Parameters

For effective analysis of surface roughness or waviness, the profile generated by the measuring instrument needs to be evaluated according to internationally recognized mathematical formulas, called parameters. The purpose of using a parameter is to apply a number that can characterize a certain aspect of the surface and hence remove the need for subjective operator assessment. It is not possible to characterize a surface completely with a single parameter, so a combination of parameters is normally used.

Parameters can be separated into four basic types:

• Amplitude parameters are measures of the vertical characteristics of the surface deviations.

• Spacing parameters are measures of irregularity spacings along the surface, regardless of the amplitude of these irregularities.

• Hybrid parameters analyze a combination of the amplitude and spacing of the surface irregularities.

• Extended parameters are not simply defined by the profile data and require further inputs or attributes.

In order to characterize a surface using parameters, between three and six parameters must be chosen, including at least one of each of the types listed above. Examples of different types of parameters and how they are applied can best be described by discussing the various types of surfaces generated by finishing methods.

Ground, Turned, and Milled Machined Surfaces. One of the most commonly used roughness amplitude parameters is the roughness average (Ra), which is used to assess the coarseness of the surface such as those produced by grinding, turning, and milling operations. The parameter Ra is defined as the mean height of the roughness profile. It does have some significant drawbacks in that many differing profiles can have the same Ra and yet have very different performance characteristics. Another averaging parameter, Rq, takes the root mean square of the profile and is more sensitive to surface variations.

Surfaces Subject to Stress. For some surfaces it is sometimes desirable to specify the maximum roughness height, Rmax, or the peak-to-valley height, Rz, rather than use the mean height given by Ra. The Rmax parameter measures the highest and lowest points of the profile and is particularly valuable where components are subjected to high stresses. Any large peak-to-valley heights may be areas likely to suffer from crack propagation. However, because Rz is very susceptible to dirt or scratches, it is an unstable parameter. It is usual to take an average of the individual peak-to-valley heights, which can be done in slightly different ways with the parameters Rtm (Rz in DIN specifications) and Rz in ISO specifications.

Sheet Steel and Peak Counting. Although roughness amplitude is very important in most applications, the spacing of the roughness peaks can be equally important. Particularly in the manufacture and use of sheet steel, surface texture control is necessary to obtain consistent lubrication when forming the sheet, avoid scoring, and prevent the texture from showing through the paint on the finished product. Amplitude parameters such as Ra are not sufficient to specify the different types of texture that can be obtained from the rolling process. The peak count parameter, Pc, allows the operator to monitor and control the roughness peak spacing as well as the amplitude, thus producing a better bonding of finishes, a more uniform finish of plating and painting, and a reduced risk of cracking during drawing or forming operations. Peak spacing is also an important factor in the performance of friction surfaces such as brake drums. The high spot count is another spacing parameter; it is used frequently by the automotive industry on cylinder liners and other surfaces where the lubrication characteristics of the topography are important.

Bearing Surfaces. The most common use of engineering surfaces is to provide a bearing surface for another component moving relative to it, resulting in wear. The material ratio parameter, Tp (in %), is commonly used to simulate the effect of wear on a surface. Material ratio is defined as the ratio of the length of the surface to the evaluation length at a specified depth in the profile. A practical example is to imagine a surface plate resting on the highest peak of a profile as illustrated in Fig. 6. As the peaks wear and the material line (the top line of the remaining profile) descends, the length of the bearing surface (the length of the profile in contact with the lapping plate) increases. The parameter Tp can thus be used to control bearing surfaces as well as surfaces requiring lubrication.

Fig. 6 Illustration showing the derivation of material ratio, tp

Plateau Honed and Lapped Surfaces. The Rk parameter uses the material ratio curve for analysis. It is commonly used to evaluate plateau honed surfaces, which are the result of multiple machining operations. Peaks are removed and large valleys are created to retain lubrication. The parameter has a wide application in the automotive field.

In using the Rk parameter five primary areas are analyzed. All measurements and parameters are scale dependent.

• Core roughness depth (Rk) is the working part of the surface that, after the initial running-in period, will carry the load and influence the life and performance of the surface.

• Reduced peak height (Rpk) is the top portion of the surface that will be worn away in the running-in period. Used in conjunction with this is the parameter Mr1, the associated percent length of the top portion of the profile that will be removed during the same process.

• Reduced valley depth (Rvk) is the lowest part of the surface that has the function of retaining the lubricant during its working life. Used in conjunction with this is the parameter Mr2, which is the associated percent length of the bottom part of the profile that will be used to retain lubricant.

Reflective, Painted, Elastic, and Wear-Resistant Surfaces. A useful hybrid parameter that has a close correlation to various surface properties is the average slope of the surface. The slope of the profile is the angle (in terms of the gradient) that it makes with a line parallel to the center line. It is defined as the mean of all the slopes at all points on the profile.

The parameter can be used to measure the actual profile length (i.e., the length that would be occupied if all the peaks and valleys were stretched out in a straight line). The steeper the average slope, the longer the actual length of the surface compared with its nominal length. The parameter is employed in painting and plating applications where the length of the surface available for keying is important. It is scale dependent.

Average slope is also useful in assessing other properties of engineering surfaces. It can be related to the hardness and elasticity of the surface, where the higher the slope, the greater the chance of surface deformation upon loading. In assessing surface reflectivity a high slope indicates a surface with low reflectivity. Also, the higher the level of average surface slope, the higher the level of friction and the greater the level of wear.

Microstructural Analysis of Finished Surfaces

Donald C. Zipperian, Buehler Ltd.

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