What Is Surface Roughness?

01 What is surface roughness?

In technical communication, many people are used to using “surface finish” index.

In fact, “surface finish” is put forward according to human visual point of view, while “surface roughness” is put forward according to the actual surface micro geometry.

In order to be in line with the international standard (ISO), the term “surface finish” is no longer used in the national standard. The term “surface roughness” should be used in the formal and rigorous expression.

Surface roughness refers to the roughness of the machined surface with small spacing and small peaks and valleys.

The distance between the two peaks or troughs (wave distance) is very small (less than 1 mm), which belongs to the micro geometry error.

Specifically, it refers to the high and low degree of micro peak valley Z and the space s condition.

Generally according to S:

  • S < 1 mm – surface roughness
  • 1 ≤ s ≤ 10mm- waviness
  • S > 10 mm- f shape

02 Formation factors of surface roughness

The surface roughness is generally formed by the machining method and other factors, such as the friction between the tool and the surface of the part in the machining process, the plastic deformation of the surface layer metal when the chips are separated, the high frequency vibration in the process system, the discharge pits in the electrical machining, etc.

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 Evaluation basis of surface roughness

1) Sampling length

The unit length of each parameter and sampling length are the length of a specified reference line for evaluating surface roughness.

Under the standard of ISO1997, the datum lengths are generally 0.08mm, 0.25mm, 0.8mm, 2.5mm and 8mm.

Sampling length Ln and evaluation length L of RA, RZ and Ry

Ra(μm)Rz.Ry(μm)L(mm)Ln=5L(mm)
≥ 0.008-0.02≥ 0.025-0.100.080.4
>0.02-0.1>0.10-0.500.251.25
>01-2.0>0.50-10.00.84
>2.0-10.0>10.0-50.02.512.5
>10.0-80.0>50.0-320840

2) Evaluation length

It consists of N datum lengths.

The surface roughness of each part of the part surface can not reflect the real parameters of the roughness in a reference length, but it needs to take N sampling lengths to evaluate the surface roughness.

Under the standard of ISO 1997, the evaluation length N is generally equal to 5.

3) Baseline

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

04 Evaluation parameters of surface roughness

1) Height characteristic parameters

Ra contour arithmetic mean deviation: arithmetic mean value of absolute value of contour offset within sampling length (Lr).

In the actual measurement, the more the number of measuring points, the more accurate the Ra is.

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

Ra is preferred in the common range of amplitude parameters.

Before 2006, there was another evaluation parameter in the national standard: “Ten point height of micro unevenness” was expressed by Rz, and the maximum height of contour was expressed by Ry.

After 2006, the ten point height of micro unevenness was cancelled in the national standard, and Rz was used to represent the maximum height of contour.

2) Characteristic parameters of spacing

The average width of the Rsm contour cell.

The average value of the micro roughness distance of the profile 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.

In the case of the same Ra value, the Rsm value is not necessarily the same, so the reflected texture will be different

The surfaces that pay attention to texture usually focus on Ra and Rsm.

The shape feature parameter of Rmr is expressed by the ratio of the contour support length to the sampling length.

The supporting length of the profile is the sum of the lengths of each section of the profile obtained by cutting the lines parallel to the central line and C away from the peak line of the profile within the sampling length.

05 Comparison table of VDI3400, Ra and Rmax

Ra index is often used in domestic production;

Rmax index is commonly used in Japan, which is equivalent to Rz index;

VDI 3400 standard is commonly used to mark surface roughness in European and American countries.

VDI indicators are often used by factories that make European mold orders. As usual, customers say that “the surface of this product is made according to VDI30”.

VDI3400 surface has a corresponding relationship with the commonly used standard Ra, many people often have to look up the data to find the corresponding value, the following table is very complete, recommended collection.

Comparison table of VDI3400 standard and Ra

VDIRaVDIRa
3400μm3400μm
001231.4
10.112241.6
20.126251.8
30.14262
40.16272.2
50.18282.5
60.2292.8
70.22303.2
80.25313.5
90.28324
100.32335
110.35345
120.4355.6
130.45366.3
140.5377
150.56388
160.63399
170.74010
180.84111.2
190.94212.6
2014314
211.124416
221.264518

Comparison table of RA and Rmax

Ra (μm)Rmax (μm)
0.10.4
0.20.8
0.41.5
0.562.4
0.83.3
1.124.7
1.66.5
2.210.5
3.212.5
4.517.5
6.324

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