Surface Texture Parameters

Some form of texture and roughness is exhibited by the surface of each component. These parameters vary due to various production methods and the structure of the part itself. It is essential to measure the surface in two and three dimensions (2D and 3D) to control the production process to the degree needed in present applications and to obtain a better understanding about the surface of a component. Surface texture parameters can be categorized as follows:

• Hybrid
• Waviness
• Spacing
• Roughness

Terminology and Surface Texture Parameters

Cutoff filter is capable of measuring the wavelength at which the surface structure is differentiated between roughness and waviness data. For precise measurements, it is very important to choose the correct filter cutoff in software; profile refers to a 2D slice through an area; areal is a 3D surface area; and evaluation length refers to the area from which data is collected.

A 2D area corresponds to the slice length as defined in the filled plot, and a 3D area corresponds to the field of view of the instrument. Cutoff length or sampling length is the area selected for assessing the roughness parameter, with the cutoff wavelength and surface unevenness spaced well beyond the sampling length being considered as waviness. Centre line or mean line refers to a straight line that is created by measuring a weighted average for each data point, resulting in equal areas above and below the line; and surface texture is the surface’s topography showing certain deviations, which are characteristics of the actual surface. It includes roughness and waviness.

Additionally, hybrid parameters are a mixture of spacing and roughness parameters. Spacing parameters refer to a measure of the horizontal or lateral periodic characteristics of the surface; roughness parameters refer to the finer, non-periodic unevenness in the surface texture during the production process; and waviness parameters refer to a biggar component of surface texture on which roughness is overlaid.

Roughness Parameters

Among the largest set of parameters, roughness parameters include Swedish height (H), areal flatness deviation (ISO flatness), total peak-to-valley profile height (P_t ISO), base roughness depth (R_3z), base roughness profile depth (R_3z ISO), arithmetical mean deviation (R_a), Kurtosis (R_ku), maximum peak-to-valley profile height.(R_max ISO), highest peak (Rp), mean peak profile height (Rpm ISO), root-mean-square (rms) roughness (R_q (rms), maximum peak-to-valley height (Rt (PV), mean peak-to-valley roughness (R_tm), mean peak-to-valley profile roughness (R_tm ISO), lowest valley (Rv), mean valley profile depth (R_vm), maximum peak-to-valley roughness (), ten-point height (R_z), average peak-to-valley profile roughness (R_z ISO), total peak-to-valley areal height (SP_t ISO), base roughness areal depth (SR_3z ISO), maximum peak-to-valley height (SR_max ISO), mean peak areal height (SR_pm ISO), mean peak- to-valley areal roughness (SR_tm ISO), mean valley areal depth (SR_vm ISO), average radial peak-to-valley areal roughness (SR_z), average peak-to-valley areal roughness (SR_z ISO), the average of individual R_z results, measured by slicing the data array in the x-axis (SR_zX), and the average of individual R_z results, measured by slicing the data array in the y-axis (SR_zY).

Figure 1. Sampling length

Figure 2. ISO flatness

Figure 3. P_t ISO

Figure 4. R_3z

Waviness Parameters

W_q is the root-mean-square roughness of all points from a plane fit to the waviness data; W_a is the average deviation or average roughness of all points from a plane fit to the waviness data; and W_y (W_max) is the maximum height of the waviness data.

Spacing Parameters

P_c (Peaks) represents the number of peaks comprised in the analysis. The average distance between peaks is peak spacing, and the number of peaks per unit area is peak density. S indicates the average spacing between local peaks across the evaluation length, and S_m is the average spacing between peaks at the mean line over the evaluation length. Summits are the number of summits comprised in the analysis; summit spacing refers to the average distance between summits; and summit density refers to the number of summits per unit area. Similarly, valleys are the number of valleys contained in the analysis. The average distance between valleys is called valley spacing, and the number of valleys per unit area is called valley density.

Hybrid Parameters

The arithmetic average surface slope of the entire data matrix is called Δ_a (Slope Ra), while Δ_q (Slope rms) refers to the geometric average slope of the whole data matrix. Δ_tm (Slope R_tm) refers to the average peak-to-valley slope of nine sample areas on the entire data matrix; I_q refers to the root-mean-square deviation of all points from a plane fit to the input data, I_a indicates the average deviation, or average surface height, of all points from a plane fit to the input data; I_y (I_max) refers to the maximum height of the input data; R_volume is the volume of the roughness data as specified by software as either above or below the surface; R_{SurfAreaRatio} refers to the ratio of the planar area taken up by the data to the roughness surface area; I_{SurfAreaRatio} means the ratio of input surface area to the planar area taken up by the data; and W_{SurfAreaRatio} refers to the ratio of input surface area to the planar area taken up by the data. Bearing ratio parameters such as Rk, Rvk, Rpk, T_p etc. are other hybrid parameters.

Conclusion

The surface texture parameters listed above are important to gain a better insight into the surface of an component and to manage the production process to the extent required in present applications.

This information has been sourced, reviewed and adapted from materials provided by Zygo Corporation.

For more information on this source, please visit Zygo Corporation.