Surface Roughness Evaluation Parameters, Measurement Methods and the Meaning of Ra

In machining, the absolutely smooth surface is an ideal state, which can never be achieved in fact.

The machined workpiece, no matter how smooth it looks, will have ups and downs on its surface.

The main reasons for this phenomenon are:

Tool marks during machining;

Plastic deformation during chip separation;

Friction between tool and machined surface;

High-frequency vibration in process system, etc.

1. Surface roughness, waviness and shape error

If you zoom in, the ups and downs of the machined part surface are actually composed of many small peaks and valleys.

According to their different geometric shape characteristics, we divide the surface topography of machined parts into surface roughness, surface waviness and shape error.

When the wave distance is less than 1mm, it belongs to micro geometry error – surface roughness.

A surface waviness with wave distance between 1 and 10 mm and periodic change.

A macroscopic geometric error with a wave distance of more than 10 and no obvious periodic change.

Compared with the waviness and shape error, the surface roughness is actually a very slight unevenness, with the wave distance less than 1mm, which can no longer be accurately identified by the naked eye, and can only be observed clearly with the help of tools.

2. Surface roughness evaluation parameters: Ra, Rz, Ry

Surface roughness is a technical index to evaluate the surface quality of parts.

The higher the surface roughness requirement (i.e., the smaller the surface roughness parameter value), the higher the processing cost.

The unit of surface roughness is micrometer( μ m) , i.e. 0.001mm.

Generally, there are three evaluation parameters as follows:

The arithmetic mean deviation Ra of the contour refers to the average of the absolute distance between each point on the contour and the centerline of the contour within a certain sampling length.

Ra can fully reflect the characteristics of the height of the surface micro geometric shape, but due to the limitation of the functions of measuring instruments, it is not necessary to be used as an evaluation parameter for surfaces that are too rough or too smooth.

Ten point average height of micro unevenness Rz: refers to the sum of the average of five maximum contour peak heights and the average of five maximum contour valley depths within the sampling length.

Rz can only reflect the peak height of the contour, but cannot reflect the sharp or flat geometric characteristics of the peak.

At the same time, if the points are different, the Rz values obtained are different, so the subjective influence of the measurer is greater.

Maximum height of contour Ry: the distance between the peak line and valley bottom line of the contour within the sampling length.

Ry is the sum of the vertical distances from the highest point and the lowest point of the ten points of micro unevenness to the center line.

Therefore, it is not as accurate as the geometric characteristics reflected by the Rz value.

It has practical significance for not allowing deep machining marks on some surfaces and for the surface quality of small parts.

3. Surface roughness drawing marking

SymbolMeaning and description
Basic symbol indicating that the surface can be obtained by any method. When the roughness parameter value or related description is not added, it is only applicable to simplified code annotation.
 The basic symbol plus a short drawing indicates that the surface is obtained by removing the material. For example: turning, milling, drilling, planing, grinding, etc.
 The basic symbol plus a small circle indicates that the surface is obtained without removing the material. For example: casting, forging, rolling, stamping, etc.

4. Surface roughness measurement method

Comparison method: directly compare the measured surface with the surface roughness sample plate, which is mostly used in workshops to evaluate workpieces with large surface roughness values.

Light cutting method: use the light cutting principle to measure with a double tube microscope. Commonly used for measuring Rz of 0.5~60 μ m.

Interferometry: measure with an interference microscope based on the principle of light wave interference. Rz and Ry values can be measured.

Needle tracing method: It is a method to measure the surface roughness Ra value by using a diamond stylus to gently scratch the surface to be measured.

5. Effect of surface roughness performance

The influence of surface roughness on parts is mainly shown in the following aspects:

Affect wear resistance

Too large or too small surface roughness is not wear-resistant.

The rougher the surface, the smaller the effective contact area between mating surfaces, the greater the pressure, the greater the friction resistance, and the faster the wear.

Too small surface roughness will also lead to increased wear.

Because the surface is too smooth to store lubricating oil, it is not easy to form an oil film between the joint surfaces, and it is easy to cause molecular bonding and increase wear.

Influence the stability of the fit

For clearance fit, the rougher the surface is, the easier it is to wear, which gradually increases the clearance during the working process;

For interference fit, because the micro convex peaks are squeezed flat during assembly, the actual effective interference is reduced and the connection strength is reduced.

Influence fatigue strength

The surface of rough parts has large wave troughs, which, like sharp notch and crack, are sensitive to stress concentration, thus affecting the fatigue strength of parts.

Affect corrosion resistance

The rough surface of the part is easy to make corrosive gas or liquid penetrate into the inner layer of the metal through the micro valleys on the surface, causing surface corrosion.

Affect sealing

Rough surfaces cannot fit tightly, and gas or liquid leaks through the gap between the surfaces.

Affect contact stiffness

Contact stiffness is the ability of the joint surface of a part to resist contact deformation under external forces. The rigidity of the machine depends to a large extent on the steel connection rigidity between the parts.

Influence the measurement accuracy

The surface roughness of the measured surface of the part and the surface of the measuring tool will directly affect the measurement accuracy, especially in precision measurement.

In addition, the surface roughness will affect the coating, thermal conductivity and contact resistance, reflective ability and radiation performance, resistance of liquid and gas flow, and current flow on the conductor surface to varying degrees.

6. Surface roughness of several common materials

Material Science

Hardness

Roughness

Photometric description

S136

54HRC

Ra0.005

Very high finish mirror effect

8407

52HRC

DF-2

58HRC

Ra0.01

Low finish without sandpaper

XW-10

60HRC

S136

300HB

Ra0.02

Lower finish but no sandpaper texture

718

300HB

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