SURFACE TEXTURE MESUREMENT

Surface Metrology  Surface metrology or surface topology refers to the geometry and texture of surfaces.  The Surface Texture is defined as the characteristics quality of an actual surface due to small departures from its general geometrical form which, occurring at regular or irregular intervals, tend to form a pattern or texture on the surface”.

Surface metrology (cont’)  Three forms of asperity (un-evenness of surface, roughness) 1. Roughness 2. Waviness 3. 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

Surface assessment  Flaw (surface defect): random irregularities such as scratches, cracks, holes, tears, inclusions, etc.  Waviness (widely spaced asperities): recurrent deviation from a flat surface.  Roughness (the finest of the asperities): closely spaced irregular deviations on a scale smaller than that of waviness.

Symbols Used to Identify Surface Finishes and Characteristics

 Surface roughness comparator  Microscope  Stylus method SURFACE EVALUATION

 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 Surface roughness comparator

Surface roughness comparator (cont’)

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

STYLUS INSTRUMENT

Stylus instrument  The stylus instrument is a 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

Two types of stylus instrument 1. True- datum or skidless instruments 2. Surface- datum or skid type instrument

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

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

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

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.

NUMERICAL VALUES FOR ASSESSMENT  Arithmetic roughness average 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.

NUMERICAL VALUES FOR ASSESSMENT (cont’) Sample Data Either arithmetic average roughness height (Ra) or root mean square (Rq)

Other standardized assessment methods 1. Root-Means-Square roughness (R q 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 2. Maximum Peak-Valley Roughness (R max or R t ) 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 3. Ten-Point Height (R z ) Averages the distance between the five peaks and five deepest valleys within the sampling length See figure B

Other standardized assessment methods (cont’) 4. Average Peak-to-Valley Roughness (R or H or H pl ) 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 5. Average Spacing of Roughness Peaks (A r or A R ) Average the distance between the peaks without regard to their height See figure D 6. 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.

7. Bearing Length Ration (T p 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 8. Leveling Depth (R p and others) Measure the height between the highest peak and the mean line See figure G 9. 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 Other standardized assessment methods (cont’)