The essence and implementation of polishing
Why do we need to perform surface processing on mechanical parts?
The surface treatment process will be different for different purposes.
1 Three purposes of surface processing of mechanical parts:
1.1 Surface processing method for obtaining part accuracy
For parts with matching requirements, the requirements for accuracy (including dimensional accuracy, shape accuracy and even position accuracy) are usually relatively high, and accuracy and surface roughness are related. To obtain accuracy, the corresponding roughness must be achieved. For example: accuracy IT6 generally requires the corresponding roughness Ra0.8.
[Common mechanical means]:
- Turning or milling
- Fine boring
- fine grinding
- Grinding
1.2 Surface processing methods for obtaining surface mechanical properties
1.2.1 Obtaining wear resistance
[Common methods]
- Grinding after hardening or carburizing/quenching (nitriding)
- Grinding and polishing after hard chrome plating
1.2.2 Obtaining a good surface stress state
[Common methods]
- Modulation and grinding
- Surface heat treatment and grinding
- Surface rolling or shot peening followed by fine grinding
1.3 Processing methods to obtain surface chemical properties
[Common methods]
- Electroplating and polishing
2 Metal surface polishing technology
2.1 Significance It is an important part of the field of surface technology and engineering, and is widely used in industrial production processes, especially in electroplating industry, coating, anodizing and various surface treatment processes.
2.2 Why are the initial surface parameters and the achieved effect parameters of the workpiece so important? Because they are the starting and target points of the polishing task, which determines how to choose the type of polishing machine, as well as the number of grinding heads, material type, cost, and efficiency required for the polishing machine.
2.3 Grinding & Polishing Stages and Trajectories
The four common stages of grinding and polishing ] : according to the initial and final roughness Ra values of the workpiece, coarse grinding - fine grinding - fine grinding - polishing. The abrasives range from coarse to fine. The grinding tool and workpiece must be cleaned every time they are changed.
2.3.1 The grinding tool is harder, the micro-cutting and extrusion effect is greater, and the size and roughness have obvious changes.
2.3.2 Mechanical polishing is a more delicate cutting process than grinding. The polishing tool is made of soft material, which can only reduce the roughness but cannot change the accuracy of size and shape. The roughness can reach less than 0.4μm.
2.4 Three sub-concepts of surface finish treatment: grinding, polishing, and finishing
2.4.1 Concept of mechanical grinding and polishing
Although both mechanical grinding and mechanical polishing can reduce surface roughness, there are also differences:
- 【Mechanical polishing】: It includes dimensional tolerance, shape tolerance and position tolerance. It must ensure the dimensional tolerance, shape tolerance and position tolerance of the ground surface while reducing the roughness.
- Mechanical polishing: It is different from polishing. It only improves the surface finish, but the tolerance cannot be reliably guaranteed. Its brightness is higher and brighter than polishing. The common method of mechanical polishing is grinding.
2.4.2 [Finishing processing] is a grinding and polishing process (abbreviated as grinding and polishing) carried out on the workpiece after fine machining, without removing or only removing a very thin layer of material, with the main purpose of reducing surface roughness, increasing surface gloss and strengthening its surface.
The accuracy and roughness of the part surface have a great influence on its life and quality. The deteriorated layer left by EDM and the micro cracks left by grinding will affect the service life of the parts.
① The finishing process has a small machining allowance and is mainly used to improve surface quality. A small amount is used to improve machining accuracy (such as dimensional accuracy and shape accuracy), but it cannot be used to improve position accuracy.
② Finishing is the process of micro-cutting and extruding the workpiece surface with fine-grained abrasives. The surface is processed evenly, the cutting force and cutting heat are very small, and a very high surface quality can be obtained. ③ Finishing is a micro-processing process and cannot correct larger surface defects. Fine processing must be performed before processing.
The essence of metal surface polishing is surface selective micro-removal processing.
3. Currently mature polishing process methods: 3.1 mechanical polishing, 3.2 chemical polishing, 3.3 electrolytic polishing, 3.4 ultrasonic polishing, 3.5 fluid polishing, 3.6 magnetic grinding polishing,
3.1 Mechanical polishing
Mechanical polishing is a polishing method that relies on cutting and plastic deformation of the material surface to remove the polished protrusions to obtain a smooth surface.
Using this technology, mechanical polishing can achieve a surface roughness of Ra0.008μm, which is the highest among various polishing methods. This method is often used in optical lens molds.
3.2 Chemical polishing
Chemical polishing is to make the microscopic convex parts of the material surface dissolve preferentially in the chemical medium over the concave parts, so as to obtain a smooth surface. The main advantages of this method are that it does not require complex equipment, can polish workpieces with complex shapes, can polish many workpieces at the same time, and is highly efficient. The core issue of chemical polishing is the preparation of the polishing liquid. The surface roughness obtained by chemical polishing is generally several tens of μm.
3.3 Electrolytic polishing
Electrolytic polishing, also known as electrochemical polishing, selectively dissolves tiny protrusions on the surface of the material to make the surface smooth.
Compared with chemical polishing, the effect of cathode reaction can be eliminated and the effect is better. The electrochemical polishing process is divided into two steps:
(1) Macro-leveling: The dissolved products diffuse into the electrolyte, and the geometric roughness of the material surface decreases, Ra 1μm.
(2) Gloss smoothing: Anodic polarization: Surface brightness is improved, Ralμm.
3.4 Ultrasonic polishing
The workpiece is placed in an abrasive suspension and placed in an ultrasonic field. The abrasive is ground and polished on the workpiece surface by the oscillation of the ultrasonic wave. Ultrasonic machining has a small macroscopic force and will not cause deformation of the workpiece, but the tooling is difficult to manufacture and install.
Ultrasonic machining can be combined with chemical or electrochemical methods. On the basis of solution corrosion and electrolysis, ultrasonic vibration is applied to stir the solution to separate the dissolved products on the workpiece surface and make the corrosion or electrolyte near the surface uniform; the cavitation effect of ultrasonic waves in the liquid can also inhibit the corrosion process and facilitate surface brightening.
3.5 Fluid polishing
Fluid polishing relies on high-speed flowing liquid and the abrasive particles it carries to brush the workpiece surface to achieve the purpose of polishing.
Commonly used methods include: abrasive jet processing, liquid jet processing, fluid dynamic grinding, etc.
3.6 Magnetic grinding and polishing
Magnetic grinding and polishing uses magnetic abrasives to form abrasive brushes under the action of a magnetic field to grind the workpiece.
This method has high processing efficiency, good quality, easy control of processing conditions, and good working conditions. With suitable abrasives, the surface roughness can reach Ra0.1μm.
Through this article, I believe you will have a better understanding of polishing. Different types of polishing machines will determine the effect, efficiency, cost and other indicators of achieving different workpiece polishing goals.
What type of polishing machine your company or your customers need should not only be matched according to the workpiece itself, but also based on the user's market demand, financial situation, business development and other factors.
Of course, there is a simple and efficient way to deal with this. Please consult our pre-sales staff to help you.
Post time: Jun-17-2024