Surface Profile Parameters
Simple Roughness Amplitude Parameters
Ra - Average Roughness
The average roughness is the area between the roughness profile and its mean line, or the integral of the absolute value of the roughness profile height over the evaluation length:
When evaluated from digital data, the integral is normally approximated by a trapezoidal rule:
Graphically, the average roughness is the area (shown below) between the roughness profile and its center line divided by the evaluation length (normally five sample lengths with each sample length equal to one cutoff):
打量的拼音The average roughness, Ra, is an integral of the absolute value of the roughness profile. It Is the shaded area divided by the evaluation length, L. Ra is the most commonly ud roughness parameter.
The average roughness is by far the most commonly ud parameter in surface finish measurement. The earliest analog roughness measuring instruments measured only Ra by drawing a stylus continuo
usly back and forth over a surface and integrating (finding the average) electronically. It is fairly easy to take the
and integrating (finding the average) electronically. It is fairly easy to take the absolute value of a signal and to integrate a signal using only analog electronics. That is the main reason Ra has such a long history It is a common joke in surface finish circles that "RA" stands for regular army, and "Ra" is the chemical symbol for Radium; only "Ra" is the average roughness of a surface. This emphasizes that the a is a subscript. Older names for Ra are CLA and AA meaning center line average and area average.. An older means of specifying a range for Ra is RHR. This is a symbol on a drawing specifying a minimum and maximum value for Ra.
Older drawings may have ud this notation to express an allowable range for Ra. This
For example, the cond symbol above means that Ra may fall between 10 µin and 20 µin. Ra is Not the End of the Story Ra does not tell the whole story about a surface. For example, here are three surfaces that all have the same Ra, but you need no more than your eyes to know that they are quite different surfaces. In some applications they will perform very differently as well.
distinguishes their different general shapes.
The three surfaces differ in the shape of the profile - the first has sharp peaks, the cond deep valleys, and the third has neither. Even if two profiles have similar shapes, they may have a different spacing between features. The following three surfaces also all have the same Ra.
eight的序数词If we want to distinguish between surfaces that differ in shape or spacing, we need to calculate other parameters for a surface that measure peaks and valleys and profile shape and spacing. The more complicated the shape of the surface we want and the more critical the ft'nction of the surface, the more sophisticated we need to be in measuring parameters beyond Ra.
Rq - Root-Mean-Square Roughness
The root-mean-square (rms) average roughness of a surface is calculated from another integral of the roughness profile:
The digital equivalent normally ud is:
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For a pure sine wave of any wavelength and amplitude Rq is proportional to Ra; it's about 1.11 times larger. Older instruments made u of this approximation by calculating Rq with analog electronics (which is easier than calculating with analog electronics) and then multiplying by 1.11 to report Rq. H
妈妈的秘密owever, real profiles are not simple sine waves, and the approximation often fails mirably. Modern instruments either digitize the profile or do not report Rq. There is never any reason to make the approximation that is proportional to Ra. Rq has now been almost completely superded by Ra In metal machining specifications. Rq still has value in optical applications where it is more directly related to the optical quality of a surface.
Rt, Rp, and Rv
The peak roughness Rp is the height of the highest peak in the roughness profile over the evaluation length (p1 below). Similarly, Rv is the depth of the deepest valley in the roughness profile over the evaluation length (v1). The total roughness, Rt, is the sum of the two, or the vertical distance from the deepest valley to the highest peak.
The three extreme parameters will succeed in finding unusual conditions: a sharp spike or burr on the surface that would be detrimental to a al for example, or a crack or scratch that might be indicative of poor material or poor processing.
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Rtm, Rpm and Rvm
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The three parameters are mean parameters, meaning they are averages of the sample lengths. For example, define the maximum height for the i-th sample length as Rpi. Then Rpm is:
Similarly,
and
梦见出差where Rvi is the depth of the deepest valley in the i-th sample length and Rti is the sum of Rvi and Rpi:
butalso
The three parameters have some of the same advantages as Rt, Rp, and Rv for finding extremes in the roughness, but they are not so nsitive to single unusual features.
Ry (or Rmax) - Maximum Roughness Height Within a Sample Length
Ry and Rmax are other names for Rti. Rmax is the older American name. Ry is the newer ISO and American name. For a standard five cutoff trace there are five different values of Ry. Ry is the maximum peak to lowest valley vertical distance within a single sample length. what's it Rymax(ISO) - Maximum Ry
