Surface Specifications ISO 21920 | Roughness | Mean Roughness Depth | Arithmetic Mean Roughness
In this video we address surface specifications according to ISO 21920. This standard defines various parameters for characterizing surface roughness. The total height Rt indicates the profile height within which all surface irregularities are contained. It corresponds to the vertical distance between the deepest valley and the highest peak within the entire evaluation length.
The maximum height Rz (formerly: mean roughness depth) corresponds to the averaged maximum height of the roughness profile over five sampling lengths, measured from the deepest valley to the highest profile peak within each individual sampling length.
The largest of the five individual profile heights was referred to as R-Max in the old standard. In the new standard, this parameter is replaced by RZX and is called the maximum height per section. This parameter is determined by the maximum profile height within a moving sampling length along the evaluation length.
The mean peak height Rp (formerly: mean smoothing depth) is obtained by averaging the individual peak heights over five sampling lengths, each measured from the profile mean line to the corresponding highest profile peak. Similarly, the mean valley depth Rv (formerly: mean groove depth) is obtained by averaging the individual valley depths over five sampling lengths, each measured from the profile mean line to the corresponding deepest valley.
With the exception of the total profile height Rt, the aforementioned parameters are defined by averaging over five sampling lengths. However, the parameters RA and RQ, determined over the profile areas, are defined in the new standard over the entire evaluation length.
The arithmetic mean height Ra (formerly: centre-line average) indicates the average deviation of the roughness profile from the mean line. Visually, this parameter corresponds to the height of a rectangular area having the same surface area as the profile peaks and valleys above and below the mean line.
In comparison to the less expressive Ra value, the root mean square height RQ gives significantly more weight to individual outliers due to the squaring of the roughness values. This parameter represents the standard deviation of the roughness profile distribution and is therefore better suited for statistical evaluations.
Now let's turn to the parameters that describe the material portion of the roughness. The core roughness depth Rk of the roughness core profile is determined as the vertical distance between the intersection points of the material ratio curve’s mean line and the diagram axes.
The reduced peak height Rpk corresponds to the height of a triangle with an area equal to that of the profile peaks above the core roughness profile, where the base width of the triangle corresponds to the material ratio Rmrk1. Similarly, the reduced valley depth Rvk is determined as the height of a triangle with an area equal to that of the profile valleys below the core roughness profile, with the base width corresponding to the material ratio Rmrk2.
The material ratio Rmrk2 corresponds to the largest material portion of the core roughness profile, while Rmrk1 corresponds to the smallest material portion.
00:00 Surface Characteristics
01:05 Surface Symbols
04:04 Entry of Surface Symbols in Drawings
06:00 Surface Roughness
07:01 1st Order: Form Deviation
07:48 2nd Order: Waviness
08:19 3rd Order: Roughness (Grooves)
08:43 4th Order: Roughness (rills, scales, peaks)
09:10 5th Order: Roughness (Microstructure)
09:30 6th Order: Lattice Structure
10:19 Stylus Profiling Method (stylus profilometer)
12:13 Determination of the maximum height of the roughness profile Rz (average roughness depth)
14:52 Maximum height per section Rzx (substitute for Rmax)
15:50 Determination of the total profile height Rt
17:41 Determination of the arithmetic mean height of the roughness profile Ra (average roughness value)
18:51 Visual determination of the arithmetic mean height
20:33 Root Mean Square Height (Standard Deviation of the Roughness Distribution)
21:16 Mean Peak Height (Smoothness Depth) and Valley Depth (Groove Depth)
22:42 Ratio of Rp to Rz
24:48 Surface Bearing Ratio Curve (Material ratio, Abbott-Firestone Curve)
25:48 Roughness Core Profile (Core Roughness Depth, Reduced Peak Height, and Valley Depth)
27:08 Material ratios RMRK1 and RMRK2 (formerly load-bearing ratios MR1 and MR2)
28:31 Periodic and Non-Periodic Surface Profiles
29:23 Mean groove width
31:05 Filtering of Wavelengths
33:55 Cut-off wavelengths (nesting index)
36:04 Setting Classes (Determination of Cutoff Wavelengths)
38:55 Summary of the roughness parameters
42:45 Example