1. CHAPTER 8 SURFACE MESUREMENT
DPT 312
METROLOGY
ROSHALIZA HAMIDON

2. OVERVIEW
Surface metrology examines the deviation between one point (or points) on a surface and another points (other points) on the same surface
Surface metrology is a concern of many branches of science and widely involved in the world of commerce and manufacturing

3. Surface metrology
Surface metrology or surface topology refers to the geometry and texture of surfaces
The condition of surface is defined by its characteristics:
Surface texture (finish)
Roundness (a function of geometry)
Material
Hardness
Surface metallurgy

4. Surface metrology (cont’)
Three forms of asperity
Roughness
Waviness
Error of form
The fourth asperity is not distinguish by wavelength; it is flaw
Lay is the direction of the asperities which in most cases means that roughness and waviness are perpendicular to each other
Vary according to the length of spacing or wavelength

5. Surface assessment Roughness – the finest of the asperities
Waviness - concern the more widely spaced ones
Flaw – surface defect
Lay – The direction of the asperities
* Asperity is defined as unevenness of surface, roughness

6. Various lay configuration
This chart categorizes the various lay configurations and shows the standardizes symbols used on drawing

7. MAIN MEASUREMENT METHOD OF SURFACE ROUGHNESS
Inspection and assessment of surface roughness of machined workpieces can be carried out by means of different measurement techniques. These methods can be ranked into the following classes:
Direct measurement methods
Comparison based techniques
Non contact methods
On-process measurement

8. Direct Measurement Method
Direct methods assess surface finish by means of stylus type devices.
Measurements are obtained using a stylus drawn along the surface to be measured:
the stylus motion perpendicular to the surface is registered.
This registered profile is then used to calculate the roughness parameters.
Disadvantage- This method requires interruption of the machine process, and the sharp diamond stylus may make micro-scratches on surfaces.

9. Comparison Based Technique
Comparison techniques use specimens of surface roughness produced by the same process, material and machining parameters as the surface to be compared.
Visual and tactile senses are used to compare a specimen with a surface of known surface finish.
Because of the subjective judgment involved, this method is useful for surface roughness Rq>1.6 micron

10. Non-contact Method
There have been some work done to attempt to measure surface roughness using non contact technique.
Here is an electronic speckle correlation method given as an example (figure 8.1).
When coherent light illuminates a rough surface, the diffracted waves from each point of the surface mutually interfere to form a pattern which appears as a grain pattern of bright and dark regions.
The spatial statistical properties of this speckle image can be related to the surface characteristics.
The degree of correlation of two speckle patterns produced from the same surface by two different illumination beams can be used as a roughness parameter.

11. Figure 8.1

12. On-process Measurement
Many methods have been used to measure surface roughness in process. For example:
Machine vision.
In this technique, a light source is used to illuminate the surface with a digital system to viewing the surface and the data being sent to a computer to be analyzed. The digitized data is then used with a correlation chart to get actual roughness values.
Inductance method.  An inductance pickup is used to measure the distance between the surface and the pickup. This measurement gives a parametric value that may be used to give a comparative roughness. However, this method is limited to measuring magnetic materials.
Ultrasound. A spherically focused ultrasonic sensor is positioned with a non normal incidence angle above the surface. The sensor sends out an ultrasonic pulse to the a personal computer for analysis and calculation of roughness parameters.

13. SURFACE EVALUATION Surface roughness comparator Microscope
Stylus method

14. Surface roughness comparator
The most common way to evaluate surface finish is to compare it visually and by feel with roughness comparison specimens having various surface finishes
It consist of composite set of surface roughness specimen standard

15. Surface roughness comparator (cont’)

16. Microscope Examination of surfaces by microscope can be informative
But it does not usually allow the heights of the asperities to be determined without destroying the test part by cutting a taper through the surface

17. Stylus Instrument Why we use stylus method??
It is the most familiar method for practical shop work
It the best demonstrate the fundamental principles of surface metrology
Standard is based on the stylus method

18. STYLUS INSTRUMENT

19. Stylus instrument
The stylus instrument widely used technique for measuring a surface profile
This technique uses a fine diamond stylus with tip size approximately 0.1 to 10 µm to transverse the surface
As the stylus tracks the surface peaks and valleys, its vertical motion is converted to a time varying electrical signal that represent surface profile
Stylus instruments operate like a phonograph pickup: the stylus is drawn across the surface and generates electrical signals that are proportional to the changes in the surface
The changes in height can be read directly with a meter or on a printed chart

20. Two types of stylus instrument
True- datum or skidless instruments
Surface- datum or skid type instrument

21. True- datum instrument
With this instrument, we draw across the surface in a very precise, mechanically controlled movement
Advantages
The resulting graph is nearly a true representation of the surface along that one line showing roughness, waviness, errors of form and flaws
Disadvantages
Very difficult to set the instrument up; must precisely align the surface being assessed with the path of the instrument

22. Figure 8.2
True-datum stylus instruments move the stylus across the part along a reference datum established by the instrument

23. Surface datum instrument
Can easily set up because they use the surface being assessed as the datum
A supporting slide (skid, a rounded member fixed to the head, a shoe, a flat pad) mounted in the head rests on the surface and slides the stylus pickup along
Skids may be located, in front of, behind, or on the opposite side of the stylus

24. Figure 8.3
Surface datum instruments create their own reference datum. This is done by supporting the stylus, T, by a member S, that is sufficiently wide to slide along the surface.

25. NUMERICAL VALUES FOR ASSESSMENT
Arithmetic roughness average
This method is also known as roughness average and by two earlier term; arithmetic average (AA) and center-line average (CLA)
The roughness average is the arithmetic average of the absolute values of the deviation from the profile height measured from the centerline along a specified sampling length
Two method for determining the value
Graphical method
Electrical averaging

26. NUMERICAL VALUES FOR ASSESSMENT (cont’)

27. Other standardized assessment methods
Root-Means-Square roughness (Ra or RMS)
Closely related to the roughness average (Ra)
Square the distances, average them, and determine the square root of the result
The resulting value is the index for surface texture comparison
Usually 11% higher than the Ra value
Maximum Peak-Valley Roughness (Rmax or Rt)
Determine the distance between the lines that contact the extreme outer and inner point of the profile
Second most popular method in industry
See figure A
Ten-Point Height (Rz)
Averages the distance between the five peaks and five deepest valleys within the sampling length
See figure B

28. Other standardized assessment methods (cont’)
Average Peak-to-Valley Roughness (R or H or Hpl)
Average the individual peak-to-valley heights
See figure C
Use the height between adjacent peaks and valleys, not measure from a center line to peak valleys
Average Spacing of Roughness Peaks (Ar or AR)
Average the distance between the peaks without regard to their height
See figure D
Swedish Height of Irregularities (R or H)
Also known as Profiljup methos
Only standard in Sweden (H) and Denmark (R)
It assume that, in wear situation, the peaks are affected by wear, but the valleys are not.

29. Other standardized assessment methods (cont’)
Bearing Length Ration (Tp and others)
Create a reference line through some of the peaks
This line is at a predetermined height from the mean line, and you have then divide the subtended length through the peaks by sampling length to arrive at the assessment value
See figure F
Leveling Depth (Rp and others)
Measure the height between the highest peak and the mean line
See figure G
Waviness Height (W)
Assess the waviness without regard to roughness by determining the peak-to-valley distance of the total profile within the sampling length

31. Example