Comprehensive overview of surface roughness

1.Principle The surface quality of machining refers to the microscopic unevenness of the machined surface of the part after machining, also called roughness. When the surface of the workpiece surface roughness Ra<0.8um is called: mirror surface. The surface quality after processing directly affects the physical, chemical and mechanical properties of the workpiece. The working performance, reliability and life of the product depend to a large extent on the surface quality of the main parts. Generally speaking, the surface quality requirements of important or critical parts are higher than those of ordinary parts. This is because parts with good surface quality will greatly improve their wear resistance, corrosion resistance and fatigue damage resistance.

Mirror surface - is the ideal state of metal cutting and the most effective means to improve the service life of mechanical parts.

Mirror surface - is a traditional synonym for very good roughness after mechanical cutting, and can clearly reflect the metal surface of the image of the object.

No matter what kind of metal processing method is used, there will always be tiny uneven knife marks on the surface of the part, and there will be staggered peaks and valleys. The surface after rough machining can be seen with the naked eye, and the surface after finishing can be seen with the naked eye. Still visible with a magnifying glass or microscope. This is the surface roughness of the part after machining, which used to be called surface finish. The parameters of surface roughness stipulated by the state are composed of height parameters, spacing parameters and comprehensive parameters.

The machining methods to obtain the mirror surface are: material removal method, non-cutting method (rolling processing).

The processing methods for removing material are: grinding, lapping, polishing, electric spark.

Non-cutting processing methods include: rolling (using mirror tools), extrusion.

In addition to graphics and dimensions, part drawings must also have some quality requirements that should be met for manufacturing parts, which are generally called technical requirements. The content of technical requirements usually includes: surface roughness, dimensional tolerance, shape and position tolerance, material and its heat treatment, surface treatment, etc. The following first introduces the surface roughness and its injection method.

Processing method comparison

Surface features    Surface roughness (Ra) value    Examples of processing methods

Visible tool marks   Ra100, Ra50, Ra25    Rough turning, rough planing, rough milling, drilling

Slight tool marks   Ra12,5,Ra6,3,Ra3,2    Fine turning, fine planing, fine milling, rough reaming, rough grinding

No traces of processing can be seen, and the processing direction can be slightly distinguished    Ra1,6,Ra0,8,Ra0,4    Finish turning, fine grinding, fine reaming, grinding

Dark glossy surface        Ra0,2,Ra0,1,Ra0,05       Grinding, honing, super fine grinding, polishing

2.The concept of surface roughness

In mechanics, roughness refers to the microscopic geometrical properties of small pitches and peaks and valleys on a machined surface. It is one of the problems of interchangeability research. Surface roughness is generally formed by the processing method used and other factors, such as the friction between the tool and the part surface during processing, the plastic deformation of the surface layer metal when the chip is separated, and the high-frequency vibration in the process system. Due to the different processing methods and workpiece materials, the depth, density, shape and texture of the traces left on the processed surface are different. Surface roughness is closely related to the matching properties, wear resistance, fatigue strength, contact stiffness, vibration and noise of mechanical parts, and has an important impact on the service life and reliability of mechanical products.

After the surface of the part is processed, it looks smooth, but it is uneven when viewed under magnification. Surface roughness refers to the micro-geometric shape features composed of small spacing and tiny peaks and valleys on the surface of the processed part, which is generally formed by the processing method or other factors. The function of the part surface is different, and the required surface roughness parameter values are also different. The surface roughness symbol should be marked on the part drawing to illustrate the surface characteristics that must be achieved after the surface is completed.

No matter which processing method is used, the surface of the part is not absolutely flat and smooth. When observed under a microscope (or magnifying glass), microscopic unevenness of peaks and valleys cannot be seen. This microscopic unevenness on the surface , It is generally formed by various factors such as the movement and friction between the tool and the part, the vibration of the machine tool and the plastic deformation of the part. The micro-geometric features of the small spacing and peaks and valleys on the surface are called surface roughness.

There are 3 types of surface roughness height parameters:

1. Profile arithmetic mean deviation Ra

Within the sampling length, the arithmetic mean of the absolute value of the distance between points on the contour line along the measurement direction (Y direction) and the reference line.

2. Ten-point height Rz of micro-roughness

It refers to the sum of the average value of the five largest profile peak heights and the average value of the five largest profile valley depths within the sampling length.

3. Maximum profile height Ry 

Within the sampling length, the distance between the highest peak line and the lowest valley bottom line of the profile.

3.The cause the main reasons for the formation of surface roughness are:

1) Tool marks during processing;

2) Plastic deformation during cutting and separation;

3) The friction between the tool and the machined surface;

4) High-frequency vibration of the process system.

4.The selection of surface roughness 

The selection of surface roughness parameter values should not only meet the functional requirements of the part surface, but also consider economic rationality. For specific selection, it can be determined by analogy with reference to existing drawings of similar parts. Under the premise of meeting the functional requirements of the part, the larger surface roughness parameter value should be selected as much as possible to reduce the processing cost. Generally speaking, the working surface, mating surface, sealing surface, friction surface with high movement speed and high unit pressure of the parts have high requirements for smoothness of the surface, and the parameter value should be smaller. For non-working surfaces, non-fitting surfaces, and surfaces with low dimensional accuracy, the parameter value should be the relationship between the parameter Ra value and the processing method and its application examples, which can be used as a reference for selection.