Surface finish metrology iss1

Surface Finish Metrology

© 2000 Taylor Hobson Ltd

Centre of Excellence

Taylor Hobson Ltd

Contents 1. Why do we need to measure Surface Finish?

2. Measurement Methods

3. Measurement Datums

4. Reproducing the Surface

5. Terminology

6. Filters

7. Parameters

8. Bearing Area (Material Ratio)

9. The Rk Parameter

10. Form Measurement

11. Calibration Methods.

12. Conics and Aspherics.

13. 3D (Areal) Measurement.

14. Drawing Indication.

Why do we need to measure Surface

Finish?

Nature of Surfaces

The microstructure of the material

Nature of Surfaces

The microstructure of the material The action of the cutting tool

Nature of Surfaces

The microstructure of the material The action of the cutting tool The instability of the cutting tool on the material

Nature of Surfaces


Errors in machine tool guideways

Nature of Surfaces


Errors in machine tool guideways Deformations due to stress patterns in the component

Unwanted Properties on a Surface

Deep valleys which may be susceptible to crack propagation

Too many peaks which may cause early surface breakdown and wear when in contact with a mating component

Excessive waviness which may cause noise or indicate machining problems

Wanted Properties on a Surface

Sufficient valleys for oil retention when lubrication is an important factor

Sufficient peaks for retention of paint and adhesives

Sufficient distribution of valleys for formability

Smooth surface profiles for reduced, noise, vibration or high reflectance.

Why Do We Need to Measure Surface Finish?

Process Control

Predicting Component Behaviour

Monitoring Component Performance


The Ideal Situation


The Reality - Process Control


Predicting Component Behaviour


Monitoring Component PerformanceBack to Contents Page

Measurement Methods

Measurement Methods

Contact Type Instruments

Non-Contact Type Instruments

How Do We Measure Surface Finish?

Comparison Plates

Contact Type Instrument

Measurement Methods

Traverse Direction (X)

Stylus Movement (Z)

Data Point Spacing (X)

Inductive Type of Transducer

Stylus

Beam

Knife Edge Pivots

Ferrite Slug (Armature)

Coil

Coil

Measurement Methods

Piezo-Electric Type of Transducer

Stylus

Beam Piezo Element

Measurement Methods

Laser

PhotoDiode

Laser Type of Transducer

Stylus

Beam

Back to Contents Page

Click Here for Further Information

Measurement Methods

Measurement Datums

Skid (surface) Datum

Skid Stylus Movement (Z)

Traverse Direction (X)

Measurement Datums

Benefits of Using a Skid Datum

• Reduces Effects of Vibration• No Surface Levelling Required • Instrument Portability • Robust Design

Measurement Datums

Effect of Skid to Stylus Pitch

Actual Height

H1 H2 Apparent Height

Measurement Datums

Effect of Skid to Stylus Pitch

Resultant Profile P-V = 0µm

P-V = 10µm

Measurement Datums

Independent Datum

Traverse Unit Datum Traverse Direction

Measurement Datums

Independent Datum

Traverse Unit Datum

Datum SkidOptical Flat

Traverse Direction

Measurement Datums


Reproducing the Surface

Stylus Tip Geometry

2µm

Conisphere Stylus Truncated Pyramid Stylus

2µm

Traverse Direction



Stylus Tip - Effects of Tip Size & Shape


Stylus Tip - Flanking


Profile produced by Stylus

Stylus Tip

B

BA

A

Stylus Tip - Flanking Back to Contents Page


Terminology

Data PointsTerminology

AliasingSampling Interval

True Signal Aliasing Signal


Terminology

Roughness, Waviness & Form

Sample Surface

Terminology


Form

Terminology


Waviness

Terminology


Roughness


Filters

Terminology

Filtering-Separates Roughness and Waviness

Surface Interaction

Filtering Using Graphical TechniquesSampling Length

Filters

Filters

Form

Sample

Cut-off

Ra,Rq,Rz etc...Roughness Filter

Filters

Form

Sample

Cut-off

Wa,Wq,Wz etc...Waviness Filter

Electronic & Digital Filter Types

• ISO 2CR Filter• 2CR PC•Gaussian

Filters

ISO 2CR Filter

ISO 2CR Filtered Profile

Filters

Unfiltered Profile


ISO 2CR Filter Effect

Modified Profile Relative

to Filtered Mean Line

Filters

Mean Line Established By Filter

2CR PC Filter2CR PC Filtered Profile

Filters

Unfiltered Profile

ISO 2CR PC Filter

Modified Profile Relative

to Filtered Mean Line

Filters

Mean Line Established By Filter

Gaussian Filter

Unfiltered Profile

Gaussian Filtered Profile

Filters

Gaussian Filter

Profile Filter

Sampling Length

X

Z

Unfiltered Profile

Mean LineCut-off

Filters


The Effects of Filtering

Unfiltered Profile

Waviness Amplitude = 100µm

Roughness Wavelength = 0.25 mm

Waviness Wavelength = 8.0 mm

Roughness Amplitude = 20µm

Filters

The Effects of 2CR Roughness Filter

8.0 mm Cut-off filter





Filters






Filters





Roughness Amplitude = 20µm Roughness Wavelength

= 0.25 mm

Filters




Roughness Amplitude = 15µm Roughness Wavelength

= 0.25 mm

Filters



Waviness Amplitude = 0µmRoughness Amplitude

= 4µm Roughness Wavelength = 0.25 mm

Filters


Relationship of Sampling, Assessment & Traverse Length (ISO 2CR)

Sampling Length (Cut-off)

Assessment (Evaluation) Length

Traverse Length

Filters

Run-up Over travel

Filter Types

• ISO 2CR- 1st 2 Cut-offs discarded• 2CR PC- 1st & Last Cut-offs discarded•Gaussian- Half 1st & Half Last Cut-off Discarded

Filters

Cut-off Selection

0.8mm Sampling Length (Cut-off)

Filters

Cut-off Selection

Filters

0.25mm Sampling Length (Cut-off)

Choosing the Correct Cut-off Value

1.25 mm

Traverse Direction

Filters

Cut-off Selection Unless otherwise indicated on a drawingthe table above should be used to

determine the cut-off

Filters

Bandwidth=Ratio of Lc/Ls

Ls

Filters


Lc

Parameters

Parameters

Analysis Types

•Roughness - Prefix R•Waviness - Prefix W•Primary - Prefix P

Parameters

Parameter Types

•Amplitude Parameters defined from Z co-ordinates

Parameters

• Amplitude Parameters defined from Z co-ordinates• Spacing Parameters defined from X co-ordinates

Parameter Types

Parameters

• Amplitude Parameters defined from Z co-ordinates• Spacing Parameters defined from X co-ordinates• Hybrid Parameters X & Z co-ordinates

Parameter Types

Parameters

Amplitude Parameters - Ra Ra

Parameters

Amplitude Parameters - Limitations of Ra Ra

Ra

Ra

Ra

Parameters

Amplitude Parameters Rq (RMS) lr=Sampling Length

Parameters

Amplitude Parameters Rt

ln=Assessment Length lr=Sampling Length

Parameters

Amplitude Parameters Rp Sampling Length

Parameters

Amplitude Parameters Rv Sampling Length

Parameters

Amplitude Parameters Rz

Rz1

Rz2

Rz3 Rz4 Rz5

Rz = Maximum peak to valley in each sample length divided by n sampling lengths

Parameters

Amplitude Parameters Rz1max, Rp1max & Rv1max

Rz1max

Rp1max

Rv1max

Parameters

Spacing Parameters HSC (High Spot Count) lr=Sampling Length (Cut-off) ln=Assessment

Length

A=Slice level B= Mean Line

Parameters

Spacing Parameters HSC (High Spot Count)

Tall narrow peaks Tall narrow peaks tend to work hardentend to work harden

Hardened peaks will eventually Hardened peaks will eventually breakbreakoff and the surface will breakdownoff and the surface will breakdown

Parameters

Spacing Parameters Rpc (Peak Count)

A=Selectable Bandwidth B= Mean Line

Parameters

Spacing Parameters Rpc (Peak Count)

Orange Peel Effect Due to Peaks on Sheet Steel Base Peel Effect Due to Peaks on Sheet Steel Base SurfaceSurface

Base Sheet Steel

Painted Surface

Parameters

Spacing Parameters Sm (Mean Spacing) lr=Sampling Length (Cut-off)

Mean Line

Parameters

Hybrid Parameters- Rdq (Pdq, Wdq) (Rms Slope)

θ

θθ θ

Parameters


Effects of Surface Slopes on vibration & noise

No Vibration/Quiet Low Frequency Rumble

High Frequency Scream

Parameters

With low surface slopes more light is reflected into the eye and hence has a good appearance

With high surface slopes less light is reflected into the eye and hence has a poor appearance



Bearing Area (Material Ratio)


Hybrid Parameters - Rmr

Upper surface defines run-in characteristics

Valleys define lubrication characteristics

Body of surface defines wear/life characteristics

Hybrid Parameters - Rmr

Lapping Plate

Bearing Line

ln=Assessment Length

Rmr= a+b+c+d+e x100 ln


Material Ratio Curve (Rmr) 0 100 %tp(%)

Level p Tp (%) at level p


0 100 %tp(%)


Material Ratio Curve (Rmr)


Material Ratio Curve (Rmr) 0


tp(%) 100 %


Amplitude Distribution Curve 0

Level p

Number of Peaks



Associated MR Curve Parameters-Rsk (Skew)

Surface with Random Amplitude Distribution- Zero SkewSurface with Random Amplitude

Distribution- Zero Skew



Surface with dominant peaks-Positive Skew



Surface with dominant valleys-negative Skew


Associated MR Curve Parameters-Rku(Kurtosis)

Rku<3

Rku=3

Rku>3



The Rk Parameter

Parameters

Rk Parameter

Surface Interaction

Material Ratio Curve (Rmr) 0 100 %tp(%)


Material Ratio curveTilted Profile

Parameters

Mean Line

Unfiltered Real Profile

Rk Parameter

Parameters

Mean Line

Unfiltered Real Profile after valley suppression

Valleys removed & kept in memory

Rk Parameter

Parameters

Final Filtered Roughness Profile

Rk Parameter

Parameters

100 %0 tp(%)

40 %

Rdc

Rk Parameter

Parameters

100 %0 tp(%)

Rdc

40 %

AB

D

E

F

C

Rk Parameter

Parameters

C

100 %0 tp(%)

Rdc

40 %

AB

D

E

F

Rk

D1

Mr2Mr1

Rk Parameter

Parameters

tp(%)

C

100 %0

Rdc

40 %

AB

D

E

F

Rk

D1

Mr2Mr1

Area 1

ECRpk

Area 2

E

D

Rvk

Rk Parameter

Parameters

100 %0 tp(%)

40 %

Rk

Mr2Mr1

Rpk

Rvk

Rk Parameter

Parameters

Rk Associated Parameters

• Rk-Core Roughness Depth• Rpk-Reduced Peak Height• Rvk-Reduced Valley Depth

• Mr1-Material Component Relative to Peaks• Mr2-Material Component Relative to Valleys• A1-Material Filled Profile Peak Area• A2-Lubricant Filled Profile Valley Area


Form Measurement

Least Squares Line

Form Measurement

Form Measurement

Straightness (LS Line)

Form Measurement

Straightness (MZ Line)

Form Measurement

Least Squares Arc

R=Calculated LS Radius r-r=LS Arc

r r

Form Measurement

Least Squares Arc - Radius Measurement

t=34.501656mm

r=110mm

l=80mm

l= Measurement Length/2

t= Chordal Heightr= Least Squares Radius

r-t

Form Measurement


t=1.833462mm

r=110mm

l=20mm



r-t

Form Measurement


t=1.833962mm

r=109.9705mm

l=20mm




Calibration Methods

Calibration Methods

Ball Standard

Traverse Direction

Software Corrections

Arcuate Errors

dZ

dX

Traverse Direction

Software Corrections

Stylus Tip Errors

Traverse Direction

Path of Stylus Tip Centre

Calibration Methods

Ra & Rz Roughness Standards

Calibration Methods Traverse Direction

2.5µm Step Height

3 Line (Step Height) Standard

Calibration Methods


A=Mean Line

B=Calibration Height

Calibration Methods


A=Level Surface

B=Pt Value

C=Unlevelled Surface



Conics & Aspherics

Conics and Aspherics

Conic Sections

Conic Constants

Ellipse: K< 0 Sphere: K= 0 Oblate Ellipse: K>0


Z2 + X2 = 1

R22 R12 Z2 + X2 = 1

R2 R2 Z2 + X2 = 1

R22 R12

Conic Constants

Hyperbola: K< -1 Parabola: K= -1


Z2 + X2 = 1

R22 R12

Z = (Ax)2

What is an Aspheric?

Conventional Spherical Lens


What is an Aspheric?

Aspheric Lens


Reasons for using an Aspheric

• Reduces Spherical Aberration• Ability to Produce Vari-focal Lenses • Reduction in Lens Size and Weight • Greater Design Freedom


How is an Asphere constructed?

Basic Conic Section (Sphere)

Z Axis

X Axis


Z2 + X2 = 1R2 R2


Aspheric Polynomial Curve

Z=a8|x|8


Z Axis

X Axis



Aspheric Profile



Z Axis

X Axis A

B C



Aspheric Profile



Z Axis

X Axis

A

B

C Aspheric Profile

Aspheric Polynomial Curves

Basic Conic Section


Standard Equation for An Aspheric Surface

cx2+a1|x| +a2| x|2 + a3| x|3 … +a20 | x|20Z(x)=

1+ 1-(K+1) c2x2

Where: X is the Radial distance from the Aspheric Axis Z is the corresponding vertical distance

a is the indexed Polynomial Coefficient C is the reciprocal of the Base Radius K is the Conic Constant of the Surface


Base Radius of Curvature


How Do We Measure an Aspheric Surface?

Optical Flat

Aspheric Axis


How Do We Analyse an Aspheric Surface?

Aspheric DataRadius=40.0mm

K= -1A4= 5.3188e-007A6=5.5231e-009A8=-1.6774e-011A10=-6.3352e-

014


Aspheric Form FitX Axis

Z Axis

AB

Aspheric Axis


Residual Error after Aspheric Form Removal

+Z Axis

-Z Axis

-X Axis

+X Axis

A

Aspheric Axis

Aspheric Axis


How Do We Analyse an Aspheric Surface?


Aspheric Parameters


Aspherics-Further Analysis

Convex Component-Concave Residual Form Error-Increase Base Radius


Convex Component-Convex Residual Form Error-Decrease Base Radius


Aspherics-Further Analysis

Smallest Rt Value Achieved-True Shape & Base Radius


Back to Contents PageAspherics-Further Analysis

3D (Areal) Measurement

3D Measurement

Advantages of 3D (Areal) over 2D

• Good Visualisation of the Surface• More Statistically Stable

• Better at Detecting & Analysing Defects • Many Methods of Representing the Data

3D Measurement

Disadvantages of 3D (Areal)

• Longer measurement cycles• Measurement Produces Large Data Files• Mainly Restricted to R & D • Sometimes Visually Subjective

3D Measurement

Contact Measurement Method

Y Axis Incremental MovementTraverse Direction

3D Measurement

Non-Contact Measurement Method

Y Axis Incremental Movement

Measurement Axis (X)

Laser CCD Detector

3D Measurement

Critical Dimensions for 3D Measurement

Profile 1

Profile 2

Z1

Z2

ΔXΔZ

ΔY

3D Measurement

Data Analysis

3D Measurement

Levelling the Data

3D Measurement

Form Removal Cylinder Liner- No Form Removed Cylinder Liner- Form Removed

3D Measurement

Viewing 3D Data Meshed Axonometric View

3D Measurement

Viewing 3D Data Photo Simulation View

Pseudo- Colour View

3D Measurement

Viewing 3D Data Continuous Axonometric View

Contour View

3D Measurement

3D Filtering

0.8mm

0.8mm

• 0.8mm Gaussian Filter

• ½ Cut-off Lost Top, Bottom Left & Right

Sampling Area

3D Measurement

3D Parameter Types

• Amplitude• Spatial• Hybrid • Functional

3D Measurement

3D Amplitude Parameters- Prefix: S

Sa =arithmetic mean of the deviations from the mean plane

Sq =RMS of the mean of the deviations from the mean plane

St =total peak to valley over the sample area

Sp =height of the highest peak to mean plane

Sv =depth of the deepest valley to mean plane

3D Measurement

3D Spatial Parameters

SPc =peak count between two selectable planes

Sds =density of summits contained in a sampling area

Std =texture direction of the surface

3D Measurement

3D Hybrid Parameters

SΔq =RMS slope of the surface

Ssc =mean summit curvature of the surface

Sdr =developed interfacial area ratio of the surface

3D Measurement

3D Functional Parameters

Stp=surface bearing area ratio

Sbi =surface bearing index

Sci =core fluid retention index

Svi =valley fluid retention index

3D Measurement

Volume/Defect Analysis

3D Measurement

Step Height Analysis

Drawing Indication

Drawing Indication

Conventional Surface Texture Symbol

Graphical Symbols for Surface Texture

6.8

Requirement for Surface Texture

1

Requirement for Surface Texture, Material Removal Required

2

Requirement for Surface Texture, Material Removal Not Permitted

3


Drawing Indication


Drawing Indication

(d) (e) 3

(c) Turned(a) 0.0025-0.8/Rz

6.8(b) 0.0025-0.8/Ra 2.2



Drawing Indication Fe/Ni 15p cr 0.0025-0.8/Rz 6.8

Drawing Indication

Bi-Lateral Tolerancing


U Ra 0.9L Ra 0.3

Drawing Indication

The 16% Rule (Default Rule)

• No more than 16% of the measured values for an upper limit should exceed the specified value

• No more than 16% of the measured values for a lower limit should be less than the specified value

Drawing Indication

The MAX Rule

RzMAX 0.9

End


Surface finish metrology iss1

Technology

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