.S.R., 36 (1965) 30 - 33. 7 D. C. Montgomery, Design and Analysis of Experiments, Wiley, New York, 2nd edn., 1984, pp. 261 - 290. 8 Y. A. Belov, Burnishing surfaces with ball type tools, Mach. Tool. U.S.S.R., 34 (1963) 23 - 26. 9 S. S. Pande and S. M. Patel, Investigations on vibratory burnishing process, Int. J. Mach. Tool Des. Res., 24 (1984) 195 - 206.
(1989)
STATISTICAL PARAMETERS N. H. LOH* Nanyang Nanyang
235
- 243
235
ANALYSES OF THE EFFECTS ON SURFACE ‘HARDNESS
OF BALL BURNISHING
and S. C. TAM
Technological Institute, School of Mechanical Avenue, Singapore 2263 (Singapore)
and Production
Engineering,
S. MIYAZAWA Mechanical (Received
Engineering May 12, 1988;
Laboratory, revised
Namiki
August
1-2, Tsukuba
19, 1988;
City (Japan)
accepted
September
21,1988)
Summary Burnishing is used increasingly as a finishing operation which gives additional advantages such as increased hardness, fatigue strength and wear resistance. Experimental work based on 34 factorial design was carried out on a vertical machining centre to establish the effects of ball burnishing parameters on the surface hardness of flat AISI 1045 specimens. Statistical analyses of the results (at 1% level of significance) show that the following parameters have significant effects on the surface hardness: depth of penetration, feed, lubricant and ball material. An interaction effect between the ball material and the linear feed was also evident. A 33% - 55% increase in hardness can be obtained.
1. Introduction The finishing of metals with a hardened surface layer has attracted the interest of researchers, e.g. those in the optomechanical industry [l]. The functional performance of a component, such as fatigue strength, load bearing capacity and wear, depends on its surface characteristics such as hardness, induced residual stresses and topography. Czichos and Habig [2] studied the tribological behaviour of medium carbon steel. They found that different wear values, wear patterns and wear mechanisms result depending on various factors: material parameters (e.g. hardness), operating variables (e.g. kinematics), geometry of the tribological system, and environmental and interfacial conditions. Studies (e.g. by Kudryavtsev [3]) have established the beneficial effects of residual compressive stresses and work hardening in improving the fatigue strength of various materials. *Author
to whom
0043-1648/89/$3.50
correspondence
should
be addressed.
@ Elsevier
Sequoia/Printed
in The Netherlands
236
Burnishing, a plastic deformation process, is commonly used to achieve good surface finish. Unlike machining processes, burnishing was also found to give several additional desirable surface characteristics, i.e. higher fatigue strength [ 41, increased surface hardness due to work hardening ]4 - 61 and higher wear resistance [4, 63. In order to utilize the increased hardness effect of the burnishing process effectively, the parameters affecting surface hardness have to be established. In this paper, a statistical systematic study of the effects of ball burnishing parameters on the final surface hardness of flat AISI 1045 specimens is reported. For the experiments to be performed efficiently, statistical techniques such as 34 factorial design were used.
2. Statistical
techniques
Factorial design is more efficient than the conventional one-factorat-a-time experiments commonly employed by researchers. It also enables the study of both the main and the interaction effects between factors. Also, if a parameter (e.g. surface hardness) needs to be maximized with respect to a combination of factors, the factorial design will give a combination near the m~imum whereas the one-factor-at-a-time procedure will not. In the 34 factorial design used, four factors (ball material, lubricant, feed and depth of penetration) were studied and they were designated alphabetically A - D. Each qualitative and quantitative factor has three TABLE 1 Experimental burnishing factors and levels Fat tor
Level
A Ball material
A0 Tungsten carbide WC (hardness: 1300 HV) Ai Ball bearing steel SUJZ (hardness: 800 HV) AZ Silicon nitride SisN4 (hardness: 2100 HV)
B Lubricant
Be Brown greasea B1 White greasea Bz Kerosene
C Feed
D Depth of penetration
80 pm Cl lOO@m C2 120pm CO
81.tm Do Dl 13pm D2 17 pm
aBrown grease (Shell ALVANIA EPl) - extreme pressure lithium soap grease, Pb-S extreme pressure additives. White grease (Grade PQ40AA-2 from American Oil and Supply Co.) - aluminium complex soap grease.
237
types of materials and three values respectively and they are commonly known as “three levels”. The three levels are designated by the digits 0, 1 and 2. The four factors and their respective levels are shown in Table 1. A more detailed treatment of factorial design can be found in ref. 7. Computation of the experimental data was carried out using Yates’ algorithm. The main effect of each factor and the effects of interactions between factors were determined using the analysis of variance (ANOVA) technique and the F-test. It was assumed that third and higher order interactions were physically impossible and could be used as an estimate of experimental error. In 34 factorial design, the sum of the squares for any main quantitative effect may be partitioned into linear and quadratic components, each with a single degree of freedom. These two components are denoted by the subscripts L and Q respectively. For example, CL denotes the main effect of the linear component of factor C, &Do denotes the interaction effect between the linear component of factor C and the quadratic component of factor D.
3. Experimental work The experimental work was conducted on a vertical machining centre with automatic tool changer. A numerically controlled program was written for the consecutive pre-machining and burnishing operations. The schematic set-up of the ball burnishing operation is shown in Fig. 1. The ball burnishing tool shown in Fig. 2 is the main element in the burnishing process. The ball is located in position and held rigidly by means of a rod and screw. The burnishing tool was held stationary and rigidly by means of a special locking mechanism attached to the machining centre. The machining centre has automatic tool length measuring and linear scale feedback systems so as to ensure accurate tool settings and workpiece movements. A Kistler piezoelectric dynamometer (see Fig. 1) was used to measure the normal and tangential forces. The forces were recorded by a pen recorder and the corresponding average forces calculated. The depth of penetration and feed terminologies are illustrated in Fig. 3 which is seen from view A of Fig. 1. The depth of penetration is the distance of the ball tip below the pre-machined surface. The feed is the horizontal distance between two successive ball centres. Both the depth of penetration and the feed were preset in the numerically controlled program. To ensure a uniform pre-machined surface for all the experimental work, the following constant pre-machining specifications were used: single carbide milling cutter of nose radius 0.8 mm, table feed rate of 150 mm mini, cutting speed of 408 m min-’ and 0.2 mm depth of cut. The burnishing operations were conducted at a constant speed of 300 mm min-’ using balls of 9.52 mm diameter.
L&king contact
Workpiece
.
locator
Dynamometer
Machine
w
bed
Machine bed movement
Fig. 1. Schematic set-up of ball burnishing operation.
-_( Burnishing
1
I_
Feed
1 ball
\ Ball
holder
sc;ew
Fig. 2. Ball burnishing tool.
View
A
Fig. 3. Schematic illustration of terminologies.
The hardnesses of the pre-machined and burnished surfaces were measured using Vickers’ hardness equipment. A pyramid diamond indenter with 136” apex angle and indentation load of 1 kgf was used. The average hardness of the pre-machined surface was 196 HV.
239
4. Results and discussion A total of 81 experimental results comprising all possible burnishing conditions are shown in Table 2. Statistical discussion on the effects of the various factors on surface hardness is based on a 1% level of significance. The ANOVA table (Table 3) shows only the s~nificant main effects and interactions, TABLE
2
Experimental
results Average
surface
hardness
Ao
(HV)
AI
A2
30
BI
B2
Bo
B1
32
Bo
31
32
DI D2
266 274 280
273 280 288
260 275 283
261 270 216
270 280 282
260 274 277
268 292 280
272 281 296
268 278 281
CI
Do Dl DZ
274 281 287
284 291 297
269 281 291
265 277 283
281 286 291
266 280 282
275 282 286
277 290 297
273 278 286
Cz
Do DI D2
279 294 297
295 300 303
272 286 296
271 282 290
284 290 296
270 281 289
274 284 294
281 294 299
281 280 285
Co
Do
TABLE
3
ANOVA Source
table 0 f uariution
Sum of squares
Degree of freedom
Mean squares
Calculated F ratio
Fo.01
Main effect A B CL DL DQ Two-factor
2 2 1 1 1
213.98 702.42 1688.96 3313.50 90.38
18.79 61.67 148.28 290.91 7.94
5.10 5.10 7.22 7.22 7.22
210.70
2
105.35
9.25
5.10
546.85
48
11.39
interaction
A% Estimated
427.95 1404.84 1688.96 3313.50 90.38
error
From the ANOVA table, it can be seen that the surface hardness of the burnished specimen is dependent on the type of ball material used.
240
The interaction effect of ball material and linear feed (A&) is also significant. Of all the main effects of the four factors on surface hardness, the ball material effect is of least significance. From the experimental results in Table 2, the tungsten carbide ball (designated by A,) gives the highest hardness value of 303 HV which is about 55% higher than the pre-machined surface hardness of 196 HV. Under the same burnishing conditions, different ball materials give different surface hardness values (see Figs. 4(a) - 4(c)) .
1::: /Is 2
2 (li250--j,, 9 Depth
of
penetration
(em)
13 Depth
of
(a)
penetration
17 (pm)
(b)
5310
2
m
250
A"
, 9
I 13 Depth
of
penetration (C)
17 (WI%)
Depth
of
penetration
(pm)
(d)
Fig. 4. Relationships between depth of penetration and surface hardness. (a) WC ball and kerosene. (b) SUJ2 ball and kerosene. (c) Si3N4 ball and kerosene. (d) WC ball and brown grease. Feed: n, 80 pm; 0, 100 pm; A, 120 pm.
4.2. Effect of lubricant From the ANOVA table, only the main effect of the lubricant type on the surface hardness is significant. A comparison of Figs. 4(a) and 4(d) shows that the effect of lubricant is dependent on the depth of penetration. For the same burnishing conditions, burnishing with brown grease gives higher hardness values than kerosene for a 9 pm depth of penetration and, at the greatest depth of penetration of 17 pm, the reverse effect seems more significant.
241
4.3. Effect of depth of penetration The ANOVA table shows that the linear effect of depth of penetration on surface hardness is the most significant factor. The general relationship between depth of penetration and hardness is shown in Fig. 4. For a particular burnishing condition, as the depth of penetration increases, the normal and tangential forces increase too, as shown in Fig. 5. The increase in forces causes an increase in work hardening and thus an increase in hardness. __ 8007 z
13
17
<
Depth of penetration (pm) Depth of penetration (pm) (b) (a) Fig. 5. Relationships between depth of penetration and burnishing forces (normal and tangential) for WC ball and brown grease. Feed: n, 80 pm; 0, 100 pm; A, 120 pm.
4.4. Effect of feed Prom the ANOVA table, the linear effect of feed is the next highly significant factor. Figure 6 shows some results of the relationship between hardness and feed. Belov [8] found that the work-hardening effect on the burnished surface is greater at lower feed and decreases with increase in feed. This is so, since at lower feed the number of times a ball deforms over the same spot is greater than at higher feed. Thus at lower feed the plastic deformation is more intensive, causing a greater increase in surface hardness. However, the reverse phenomenon seemed to occur here as shown in Fig. 6. It was observed that, with an increase in feed from 80 pm to 120 pm, the surface hardness increases. This trend was also found by Pande and Pate1 [9] in their work on vibratory ball burnishing on a lathe machine, in the feed rate range of 0.05 - 0.11 mm rev-‘. The reverse phenomenon could be explained by observing the relationships between the forces and feed as shown in Fig. 7. For the same burnishing conditions, as the feed increases, the normal and tangential forces increase too, causing the surface hardness to increase. In this particular case, the increase in hardness due to the increase in force is greater than the reduction in hardness due to higher feed. It is also obvious from Table 3 that the effect of depth of penetration on surface hardness is much more significant than the effect of feed on hardness.
242
270: E
3
* 250-A" Feed
, 80
(@m)
Feed
100 (pm)
(b)
(a)
G '
1”
250L8p
v) 250~$1 120 Feed
80
(pm)
Feed
100 (pin)
Cd)
(C)
Fig. 6. Relationships between feed and surface hardness. (a) WC ball and kerosene. (b) SUJ2 ball and kerosene. (c) SisN4 ball and kerosene. (d) WC ball and brown grease. Depth of penetration: w, 9 pm; 0, 13 pm; A, 17 pm.
800-
A 100 (a)
8o
Feed
(pm)
120
(b)
So
Feed
100 (pm)
120
Fig. 7. Relationships between feed and burnishing forces (normal and tangential) for WC ball and brown grease. Depth of penetration: w, 9 pm; 0, 13 pm; A, 17 pm.
5. Conclusions The use of factorial design enables the following conclusions, at a 1% significance level, to be drawn.
243
(1) The main effects on surface hardness of ball material, lubricant, feed and depth of penetration were significant. (2) An interaction effect between the ball material and the linear feed was also evident. The experimental work shows that an increase in surface hardness of 260 HV to 303 HV, an improvement of about 33% - 55%, can be obtained by the ball burnishing process.
Acknowledgments The authors would like to thank the Nanyang Technological Institute, the Ministry of International Trade and Industry (MIT1 of Japan) and the Mechanical Engineering Laboratory (Tsukuba City, Japan) for supporting this research programme on ball burnishing.
References 1 I. Ya. Vakhrameyev and P. G. Balyura, Metal finishing with a ball tool, Sov. J. Opt. Technol., 41 (1974) 168 - 170. 2 H. Czichos and K. Habig, Wear of medium carbon steel - a systematic study on influence of materials and operating parameters. In Y. Tamai (ed.), Proc. JSLE Znt. Tribology Conf, Tohyo, July 8 - 10, 1985, Elsevier, Amsterdam, 1985, pp. 879 884, 3 I. V. Kudryavtsev, Strengthening machine parts by surface plastic deformation, Russ. Eng. J., 50 (1970) 8 - 12. 4 D. D. Papshev and Yu G. Golubev, Effectiveness of surface work-hardening of titanium alloy components, Russ. Eng. J., 52 (1972) 48 - 51. 5 D. D. Papshev, Effectiveness of method for finishing-strengthening treatment, Sou. Eng. Res., 3 (1983) 34 - 47. 6 P. G. AIekseev, The wear resistance of burnished flat surfaces, Mach. Tool. U.S.S.R., 36 (1965) 30 - 33. 7 D. C. Montgomery, Design and Analysis of Experiments, Wiley, New York, 2nd edn., 1984, pp. 261 - 290. 8 Y. A. Belov, Burnishing surfaces with ball type tools, Mach. Tool. U.S.S.R., 34 (1963) 23 - 26. 9 S. S. Pande and S. M. Patel, Investigations on vibratory burnishing process, Int. J. Mach. Tool Des. Res., 24 (1984) 195 - 206.