What are the 2 basic components of a surface texture measurement (contact) equipment?
- Stylus with diamond tip - drag along the surface
- Skid - provides a measurement datum
What is Stylus shape for surface texture?
Typically conical with stylus with spherical tip
- the size and shape of the stylus with affect the accuracy of the measurement in penetration into the valleys and re-entrant features
- spherical form can tend to round off peaks
What are Re-entrant features?
Those which are typified by pockets which the stylus, in these cases the stylus will lose contract with the surface profile. These are often found in cast materials
What is Stylus Dynamics?
An important issue is whether the stylus correctly tracks across the surface and does not “lift off”. This has been called “trackability”.
What is Stylus deflection?
- Stylus deflection does not hold entirely true, because of the contact of the stylus tip with the surface of the work-piece.
- The cantilever beam assumes a free end. A contacting end is not represented by the cantilever beam. Hence the model is only an approximation.
The trackability of the system (surface texture) in mechanical terms is a function of the: 4 things
- Inertia of the pivoting system
- Static stylus force on the workpiece surface
- Stylus traverse speed
- Amplitude and wavelength of the surface features.
What happens with the Static Two Body Interaction of a styus and work piece?
- Contact between the stylus tip and the surface must include some force on the stylus to keep it in contact with the surface.
- The consequence of very small contact areas and force is the issue of elastic or even plastic deformation on a localised scale.
- The key to estimating the contact deformation is calculating the nominal areas of contact, which is generally achieved using theory developed by Heinrich Hertz.
PHASE GRATING INTERFEROMETRIC GAUGE
The phase grating interferometric gauge typifies a common modern solution to stylus displacement measurement.
* Typical range to resolution ratios are in the order of 780,000. Consequently maximum resolution is available with maximum range.
What GAUGEs are used to determine the postition of surface texture equipment? 2
An important aspect of the measurement of the surface texture is having some form of reference. This can be achieved two ways; either an independent reference datum, or by the use of a skid.
What is a SKID BASED REFERENCE?
- Systems which involve a skid, use a large plate (in comparison to the size of the stylus) which travels across the same surface line as the stylus. There may be a phase lag between the features being seen by the stylus, and the position of the skid.
- Suppression of peaks and features is possible, if the waviness spacing is of the order of the skid/stylus spacing.
What is an INDEPENDENT DATUM?
An alternative approach to using a skid on the surface, is to have an independent glass datum surface, which will always give a flat reference line/surface.
- This will remove any issue of indeterminate filtering effects caused by the skid/stylus spacing.
- weak though, potential for damage easier can be knocked
What is FILTERING?
Filtering is required to separate roughness wavelengths from waviness wavelengths
How to determine FILTER QUALITY?
A ”filter cut-off value” (lambda-c or lc) determines the wavelength at which roughness is separated from waviness.
What is a Filtering cut off?
It is sizing what you want to accept or what you want to exclude A roughness filter cut-off with a numeric value of 0.8mm will allow wavelengths below 0.8mm to be assessed with wavelengths above 0.8mm being reduced in amplitude. A waviness filter cut-off with a numeric value of 0.8mm, wavelengths above 0.8mm will be assessed with wavelengths below 0.8mm being reduced in amplitude.
* To produce a good statistical analysis of the surface texture marks, a cut-off should be selected in the order of 10 times the wavelengths under consideration.
SUMMARY OF THE 8 OPERATIONS for surface texture
- Raw Data Set
- Removal of Form Components
- Leveling of Data
- Raw (P) Parameters Calculated (if necessary)
- Selection of Filter Type
- Selection of Filter Parameters (key one being cut-off lc)
- Roughness and/or Waviness Frequencies / Wavelengths
- Roughness (R) and/or Waviness (W) Parameters Calculated
