The most common surface roughness parameter to measure the quality of a surface is the average surface roughness (Ra). But while moving on to specific application, we generally refer to alternate surface roughness parameters like (Rq, Rp, Rv, Rskew, Rkurt etc.). How do I differentiate which surface roughness to consider and when?
The surface roughness parameters depend on the measuring method used and are adapted in their calculation to mechanical processing methods. In most cases, the arithmetic mean value is used over a defined measuring distance. This mean value is very variable in micro-machining because the measuring section has to be composed of many partial sections. For comparisons, it must also be assumed that the same measuring tip radii are used. A basic prerequisite would also be that the processing requires a directed processing. This condition is necessary to meet the conversions. For the EDM the random crater formation does not give this direction, so the conversions are not very useful. When machining workpieces such as hard metal, where material parts can fall out, the conversion is no longer valid at all.
It must therefore be borne in mind that the same measuring conditions must be observed in order to make a reasonable comparison possible. On conversions I would basically renounce.
@Schulze sir, when we trace the path using touch probe or optical scan, we actually get all the values of surface roughness parameters (Ra, Rq, Rt, Rp, Rv, Rskew, Rkurt etc.). I wanted to know which of the parameters are useful for particular application.
Tribeni, This is an example of measured surface roughness for glass and fused silica diffusers. Ra and Sz are roughly related to # numbers. I think, rms surface roughness and autocorrelation length will better specify their optical performance. Regards, Shigeo
As usual, this depends very much on typical methods of analysis in your field of research. In most cases, this will be the RMS roughness Rq (also ISO 4287-1). However, RMS assumes a Gaussian height distribution. Rskew and Rkurt help you to determine, how well your experiment matches this condition. Thus they are directly related to Rq. The only relevant alternative is Ra, which is less prone to outliers. Just try both on your specific sample. I never used Rv and Rp. In AFM, they usuall values of noise artifacts.
In any case you should consider the general remarks of H.-P. Schulze: tips with similar radii, same measurement conditions (e.g. image size) and alway a good image quality (no "shadow"-effects or tip artifacts).
http://www.imagemet.com/WebHelp6/Default.htm#RoughnessParameters/Roughness_Parameters.htm
I think the questions is. where to use the SR parameters for which aplication. for example in machining we use Ra and Rz practicaly, Why? which parameters can be used in sealing surface, coating surface or frictional surface for example in honning pipe or hydrolic cylinder or lapped surface or painting surface. is there any knowledge about this questions. ?
Dear Tribeni.
The most common surface roughness parameter is Ra, the arithmetic roughness average of the surface. After Ra, perhaps the next most common measurement parameter for gears is Rz.
Ra and Rq
The majority of the methods for linking surface fatigue or failure to surface roughness use Ra or the similar measure, Rq, the root mean square of the surface roughness. Surfaces with the same Ra or Rq values can vary widely in their surface topology and, therefore, diverge widely in performance.
Ra and Rz
Ra and Rz are different parameters of roughness. Ra is the average roughness of a surface. Rz is the difference between the tallest "peak" and the deepest "valley"in the surface. “Rz is calculated by measuring the vertical distance from the highest peak to the lowest valley within five sampling lengths, then averaging these distances. Rz averages only the five highest peaks and the five deepest valleys—therefore, extremes have a much greater influence on the final value.”
Rz, Rt, and Rsk
Rz is the average of the tallest peak to the depth of the lowest valley from each “cutoff” or subsection of a surface measurement. Rz offers advantages over parameters such as Rt, which is simply the height of the single “tallest” peak to the depth of the “deepest” valley in a surface trace — as it incorporates more of the surface.
Rsk classifies both the magnitude and direction of surface roughness such that positive numbers represent surfaces that consist predominately of peak asperities, and negative numbers represent surfaces that consist primarily of valleys. Rsk is an excellent parameter for measuring surfaces that are expected to have peaks or valleys. The common interpretation of Rsk as applied to gear flanks would be that a negative value is good and a positive value is bad. However, if a surface has been largely planarized such that few peaks or valleys remain, then Rsk returns to nearly zero.
Ashish
Dear All
Actually i am having doubt about the analysis parameters of SR.
If we measure the Rz , Ra, Rv values then what is the exact reason for their variation with the varying parameters of the film prepared.. In some cases it is decreased and in some it is increased.. it's depends on depth profile?Please clarify.
Thank you
Dear Kiruthiga, There are many responsible factors magnitude and direction, depth of profile, surface topology , texture morphology, temperature, environment varying the surface roughness parameters. As temperature increases surface roughness parameters decreases and it is also understood temperature varies on the surface. Irregularities surface site is another reason...
Ashish
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