The meaning of surface roughness evaluation parameters ra and rz

01 What is surface roughness?

In technical communication, many people are used to using the “surface finish” indicator. In fact, “surface finish” is put forward according to human vision, while “surface roughness” is put forward according to the actual micro geometric shape of the surface. In order to be in line with the international standard (ISO), the term “surface finish” is no longer used in the national standard, and the term “surface roughness” should be used in formal and rigorous expressions.

Surface roughness refers to the roughness of machined surfaces with small spacing and small peaks and valleys. The distance (wave distance) between two wave peaks or two wave troughs is very small (below 1mm), which belongs to micro geometric shape error.

Specifically, it refers to the height and spacing S of micro peaks and valleys. Generally, the score is S:

S ＜ 1mm is the surface roughness

1 ≤ S ≤ 10mm is the waviness

F shape if S ＞ 10mm

02 Formation factors of surface roughness

The surface roughness is generally formed by the processing method adopted and other factors, such as the friction between the tool and the part surface during the processing, the plastic deformation of the surface layer metal when the chip is separated, the high-frequency vibration in the process system, and the discharge pits in the electrical machining. Due to different processing methods and workpiece materials, the depth, density, shape and texture of the traces left on the machined surface are different.

03 Assessment basis of surface roughness

1) Sampling length

The unit length of each parameter. The sampling length is the length of a reference line specified for evaluating the surface roughness. Under the ISO1997 standard, 0.08mm, 0.25mm, 0.8mm, 2.5mm and 8mm are generally used as reference lengths.

The difference between the surface roughness Rz and Ra: Ra: the arithmetic mean deviation of the evaluation profile The arithmetic mean deviation of the evaluation profile Ra refers to the arithmetic mean value of the absolute value of the ordinate Z within a sampling length, which is recorded as Ra.

The value of Ra can objectively reflect the micro geometric characteristics of the measured surface. The smaller Ra is, the smaller the amplitude of micro peaks and valleys on the measured surface is, the smoother the surface is; On the contrary, it indicates that the rougher the measured surface is.

The Ra value is measured with a stylus inductive profiler. Limited by the stylus radius and the measuring principle of the instrument, it is applicable to Ra values between 0.025 and 6.3 μ  M surface.

Rz: maximum height of contour, micro unevenness, ten point height

Rz: the sum of the average values of the five largest contour peak heights and the average values of the five largest contour valley depths within the sampling length.

Main parameters and definitions of Ra, Rz, Ry and other surface roughness (also known as surface finish)

2) Assessment length

Consists of N datum lengths. The surface roughness of each part of the surface of a part cannot truly reflect the true parameters of the roughness in a datum length, but needs to take N sampling lengths to evaluate the surface roughness. Under the ISO1997 standard, the evaluation length is generally N equal to 5.

3) Baseline

The datum line is the contour centerline used to evaluate the surface roughness parameters.

04 Surface roughness evaluation parameters

1) Height characteristic parameters

Ra contour arithmetic mean deviation: the arithmetic mean value of the absolute value of the contour offset within the sampling length (lr). In actual measurement, the more measuring points, the more accurate Ra.

Rz Maximum height of contour: the distance between the contour peak line and the valley bottom line.

Ra is preferred within the common range of amplitude parameters. Before 2006, there was also an evaluation parameter in the national standard that “ten point height of micro unevenness” was represented by Rz, and the maximum height of the contour was represented by Ry. After 2006, the national standard canceled the ten point height of micro unevenness, and Rz was used to represent the maximum height of the contour.

2) Spacing characteristic parameters

Average width of Rsm contour cells. The average value of the profile micro unevenness spacing within the sampling length. The micro unevenness distance refers to the length of the contour peak and the adjacent contour valley on the central line. With the same Ra value, the Rsm value is not necessarily the same, so the reflected texture will be different. Surfaces that pay attention to texture usually pay attention to Ra and Rsm.

The Rmr shape feature parameter is expressed by the contour support length ratio, which is the ratio of the contour support length to the sampling length. The supporting length of the contour is the sum of the length of the sections obtained by cutting the contour with a straight line parallel to the centerline and c away from the contour peak line within the sampling length.