Surface Metrology

Jay Raja

Center for Precision MetrologyUNC Charlotte

CPM - UNC -Charlotte

What's significant in roughness?

Surface roughness is a second-order effect. The only point of measuring and quantifying it is to relate it to function. Which of the bewildering array of magic parameters currently offered by instrument manufacturers is the right one to do this? The practical engineer does not want his time wasted with mathematical caveats or statistical technicalities: he wants a straight answer to this question and he wants it now. Unfortunately the only answer it seems we can give him at the moment is: "All of them".

We can perhaps see the difficulty more clearly by analogy. There is a surface with which we are all intimately familiar and whose topography influences every detail of our everyday lives: the surface of the Earth itself. If we had to travel to some distant country on business, or seek out a holiday resort, or select a site for a new house, what sort of topographical information would be useful, and how would we specify it in terms of existing roughness parameters?

The average roughness, measured at a cutoff of a few miles, would certainly enable us to distinguish between, say, the Netherlands and the Alps, but not between, say, the rolling coastal hills of California and the savage profiles of the Sierra Nevada'. The peak-to-valley roughness or the ten-point height would tell us if we were on a land dotted with buttes, like Monument Valley, or fissured by gorges like the Grand Canyon, but not which was which. To make this distinction we would need to know in addition the skewness of the height distribution. At the other end of the scale, the mean slope or the peak curvature, measured at the right cutoff, would tell us whether or not we needed climbing boots.

If we progressively lowered the long-wavelength cutoff while making successive measurements of roughness we would find out how hilly the roads were, then how bumpy /they were, and finally how good a grip they would give to our tyres*. If carried out at enough cutoffs, this procedure amounts to sweeping the surface power spectrum with a tunable band-pass filter. Does the power spectrum, then, contain all the information we need? Unfortunately not: it turns out that the shape of the power spectrum is relatively insensitive to the shape of the height distribution. So if the sea level around the shores of Britain dropped by

hundreds of feet, reuniting most of the islands of Western Scotland to the mainland, the normalized power spectrum of a profile across the Highlands would look much the same. Thus the coming generation of optical measuring devices which essentially perform direct Fourier transforms on the surface roughness may not be very much more helpful for certain purposes than existing stylus instruments.

In everyday life we are not too bothered by our inability to quantify our surrounds with a single parameter. We simply look at a map drawn on a scale appropriate to our needs and use our accumulated experience of the Earth's topography backed by the unrivalled aptitude of the human brain for pattern recognition, We could do almost as well if the only information we had were related to a section through the surface; when I look at the ridge line of the Black Cuillins of Skye I do not need to do any sums to know that I cannot drive my car along it.

This is to suggest that as much as possible of the information present in a surface profile should be used to relate the surface to its function. An approach along these lines has been used successfully for many years by a large European car manufacturer, who defines an acceptable surface as, effectively, one whose profile looks "right" to an inspector with 20 years' experience. The investment of many man-years of training formerly needed to make such an approach successful put it beyond the general reach.

The advent of the microprocessor, however, now allows the use of real-time on-line pattern recognition techniques. One approach that has already proved successful is: digitise surface profiles of a number of components which experience shows are functionally 'good' and 'bad'; extract a number of parameters; and look for a 'pattern' of suitably weighted parameters which differentiates between 'good' and 'bad'. This process should be relatively easy for mass-produced components, though more thought may have to be given in the case of some of the artefacts described by previous contributors to this column. I would not wish to press too hard the case for turning out 100 telescope mirrors to allow a sufficiently long run for adequate statistical analysis of their roughness problems!

EMBED PBrush

Why Measure Surface Texture

Functionally Important Process Control Quality Control Research and Development Contractual Obligation

Engineered Surfaces Plateau Honed Surface Texture Rollers to Transport Film Computer Hard Disk Print Rollers Consumer Products

Plateau Honed Surface TextureTexture Component Product ProcessValley Texture Oil Retention. Abrasive Selection, Pressure & Process Parameters

Plateau Texture Sliding Friction Abrasive Selection, Pressure & Process Parameters

Plateau Placement Wear Resistance, Plateau Pressure

Load Capacity, & and Cycle Time Oil Retention

Disk Drives and Suspension DevicesHUTCHINSON TECHNOLOGY

Flying Height

Supporting the flying Heights

Conventional Sand Paper 120XExample of Trizact Abrasives 250X

3M Precision Surface

Components of Surface Topography Roughness

Waviness

Form Error

Lay

Components of Surface Topography

Roughness - Closely Spaced irregularities (cutting tool marks, grit of grinding wheel)

Waviness - More Widely Spaced Irregularities (Vibration and Chatter)

Error of Form - Long Period or Non-Cyclic Deviations (errors in ways or Spindle, uneven wear)

Flaws - Discrete, Infrequent Irregularities (cracks, pits, scratches)

Roughness and Waviness Comprise the Surface Texture

Cutting process creates spiral toolmarks - roughness Vibrations, both in machine tool and the environment, result in waviness Out-of-straightness of the too carriage movement creates error of form Total Profile - combination of all components superimposed.Lathe

Measurement and Characterization of Surface ProfilesMeasurement of ProfilesSensorsInstrumentsDatumsCalibration and UncertaintyNumerical characterizationPreprocessingFilteringParametersFunction

Classification of Instruments Need for Classification

Classification Chart

Instrument Characteristics

Source: ASME B46 (Draft)Classification of Common Instruments for Measurement of Surface texture

Classification SchemeTYPE I: Profiling Contact Skidless instrumentsProperties Measuring range often includes very smooth and rough surfaces. Measure roughness and may measure waviness and error of form with respect to an external datum. May have a selection of filters and parameters for data analysis. For stylus-type transducers, tips are often changeable and may range from sub- micrometer diamond styli to ball tips with radii of several millimeters.

Can generate filtered or unfiltered profiles.

Capable of either unfiltered profiling or topographical analysis(area profiling), or both.

Examples Skidless stylus-type adapted with LVDT (Linear Variable Differential Transformer) vertical measuring transducer. Skidless stylus-type using an interferometric transducer. Skidless stylus-type using an capacitance transducer.

Components of a Stylus Instrument Transducer

Stylus tip

Profile Distortion

Reference/Drive Unit

Scanning electron microscope photograph, enlargement 280.1.Source:surface TestingLaboratory of the Federal Physical-Technical Institute (BTP)Scanning a Ground SurfaceThe xy-100 measuring table with integrated video system

Source: R. E. Reason

Source: L. Mummery, Hommelwerke

Effect of Skid

Example of Profile Distortion

Preprocessing and Filtering Form Removal Filtering of Profiles 2RC and ISO Filters Bandwidth and cut-off

Constituents of Surface Texture From: ASME B46

Surface Texture Measurement and CharacterizationProfile MeasurementPreprocessingFilteringNumerical Evaluation- Roughness: Ra - Waviness: Wa- Profile: Pa

Source: ASME B46

Curved Nominal ProfileStraight-Line Nominal ProfileForm Removal

Source: L. Mummery, Hommelwerke

Sampling Length ()Assessment Length (L)Traversing Length

Selection of cut-offsSource: Tabenkin, Federated Products Corp.

Typical Finishing Process

(mm)

0.25

(mm)

0.8

(mm)

2.5

(mm)

8.0

(mm)

25.0

Milling

X

X

X

Boring

X

X

X

Turning

X

X

Grinding

X

X

X

Planing

X

X

X

Reaming

X

X

Broaching

X

X

Diamong Boring

X

X

Diamond Turning

X

X

Honing

X

X

Lapping

X

X

Superfinishing

X

X

Buffing

X

X

Polishing

X

X

Shapping

X

X

X

Electro-Discharge-Machining

X

X

Burnishing

X

X

Drawing

X

X

Extruding

X

X

Moulding

X

X

Electro-Polishing

X

X

Wavelength Transmission LimitsSource: ANSI B46.1 - 1985

2RC Filter Transmission Characteristics

Source: R. E.ReasonDistortion produced by the standard filter of waveforms whose period is only slightly smaller than the cut-off.

Gaussian Filter

Gaussian Transmission Characteristicstogether with the Nominal Characteristic of a 2m Stylus

Transmission band for roughness and waviness profilesISO/DIS 4287-1.2:1994(E)

Source: ASME B46Standard cut-off for Gaussian Filters and Associated cut-off Ratios* With a nonstandard stylus of 0.5 mm, the cut-off ratio for lc = 0.08 may be equal to 100 provided ls = 0.8 mm and the maximum point spacing = 0.16mm.

+ With a nonstandard stylus of 0.5 mm, the cut-off ratio for lc = 0.25 may be equal to 300 provided ls = 0.8 mm and the maximum point spacing = 0.16mm

c

mm (inch)

s

m (inch)

c:s

rtip

m (inch)

Max Sample Spacing

m (inch)

0.08 (.003)

2.5 (0.0001)

30

2 (0.00008) or less*

0.5 (0.00004)

0.25 (0.01)

2.5 (0.0001)

100

2 (0.00008) or less+

0.5 (0.00002)

0.8 (0.03)

2.5 (0.0001)

300

2 (0.00008) or less

0.5 (0.00002)

2.5 (0.10)

8 (0.0003)

300

5 (0.0002) or less

1.5 (0.00006)

8 (0.3)

25 (0.001)

300

10 (0.0004) or less

5 (0.0002)

Cut-off Values for Profiles using Sm

Sm

Cuttoff Length*

Evaluation Length

Over

Up to

(including)

mm (x0.001 in)

mm (x 0.001in)

mm (inch)

mm (inch)

0.013 (0.5)

0.04 (1.6)

0.08 (0.003)

0.40 (0.016)

0.040 (1.6)

0.13 (5)

0.25 (0.01)

1.25 (0.05)

1.3 (5)

0.40 (16)

0.80 (0.03)

4.0 (0.16)

0.4 (16)

1.3 (50)

2.5 (0.10)

12.5 (0.5)

1.3 (50)

4.0 (160)

8.0 (0.3)

40.0 (1.6)

Cut-off Values for Non-Periodic Profiles using Ra

Ra

Cuttoff Length

Evaluation Length

Over

Up to

(including)

mm (min)

mm (min)

mm (inch)

mm (inch)

0.02 (0.8)

0.08 (0.003)

0.40 (0.016)

0.02 (0.8)

0.10 (4)

0.25 (0.01)

1.25 (0.05)

0.10 (4)

2.0 (80)

0.80 (0.03)

4.0 (0.16)

2.0 (80)

10 (400)

2.5 (0.10)

12.5 (0.5)

10 (400)

8.0 (0.3)

40.0 (1.6)

Surface Texture Symbols 0.030^ 0.015 cut-offLayMax RoughnessSpacing0.002.-26332Max Waviness WidthMax Waviness HeightMax RaMin Ra

Lay Symbols ASME Y14.36

Wavelength Domain in MetrologyJay Raja - University of North Carolina at Charlotte

Historical Surface Metrology Measurement Spectrum

Current Metrology Measurement spectrum

Straightness Profile - Stylus Instrument A

Straightness Profile - Stylus Instrument B

Straightness Profile - Cylindricity Instrument

Straightness Profile - Scanning CMM

Comparison of Straightness values

Wavelength Domain and Process Control

Wavelength Domain and Functional Performance

Separation of Roughness, Waviness and Form

Classification of Surface Texture Measurement Methods Contacting Stylus Phase Shifting InterferometryVertical Scanning (White Light) InterferometryFocus Sensing Confocal MicroscopyChromatic Length Aberration Confocal MicroscopyStructured Light and Triangulation SEM StereoscopyScanning Tunneling MicroscopyAtomic Force MicroscopyOptical Difference Profiling Angle Resolved SEM