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Functional needs, machining conditions, and economics of surface finishing
Functional needs, machining conditions, and economics of surface finishing* J. Biellet translated and edited by T,V. Vorburger and V.B. Roy¢ Two applications o f surface finish technology to industrial problems are discussed. The first problem involves the deterioration of tools used to turn large numbers of parts. Preliminary observations indicated that waviness of the tool cutting surface impeded chip flow over the tool and reduced usable life. Changing the tool finishing conditions reduced waviness considerably and increased tool life between sharpenings from 8000 to 53 000 parts. The second case involves the degradation o f flat steel tracks for rolling needle bearings in a moulding machine. Waviness o f both the newly ground and refinished surfaces led to their rapid deterioration In addition, the hardness of the steel was also considered to be too l o w for this application. By improving the finishing process, the life of a set of guides was improved from typically 1500 hours to 5000 hours. For this case, the financial implications o f the results are also discussed.
Keywords: surface roughness measurement, turning, cutting tools, polishing, bearings
Two distinct experimental studies are described in this paper: a study of the cutting faces of tools for automatic lathes and their functional performance with respect to sliding friction of chips; and a study of the surface typology of steel guides and their functional performance with respect to rolling friction against needle bearings. The studies were carried out in accordance with Point 2.1 in the document NF E-05-017, one of the approved standards** on surface finish in France TM.
Cutting tools and sliding friction In collaboration with the Jaeger Company of Chalons-sur-Marne, an experiment was conducted to study the roundness deviations of parts produced on multi-spindle lathes as a function of time. In the course of this work, a number of observations were collected, some relating to the cutting faces of the tools that were used to turn the parts. Analysis of these data has led to improved tool life. First experiment
The initial experimental conditions were: * A version of this article was originally published by Bielle in French in Mecanique Materiaux Electricite, No. 286, 27 (1973). Vorburger and Roy have rewritten the article for publication in English and in cooperation with Bielle have added some new details not found in the original. ?ENSAM, Chabons-sur-Marne, France. ¢ National Bureau of Standards, Gaittersburg, Maryland 20899, USA. ** For a better understanding of this article, the reader should refer to the four documentary standards (Refs 1-4) and in particular to the definitions of parameters and terms relevant to surface finish.
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Workpiece material5: Brass UZ39 Pb 0.8; Machine: Newbritain* six spindle lathe; Cutting tool composition6: Steel S.6.5.2Z80 WDV060502; Cutting conditions: Tool speed D 165 mm/min, feed - - 0.20 mm/revolution; Sharpening procedure: Rough sharpen with a grinding wheel (38 A60 MVG/Norton), final sharpen with a diamond grinding wheel (resin JK SMIT DT 1000TF SO B5.15) which is allowed to sweep back and forth for 10 minutes on the cutting face of the tool. The resulting typology of the cutting face of the tool (Fig 1) has machining marks that are circular, evenly distributed, and crossed. Fig 1 also shows the directions of motion of the tool feed and the workpiece. Observations
This tool produced 8000 parts conforming to the design specifications, after which surface profiles were measured using the stylus technique. Fig 2 shows a surface profile in the plane Xp, Yp oriented perpendicular to the edge of the tool, that is, parallel to the flow of the metal on the cutting face of the tool. Fig 2 shows two zones: Zone 2 is the surface profile resulting from sharpenin 9 with the diamond wheel. The parameters R and W', estimated on the graph, are 0.48/Jm and 1.5/Jm respectively. Zone 1 is the surface profile that results after using the * Certain commercial equipment or products are identified in the paper in order to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by ENSAM or NBS nor does it imply that the equipment or products are necessarily the best available for the purpose.
0141--6359/85/010031---07(~) 1985 Butterworth & Co (Publishers) Ltd
31
Bielle
-
Functional needs, machining conditions, and economics of surface finishing
-
tool to make 8000 parts. Zone 1 contains information on the range of the interaction between the chip and the cutting face (-0.35 mm) and on the dominant surface parameter affecting function. In fact, the waviness Wwith its characteristic spacing Aw is likely to be the parameter which opposes the flow of the metal, resulting in crater formation as shown in Zone 1 of Fig 2.
Initial conclusion If waviness is the dominant factor limiting tool life, then it is necessary to understand its generation and methods of controlling it.
Fig 4 Diagram of a tool redesigned with a recess in the cutting face. The dimensions are in mm
I
xo
Fig 5 Surface profile AA' measured after sharpening of the cutting face of the tool shown in Fig 4
~YP
jw
/ Fig I Motions of the chip, work piece, and tool during a machining operation. Also shown is the surface typology of the cutting face, whose grooves can be characterized as circular, crossed, and evenly distributed I
Lo Weviness__~ ] s,oecing [
I-Xp
|
Iyp Xp"
~,,--0.35mm--~
Fig 3 shows the variation of the grinding wheel-to-tool contact area for a wheel diameter of 140 mm. At each moment, variation in the area of contact between the wheel and the tool causes variations in the pressure; when the pressure is high enough, the abrasive particles produce cutting. The grinding time of 10 minutes in the final stage of sharpening may reduce the amplitude of the waviness but does not decrease it enough. Moreover, the finishing wheel works in a zone which will never interact with the chip.
Prognosis 100
Zone 2
Zone 1
J
Fig 2 Surface profile of the cutting face of a tool after machining 8000 parts. The cutting edge is shown at the right. The surface profile was measured with a flat reference plane and without skids or electrical filters
Top view of Rotation of outlineof tool grinding wheel
i i I \
t
.J //
~<~
rmdmg marks
Fig 3 Motions of the grinding wheel on the tool cutting face 32
Second experiment The active part of the tool was limited to a lip extending 3 mm from the cutting edge atthe back. Fig 4 shows the new design minimizing contact area variations. In addition, the lip of the cutting face was sharpened under the following conditions: using the same grinding wheel as before; the same sharpening machine; grinding t i m e - - 20 minutes; light lubrication with Diacool.
\
It would seem necessary to reduce the area of contact between the wheel and the tool and to minimize contact area fluctuations during the final sharpening process. This would cause the pressure to be constant, the waviness would disappear, and only the roughness should remain.
Fig 5 shows the resulting profile for the Xp, Yp plane in Fig 4. Fig 6 shows segments ofthe profiles of Fig 2 for comparison with Fig 5. It is important to note the differences in the Yscale between Figs 2 and 5.
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Bielle m Functional needs, machining conditions, and economics of surface finishing These new sharpening conditions resulted in reduced waviness and a reduction in the roughness R from 0.48/~m to 0.05/~m, for a reduction by a factor of 9.6 of the third order structure l's.
Wheel . rotation , " , /f,._~"~, [ ~, ~ / 4 .-,, / I
Observations and second conclusion
•
The reduction in the number of tool changes (15 minutes halt in the lathe operation for each tool change) gave 37% increase in production. Sharpening time was doubled, but the number of sharpenings was reduced by a factor of 6.6, giving savings of about 70% of the cost of sharpening.
At the time of the original study, selection of wheel type to match the newly reduced area of the cutting surface of the tool and the minimum dwell time for the finishing cuts during sharpening remained to be determined. Optimum conditions of sharpening were studied in collaboration with the engineering department of the Jaeger Company.
Verification o f the hypotheses For this experimental verification (Fig 7) a single piece of special carbon steel having a 12 mm square cross-section was cut into two tools which were then sharpened simultaneously with the Xp
It
,
With these new sharpening conditions, the tool produced 53 000 in tolerance components. The suppression of the waviness on the cutting face and the reduction of the R parameter produced the following results: •
'
~ !
I
I
R,,,65mm
l"
U,~IIt"
I
',,
lil..I i; LJ a,ino,eeo Fig 7 Schematic diagram showing the simultaneous grinding of two tools using an alternating feed stroke. Tool 2 has a recess in the cutting face A
0"51LP~
B
AB,.-I
Fig 8 Surface profile obtained for Tool 1. The lower curve is a continuation of the upper curve. Z is the zone used for calculating the parameter AR ,,FZ
l
x°
0.5JL.~ 100
100
o2 t
Fig 6 A section of the profile of Fig 2. In the lower part of a smaller section is expanded to have approximately the same scale as that of Fig 5 for comparison
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Fig 9 Surface profile obtained for Tool 2. Z is the zone used for calculating the parameter AR same cup wheel - - a rough wheel with the characteristics 38A 6OL 5VG. One pass of 20/~m depth, was followed by 10 sweeps back and forth at a mean speed of 400 mm/min using the alternating motion shown in Fig 7. At no time was the wheel in contact with both tools. Tool 1 was sharpened on the whole cutting face, that is, over a width of 5.1 mm. Tool 2 was sharpened on a 2 mm wide lip adjacent to the cutting edge. Fig 8 shows the XpYp profiles for Tool 1 and Fig 9 an XpYp profile for Tool 2. Fig 10 shows that on the XpYp plane of this surface typology, one can see an increasing spacing between the striations. Therefore, as the metal slides across the cutting face, it encounters the largest wavelength near the cutting edge. An examination of the profiles of Figs 8 and 9 enables one to calculate the parameters shown in Table 1 for an evaluation length originating at the cutting edge and ending 2 mm in from there. Under identical sharpening conditions, Tool 2 has the lower values of R, AR, W, and Aw. Since the only variable is the sharpened area, the observations above are
33
Bielle ~ Functional needs, machining conditions, and economics of surface finishing
~
l
verified once again for these conditions of rough grinding.
X'p
W e a r in r o l l i n g
AR2 AR1
A local plastic forming company suffered frequent production halts for repair on one assembly line (about every 1500 hours of operation). The cause of these interruptions stemmed from the deterioration of the rolling tracks for flat needle bearing assemblies. With this type of machine, the two halves of the mould close on the plastic blank which is forced outwards against the mould by compressed air to give the final shape. The closing and opening movement of the two halves rely on steel guides (Fig 11 ). These guides constrain the motion of the mould halves using needle bearing assemblies which roll along their surfaces. Each machine is equipped with: 4 large (740 x 80 x 27 ram)guides weighing 12.4 kg, 8small (370 x 50 x 27 mm) guides weighing 3.8 kg, 16 bearing assemblies with needle bearings 3.5 mm in diameter and 16 mm long. The mean speed of the needles, which roll along the surfaces of the guides, is 0.3 m/s. The replacement of a set of guides requires two operators for each machine for two 8-hour day shifts. This would mean a 48-hour loss of production for these machines, which normally work continuously for three shifts. Along with this information, two guides, one brand new from the manufacturer and one which had been surfaced by a subcontractor, and some worn guides were supplied. The requirement was to determine the cause of the deterioration of the surfaces of the slideways.
Xp
Fig 10 Typology of a tool cutting face showing how the roughness spacing varies with position Table 1 Tool profiles
Parameters
Tool no 1
Tool no 2
R,/Jm A R,/Jm W,/Jm Aw,/zm
0.18 41 1.2 160
0.092 29 0.6 55
Initial o b s e r v a t i o n s --
Small guide Large guide
Tests were performed on the brand new guide, the guide that was refinished by the subcontractor, and the worn guides. On the new guide the typology is obtained by an end grinding process. Fig 12
Fig 11 Photograph of moulding machine showing both the large and small guides
Need le bemiring lyp
'1
Guide
Ix
250
\ I
Cut off : 0 . 2 5 m m
Re = O . 6 5 p m
Rp= 1.251Jm
Xp
Rt = 4 . 2 1 a m
Fig 12 Surface profile of a new steel guide taken parallel to the direction of rolfing. The envelope profile (hand drawn) is also shown. The measurements were taken with an electrical filter cut-off of O.25 ram. The resulting values for Ra, Rp and Rt are indicated. The needle bearing is not drawn to the same scale as the surface profile
34
PRECISION ENGINEERING
Bielle
Functional needs, machining conditions, and economics of surface finishing Knife-edge shows an XpYp profile taken perpendicular to the stylus Profile 2 axis of the needle bearing and Fig 13 a profile taken ---,,-~I [ / W o r n oreo parallel to the axis of the needle bearing. In both cases one can distinguish a waviness characteristic of machining by a cup wheel. Results of the tests on the used guide are shown in Figs 14 and 15. Stylus profile 1 was traced next to a worn area marked by the needle bearing but on a new, unworn area. The profile is shown in Fig 15. Stylus profile 2 was traced parallel to 1, but Fig 16 Positions of surface profiles measured on a on the worn area. It will be noted that the damage refinished guide by a knife-edge stylus in profile 2 was produced at a place that is adjacent to the bottom of a strong undulation shown in Pathof needle bearing profile 1. Figs 16 and 17 show results of the tests on a guide refinished by the subcontractor. These guides were ground tangentially. If one wishes to trace parallel to the grinding marks, that is, parallel to the rolling direction of the needle bearings, the measurement becomes more difficult. For this ,o0 'V' I operation, we used a knife-edge stylus with an edge 3 mm long and a radius of 5/~m. The length Fig 17 Profiles described in Fig 16 of the stylus tip was then a fraction of the length of one needle bearing (Fig 16). Comparison of profiles The hardness of these guides was also tested. 1 and 2 of Fig 17 suggests again how the marks on It varied from 52 to 55 HRC (Rockwell hardness), a the guides made by the needle bearings have a range which seems fairly low for a needle bearing tendency to be formed at the bottom of undulatrack. tions on the original ground surface. -
-
y•pXp
Vt//l_ Y
Diagnosis and prognosis I 100
• Xp
~%
Fig 13 Surface profile of a new steel guide taken perpendicular to the rolling direction, ie parallel to the bearing axis Xp_ Yp ~ ~
Profile 2 / jArea marked by needle bearings End grinding, ~ circularmarks
~ 'Profile 1 Measurement zone Fig 14 Positions of two surface profiles measured on a steel guide after 1500 hours of operation ~='
Xp
5OO Xp
Profile1
Path of
Profile2
0.5~_~ 50O
Mark
from needle bearing
Fig 15 Profiles described in Fig 14 PRECISION ENGINEERING
needle
For both cases of grinding, ie the new guides with end ground surfaces and the guides tangentially ground by the subcontractor, one notes a waviness inherent in the machining process. This waviness promotes the marking of the guides, which is perhaps accentuated by the lack of hardness. Therefore, two problems present themselves: to increase the hardness of the guides and to eliminate the waviness from grinding. There were two constraints: several machines were equipped with guides that were still reusable after regeneration of their surfaces, and some reserve guides still remain in the storeroom. In future, therefore, it would be desirable to manufacture guides from specially hardened and annealed steel to increase the hardness to 60 HRC. In the interim, however, it was necessary to use the existing guides with the waviness eliminated.
Second experiment A large guide was tested for 1500 hours with one rolling surface ground by the subcontractor and the second rolling surface polished on marble for five minutes with alumina grit having a size of 1800 per inch (Fig 18). On each surface, stylus traces were taken before and after it was placed in service. After operation for 1500 hours, the surface of the guide which was tangentially ground by a subcontractor was measured; the profiles are shown in Fig 19. The surface of the other guide is shown in Fig 20. The polishing yields marks with a random orientation. Note that the third order surface structure effectively disappears. Only the aperiodic fourth order structures remain, which are certainly 35
Bielle - - Functional needs, machining conditions, and economics of surface finishing valuable in retaining .ubricant. The roughness spacings correspond approximately to the size of alumina grains of Fig 18. The polished guide behaved very well, but the ground guide reveals the beginnings of tracks made by the needle bearings. It seems that the roughness due to polishing (R = 0.35/Jm) permitted the surface to perform satisfactorily, since the roughness changed after 1500 hours to R = 0.20/~m. The deepest marks (fourth order) with an average value of 0.8pm did not present any drawbacks a priori. It is possible for us to have the guide surfaces regenerated in this manner under a subcontract; as a first estimation this should easily double the running time ofthe guides.
Continued experiments A set of 8 small and 4 large guides was produced by grinding and polishing and put into service to run to failure on one production machine. At 4500 hours, a breakdown of another device caused a halt in the machine and a disassembly of the guides. Upon inspection, they showed no trace of abnormal wear, not a mark. After changing the needle bearings, they exceeded 5000 hours of operation without apparent wear and tear**. Economic implications The price of a forming machine is about FF 400 000. This is depreciated over 3 years from the gross income, which must also cover monthly salaries, commercial expenses, financial expenses, and taxes on the capital. In this type of manufacturing, there is at most a profit margin of 2% for reinvestment. The percentage of the gross absorbed each year by replacing the guides after every 1500 hours of operation and by changing the guides every 5000 hours can now be calculated: Replacing guides every 1500 hours and for a 3 year depreciation period: 7 guide set regrinds 2 new guides due to the necessity to change the set after 3 regenerations by grinding ie FF 7324 x 2 9 disassemblies at FF 400 9 halts in production of 20 hours yielding a loss in the gross income Total loss in the gross income Loss per year
Fig 18 Grit used in polishing the second rolling surface Tangential grinding, befo¢e use
100 After 1500 hours use
100
Fig 19 Profiles of the surface that was tangentially ground. The profiles were measured parallel to the needle bearing axes and perpendicular to the grinding marks. A conventional stylus was used
1 820 14 648 3 600 10 800 30 868 10 289
If by polishing, guide life is increased to 5000 hours: 2 refurbishments 2 disassemblies 2 polishings at FF 83 2 production halts of 20 hours Total loss in the profit margin Loss per year
520 800 166 2400 3886 1295
Income is FF 0.05 per part, and the machine must run for 5000 hours per year, according to the planning estimate; producing 1200 parts per hour. Therefore, the total income per year is FF 300 000. In the first case, the percentage loss in the income is 3.42% and in the second case 0.43%. Results
In such high volume manufacturing, the maximum profit available for reinvestments is about 2%. One can see then that in the first case, the machines operate at a loss of 1.42% (2-3.42%). In the second case, there is a profit margin of 1.57%.
IO0
After 1500 hours use
Fig 20 Profiles of the surface that was rough ground by a subcontractor, then polished
36
** Since the original article was published, the moulding machine continued to operate and, in fact, achieved more than 50 000 hours of operation without changing or rejuvenation o f the guides. The needle bearings were changed every 15 000 hours.
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Bielle - - Functional needs, machining conditions, and economics of surface finishing
These studies were not part of a formal research programme but aimed, above all, to exemplify the role of standardization, the importance of a judicious choice of the manufacturing process relevant to the mechanical function of the surface, and the benefit of the effective use of measurement results. When applied to industrial metrology, the guiding objectives of the French standard of surface finish are to some degree achieved. In particular, it appears that: • • • •
no profile parameter can characterize the surface by itself; waviness is a parameter which escapes visual tactile examination; waviness is important in a number of functions; to use high pass filters to eliminate the waviness during the measurement and to avoid taking it into account is dangerous.
The guides were produced using the available processes. It is possible that superfine grinding would have brought identical results. Functional requirements, manufacturing specifications, and financial considerations are the themes which have been chosen for emphasis here. Applying the directives of Point 2.1 of the standard NF E 05.017, 'Determination of Measured Surfaces' (which is summarized as inspection, diagnosis, prognosis), has distinguished: 1. 2. 3. 4.
The functional requirements by systematic testing and operation; The specifications: establishing a set of parameters representative of the function; Manufacturing: establishing a baseof machining techniques that permit reproducibility of the surfaces; Financial considerations: verification of the profitability of measurement operations.
of measurement at FF 50, and we take into account lost time, the cost of transportation, the cost of meetings, the cost of measuring the hardness of the guides, etc, the firm estimates that it saved a minimum of 5% of the gross income. In the present economic situation, the fraction of the profit margin that an organization reserves for earnings or for investment is necessarily limited. Problems with financial consequences resulting from technological problems should never be resolved by empiricism. In this setting, metrology points the way to success for the organization.
Acknowledgements Professor Bielle thanks MM. Affigliati and Knoepfli, the principal technicians, for their contributions to these studies.
References 1 Surface Texture of Products. Regulations. 1 - - General, Terminology, Definitions. Association Francaise de
Normalisation (AFNOR), Norme Francaise (NF) E 05-015, (Paris, 1972) 2 Surface Texture of Products. Regulations. 2 - - Specifications of Surface Texture on Drawings. AFNOR, NF E 05-016, (Paris,
1972) 3 Surface Texture of Products. Regulations. 3 - - Determination of Measured Surfaces. AFNOR, NF E 05-017, (Paris, 1972) 4 Surface Texture of Products. Regulations. 4 - - Economic Aspects. AFNOR, NF E 05-018, (Paris, 1969) 5 Similar terminology for brass composition appears in: Inter-
national Metallic Materials Cross Reference, 2nd edition, Arcuri J.V. and Potts D.L. Eds, (General Electric, Schenectady,
New York, 1983)22.19 6 Similar terminology for steel composition appears in Ross R.B., Metallic Materials Specification Handbook, 3rd edition
(E. & F.N. Spon, London, 1980)479
Conclusion In the case of the second study, there was one hour of measurement work. If we set the cost of an hour
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7 Fahl C.F. Motif combination - - A new approach to surface profile analysis. Wear, 1982,83, 165 8 Scheffer B. Comparaison de Diff6rentes Normes Nationales
(Rdgie Nationale des Usines Renault, Billancourt, 1989) pp 7-28f
37
Keywords: surface roughness measurement, turning, cutting tools, polishing, bearings
Two distinct experimental studies are described in this paper: a study of the cutting faces of tools for automatic lathes and their functional performance with respect to sliding friction of chips; and a study of the surface typology of steel guides and their functional performance with respect to rolling friction against needle bearings. The studies were carried out in accordance with Point 2.1 in the document NF E-05-017, one of the approved standards** on surface finish in France TM.
Cutting tools and sliding friction In collaboration with the Jaeger Company of Chalons-sur-Marne, an experiment was conducted to study the roundness deviations of parts produced on multi-spindle lathes as a function of time. In the course of this work, a number of observations were collected, some relating to the cutting faces of the tools that were used to turn the parts. Analysis of these data has led to improved tool life. First experiment
The initial experimental conditions were: * A version of this article was originally published by Bielle in French in Mecanique Materiaux Electricite, No. 286, 27 (1973). Vorburger and Roy have rewritten the article for publication in English and in cooperation with Bielle have added some new details not found in the original. ?ENSAM, Chabons-sur-Marne, France. ¢ National Bureau of Standards, Gaittersburg, Maryland 20899, USA. ** For a better understanding of this article, the reader should refer to the four documentary standards (Refs 1-4) and in particular to the definitions of parameters and terms relevant to surface finish.
PRECISION ENGINEERING
Workpiece material5: Brass UZ39 Pb 0.8; Machine: Newbritain* six spindle lathe; Cutting tool composition6: Steel S.6.5.2Z80 WDV060502; Cutting conditions: Tool speed D 165 mm/min, feed - - 0.20 mm/revolution; Sharpening procedure: Rough sharpen with a grinding wheel (38 A60 MVG/Norton), final sharpen with a diamond grinding wheel (resin JK SMIT DT 1000TF SO B5.15) which is allowed to sweep back and forth for 10 minutes on the cutting face of the tool. The resulting typology of the cutting face of the tool (Fig 1) has machining marks that are circular, evenly distributed, and crossed. Fig 1 also shows the directions of motion of the tool feed and the workpiece. Observations
This tool produced 8000 parts conforming to the design specifications, after which surface profiles were measured using the stylus technique. Fig 2 shows a surface profile in the plane Xp, Yp oriented perpendicular to the edge of the tool, that is, parallel to the flow of the metal on the cutting face of the tool. Fig 2 shows two zones: Zone 2 is the surface profile resulting from sharpenin 9 with the diamond wheel. The parameters R and W', estimated on the graph, are 0.48/Jm and 1.5/Jm respectively. Zone 1 is the surface profile that results after using the * Certain commercial equipment or products are identified in the paper in order to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by ENSAM or NBS nor does it imply that the equipment or products are necessarily the best available for the purpose.
0141--6359/85/010031---07(~) 1985 Butterworth & Co (Publishers) Ltd
31
Bielle
-
Functional needs, machining conditions, and economics of surface finishing
-
tool to make 8000 parts. Zone 1 contains information on the range of the interaction between the chip and the cutting face (-0.35 mm) and on the dominant surface parameter affecting function. In fact, the waviness Wwith its characteristic spacing Aw is likely to be the parameter which opposes the flow of the metal, resulting in crater formation as shown in Zone 1 of Fig 2.
Initial conclusion If waviness is the dominant factor limiting tool life, then it is necessary to understand its generation and methods of controlling it.
Fig 4 Diagram of a tool redesigned with a recess in the cutting face. The dimensions are in mm
I
xo
Fig 5 Surface profile AA' measured after sharpening of the cutting face of the tool shown in Fig 4
~YP
jw
/ Fig I Motions of the chip, work piece, and tool during a machining operation. Also shown is the surface typology of the cutting face, whose grooves can be characterized as circular, crossed, and evenly distributed I
Lo Weviness__~ ] s,oecing [
I-Xp
|
Iyp Xp"
~,,--0.35mm--~
Fig 3 shows the variation of the grinding wheel-to-tool contact area for a wheel diameter of 140 mm. At each moment, variation in the area of contact between the wheel and the tool causes variations in the pressure; when the pressure is high enough, the abrasive particles produce cutting. The grinding time of 10 minutes in the final stage of sharpening may reduce the amplitude of the waviness but does not decrease it enough. Moreover, the finishing wheel works in a zone which will never interact with the chip.
Prognosis 100
Zone 2
Zone 1
J
Fig 2 Surface profile of the cutting face of a tool after machining 8000 parts. The cutting edge is shown at the right. The surface profile was measured with a flat reference plane and without skids or electrical filters
Top view of Rotation of outlineof tool grinding wheel
i i I \
t
.J //
~<~
rmdmg marks
Fig 3 Motions of the grinding wheel on the tool cutting face 32
Second experiment The active part of the tool was limited to a lip extending 3 mm from the cutting edge atthe back. Fig 4 shows the new design minimizing contact area variations. In addition, the lip of the cutting face was sharpened under the following conditions: using the same grinding wheel as before; the same sharpening machine; grinding t i m e - - 20 minutes; light lubrication with Diacool.
\
It would seem necessary to reduce the area of contact between the wheel and the tool and to minimize contact area fluctuations during the final sharpening process. This would cause the pressure to be constant, the waviness would disappear, and only the roughness should remain.
Fig 5 shows the resulting profile for the Xp, Yp plane in Fig 4. Fig 6 shows segments ofthe profiles of Fig 2 for comparison with Fig 5. It is important to note the differences in the Yscale between Figs 2 and 5.
PRECISION ENGINEERING
Bielle m Functional needs, machining conditions, and economics of surface finishing These new sharpening conditions resulted in reduced waviness and a reduction in the roughness R from 0.48/~m to 0.05/~m, for a reduction by a factor of 9.6 of the third order structure l's.
Wheel . rotation , " , /f,._~"~, [ ~, ~ / 4 .-,, / I
Observations and second conclusion
•
The reduction in the number of tool changes (15 minutes halt in the lathe operation for each tool change) gave 37% increase in production. Sharpening time was doubled, but the number of sharpenings was reduced by a factor of 6.6, giving savings of about 70% of the cost of sharpening.
At the time of the original study, selection of wheel type to match the newly reduced area of the cutting surface of the tool and the minimum dwell time for the finishing cuts during sharpening remained to be determined. Optimum conditions of sharpening were studied in collaboration with the engineering department of the Jaeger Company.
Verification o f the hypotheses For this experimental verification (Fig 7) a single piece of special carbon steel having a 12 mm square cross-section was cut into two tools which were then sharpened simultaneously with the Xp
It
,
With these new sharpening conditions, the tool produced 53 000 in tolerance components. The suppression of the waviness on the cutting face and the reduction of the R parameter produced the following results: •
'
~ !
I
I
R,,,65mm
l"
U,~IIt"
I
',,
lil..I i; LJ a,ino,eeo Fig 7 Schematic diagram showing the simultaneous grinding of two tools using an alternating feed stroke. Tool 2 has a recess in the cutting face A
0"51LP~
B
AB,.-I
Fig 8 Surface profile obtained for Tool 1. The lower curve is a continuation of the upper curve. Z is the zone used for calculating the parameter AR ,,FZ
l
x°
0.5JL.~ 100
100
o2 t
Fig 6 A section of the profile of Fig 2. In the lower part of a smaller section is expanded to have approximately the same scale as that of Fig 5 for comparison
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Fig 9 Surface profile obtained for Tool 2. Z is the zone used for calculating the parameter AR same cup wheel - - a rough wheel with the characteristics 38A 6OL 5VG. One pass of 20/~m depth, was followed by 10 sweeps back and forth at a mean speed of 400 mm/min using the alternating motion shown in Fig 7. At no time was the wheel in contact with both tools. Tool 1 was sharpened on the whole cutting face, that is, over a width of 5.1 mm. Tool 2 was sharpened on a 2 mm wide lip adjacent to the cutting edge. Fig 8 shows the XpYp profiles for Tool 1 and Fig 9 an XpYp profile for Tool 2. Fig 10 shows that on the XpYp plane of this surface typology, one can see an increasing spacing between the striations. Therefore, as the metal slides across the cutting face, it encounters the largest wavelength near the cutting edge. An examination of the profiles of Figs 8 and 9 enables one to calculate the parameters shown in Table 1 for an evaluation length originating at the cutting edge and ending 2 mm in from there. Under identical sharpening conditions, Tool 2 has the lower values of R, AR, W, and Aw. Since the only variable is the sharpened area, the observations above are
33
Bielle ~ Functional needs, machining conditions, and economics of surface finishing
~
l
verified once again for these conditions of rough grinding.
X'p
W e a r in r o l l i n g
AR2 AR1
A local plastic forming company suffered frequent production halts for repair on one assembly line (about every 1500 hours of operation). The cause of these interruptions stemmed from the deterioration of the rolling tracks for flat needle bearing assemblies. With this type of machine, the two halves of the mould close on the plastic blank which is forced outwards against the mould by compressed air to give the final shape. The closing and opening movement of the two halves rely on steel guides (Fig 11 ). These guides constrain the motion of the mould halves using needle bearing assemblies which roll along their surfaces. Each machine is equipped with: 4 large (740 x 80 x 27 ram)guides weighing 12.4 kg, 8small (370 x 50 x 27 mm) guides weighing 3.8 kg, 16 bearing assemblies with needle bearings 3.5 mm in diameter and 16 mm long. The mean speed of the needles, which roll along the surfaces of the guides, is 0.3 m/s. The replacement of a set of guides requires two operators for each machine for two 8-hour day shifts. This would mean a 48-hour loss of production for these machines, which normally work continuously for three shifts. Along with this information, two guides, one brand new from the manufacturer and one which had been surfaced by a subcontractor, and some worn guides were supplied. The requirement was to determine the cause of the deterioration of the surfaces of the slideways.
Xp
Fig 10 Typology of a tool cutting face showing how the roughness spacing varies with position Table 1 Tool profiles
Parameters
Tool no 1
Tool no 2
R,/Jm A R,/Jm W,/Jm Aw,/zm
0.18 41 1.2 160
0.092 29 0.6 55
Initial o b s e r v a t i o n s --
Small guide Large guide
Tests were performed on the brand new guide, the guide that was refinished by the subcontractor, and the worn guides. On the new guide the typology is obtained by an end grinding process. Fig 12
Fig 11 Photograph of moulding machine showing both the large and small guides
Need le bemiring lyp
'1
Guide
Ix
250
\ I
Cut off : 0 . 2 5 m m
Re = O . 6 5 p m
Rp= 1.251Jm
Xp
Rt = 4 . 2 1 a m
Fig 12 Surface profile of a new steel guide taken parallel to the direction of rolfing. The envelope profile (hand drawn) is also shown. The measurements were taken with an electrical filter cut-off of O.25 ram. The resulting values for Ra, Rp and Rt are indicated. The needle bearing is not drawn to the same scale as the surface profile
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Bielle
Functional needs, machining conditions, and economics of surface finishing Knife-edge shows an XpYp profile taken perpendicular to the stylus Profile 2 axis of the needle bearing and Fig 13 a profile taken ---,,-~I [ / W o r n oreo parallel to the axis of the needle bearing. In both cases one can distinguish a waviness characteristic of machining by a cup wheel. Results of the tests on the used guide are shown in Figs 14 and 15. Stylus profile 1 was traced next to a worn area marked by the needle bearing but on a new, unworn area. The profile is shown in Fig 15. Stylus profile 2 was traced parallel to 1, but Fig 16 Positions of surface profiles measured on a on the worn area. It will be noted that the damage refinished guide by a knife-edge stylus in profile 2 was produced at a place that is adjacent to the bottom of a strong undulation shown in Pathof needle bearing profile 1. Figs 16 and 17 show results of the tests on a guide refinished by the subcontractor. These guides were ground tangentially. If one wishes to trace parallel to the grinding marks, that is, parallel to the rolling direction of the needle bearings, the measurement becomes more difficult. For this ,o0 'V' I operation, we used a knife-edge stylus with an edge 3 mm long and a radius of 5/~m. The length Fig 17 Profiles described in Fig 16 of the stylus tip was then a fraction of the length of one needle bearing (Fig 16). Comparison of profiles The hardness of these guides was also tested. 1 and 2 of Fig 17 suggests again how the marks on It varied from 52 to 55 HRC (Rockwell hardness), a the guides made by the needle bearings have a range which seems fairly low for a needle bearing tendency to be formed at the bottom of undulatrack. tions on the original ground surface. -
-
y•pXp
Vt//l_ Y
Diagnosis and prognosis I 100
• Xp
~%
Fig 13 Surface profile of a new steel guide taken perpendicular to the rolling direction, ie parallel to the bearing axis Xp_ Yp ~ ~
Profile 2 / jArea marked by needle bearings End grinding, ~ circularmarks
~ 'Profile 1 Measurement zone Fig 14 Positions of two surface profiles measured on a steel guide after 1500 hours of operation ~='
Xp
5OO Xp
Profile1
Path of
Profile2
0.5~_~ 50O
Mark
from needle bearing
Fig 15 Profiles described in Fig 14 PRECISION ENGINEERING
needle
For both cases of grinding, ie the new guides with end ground surfaces and the guides tangentially ground by the subcontractor, one notes a waviness inherent in the machining process. This waviness promotes the marking of the guides, which is perhaps accentuated by the lack of hardness. Therefore, two problems present themselves: to increase the hardness of the guides and to eliminate the waviness from grinding. There were two constraints: several machines were equipped with guides that were still reusable after regeneration of their surfaces, and some reserve guides still remain in the storeroom. In future, therefore, it would be desirable to manufacture guides from specially hardened and annealed steel to increase the hardness to 60 HRC. In the interim, however, it was necessary to use the existing guides with the waviness eliminated.
Second experiment A large guide was tested for 1500 hours with one rolling surface ground by the subcontractor and the second rolling surface polished on marble for five minutes with alumina grit having a size of 1800 per inch (Fig 18). On each surface, stylus traces were taken before and after it was placed in service. After operation for 1500 hours, the surface of the guide which was tangentially ground by a subcontractor was measured; the profiles are shown in Fig 19. The surface of the other guide is shown in Fig 20. The polishing yields marks with a random orientation. Note that the third order surface structure effectively disappears. Only the aperiodic fourth order structures remain, which are certainly 35
Bielle - - Functional needs, machining conditions, and economics of surface finishing valuable in retaining .ubricant. The roughness spacings correspond approximately to the size of alumina grains of Fig 18. The polished guide behaved very well, but the ground guide reveals the beginnings of tracks made by the needle bearings. It seems that the roughness due to polishing (R = 0.35/Jm) permitted the surface to perform satisfactorily, since the roughness changed after 1500 hours to R = 0.20/~m. The deepest marks (fourth order) with an average value of 0.8pm did not present any drawbacks a priori. It is possible for us to have the guide surfaces regenerated in this manner under a subcontract; as a first estimation this should easily double the running time ofthe guides.
Continued experiments A set of 8 small and 4 large guides was produced by grinding and polishing and put into service to run to failure on one production machine. At 4500 hours, a breakdown of another device caused a halt in the machine and a disassembly of the guides. Upon inspection, they showed no trace of abnormal wear, not a mark. After changing the needle bearings, they exceeded 5000 hours of operation without apparent wear and tear**. Economic implications The price of a forming machine is about FF 400 000. This is depreciated over 3 years from the gross income, which must also cover monthly salaries, commercial expenses, financial expenses, and taxes on the capital. In this type of manufacturing, there is at most a profit margin of 2% for reinvestment. The percentage of the gross absorbed each year by replacing the guides after every 1500 hours of operation and by changing the guides every 5000 hours can now be calculated: Replacing guides every 1500 hours and for a 3 year depreciation period: 7 guide set regrinds 2 new guides due to the necessity to change the set after 3 regenerations by grinding ie FF 7324 x 2 9 disassemblies at FF 400 9 halts in production of 20 hours yielding a loss in the gross income Total loss in the gross income Loss per year
Fig 18 Grit used in polishing the second rolling surface Tangential grinding, befo¢e use
100 After 1500 hours use
100
Fig 19 Profiles of the surface that was tangentially ground. The profiles were measured parallel to the needle bearing axes and perpendicular to the grinding marks. A conventional stylus was used
1 820 14 648 3 600 10 800 30 868 10 289
If by polishing, guide life is increased to 5000 hours: 2 refurbishments 2 disassemblies 2 polishings at FF 83 2 production halts of 20 hours Total loss in the profit margin Loss per year
520 800 166 2400 3886 1295
Income is FF 0.05 per part, and the machine must run for 5000 hours per year, according to the planning estimate; producing 1200 parts per hour. Therefore, the total income per year is FF 300 000. In the first case, the percentage loss in the income is 3.42% and in the second case 0.43%. Results
In such high volume manufacturing, the maximum profit available for reinvestments is about 2%. One can see then that in the first case, the machines operate at a loss of 1.42% (2-3.42%). In the second case, there is a profit margin of 1.57%.
IO0
After 1500 hours use
Fig 20 Profiles of the surface that was rough ground by a subcontractor, then polished
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** Since the original article was published, the moulding machine continued to operate and, in fact, achieved more than 50 000 hours of operation without changing or rejuvenation o f the guides. The needle bearings were changed every 15 000 hours.
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Bielle - - Functional needs, machining conditions, and economics of surface finishing
These studies were not part of a formal research programme but aimed, above all, to exemplify the role of standardization, the importance of a judicious choice of the manufacturing process relevant to the mechanical function of the surface, and the benefit of the effective use of measurement results. When applied to industrial metrology, the guiding objectives of the French standard of surface finish are to some degree achieved. In particular, it appears that: • • • •
no profile parameter can characterize the surface by itself; waviness is a parameter which escapes visual tactile examination; waviness is important in a number of functions; to use high pass filters to eliminate the waviness during the measurement and to avoid taking it into account is dangerous.
The guides were produced using the available processes. It is possible that superfine grinding would have brought identical results. Functional requirements, manufacturing specifications, and financial considerations are the themes which have been chosen for emphasis here. Applying the directives of Point 2.1 of the standard NF E 05.017, 'Determination of Measured Surfaces' (which is summarized as inspection, diagnosis, prognosis), has distinguished: 1. 2. 3. 4.
The functional requirements by systematic testing and operation; The specifications: establishing a set of parameters representative of the function; Manufacturing: establishing a baseof machining techniques that permit reproducibility of the surfaces; Financial considerations: verification of the profitability of measurement operations.
of measurement at FF 50, and we take into account lost time, the cost of transportation, the cost of meetings, the cost of measuring the hardness of the guides, etc, the firm estimates that it saved a minimum of 5% of the gross income. In the present economic situation, the fraction of the profit margin that an organization reserves for earnings or for investment is necessarily limited. Problems with financial consequences resulting from technological problems should never be resolved by empiricism. In this setting, metrology points the way to success for the organization.
Acknowledgements Professor Bielle thanks MM. Affigliati and Knoepfli, the principal technicians, for their contributions to these studies.
References 1 Surface Texture of Products. Regulations. 1 - - General, Terminology, Definitions. Association Francaise de
Normalisation (AFNOR), Norme Francaise (NF) E 05-015, (Paris, 1972) 2 Surface Texture of Products. Regulations. 2 - - Specifications of Surface Texture on Drawings. AFNOR, NF E 05-016, (Paris,
1972) 3 Surface Texture of Products. Regulations. 3 - - Determination of Measured Surfaces. AFNOR, NF E 05-017, (Paris, 1972) 4 Surface Texture of Products. Regulations. 4 - - Economic Aspects. AFNOR, NF E 05-018, (Paris, 1969) 5 Similar terminology for brass composition appears in: Inter-
national Metallic Materials Cross Reference, 2nd edition, Arcuri J.V. and Potts D.L. Eds, (General Electric, Schenectady,
New York, 1983)22.19 6 Similar terminology for steel composition appears in Ross R.B., Metallic Materials Specification Handbook, 3rd edition
(E. & F.N. Spon, London, 1980)479
Conclusion In the case of the second study, there was one hour of measurement work. If we set the cost of an hour
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7 Fahl C.F. Motif combination - - A new approach to surface profile analysis. Wear, 1982,83, 165 8 Scheffer B. Comparaison de Diff6rentes Normes Nationales
(Rdgie Nationale des Usines Renault, Billancourt, 1989) pp 7-28f
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