Measurement 44 (2011) 611–619

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Measurement

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Investigations on machined metal surfaces through the stylus typeand optical 3D instruments and their mathematical modeling with thehelp of statistical techniques

P. Demircioglu a, M.N. Durakbasa b,⇑a Adnan Menderes University, Faculty of Engineering, Department of Mechanical Engineering, 09010 Aydin, Turkeyb Vienna University of Technology, Institute for Production Engineering and Laser Technology, Department of InterchangeableManufacturing and Industrial Metrology, Karlsplatz 13/311, 1040 Wien, Vienna, Austria

a r t i c l e i n f o a b s t r a c t

Article history:Received 10 November 2009Accepted 14 December 2010Available online 19 January 2011

Keywords:Surface metrologySurface roughnessComparative studyStylus and 3D optical methodsStatistical analysis

0263-2241/$ - see front matter � 2010 Elsevier Ltddoi:10.1016/j.measurement.2010.12.001

⇑ Corresponding author.E-mail address: durakbasa@mail.ift.tuwien.ac.at

The measurement of roughness on machined metal surfaces is of considerable importanceto manufacturing industries as the roughness of a surface has a significant influence on itsquality and function of products. In this paper, an experimental approach for surfaceroughness measurement has been based on the comparison of roughness values takenfrom the stylus and optical type instruments on the machined metal surfaces (turning,grinding and milling) is presented.

Following this experimental study, all measured surface roughness parameters havebeen analyzed by using Statistical Package for Social Science (SPSS 15.0) statistically andmathematical models for the two most important and commonly used roughness param-eters Ra and Rz have been developed so that Ra = Ra (F, P, C) and Rz = Rz (F, P, C, M), whereas Fexpresses feed, P periodicity, C contrast and M the type of material. The statistical resultsfrom numerous tests showed that there has been a correlation between the surface rough-ness and the properties of the surface topography and there have been slight differencesamong three measurement instruments on machined metal surfaces in this experimentalstudy.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Surface roughness plays a vital role in determining thedesired quality of a machined metal surface for today’sengineering industry. The quality of assessment of engi-neering surfaces with respect to their functional and opti-cal properties for different loading conditions is influencedby roughness parameters characterizing basically the sur-face microtopography [1]. It is traditionally defined bytwo parameters: arithmetical mean deviation of the as-sessed profile Ra and average maximum height of assessedprofile Rz as they are one of the most commonly used andaccepted by researchers and in industry as well. Surface

. All rights reserved.

(M.N. Durakbasa).

roughness inspection is one of the essential quality controlprocesses carried out to ensure that manufactured partsconform to specified standards [2].

The surface parameter used to evaluate surface rough-ness in this experimental study is the roughness average(Ra), the most widely used parameter for surface texture.The roughness average is the area between the roughnessprofile and its central line, or the integral of the absolutevalue of the roughness profile height over the samplinglength. Determination of Ra is normally computed by thesoftware but can be derived using the following formula:

Ra ¼1lr

Z lr

0zðxÞj jdx ð1Þ

where is z(x) is the profile deviation from the mean lineand lr is the sampling length.

612 P. Demircioglu, M.N. Durakbasa / Measurement 44 (2011) 611–619

The more common parameter for roughness is maxi-mum height of profile (Rz). Rz is calculated by measuringthe vertical distance from the highest peak to the lowestvalley within five sampling lengths, then averaging thesedistances. Rz averages only the five highest peaks and thefive deepest valleys – therefore extremes have a muchgreater influence on the final value.

The goal of this research work is to obtain mathematicalmodels of Ra and Rz estimating the coefficients of the linearequation, involving a few independent variables (feed inmm, periodicity, type of material, contrasting, type of pro-duction process, etc.) with an analysis of variance (ANOVA)and regression analysis.

2. Surface topography techniques for the comparativestudy

Conventionally, surface roughness measurement hasbeen performed by using a stylus instrument [3,4]. Whena stylus traverses a surface, the vertical motion of the sty-lus is converted by way of a pick-up into an electrical sig-nal. The pick-up is generally a linear variable differentialtransducer (LVDT). The electric signal is amplified and pro-cessed or converted into a digital signal via an A/D con-verter and then analysed using a computer. A schematicdiagram of such a system is shown in Fig. 1.

In the diagram shown, the stylus is held stationarywhile the specimen surface is moved in a raster scan usingprecision X, Y-tables. The movement of the table is con-trolled via a computer, allowing numerous combinationsof area size and data sample spacing to be selected [6].

The stylus measurement method is a contact type, themain drawback of which is that the loaded stylus can dam-age or scratch the surface being measured, especially onsoft surfaces [7]. The transducer and stylus tips are oftenfragile, hence the instrument must be applied in a fairlyvibration free environment. Consequently, this direct con-

Fig. 1. Schematic diagram illustrating the major constituents of

tact measurement method is not suitable to be used on atest object undergoing a machining process simulta-neously [8].

These two main drawbacks of the stylus-based surfacemeasurement technique make it necessary to developnon-contact optical methods that can be used for in-pro-cess measurement and the measurement of soft surfaces[9]. Stylus and optical type profilometers are, in a sense,different, that is; while traditional stylus method is usedfor height information, the optical method refers to areal.Optical technique as a complement to the stylus instru-ment, a non-destructive and non-contact method, appearsto be a suitable alternative for carrying out measurementof surface quality including surface roughness [10].

New breakthroughs by the instrumentations have beenmade in recent years, to establish high-tech instrumentswhich can acquire a 3D surface structure of the preciselymachined surfaces to fulfill the requirements for the appli-cation in industrial environment. In this experimentalstudy, both surface measuring systems will be examinedwith great many practical applications [11].

The measurements of optical systems were carried outby two different instruments in this experimental study.One is a new non-contact optical surface characterizationtechnique called focus variation used by the infinite focusmicroscope (IFM) to build true color 3D images of surfacesand microscopic structures. Its operating principle com-bines the small depth of focus of an optical system withvertical scanning to provide topographical and color infor-mation from the variation of focus. The system deliversdense measurements over large areas with a density of 2Mio – 25 Mio measurement points and a high vertical res-olution up to 20 nm [12].

This non-destructive method utilizes coaxial white lightwhich is provided by a light source delivered through abeam splitter to a series of selectable, infinity-corrected,high-Numerical Aperture (N.A.) objectives contained in a

a stylus-type of surface texture measuring instrument [5].

Fig. 2. Schematic visualization of the focus-variation technology [12].

Fig. 3. Schematic representation of the principle of a scanning typeconfocal laser microscope [13].

P. Demircioglu, M.N. Durakbasa / Measurement 44 (2011) 611–619 613

six-place nosepiece. The specimen’s reflected light is pro-jected through the beam splitter onto a color digital sensor(Fig. 2).

The other one is a scanning type confocal laser micro-scope with high resolution, high contrast and drasticallyenhanced resolution in light axis direction via confocal op-tics. Confocal optics are designed to have almost infinitesmall depth of focus. That’s why, not only the variation offocus, but also the absolute maximum value is analysed[10]. The scanning type confocal laser microscope targetslaser beam at a very small spot with objective lens andscans over the specimen in X–Y directions. It then capturesa light from specimen with detector and outputs the imageof specimen on monitor [13]. A schematic diagram of sucha system is illustrated in Fig. 3.

In this experimental study, measurements by stylustype instrument were performed with Form Talysurf Intra50, which is skidless and can be used for waviness, profileand other parameters such as material ratio with absoluteconfidence in the measurement results and measurementsby optical 3D surface measurement devices, the infinitefocus microscope and the confocal laser scanningmicroscope.

3. Conditions of experiment

In this experimental study, the measurements weremade with commercially available Form Talysurf Intra 50by Taylor Hobson GmbH for the tactile surface evaluationwith a high resolution, in 1.0 mm range 16 nm. By its soft-ware ‘‘ultra’’ it is possible to analyze and monitor opera-tions. It was used 60 mm stylus arm length, 2 lm radiusconisphere diamond stylus tip size, 1 mN force(speed = 1 mm/s) and Gaussian filter in all measurementsby the stylus instrument [2,14,15]. Each measurement

was redone six times. During the process of measuring,the cutoff length was taken 0.8 mm and the samplinglength 4 mm according to the ISO standards. The analyseswere then performed for all specimens for several rough-ness parameters which are Ra and Rz parameters givingus much of the idea [16,17]. Altogether 90 experimentswere conducted in order to allow performing ANOVA andregression analysis using SPSS 15, capable of predictingprecise surface roughness. The measurements were takenwith commercially available instruments.

4. Experimental work

The precise measurement and evaluation processes ofthis research work have been carried out in Vienna Univer-sity of Technology, Interchangeable Manufacturing andIndustrial Metrology, Nanometrology Laboratory. Theexperiments were done by preparing flat specimens withdifferent machining processes such as turning, grindingand milling. Fifteen flat samples with periodic and randomprofiles of different roughness value classes, having shinyand browned surfaces were used in this experimentalwork. The samples were classified into two groups as peri-odic and random surface profiles in this research work. Thereason for having two different groups of profiles is thatthe profile is an important concern when compared the de-vices. Six consecutive measurements were taken for eachcondition. The same alignment system was applied for allspecimens. Obtained results from both, stylus and opticalsurface measuring instruments for the surface evaluationunder laboratory conditions are presented in this paper[10].

614 P. Demircioglu, M.N. Durakbasa / Measurement 44 (2011) 611–619

4.1. Specimens with periodic surface profiles

Three face turning aluminum shiny specimens, threeface turning steel shiny samples and three face turningsteel browned workpieces with periodic surface profileswere measured by three instruments. Comparability forperiodic surfaces will be verified as a result of the experi-

Fig. 4. Face turning steel shiny sample 1 [10].

Fig. 5. Diagrams of roughness profile belonging to face turning steel shiny samp(on the right), the infinite focus microscope (in the middle) and the confocal las

Face Turning Steel Shiny Sam

Rou

ghne

ss V

alue

s (µ

m)

0,000

1,000

2,000

3,000

4,000

5,000

6,000

7,000

Face Turning Steel Shiny Sa

Rou

ghne

ss v

alue

s (

m)

210215220225230235240245250255260

µ

1 (o1

)

2 (o1

)

3 (o1

)

4 (o1

)

5 (o1

)

6 (o1

)

Mea

n (o1

)1 (

c) 2 (

c)3 (

c) 4 (

c) 5 (

c)

1 (o1

)

3 (o1

)

5 (o1

)

Mea

n (o1

)2 (

c)4 (

c )6 (

c)

1

Fig. 6. Comparisons of the roughness values belonging to face turning steel shsystems in terms of the parameter RSm, Ra and Rz [10].

ments with nine specimens. By using the same alignmentsystem, six successive measurements were made on ninesamples processed with face turning by means of threeinstruments. Face turning steel shiny sample 1 has periodicsurface profiles as shown in Fig. 4. The diagrams of rough-ness profile belonging to the sample taken from the stylustype device and two optical instruments were obtained asshown in Fig. 5.

Comparisons of the roughness values belonging to theflat specimens taken from the tactile system and two opti-cal systems are shown in Fig. 6, where o1 is the abbrevia-tion of the infinite focus microscope, c is the abbreviationof stylus instrument and o2 is the abbreviation of the con-focal laser scanning microscope. The flat specimens havingperiodic surface profiles have also been evaluated in termsof RSm as well as Ra and Rz parameters according to ISO4288: 1998 [2].

For this sample, it was taken comparable results formean values of Ra parameters. (For the stylus system,

le 1 respectively obtained them from the contact stylus measuring systemer scanning microscope (on the left) [10].

ple 1

Ra

Rz

mple 1

RSm (µm)

6 (c)

Mea

n (c)

1 (o2

)

2 (o2

)

3 (o2

)

4 (o2

)

5 (o2

)

6 (o2

)

Mea

n (o2

)

(o2)

3 (o2

)

5 (o2

)

Mea

n (o2

)

iny sample 1 taken from the contact stylus and two optical measuring

P. Demircioglu, M.N. Durakbasa / Measurement 44 (2011) 611–619 615

1.031 lm; for the infinite focus microscope, 1.113 lm; forthe confocal laser scanning microscope, 1.111 lm were ta-ken). The standard deviation of the data belonging to theface turning steel shiny sample 1 is also similar. It was alsotaken compatible results for RSm and Rz values according tothe ISO standards.

4.2. Specimens with random surface profiles

Two surface grinding steel browned samples, twoperipheral milling steel browned samples and two frontmilling steel browned samples with random profiles were

Fig. 7. Peripheral milling steel browned sample 1 [10].

Fig. 8. Diagrams of roughness profile belonging to peripheral milling steel brownsystem (a), the infinite focus microscope (b) and the confocal laser scanning mi

Peripheral Milling Steel Br

Rou

ghne

ss V

alue

s (µ

m)

0,000

1,000

2,000

3,000

4,000

5,000

6,000

7,000

1 (o1

)

2 (o1

)

3 (o1

)

4 (o1

)

5 (o1

)

6 (o1

)

Mea

n (o1

)1 (

c) 2 (

c)3 (

c) 4 (

c) 5 (

c)6

Fig. 9. Comparisons of the roughness values belonging to peripheral millingmeasuring systems in terms of the parameter Ra and Rz [10].

used in order to strengthen this study. In this group of pro-files there will be couples with roughness class differenceof one. Comparability for six specimens with random sur-faces will be verified as a result of the experiments. Thediagrams of roughness profile and values belonging toperipheral milling steel browned sample 1 (Fig. 7) takenfrom both systems were obtained as shown in Fig. 8. Com-parisons of roughness values taken from both measure-ment systems in terms of parameters Ra and Rz are givenin Fig. 9.

For the peripheral milling steel browned sample 1, itwas taken comparable results for mean values of Ra param-eters (For the stylus system, 1.031 lm; for the infinite fo-cus microscope, 1.113 lm; for the confocal laser scanningmicroscope, 1.111 lm were taken.). It was also taken com-patible results for Rz values according to the ISO standards.

5. Experimental results

During this experimental work, all measured surfaceroughness parameters have been analysed by using SPSS15 statistically. Data for each test were statistically ana-lyzed by the help of regression analysis, which is the mostpopular empirical methods. The regression analysis deter-mines which factors and interactions are significant.

ed sample 1 respectively obtained them from the contact stylus measuringcroscope (c) [10].

owned Sample 1

Ra

Rz

(c)

Mea

n (c)

1 (o2

)

2 (o2

)

3 (o2

)

4 (o2

)

5 (o2

)

6 (o2

)

Mea

n (o2

)

steel browned sample 1 taken from the contact stylus and two optical

616 P. Demircioglu, M.N. Durakbasa / Measurement 44 (2011) 611–619

A One-way Analysis of variance (Oneway ANOVA) hasalso been used (a = 0.05) to test the significant differencebetween measurement systems. When the Oneway ANO-VA has been applied so as to test the equality of threeinstruments at one time by using variances (feed in mm,periodicity, type of material, contrasting, type of produc-tion process, etc.), a comparison of them was done employ-ing a Post-Hoc test to identify which groups weresignificantly different from others assuming a 95% of con-fidence level. These tests showed that there has been aslight difference between the contact stylus surface mea-surement system and two non-contact optical measure-ment systems on engineering surfaces statistically.

In Figs. 10 and 11, the distribution curves (FrequencyHistograms) of the descriptive statistical results in termsof Ra and Rz parameters were presented.

Finally, ‘‘Linear Regression’’ of Ra and Rz parameters esti-mated the coefficients of the linear equation, involving a few

Fig. 10. The distribution curves (Frequency Histograms) of the descriptive statistisample 1, face turning steel shiny sample 1 and face turning steel browned sam

Fig. 11. The distribution curves (Frequency Histograms) of the descriptive statbrowned sample 1, peripheral milling steel browned sample 2, front milling steand Rz parameters.

independent variables (feed in mm, periodicity, type ofmaterial, contrasting, type of production process, etc.), thatbest predicted the value of the dependent variable. Andtwo mathematical models giving the values of Ra and Rz

roughness parameters have been established in terms offeed in mm, the periodicity, contrasting and type of materialas a result of the statistical analyses as shown below.

According to Table 1, Ra depends on periodicity, thetype of material, contrast, the type of production processes,feed and the type of machine as its independent variablesbut feed, periodicity and contrast predicted the value of theRa parameter in a best way. The linear equation of Ra

parameter is given below. In Fig. 12, the distribution curvesof Ra with descriptive statistic values for measurementdata were presented.

Ra ¼ �1:314þ 9:097 � F þ 1:279 � P � 0:188 � C ð2Þ

whereas F feed, P periodicity and C contrasting.

cal results belonging to the measurements of face turning aluminum shinyple 1 in terms of Ra and Rz parameters.

istical results belonging to the measurements of peripheral milling steelel browned sample 1, front milling steel browned sample 2 in terms of Ra

P. Demircioglu, M.N. Durakbasa / Measurement 44 (2011) 611–619 617

According to Table 2, Rz depends on periodicity, the typeof material, contrast, the type of production processes, feedand the type of machine as its independent variables butfeed, periodicity, contrast and the type of material pre-dicted the value of the Ra parameter in a best way. The lin-ear equation of Rz parameter is given below. In Fig. 13, thedistribution curves of Rz with descriptive statistic valuesfor measurement data were presented.

Rz¼�3:258þ38:059�Fþ4:495�Pþ0:906�M�1:500�C ð3Þ

whereas F feed, P periodicity, C contrasting and M type ofmaterial.

Table 1The coefficients of the linear equation, involving a few independent variables (feeetc.), that best predicted the value of the dependent variable Ra.

Model Unstandardized coeffic

B Std.

1 (Constant) �1.314 0.24Periodic surface profiles 1.279 0.28The type of material 0.059 0.08Contrast �0.188 0.08The type of production processes �0.024 0.10Feed 9.097 0.12The type of machine �0.025 0.03

Fig. 12. The distribution curves of Ra with descriptiv

Table 2The coefficients of the linear equation, involving a few independent variables (feeetc.), that best predicted the value of the dependent variable Rz.

Model Unstandardized coeffici

B Std. e

1 (Constant) �3.258 .960Periodic surface profiles 4.495 1.113The type of material .906 .342Contrast �1.500 .342The type of production processes .067 .419Feed 38.059 .487The type of machine �.051 .142

Dependent variable: Rz.

The measurement results for 15 flat specimens werevery consistent with each other. This showed that therewas an inner accuracy of the three measurement systems.Furthermore, it could be seen that measured values werecomparable and the differences between the methodswere small. The measurement results taken from the tac-tile instrument in terms of Ra parameters turned out tobe extremely well-matched with those taken from twooptical instruments, because Ra value refers to a mean va-lue. However Rz value refers to the height of a profile,between the minimum and maximum points of the profile.Rz is calculated by measuring the vertical distance from the

d in mm, periodicity, type of material, contrast, type of production process,

ients Standardized coefficients T Sig.

Error Beta

4 �5.396 0.0003 0.234 4.525 0.0007 0.010 0.680 0.4977 �0.037 �2.166 0.0316 �0.011 �0.227 0.8214 1.097 73,633 0.0006 �0.008 �0.687 0.493

e statistic values for measurement data [10].

d in mm, periodicity, type of material, contrast, type of production process,

ents Standardized coefficients T Sig.

rror Beta B Std. error

�3.395 .001.194 4.038 .000.036 2.648 .009�.070 �4.386 .000.007 .159 .8741.082 78.203 .000�.004 �.357 .721

Fig. 13. The distribution curves of Rz with descriptive statistic values for measurement data [10].

0

1

2

3

4

5

6

7

8

9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

The no of measured samples with flat surfaces

Rou

ghne

ss v

alue

s (µ

m)

SaRa

Fig. 14. Comparisons of Ra and Sa parameters belonging to the samples having flat surfaces taken from the contact stylus measurement system and twonon-contact optical systems [10].

618 P. Demircioglu, M.N. Durakbasa / Measurement 44 (2011) 611–619

highest peak to the lowest valley within the samplinglengths, then averaging these distances. Rz averages onlythe few highest peaks and the deepest valleys, thereforeextremes have a greater influence on the final value. Thisis the reason why the measurement results of the samplestaken from the non-contact instruments in terms of Rz

parameters came out slightly higher than the contactinstrument. The differences between three measuringinstruments were small because these 15 flat samples haveordinary engineering surfaces.

In this study, the 3D surface parameters were also mea-sured. For evaluation of surface roughness as the mostpopular 3D surface parameter Sa was chosen for compari-son. Comparisons of Ra and Sa parameters belonging tothe samples having flat surfaces taken from the contactstylus measurement system and two non-contact opticalsystems are shown in Fig. 14. As it can be seen from figure,measurement results for Sa turned out to be quite similarto those for Ra parameter, because Ra is the 2D counterpartof the 3D descriptor Sa. Both Ra and Sa reflect the arithmeticmean of the absolute values of the surface point departuresfrom the mean plane within the sampling area [18].

6. Conclusion

This paper is an experimental study of the roughnessanalyses of the conventionally machined samples with flatsurfaces in order to compare the data obtained from thecontact stylus measurement device with two non-contactoptical surface measurement instruments and the capabil-ities of three measurement systems were noted in terms oftheir similarities and differences. The major disadvantageof using a stylus instrument is that it requires direct phys-ical contact, which limits the measuring speed. In addition,the instrument readings are based on a limited number ofline samplings, which may not represent the real charac-teristics of the surface. This kind of deviation may causeserious errors in the surface quality assessment especiallywhen the surface profile is periodic.

It is observed that three devices are giving comparableresults if the surface has a good reflection value, is not veryfine machined surface with a periodic profile and notruined or scratched. According to the presumptions, theproblem with the fine machined samples having periodicprofiles is that contact system can not detect the extreme

P. Demircioglu, M.N. Durakbasa / Measurement 44 (2011) 611–619 619

values of the surface and give a result due to general fineprofile. But optical systems can detect precise value ofthe profile so that the results are getting scattered. At thispoint, when the inadequacy of stylus was compared to alight beam, it has been noted that stylus was inefficient be-cause of its geometrical form. Of course no man buildingstylus can reach the thickness of a light beam in today’stechnology.

The next step of this research work is to compare spher-ical specimens to understand how the contact stylus andthe optical measurement instruments react to sphericalsamples in terms of their geometrical shapes. That is, con-tact stylus surface measurement instrument has some lim-itations by measuring samples with spherical surfaces.

Further works could be compared the data of the 3Droughness parameters obtained from the stylus type profi-lometer with those from the optical systems. 3D surfaceparameters give more precise picture of the surface, so itis possible to evaluate more precise surface roughnessparameters according to used machining technique.Although surface topography has been studied in great de-tail in terms of 2D parameters, a comprehensive compari-son on the nature of 3D surface topography generated bydifferent machining styles at different machining condi-tions is still missing. Additionally, the relationship be-tween 3D surface topography and functional aspects ispoorly understood. Because of this reason, the followingstep of this paper is to analyze the 3D surface topographyof the machined surfaces and compare the measurementsystems in terms of 3D surface roughness parameters.

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