What is the Grinding Process and Grinding Wheel?
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What is the Grinding Process and Grinding Wheel?

A grinding wheel is a tool for grinding. It is composed of coarse-grained abrasive compounds bonded together. After forming a solid round object, grinding and abrasive processing can be performed on a grinding machine. Grinding is a commonly used processing method, which can obtain higher workpiece density and surface quality. Different grinding methods will produce different grinding defects. The most influential factor is the grinding wheel.
Published: Jun 27, 2022
What is the Grinding Process and Grinding Wheel?

What is a Grinding Wheel?

Grinding is a processing method. Different from cutting, grinding uses a grinding wheel, which uses the abrasive particles on the tool to process the surface of the workpiece under a certain pressure. The composition of the grinding wheel includes an abrasive, bonding agent, and pores, which have different functions. The abrasive is the part that directly cuts the workpiece, the bonding agent is responsible for grasping the abrasive, and the pores can dissipate heat and remove chips. Under the action of grinding, the mechanism overcomes the chain force inside the solid material, and the size and shape of the ground material will change.

Grinding is a micro-processing method. Grinding produces relative motion between the machined surface of the workpiece and the grinding tool with the help of a grinder and an abrasive and exerts a certain pressure to remove the workpiece from the workpiece. Remove tiny surface raised layers to obtain low surface roughness and high dimensional accuracy, geometric shape accuracy, etc. In-mold manufacturing, it is widely used in precision die-casting molds, plastic molds, and automotive cover molds that require high product appearance quality.

There are types of grinding wheels, and they have various sizes and shapes. The different abrasives, bonding agents, and manufacturing processes have their scope of application. If the grinding wheel is not properly evaluated, it maybe directly affects the machining accuracy, surface roughness, and production efficiency. Therefore, it is necessary to look at various conditions and situations to select a suitable grinding wheel, and finally obtain satisfactory results.

What are the Basic Principles of Grinding?

  1. Physical Action:
    When grinding, the grinding surface of the grinding tool is evenly coated with abrasive. If the hardness of the grinding tool material is lower than that of the workpiece, when the grinding tool and the workpiece move relative to each other under the action of pressure, the grinding agent has sharp edges and corners and high hardness. Particles, some will be pressed into the grinding tool surface to produce cutting action, and some will roll or slide between the grinding tool and the workpiece surface to produce sliding. These particles are like countless cutting edges, which produce a small amount of cutting action on the surface of the workpiece, and evenly cut a thin layer of metal from the surface of the workpiece. At the same time, under the action of grinding pressure, the passivated abrasive grains produce micro-extrusion plastic deformation on the machined surface by extruding the peak point of the machined surface, so that the workpiece gradually obtains high dimensional accuracy and low surface roughness.
  2. Chemical Action:
    When an abrasive such as chromium oxide and stearic acid is used, a chemical action occurs between the abrasive and the machined surface of the workpiece during the grinding process, resulting in a very thin oxide film, which is easily ground off. The grinding process is the process of continuous generation and erasing of the oxide film, many cycles are repeated so that the roughness of the processed surface is reduced.

Grinding Characteristics:

Grinding can play a role in high-speed machining when processing soft or easy-to-machine materials and can conform to complex workpiece shapes. Grinding has several advantages, such as abrasives that are harder than workpieces, so it can process harder materials. It is the abrasive particles that can be continuously generated, and the tool needs to be replaced after passivation, but the grinding wheel has a self-sharpening effect, the grinding speed is fast, and the simultaneous action of countless tool tips can have a high processing rate.

  1. Low surface roughness:
    Grinding belongs to micro-feed grinding, and the cutting depth is small, which is beneficial to reducing the surface roughness value of the workpiece. The machined surface roughness can reach Ra0 .01μm.
  2. High dimensional accuracy:
    The grinding adopts extremely fine micro-powder abrasives, and the machine tool, the grinding tool, and the workpiece are in an elastic floating working state. Under the action of low speed and low pressure, the convex peaks of the machined surface are ground successively, and the machining accuracy can reach 0. 1μm~0.01μm.
  3. High shape accuracy:
    The workpiece is basically in a free state during grinding, the force is uniform, the movement is stable, and the movement accuracy does not affect the shape and position accuracy. The cylindricity of the machined cylinder can reach 0.1 μm.
  4. Improve the mechanical properties of the surface of the workpiece:
    The cutting heat of grinding is small, the deformation of the workpiece is small, the metamorphic layer is thin, and there will be no micro-cracks on the surface. It can reduce the surface friction coefficient and improve wear resistance and corrosion resistance. There is residual compressive stress on the surface of grinding parts, which is beneficial to improving the fatigue strength of the workpiece surface.
  5. The requirements for grinding tools are not high:
    The grinding tools and equipment used for grinding are generally simple and do not require extremely high precision. The material of the grinding tool is generally softer than the workpiece, and it will be worn during grinding and should be repaired and replaced in time.
Grinding Sensing:

The traditional grinding method uses torsion force and vibration to observe, but because the characteristics of the grinding wheel in contact with the workpiece are the dense random structure created by the emery and the binder, the grinding force during the grinding process is complicated, and it makes difficult to see significant conditions in torque and vibration sensing than direct force sensing.

How to Know the Status of Grinding?

When doing grinding, you may know that the grinding is not good, and there will be a larger and less stable grinding force, but there are many factors for poor grinding that it is impossible to judge, such as the threshing of the grinding wheel, out-of-roundness, and inability to repair sand. Good, poor fixture locking, jumping flowers, uneven grinding force, and even can directly reflect the surface roughness between grinding. With the assistance of the smart knife handle, the grinding condition can be clearly understood, avoiding the problem of finding out the problem after the processing is completed, but requiring heavy work, resulting in higher time and cost.

How to Choose the Right Grinding Wheel?

There are many types of grinding wheels including a variety of sizes and shapes. Like tools, they have different applicable ranges according to different materials and performance. If the selection is incorrect, it will directly affect the machining accuracy, surface roughness, and manufacturing efficiency.

How to Choose Abrasive?

In the selection of abrasives, it can be selected according to the workpiece and the heat treatment method. If the workpiece has high tensile strength, abrasives with high toughness can be selected. For workpieces with low hardness and high elongation, brittle abrasives can be used.

  • Brown corundum grinding wheel: This kind of grinding wheel has high hardness and high toughness. It is suitable for grinding metal workpieces with high tensile strength (carbon steel, alloy steel, etc.). This kind of abrasive has good grinding performance and wide adaptability. It is commonly used It is cheap and widely used to remove coarse grinding with a large allowance.
  • White corundum grinding wheel: The hardness is higher than that of brown corundum, and the toughness is lower. When grinding, the abrasive grains are easily broken. Therefore, the heat generated is small, which is suitable for fine grinding of high carbon steel, high-speed steel, or thin-walled parts, etc., but the cost is higher than that of brown corundum.
  • Black silicon carbide grinding wheel: This kind of grinding wheel is brittle and sharp, and its hardness is higher than that of white corundum. It is suitable for grinding workpieces with low mechanical strength, such as cast iron, aluminum, or refractory materials.
  • Green silicon carbide grinding wheel: This kind of grinding wheel is more brittle than black carbide, with sharp abrasive grains and good thermal conductivity. It is suitable for grinding hard and brittle materials such as cemented carbide and optical glass.
  • Chrome steel jade grinding wheel: Suitable for grinding tools, measuring tools, instruments, and other workpieces with high surface processing quality requirements.
  • Microcrystalline corundum grinding wheel: Suitable for grinding stainless steel, bearing steel, etc., for forming grinding, plunge grinding, and mirror grinding.
  • Zirconium steel jade grinding wheel: Suitable for grinding austenitic stainless steel, titanium alloy, heat-resistant alloy, etc., and is inclined to heavy-duty grinding.
Granularity:

The particle size refers to the particle size of the abrasive, and the size is usually expressed by the particle size number. When grinding with coarser particle size, the production efficiency is higher, but the surface of the workpiece will be rougher, while the finer particle size has better roughness but lower productivity. If the roughness requirements are met, a coarse-grained grinding wheel can be selected to maintain high production efficiency.

Hardness:

Hardness refers to the difficulty of the grinding wheel abrasive particles falling off under the action of external force. According to the processing requirements of different workpieces, the grinding wheel will be divided into different hardness levels. If the selected grinding wheel is too hard, the dull abrasive particles will not easily fall off, resulting in Blockage and grinding heat increase, making the workpiece easy to burn, reducing efficiency, and ultimately affecting the surface quality of the workpiece. If you choose a too soft grinding wheel, it will fall off when the abrasive grains are still sharp, increasing the wear of the grinding wheel, affecting the accuracy of the workpiece, and losing the correct shape. Therefore, when selecting the hardness of the grinding wheel, it is necessary to evaluate the contact area between the grinding wheel and the workpiece, the shape of the workpiece, the grinding method, the cooling method, or the type of bonding agent of the grinding wheel.

Binder Selection:

The most common binders are ceramic binders and resin binders. The former is an inorganic binder with stable chemical properties, heat resistance, and good corrosion resistance. The grinding wheel using this binder has high grinding efficiency and low wear and can maintain the grinding wheel. Geometric shape, a wide range of applications. However, the disadvantage of the vitrified bond grinding wheel is that it is brittle and cannot accept severe vibration. The resin bond is an organic bond, with high strength, certain elasticity, low heat resistance, good self-sharpening, and simple production and short process period, and its application range is second only to the ceramic bond.

Organization Selection:

The organization refers to the percentage of the abrasive grains in the grinding wheel to the volume of the grinding wheel. The tighter the grinding wheel, the better the surface of the workpiece can be ground, and the looser the grinding wheel, the larger the gap can ensure to accommodate the abrasive debris and avoid the blockage of the grinding wheel. The structure can be selected according to the material of the workpiece. When grinding soft metals, it is easy to block, and the grinding wheel with a loose structure can be selected. When performing more precise grinding, to ensure better roughness, the grinding wheel with a tight structure can be selected.

What are the Classifications of Grinding?

  • According to the degree of automation of the grinding process:
    • Manual Grinding: The relative movement of the workpiece and the grinding tool is manually operated. The processing quality depends on the skill level of the operator, which is labor-intensive and low-efficiency. It is suitable for various surfaces of metal and non-metal workpieces. Parts such as local narrow slits, slots, deep holes, blind holes, and dead corners on mold-forming parts are still mainly ground by hand.
    • Semi-mechanical Grinding: One of the workpieces and laps uses simple mechanical movements and the other uses manual operations. The processing quality is still related to operator skills, and the labor intensity is reduced. It is used for grinding the inner and outer cylindrical surfaces, plane, and conical surfaces of workpieces. Commonly used when grinding mold parts.
    • Mechanical Grinding: The movement of the workpiece and grinding tool adopts mechanical movement. The processing quality is guaranteed by mechanical equipment, and the work efficiency is relatively high. But it can only be applied to the grinding of parts with less complex surface shapes.
  • According to the Conditions of Use of Abrasives:
    • Wet Grinding: During the grinding process, the abrasive is smeared on the surface of the grinding tool, and the abrasive rolls or slides between the grinding tool and the workpiece, forming a cutting effect on the surface of the workpiece. The machining efficiency is high, but the geometric shape and dimensional accuracy, and gloss of the machined surface are not as good as dry grinding and are mostly used for rough grinding and semi-finishing planes and inner and outer cylindrical surfaces.
    • Dry Grinding: Before grinding, the abrasive particles are evenly pressed into the working surface of the grinding tool to a certain depth, which is called sand embedding. During the grinding process, the grinding tool and the workpiece maintain a certain pressure and move relative to a certain trajectory to achieve micro-cutting, thereby obtaining high dimensional accuracy and low surface roughness. When dry grinding, no or only a small amount of lubricating abrasive is applied. Generally used for fine grinding planes, the production efficiency is not high.
    • Semi-dry Grinding: A paste-like grinding paste is used, like wet grinding. When grinding, according to the requirements of workpiece machining accuracy and surface roughness, apply the abrasive paste in time. It is suitable for the rough and fine grinding of various workpieces.
In conclusion

Before selecting a grinding wheel, it should be evaluated according to various conditions, such as abrasive, particle size, hardness, binder, structure, etc., plus different workpiece materials, grinding methods, and cooling methods. Although many factors need to be considered, however, make full preparations before grinding, and even conduct a quality inspection with the assistance of smart knife handles during grinding to ensure that the final quality meets the requirements.

Published by Jun 27, 2022 Source :machsync

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