- Email: [email protected]
computer system describing three-dimensional engineering surfaces
295
Applications
A Stylus/Computer System Describing Three-Dimensional Engineering Surfaces H.E. George, R.F. Babus'Haq, P.W. O'Callaghan and S.D. Probert Department of Applied Energy, School of Mechanical Engineering, CranfieM Institute of Technology, Bedford MK43 OAL, United Kingdom A highly accurate system has been developed for providing (i) three-dimensional assessments of engineering surfaces, (ii) measurements of surface degradations due to wear or other deformation processes, and (iii) predictions of the true area of contact formed between two abutting solid surfaces. It is based on a stylus profile-tracing instrument and incorporates an automatically-controlled parallel-profile digitising stage. A software package has been developed to facilitate the processing, manipulation and visuafisation of the numerical descriptors obtained for the surfaces. Examples of typical quantitative and qualitative full-colour surface representations, which emerge from the system, are presented. Such a surfaceassessment capability has a major role to play in both research and industrial quality-assurance testing.
Keywords: Surface topography, Area of contact, Computeraided design.
Hakim E. George is a graduate in Engineering Science from Exeter University. He is an expert in Computer Technology in Software Engineering, and as such he possesses specialist knowledge in the areas of Image Processing, Parallel Computation, Human Computer Interaction, and Secure System Engineering. Currently, Mr. George is a PhD Research Student at Cranfield Institute of Technology.
Elsevier Computers in Industry 13 (1990) 295-304 0166-3615/90/$3.50 © 1990 - Elsevier Science Publishers B.V.
I. Surface Analysis The surface texture of an engineering component is important because it is affected by the machining process: any changes in the conditions of either the component, tool or machine will influence the texture of the component which is being produced. There is therefore, a need to ..............
Ramiz F. Babus'Haq received his BSc Honours Degree in Mechanical Engineering in 1976 and his MSc in Air conditioning and Refrigeration in 1978 from the University of Technology. He joined Cranfield in 1982, and obtained his PhD. in Applied Energy in 1986. Dr. Babus'Haq is now Research Officer in Thermal Systems Engineering at Cranfield and involved in re~ ~ ~ search projects in the fields of Cooling of Electrical/Electronic Components, C H P / D H C , CAE, Surface Analysis, and Thermal Contact Resistance. He is a chartered Engineer, and author of a text-book and more than fifty technical papers. Paul O'Callaghan is the Reader in Thermal Systems Engineering in the Department of Applied Energy. After a period in industry and at University college, Swansea, he joined Cranfield in 1972. Dr. O'Callaghan is a director of MSc. courses in Energy conservation and organises various industrial short courses. He conducts research and consultancy in energy-related fields, ranging from Industrial Energy Management to the Thermal Analysis of Electronic Packages. He is a Chartered Engineer and the author of three text-books and over hundred technical papers. S . Douglas Probert read Mathematics and Physics at Oxford and Aeronautical Engineering at Cambridge Universities. Since 1972 he has been Professor of Applied Energy in the School of Mechanical Engineering at Cranfield. Prof. Probert is a chartered Engineer, the Editor of the international monthly journal Applied Energy and a member of numerous Institutions and Committees. He is the recipient of several prizes and the author or joint author of more than six hundred technical papers.
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examine surface profiles in detail in order to detect realistically such changes and link them to a particular property (e.g. shape or hardness) of the tool employed [8]. For instance, in the disk drive of a computer, if the local surface roughness exceeds a critical value, the read-write head may contact the surface of the magnetic disk and so damage its memory function [11]. However, in the nuclear industry, it is vital to ensure that good thermal contact ensues between the nuclear-fuel elements and their containers in order to avoid local overheating of the elements, and so relative distortions with respect to the adjacent cladding. An experienced engineer will be able to assess qualitatively, to a surprising degree of accuracy, the finish of a machined surface merely by looking at or feeling it. However, such human sensitivities and dependabilities of the human eyes and fingers are limited and vulnerable to variation due for instance to tiredness, and so industry has equipped itself with more reliable measuring devices. Of the numerous roughness measurement methods available, the simple stylus technique is the most common [1-4]. Many parameters have been used or proposed for characterising quantitatively the distribution of surface asperities (sometimes referred to as "heights"), which may be obtained from a profile trace. However, a single profile will not describe comprehensively and unequivocally the topographical features of a surface. Consequently, to achieve this, it is necessary to record data from an area of the surface [10].
2. Area of Contact The true area of actual contact between two abutting solids is only a small fraction of the nominal area. Its magnitude depends upon the normal load applied to the joint and the effective surface hardness of the softer of the two surfaces involved. It is therefore, to a first approximation, independent of the nominal area of the interface. The accurate estimation of the total true area and assessment of the characteristics of the contact configurations are crucial for studies involving interfacial cohesion, static friction, cold welding, the electrical and thermal conductances of contacts, face seals, lubrication, bearing design, wear, scuffing, and surface damaged [5].
Computers in lndustrv
Depending upon the topographies and properties of the contacting surfaces and the applied loading, surface deformations at the joint may be elastic, plastic, or elastoplastic and complicated by thermoelastic effects. Moreover, because engineering surfaces tend to exhibit waviness in addition to small-scale roughness, the substrata beneath the asperities usually deform elastically, so bringing new clusters of micro-asperities into contact. In contacts between rough surfaces, the numbers of plastic asperity contacts increase as a result of the substrate flattening elastically as loading increases. Although the majority of engineering joints deform elastoplastically, most research investigations have concentrated upon examining the plastic contacts which occur where surface asperities meet [5].
3. Scope of the Present Investigation Progress has been made with respect to the choice of parameters which uniquely describe a surface, but careful consideration must be given to the use of these with respect to each particular application. For instance, certain surface descriptive parameters (such as skewness [Rsk ] and kurtosis [Rku ] of the height distribution) relevant to lubricant retention in bearing surfaces are not in themselves sufficient for predicting the contact's behaviour. This needs a knowledge of other surface descriptors, such as asperity-flank slopes and the radii of curvature of the asperity peaks. Reliable, high-sensitivity equipment for measuring these parameters has been invented, developed and produced, some of it commercially [6]. The aim of the present study is to obtain comprehensive and valid three-dimensional topographical descriptions of engineering surfaces, using a stylus-based measuring system for producing parallel-profile traces. A software package has been developed [7] in order to present the surface topographies to research workers or customers as coloured-isometric and contour maps.
4. The Surfaee-Topography Instrument The system developed is based on a Talysurf 4 ( R a n k - T a y l o r - H o b s o n ) profile-tracing instrument. It incorporates an automatically controlled,
Computers in Industry
H.E. George et al. / 3-D Engineering Surfaces
297
Fig 1. Surface-assessment test rig.
three-dimensional relocating stage, as well as microcomputer-based data-handling and processing facilities. The relocation stage (see Fig. 1) consists of two moving tables, one which moves in a direction parallel to the locked stylus arm, and the other, after completion of a trace, shifts a small predetermined amount parallel to the mean plane of the surface but orthogonal to the arm, thereby enabling parallel tracing to be performed. The former is driven through a pulley arrangement and reduction gearbox by means of a d.c. motor. Slow speeds of traverse (i.e. 1 m m / s or 0.25 m m / s ) can be chosen according to the degree of surface roughness likely to be encountered and a fast-return speed is used in the reverse direction, i.e. after the stylus has been raised off the surface. The straightness accuracy of the slide-way is to
within 1 ~tm over the complete 150 m m range of traverse. An optical encoder, coupled into the reduction gearbox, enables accurate positioning of the traversing table to be achieved. The sampling interval along the surface, has a value of 3.865 ~m or any multiple of this value up to 77.3 ~tm. The indexing table, mounted on the lower traversing table, is driven by a stepper motor. This gives a minimal step-interval, between the parallel traces, of 2.501 ~m + 0.034 ~m over a 25 m m m a x i m u m traverse. The fast return with the stylus above (i.e. not in contact with) the surface is achieved by a motordriven lifting arm connected to a light stirrup placed around the stylus-carrying arm. N o restrictions with respect to the normal operation of the stylus instrument (i.e. single traces and meter cutoff options) have been incorporated into the de-
298
Applications
Computers in Industry
sign. D i s c o n n e c t i o n of the lifting s t i r r u p a n d u n l o c k i n g of the stylus a r m e n a b l e the i n s t r u m e n t to b e used i n its n o r m a l m o d e . A k i n e m a t i c reloca-
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t i o n facility, u s i n g the t h r e e - b a l l t e c h n i q u e , is inc o r p o r a t e d o n top of the t r a v e r s i n g table. R e l o c a tions of this t a b l e a n d the i n d e x i n g t a b l e to w i t h i n
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Computers in Industry
+ 1 ~m are accomplished by being guided by signals presented on the positional display units on the associated electronic-control console. The latter consists of several modules, each interlinked to provide a fully automatic operation cycle. Presetting the required sampling/parallel-step intervals and number of traces, together with the traverse positions at which it is decided that sampling should commence and end, are made on the console. The output from the stylus head is fed directly to a North Star Horizon micro-computer. This contains 48 kilobytes of random access memory, a Z80 micro-processor and dual floppy-discs. There are eight i n p u t / o u t p u t ports on the interface: these are a single digital channel and seven analogue channels. The latter consist of seven eight-bit ADC input channels: records are made on floppy discs. The data handling is software-controlled; the unit possessing a capacity of 16,000 readings per stylus trace and up to 250 ensemble averages. At this stage, some standard surface parameters may be evaluated on the micro-computer, which provides an output to a line printer. To obtain detailed statistical analyses and a qualitative representation of the surface-zone examined in the form of colour isometric and contour plots, the data are transferred via a direct link to an IBM PC-AT micro-computer using a graphics package developed in-house. The sensitivity of this surface-mapping technique is limited by two factors: (i) the finite dimensions of the ultra-lightweight stylus tip prevents penetration into the finer surface valleys; and (ii) the speed of the traverse of the stylus over the surface must be sufficiently slow (less than 1 m m / s ) so as to inhibit the stylus from "bouncing", and thereby indicating false topographies. The mechanical factor (ii) is the overall limitation as to how fast the data collection can be completed; the electronic computing being achieved relatively rapidly [9].
5. Measurements and Predictions 5.1. Surface Parameters
Surface traces obtained during engineeringsurface assessments (such as the examination of die wear; analyses of shot-peened surfaces; inves-
H.E. George et al. / 3-D Engineering Surfaces
299
tigation of the wear of a porous bearing; as well as volume measurement of the craters formed in a surface as a result of solid particle erosion) have been produced by the system in order to provide visual displays of the surface topographies of the specimens under consideration. Various data files have been created in order to be compatible with the surface-analysis programme packages. The standard surface parameters (e.g. average roughness [Ra] ) and special functions (e.g. bearing-ratio curve [tp]) were computed by averaging the profile data taken from the parallel tracings over the surface. The parameters include: root-mean-square roughness [Rq]; maximum peak-to-valley height [Rt]; ISO ten-point height [Rz]; arithmetic mean slope; root-mean-square profile slope [Aq]; average wavelength [)~q]; skewness [Rsk]; kurtosis [Rku]; high-spot count and spacing [S]; mean-peak [Rpk ] and mean-valley [R vk] lengths; maximum asperity-height above the mean-surface plane [Rp]; and maximum depth below the mean-surface plane [R v]. The special functions include; probability distribution and asperity densities; auto-correlation; and powerspectrum density [6]. Surface traces of the considered specimen can be produced before and after it has been subjected to a prescribed loading pattern, in order to identify the permanent deformations that resulted. The relocation of the surface on the measuring stage can be achieved to an accuracy of less than 1 ~tm. This ensures that the representation after the surface treatment is truly that for the same area as was assessed previously, i.e. prior to experiencing the deformation. 5.2. Isometric and Contour Plots
The surface topographies of the specimens can also be represented, in colours, as isometric and contour plots. For instance, the isometric views for a ceramic surface and the eagle's head on a quarter-dollar coin shown in Figs. 2(a) and 3(a) respectively, cover 3.20 mm by 4.95 mm of nominal area, made up from 128 parallel traces at step intervals of 25.01 ~m; each trace contains 128 samples taken at 38.65 ~m increments along the machined surfaces considered in this instance. The various colours of the parallel traces correspond to the height scale indicated. Thus, they aid in highlighting the surface characteristics under consider-
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ation, a n d hence, show clearly any waviness present which is usually greater the larger the sampling intervals employed. Nevertheless, these iso-
metric plots can be sliced, along the tracing, at the surface height required, thus specific characteristics of the surface at a n y level, can be o b t a i n e d
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(see Figs. 4 a n d 5 as typical examples). Moreover, all the isometric plots can be inverted (i.e. valleys will be peaks), so that any h i d d e n areas of the
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surface u n d e r c o n s i d e r a t i o n can be identified (an a p p l i c a t i o n widely used in l u b r i c a n t retention), see Fig. 5.
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Fig. 6. Isometric plots of a stator-core of an electric machine (a) before and (b) after extracting its underlying form (c).
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Fig. 7. The contact between two solid surfaces.
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Also presented in Figs. 2 and 3 are contour plots of the surfaces under consideration (originally in colour). Each plot is computer-enhanced to contain a larger range of colours thereby representing the variation of the surface heights more clearly and therefore facilitating observations of any waviness of the surface. The four presentations display the surface topographies corresponding to the colour lines drawn across the contour plot. These lines can be manoeuvred throughout the plot.
Computers in Industo'
are likely to become increasingly needed in future in terms of three-dimensional surface mapping and automatic recognition of details for the control of the manufacturing process as well as for monitoring the condition of the machine and tool. Future research efforts need to be concentrated upon the examination of thermoelastic behaviour of rectangular bolted or edge-clamped joints encountered in engineering practice (e.g. in electronic packages, rotating electrical machiness, space vehicles, solar collectors and control systems).
5.3. True Areas of Contact Despite meticulous manufacture, even under strictly controlled laboratory conditions, a nonwavy surface is difficult to prepare. Also, few surface topographies in practice are completely random. Moreover, any misalignment to the surfaces under consideration during their scanning, will introduce a low order of form beneath each surface (see Fig. 6). However, this state of waviness must be removed from both surfaces before pressing them together. This can be done by subtracting a two-dimensional polynomial regression from the face of each surface (see Fig. 6). After extracting the underlying form from the three-dimensional images of the two surfaces, they were placed in contact under a specific loading (see Fig. 7). The magnitudes of macroscopic contact areas may be predicted from a knowledge of the contact patterns and the pressure distributions enable plastically deformed microcontact configurations to be quantified.
6. Conclusions The stylus-based profile-tracing instrument employed provides accurate measurements as well as colour representations of the surfaces in three-dimensions. Such a technique facilitates quantifying any surface degradations (which occur due to wear or other deformation processes) as the relocation facility will accurately assess the surface topographies of the same area before and after it suffers any damage. A software package has been developed to analyse the numerical descriptors of the surfaces obtained as well as predicting the real area of contact if these surfaces are pressed together under any specific loading. Such techniques
Acknowledgements The authors wish to thank the Science and Engineering Council, UK, for providing financial support. References [1] R.F. Babus'Haq, S.D. Probert, P.W. O'Callaghan and G.N. Evans, "Peaks and troughs of surface measurements", Prof Eng., Vol. 1, No. 2, 1988, pp. 52-53. [2] L. De Chiffre and H. Strobaek Nielsen, "A digital system for surface roughness analysis of plane and cylindrical parts", Precis. Eng., Vol. 9, No. 2, 1987, pp. 59-64. [3] M. Fripan and H.E. Exner, "'Three-dimensional orientation and roughness of surfaces", Acta Stereol., Vol. 3. No. 2, 1984, pp. 181-186. [4] J. Mignot and C. Gorecki, "Measurement of surface roughness: comparison between a defect-of-focus optical technique and the classical stylus technique", Wear, Vol. 87, 1983, pp. 39-49. [5] P.W. O'Callaghan and S.D. Probert, "Prediction and measurement of true areas of contact between solids", Wear, Vol. 120, No. 1, 1987, pp. 29-49. [6] P.W. O'Callaghan, R.F. Babus'Haq and S.D. Probert, "Surface-topography assessment: precursor to the prediction of pressed-contact behaviour", Trans. Inst. Meas. Control, Vol. 10, No. 4, 1988, pp. 207 217. [7] P.W. O'Callaghan, R.F. Babus'Haq, S.D. Probert and G.N. Evans, "Three-dimensional surface-topography assessments using a stylus/computer system", Int. J. Cornput. Appl. Technol., Vol. 2, No, 2, 1989, pp. 101-107. [8] J. Raja and D.J. Whitehouse, "Applications of complex demodulation techniques to surface analysis", Precis. Eng., Vol. 5, No. l, 1983, pp. 17 21. [9] B. Snaith, M.J. Edmonds and S.D. Probert, "Use of a profilometer for surface mapping", Precis. Eng., Vol. 3, 1981, pp. 87 90. [10] K.J. Stout, "Understanding surface metrology", Surface Finish and Function '87 Meeting, Northampton, England, 1987. [11] T. Tsukada and T. Kanada, "Evaluation of two- and three-dimensional surface roughness profiles and their confidence", Wear, Vol. 109, 1986, pp. 69-78.
Applications
A Stylus/Computer System Describing Three-Dimensional Engineering Surfaces H.E. George, R.F. Babus'Haq, P.W. O'Callaghan and S.D. Probert Department of Applied Energy, School of Mechanical Engineering, CranfieM Institute of Technology, Bedford MK43 OAL, United Kingdom A highly accurate system has been developed for providing (i) three-dimensional assessments of engineering surfaces, (ii) measurements of surface degradations due to wear or other deformation processes, and (iii) predictions of the true area of contact formed between two abutting solid surfaces. It is based on a stylus profile-tracing instrument and incorporates an automatically-controlled parallel-profile digitising stage. A software package has been developed to facilitate the processing, manipulation and visuafisation of the numerical descriptors obtained for the surfaces. Examples of typical quantitative and qualitative full-colour surface representations, which emerge from the system, are presented. Such a surfaceassessment capability has a major role to play in both research and industrial quality-assurance testing.
Keywords: Surface topography, Area of contact, Computeraided design.
Hakim E. George is a graduate in Engineering Science from Exeter University. He is an expert in Computer Technology in Software Engineering, and as such he possesses specialist knowledge in the areas of Image Processing, Parallel Computation, Human Computer Interaction, and Secure System Engineering. Currently, Mr. George is a PhD Research Student at Cranfield Institute of Technology.
Elsevier Computers in Industry 13 (1990) 295-304 0166-3615/90/$3.50 © 1990 - Elsevier Science Publishers B.V.
I. Surface Analysis The surface texture of an engineering component is important because it is affected by the machining process: any changes in the conditions of either the component, tool or machine will influence the texture of the component which is being produced. There is therefore, a need to ..............
Ramiz F. Babus'Haq received his BSc Honours Degree in Mechanical Engineering in 1976 and his MSc in Air conditioning and Refrigeration in 1978 from the University of Technology. He joined Cranfield in 1982, and obtained his PhD. in Applied Energy in 1986. Dr. Babus'Haq is now Research Officer in Thermal Systems Engineering at Cranfield and involved in re~ ~ ~ search projects in the fields of Cooling of Electrical/Electronic Components, C H P / D H C , CAE, Surface Analysis, and Thermal Contact Resistance. He is a chartered Engineer, and author of a text-book and more than fifty technical papers. Paul O'Callaghan is the Reader in Thermal Systems Engineering in the Department of Applied Energy. After a period in industry and at University college, Swansea, he joined Cranfield in 1972. Dr. O'Callaghan is a director of MSc. courses in Energy conservation and organises various industrial short courses. He conducts research and consultancy in energy-related fields, ranging from Industrial Energy Management to the Thermal Analysis of Electronic Packages. He is a Chartered Engineer and the author of three text-books and over hundred technical papers. S . Douglas Probert read Mathematics and Physics at Oxford and Aeronautical Engineering at Cambridge Universities. Since 1972 he has been Professor of Applied Energy in the School of Mechanical Engineering at Cranfield. Prof. Probert is a chartered Engineer, the Editor of the international monthly journal Applied Energy and a member of numerous Institutions and Committees. He is the recipient of several prizes and the author or joint author of more than six hundred technical papers.
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examine surface profiles in detail in order to detect realistically such changes and link them to a particular property (e.g. shape or hardness) of the tool employed [8]. For instance, in the disk drive of a computer, if the local surface roughness exceeds a critical value, the read-write head may contact the surface of the magnetic disk and so damage its memory function [11]. However, in the nuclear industry, it is vital to ensure that good thermal contact ensues between the nuclear-fuel elements and their containers in order to avoid local overheating of the elements, and so relative distortions with respect to the adjacent cladding. An experienced engineer will be able to assess qualitatively, to a surprising degree of accuracy, the finish of a machined surface merely by looking at or feeling it. However, such human sensitivities and dependabilities of the human eyes and fingers are limited and vulnerable to variation due for instance to tiredness, and so industry has equipped itself with more reliable measuring devices. Of the numerous roughness measurement methods available, the simple stylus technique is the most common [1-4]. Many parameters have been used or proposed for characterising quantitatively the distribution of surface asperities (sometimes referred to as "heights"), which may be obtained from a profile trace. However, a single profile will not describe comprehensively and unequivocally the topographical features of a surface. Consequently, to achieve this, it is necessary to record data from an area of the surface [10].
2. Area of Contact The true area of actual contact between two abutting solids is only a small fraction of the nominal area. Its magnitude depends upon the normal load applied to the joint and the effective surface hardness of the softer of the two surfaces involved. It is therefore, to a first approximation, independent of the nominal area of the interface. The accurate estimation of the total true area and assessment of the characteristics of the contact configurations are crucial for studies involving interfacial cohesion, static friction, cold welding, the electrical and thermal conductances of contacts, face seals, lubrication, bearing design, wear, scuffing, and surface damaged [5].
Computers in lndustrv
Depending upon the topographies and properties of the contacting surfaces and the applied loading, surface deformations at the joint may be elastic, plastic, or elastoplastic and complicated by thermoelastic effects. Moreover, because engineering surfaces tend to exhibit waviness in addition to small-scale roughness, the substrata beneath the asperities usually deform elastically, so bringing new clusters of micro-asperities into contact. In contacts between rough surfaces, the numbers of plastic asperity contacts increase as a result of the substrate flattening elastically as loading increases. Although the majority of engineering joints deform elastoplastically, most research investigations have concentrated upon examining the plastic contacts which occur where surface asperities meet [5].
3. Scope of the Present Investigation Progress has been made with respect to the choice of parameters which uniquely describe a surface, but careful consideration must be given to the use of these with respect to each particular application. For instance, certain surface descriptive parameters (such as skewness [Rsk ] and kurtosis [Rku ] of the height distribution) relevant to lubricant retention in bearing surfaces are not in themselves sufficient for predicting the contact's behaviour. This needs a knowledge of other surface descriptors, such as asperity-flank slopes and the radii of curvature of the asperity peaks. Reliable, high-sensitivity equipment for measuring these parameters has been invented, developed and produced, some of it commercially [6]. The aim of the present study is to obtain comprehensive and valid three-dimensional topographical descriptions of engineering surfaces, using a stylus-based measuring system for producing parallel-profile traces. A software package has been developed [7] in order to present the surface topographies to research workers or customers as coloured-isometric and contour maps.
4. The Surfaee-Topography Instrument The system developed is based on a Talysurf 4 ( R a n k - T a y l o r - H o b s o n ) profile-tracing instrument. It incorporates an automatically controlled,
Computers in Industry
H.E. George et al. / 3-D Engineering Surfaces
297
Fig 1. Surface-assessment test rig.
three-dimensional relocating stage, as well as microcomputer-based data-handling and processing facilities. The relocation stage (see Fig. 1) consists of two moving tables, one which moves in a direction parallel to the locked stylus arm, and the other, after completion of a trace, shifts a small predetermined amount parallel to the mean plane of the surface but orthogonal to the arm, thereby enabling parallel tracing to be performed. The former is driven through a pulley arrangement and reduction gearbox by means of a d.c. motor. Slow speeds of traverse (i.e. 1 m m / s or 0.25 m m / s ) can be chosen according to the degree of surface roughness likely to be encountered and a fast-return speed is used in the reverse direction, i.e. after the stylus has been raised off the surface. The straightness accuracy of the slide-way is to
within 1 ~tm over the complete 150 m m range of traverse. An optical encoder, coupled into the reduction gearbox, enables accurate positioning of the traversing table to be achieved. The sampling interval along the surface, has a value of 3.865 ~m or any multiple of this value up to 77.3 ~tm. The indexing table, mounted on the lower traversing table, is driven by a stepper motor. This gives a minimal step-interval, between the parallel traces, of 2.501 ~m + 0.034 ~m over a 25 m m m a x i m u m traverse. The fast return with the stylus above (i.e. not in contact with) the surface is achieved by a motordriven lifting arm connected to a light stirrup placed around the stylus-carrying arm. N o restrictions with respect to the normal operation of the stylus instrument (i.e. single traces and meter cutoff options) have been incorporated into the de-
298
Applications
Computers in Industry
sign. D i s c o n n e c t i o n of the lifting s t i r r u p a n d u n l o c k i n g of the stylus a r m e n a b l e the i n s t r u m e n t to b e used i n its n o r m a l m o d e . A k i n e m a t i c reloca-
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t i o n facility, u s i n g the t h r e e - b a l l t e c h n i q u e , is inc o r p o r a t e d o n top of the t r a v e r s i n g table. R e l o c a tions of this t a b l e a n d the i n d e x i n g t a b l e to w i t h i n
981
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Z78I
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F i le=b ×.S :+ Y . S =+ tJ.S =+ W.B : +
0 ~.38
li Min :
-31.2SpM
Mag :
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SBB. OX
Max :
1-
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Computers in Industry
+ 1 ~m are accomplished by being guided by signals presented on the positional display units on the associated electronic-control console. The latter consists of several modules, each interlinked to provide a fully automatic operation cycle. Presetting the required sampling/parallel-step intervals and number of traces, together with the traverse positions at which it is decided that sampling should commence and end, are made on the console. The output from the stylus head is fed directly to a North Star Horizon micro-computer. This contains 48 kilobytes of random access memory, a Z80 micro-processor and dual floppy-discs. There are eight i n p u t / o u t p u t ports on the interface: these are a single digital channel and seven analogue channels. The latter consist of seven eight-bit ADC input channels: records are made on floppy discs. The data handling is software-controlled; the unit possessing a capacity of 16,000 readings per stylus trace and up to 250 ensemble averages. At this stage, some standard surface parameters may be evaluated on the micro-computer, which provides an output to a line printer. To obtain detailed statistical analyses and a qualitative representation of the surface-zone examined in the form of colour isometric and contour plots, the data are transferred via a direct link to an IBM PC-AT micro-computer using a graphics package developed in-house. The sensitivity of this surface-mapping technique is limited by two factors: (i) the finite dimensions of the ultra-lightweight stylus tip prevents penetration into the finer surface valleys; and (ii) the speed of the traverse of the stylus over the surface must be sufficiently slow (less than 1 m m / s ) so as to inhibit the stylus from "bouncing", and thereby indicating false topographies. The mechanical factor (ii) is the overall limitation as to how fast the data collection can be completed; the electronic computing being achieved relatively rapidly [9].
5. Measurements and Predictions 5.1. Surface Parameters
Surface traces obtained during engineeringsurface assessments (such as the examination of die wear; analyses of shot-peened surfaces; inves-
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tigation of the wear of a porous bearing; as well as volume measurement of the craters formed in a surface as a result of solid particle erosion) have been produced by the system in order to provide visual displays of the surface topographies of the specimens under consideration. Various data files have been created in order to be compatible with the surface-analysis programme packages. The standard surface parameters (e.g. average roughness [Ra] ) and special functions (e.g. bearing-ratio curve [tp]) were computed by averaging the profile data taken from the parallel tracings over the surface. The parameters include: root-mean-square roughness [Rq]; maximum peak-to-valley height [Rt]; ISO ten-point height [Rz]; arithmetic mean slope; root-mean-square profile slope [Aq]; average wavelength [)~q]; skewness [Rsk]; kurtosis [Rku]; high-spot count and spacing [S]; mean-peak [Rpk ] and mean-valley [R vk] lengths; maximum asperity-height above the mean-surface plane [Rp]; and maximum depth below the mean-surface plane [R v]. The special functions include; probability distribution and asperity densities; auto-correlation; and powerspectrum density [6]. Surface traces of the considered specimen can be produced before and after it has been subjected to a prescribed loading pattern, in order to identify the permanent deformations that resulted. The relocation of the surface on the measuring stage can be achieved to an accuracy of less than 1 ~tm. This ensures that the representation after the surface treatment is truly that for the same area as was assessed previously, i.e. prior to experiencing the deformation. 5.2. Isometric and Contour Plots
The surface topographies of the specimens can also be represented, in colours, as isometric and contour plots. For instance, the isometric views for a ceramic surface and the eagle's head on a quarter-dollar coin shown in Figs. 2(a) and 3(a) respectively, cover 3.20 mm by 4.95 mm of nominal area, made up from 128 parallel traces at step intervals of 25.01 ~m; each trace contains 128 samples taken at 38.65 ~m increments along the machined surfaces considered in this instance. The various colours of the parallel traces correspond to the height scale indicated. Thus, they aid in highlighting the surface characteristics under consider-
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Computers in Industry
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Fig. 6. Isometric plots of a stator-core of an electric machine (a) before and (b) after extracting its underlying form (c).
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Fig. 7. The contact between two solid surfaces.
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Also presented in Figs. 2 and 3 are contour plots of the surfaces under consideration (originally in colour). Each plot is computer-enhanced to contain a larger range of colours thereby representing the variation of the surface heights more clearly and therefore facilitating observations of any waviness of the surface. The four presentations display the surface topographies corresponding to the colour lines drawn across the contour plot. These lines can be manoeuvred throughout the plot.
Computers in Industo'
are likely to become increasingly needed in future in terms of three-dimensional surface mapping and automatic recognition of details for the control of the manufacturing process as well as for monitoring the condition of the machine and tool. Future research efforts need to be concentrated upon the examination of thermoelastic behaviour of rectangular bolted or edge-clamped joints encountered in engineering practice (e.g. in electronic packages, rotating electrical machiness, space vehicles, solar collectors and control systems).
5.3. True Areas of Contact Despite meticulous manufacture, even under strictly controlled laboratory conditions, a nonwavy surface is difficult to prepare. Also, few surface topographies in practice are completely random. Moreover, any misalignment to the surfaces under consideration during their scanning, will introduce a low order of form beneath each surface (see Fig. 6). However, this state of waviness must be removed from both surfaces before pressing them together. This can be done by subtracting a two-dimensional polynomial regression from the face of each surface (see Fig. 6). After extracting the underlying form from the three-dimensional images of the two surfaces, they were placed in contact under a specific loading (see Fig. 7). The magnitudes of macroscopic contact areas may be predicted from a knowledge of the contact patterns and the pressure distributions enable plastically deformed microcontact configurations to be quantified.
6. Conclusions The stylus-based profile-tracing instrument employed provides accurate measurements as well as colour representations of the surfaces in three-dimensions. Such a technique facilitates quantifying any surface degradations (which occur due to wear or other deformation processes) as the relocation facility will accurately assess the surface topographies of the same area before and after it suffers any damage. A software package has been developed to analyse the numerical descriptors of the surfaces obtained as well as predicting the real area of contact if these surfaces are pressed together under any specific loading. Such techniques
Acknowledgements The authors wish to thank the Science and Engineering Council, UK, for providing financial support. References [1] R.F. Babus'Haq, S.D. Probert, P.W. O'Callaghan and G.N. Evans, "Peaks and troughs of surface measurements", Prof Eng., Vol. 1, No. 2, 1988, pp. 52-53. [2] L. De Chiffre and H. Strobaek Nielsen, "A digital system for surface roughness analysis of plane and cylindrical parts", Precis. Eng., Vol. 9, No. 2, 1987, pp. 59-64. [3] M. Fripan and H.E. Exner, "'Three-dimensional orientation and roughness of surfaces", Acta Stereol., Vol. 3. No. 2, 1984, pp. 181-186. [4] J. Mignot and C. Gorecki, "Measurement of surface roughness: comparison between a defect-of-focus optical technique and the classical stylus technique", Wear, Vol. 87, 1983, pp. 39-49. [5] P.W. O'Callaghan and S.D. Probert, "Prediction and measurement of true areas of contact between solids", Wear, Vol. 120, No. 1, 1987, pp. 29-49. [6] P.W. O'Callaghan, R.F. Babus'Haq and S.D. Probert, "Surface-topography assessment: precursor to the prediction of pressed-contact behaviour", Trans. Inst. Meas. Control, Vol. 10, No. 4, 1988, pp. 207 217. [7] P.W. O'Callaghan, R.F. Babus'Haq, S.D. Probert and G.N. Evans, "Three-dimensional surface-topography assessments using a stylus/computer system", Int. J. Cornput. Appl. Technol., Vol. 2, No, 2, 1989, pp. 101-107. [8] J. Raja and D.J. Whitehouse, "Applications of complex demodulation techniques to surface analysis", Precis. Eng., Vol. 5, No. l, 1983, pp. 17 21. [9] B. Snaith, M.J. Edmonds and S.D. Probert, "Use of a profilometer for surface mapping", Precis. Eng., Vol. 3, 1981, pp. 87 90. [10] K.J. Stout, "Understanding surface metrology", Surface Finish and Function '87 Meeting, Northampton, England, 1987. [11] T. Tsukada and T. Kanada, "Evaluation of two- and three-dimensional surface roughness profiles and their confidence", Wear, Vol. 109, 1986, pp. 69-78.