Surface Texture Parameters Flashcards
Surface texture length can be defined in 3 ways?
- Traverse length - includes a run-up distance and an over-travel distance to exclude both mechanical and electrical transients from the measurement.
- Assessment length- the distance over which surface texture measurements are made
- Sampling length - length over which a single assessment of a parameter is made
Reference line for surface texture
- Quantification of the surface texture requires a datum within the measured profile to which measurements can be related.
- For practical purposes, the reference adopted in most cases is the “centre line” or “mean line” of the profile
- Production of most consistent “mean line” relies on a Least Squares calculation.
- This is a reference line which is positioned so that the sum of the squares of the deviations from the line are minimised.
Traces of surface texture parameters 3 types
Magnitude, Amplitude, Spacing
Magnitude parameters
Examine the average departure of the profile line from the centre line.
Amplitude parameters
Examine the peaks and valleys of the profile and their
respective heights and depths.
Spacing Parameters and the Spacing Parameter
Examine the frequency of peaks & valleys along a profile line.
The mean width of the profile elements (RSm) can be calculated, by taking into account all of the identified element widths (Xs) within a sampling length
QUANTIFYING SURFACE TEXTURE into 3 categories?
ROUGHNESS - The irregularities in the surface texture arising from the inherent action of the cutting process (cutting tool on the component).
WAVINESS - The irregularities caused mainly due to machine vibration and deflections under cutting force.
ERRORS OF FORM - The departure from the geometrical shape.
ROUGHNESS AVERAGE
The most common form of magnitude parameter is the Roughness Average (Ra) value.
* It is the most commonly used in surface texture analysis, and has previously been known as the Centre Line Average (CLA) and the Arithmetic Average (USA).
Limitations of the Ra value
- A primary issue of the Ra parameter is that it does not identify the types of features that have been measured.
- Surfaces of substantially differing surface form can produce similar Ra values at the end of the quantitative analysis.
- This is one good reason why one parameter description of a surface will not provide you with a 100% description of the surface
SAMPLING LENGTHS & CUT-OFFS
The effect of different sampling lengths and the resultant answers which can be different and lead to misinterpretation.
The “Cut-Off” is the instrument equivalent of the metrology sampling length.
* The distinction between sampling length and cut-off length is very important:
- Sampling length is a physical length of the surface.
- Cut-off is the means whereby the profile is made to simulate the effect of restricting the assessment to the sampling length.
WHAT IS A PEAK ?
To be considered as a peak, the profile trace must cross the mean line twice
PEAK HEIGHT
The maximum profile peak height (Rp) is the largest profile peak height (Zp) within a sampling length.
VALLEY DEPTH
The maximum valley depth height (Rv) is the largest profile valley depth (Zv) within a sampling length.
MAXIMUM HEIGHT
The maximum height (Rz) of the profile is the sum of height of the largest profile peak height (Zp) and the largest profile valley depth (Zv) within a sampling length.
MEAN HEIGHT
The mean height of the profile elements (Rc) is the mean value of the profile element heights (Zt) within a sampling length.
TOTAL HEIGHT
The total height of the profile (Rt) is the sum of the height of the largest profile peak height (Zp) and the largest profile valley depth (Zv) within the evaluation length
What is the Material Ratio parameter?
The material ratio parameter which simulates the effect of wear on a surface is being found to be of increasing use in industry.
As the peaks wear, the bearing line descends through the surface profile, and consequently the bearing line increases.
What does MATERIAL RATIO curve predict?
The Material Ratio curveparameter only simulates the effect of wear, and cannot replace experimentation designed to actively measure the same data.
Slope parameter
One of the primary uses of the slope parameters is to assess the developed or actual length of the profile, which can be simulated or calculated by stretching all of the peaks and valleys out into one long straight line.
ENVELOPE METHODS
The parameters described so far have all relied on the positioning of a reference line, and subsequent assessment from this reference line.
* There are other techniques which only use selected parts of the surface profile, such as extreme peak and valleys, in order to contribute to the positioning of a reference line
16% rule
The surface is considered acceptable if not more than 16% of all of the measured values of the selected parameter (based on an evaluation length), exceed the value specified within the technical documentation.
This 16% rule applies to lower limits and upper limits of specified tolerances
Surface Texture measurement standard A
Depth measurement standard
Type A standards are for calibrating the vertical profile component of stylus instruments.
Surface Texture measurement standard B
Tip condition measurement standard
Type B standards are primarily for calibrating the condition of the stylus tip
Surface Texture measurement standard C
Spacing measurement standard
Type C standards are used for primarily used for calibrating vertical profile elements, but may also be used for calibrating horizontal components.
