Bonded Abrasives and Corundum Abrasive

Classification and performance of bonded abrasives

1. What is bonded abrasives

Bonded abrasive is a kind of abrasive. It is a tool with a certain shape and grinding ability made of abrasive (grinding material) and bond. Usually, ceramic, resin, rubber, rhombic bittern and other bonds are used to consolidate the abrasive particles into abrasives with certain shapes and strengths.

2. Classification and performance of bonded abrasives

Bonded abrasives are divided into ordinary bonded abrasives (corundum, silicon carbide) and superhard bonded abrasives (diamond, cubic boron nitride) according to different abrasives. According to different binders, bonded abrasives are divided into ceramic abrasives, resin binders, rhombolite binders, rubber binders and metal binders. The commonly used bonded abrasives mainly include the following:

2.1 Grinding wheel

(1) Classification: vitrified grinding wheels, resin grinding wheel, rubber grinding wheel, ginger bitter earth grinding wheel.

(2) Raw materials: natural abrasives, brown aluminium oxide, white corundum, zirconium corundum, single crystal corundum, pink corundum, microcrystalline corundum, sintered corundum, green silicon carbide, black silicon carbide, ceramic abrasives, mixed abrasives and other abrasives.

(3) Grinding methods: surface grinding, external grinding, internal grinding, centerless grinding, end grinding, tool grinding and slow feed.

(4) Grinding wheel shape: parallel grinding wheel, cylindrical grinding wheel, cup-shaped grinding wheel, bowl-shaped grinding wheel, dished grinding wheel, conical grinding wheel, bevel grinding wheel, annular grinding wheel, single-sided convex grinding wheel, double-sided concave grinding wheel, double-sided convex grinding wheel and special-shaped grinding wheel.

(5) Processing conditions: dry grinding, wet grinding, general.

(6) Application industries: stone industry, steel industry, automobile industry, bearing industry, gear industry, tool processing, aerospace, steam turbine manufacturing, construction industry, mold industry, woodworking industry, general manufacturing, photoelectric industry, hardware industry, glass industry, ceramic industry, jewelry industry, magnetic materials, mineral industry, bioengineering and other industries.

2.2 Cutting plate / Grinding plate

(1) Resin cutting piece: the abrasive is consolidated and formed with resin bond, which - generally has two shapes: parallel cutting and cymbal cutting.

(2) Rubber cutting piece: using rubber bond, it also has two shapes: parallel cutting and cymbal cutting.

(3) Grinding disc: with resin as a binder, there are two kinds of cymbal grinding wheel and conical cymbal.

2.3 Grinding head

There are many kinds of grinding heads, including ceramic grinding heads and rubber grinding heads. Suppliers can choose according to different abrasives and binders; The grinding head has many shapes, and each company has no unified code for the shape of the grinding head.

2.4 Oilstone and grindstone

Oilstone is a strip-shaped consolidated abrasive tool made of abrasive and binder, which is named because it is usually lubricated when in use. It is generally used for manual grinding of tools and parts, and can also be clamped on the machine tool for honing and superfinishing.

(1) Natural oilstone: processed with natural abrasive, it has many shapes, such as round oilstone, semi-circular oilstone, rectangular oilstone, square oilstone, triangular oilstone and so on.

(2) Fiber oilstone: processed with ceramic abrasives. There are four kinds of binders - resin, ceramic, rubber and rhombic bittern.

(3) Ruby oilstone: it is processed with sintered corundum as abrasive. It can be used for dry grinding, wet grinding and general use.

(4) Corundum oilstone: brown corundum, white corundum, zirconium corundum and single crystal corundum, pink corundum, microcrystalline corundum and other corundum abrasives.

(5) Silicon carbide oilstone: processed with silicon carbide abrasive as raw material.

2.5 Grinding block

Including brown corundum grinding block, silicon carbide grinding block and other common grinding blocks.

2.6 Sand tile

The sand tile is a block fixed abrasive tool made of abrasive and bond, most of which are made of corundum or silicon carbide abrasive and resin bond, and a small amount of which are made of ceramic or rhombic clay bond. It is usually used after several sand tiles are fixed on the grinding wheel chuck and embedded into a group. There is generally a certain interval between each sand tile to facilitate heat dissipation and chip removal.

3. Manufacturing technology of bonded abrasives

High efficiency grinding technology is a necessary condition of advanced manufacturing technology, which completely solves the machining limitations of high precision and low efficiency of traditional grinding. While obtaining high efficiency and high precision, it can process various materials and shapes with high surface integrity and reduce the cost. It is of great significance to speed up the research and application of grinding technology and improve the development and application level of the existing machinery processing industry.

The high efficient grinding wheel should have good wear resistance, high dynamic balance accuracy, crack resistance, good damping characteristics, high stiffness and good thermal conductivity, and its mechanical strength must be able to withstand the cutting force during high efficiency grinding. High efficiency grinding wheels can use corundum, silicon carbide, CBN and diamond abrasives. Under the conditions of appropriate binders and advanced manufacturing technology, the service speed of the produced abrasive tools can reach 125m / s.

The existing vitrified bonded grinding wheels can resist the corrosion of water, oil, acid and alkali, and can maintain the correct geometry. It has large porosity, high grinding rate and high strength, but it has poor toughness, elasticity and vibration resistance, and can not bear lateral force. For the grinding of v-wheel < 35m / s, this bond is the most widely used and can be made into various grinding tools, which is suitable for forming grinding and grinding threads, gears, crankshafts, etc.

The existing resin bond has high strength and elasticity, is not afraid of impact, and can work at high speed. It has the function of friction polishing, but its firmness and heat resistance are worse than those of ceramic binder. It is not resistant to acid and alkali, has small porosity and is easy to be blocked; The high-speed grinding of V wheel > 50M / s can make the grinding groove of flake grinding wheel, the rake face of edge grinding tool and high-precision grinding.

Inorganic polymer bonded abrasives have high strength, so they have high service speed. They are generally suitable for high-speed cutting, raw grinding and heavy load grinding. In addition, inorganic polymer bond grinding has a certain elasticity, high heat resistance and good grinding self sharpening.

In this way, compared with traditional resin bonds, the sharp angle retention and shape retention of the grinding wheel are good. It is also suitable for fine grinding, such as thread grinding, forming grinding and edge grinding. It can be said that inorganic polymer binder has the properties of both ceramic abrasive tools and resin abrasive tools, and avoids their defects at the same time.

The following are the specific design principles and methods of grinding tools made of inorganic polymer bond, high-speed and heavy load grinding tools made of inorganic polymer composite materials, raw grinding tools, strong grinding tools, high-efficiency deep grinding tools, slow feed grinding tools, fast point grinding tools, high-efficiency grinding tools, spindle and bearing grinding tools, high-efficiency grinding machine grinding tools, rail grinding tools Some superficial views on the process of belt grinding are put forward.

4. Design and manufacture of grinding wheel

In the design and manufacture of grinding wheel, the following problems should be considered: Firstly, the performance of the processed material and the accuracy of the machined surface of the material should be considered, and then the appropriate abrasive, particle size and mixed abrasive should be selected for grinding.

The second is the various stress conditions, impact resistance, speed, temperature, processing material and service life of the grinding wheel under the working condition of the grinding wheel, as well as the selection of appropriate binding agents and auxiliary materials, and consider the mutual matching and scientific technology. The strength of abrasives depends on the performance of the binder, the manufacturing process of abrasives and the specification of abrasives.

The factors that affect the strength of bonded abrasives include: the type and particle size of abrasive, the type and performance of binder, the hardness, structure, density, mixing, forming process and curing process conditions of abrasive tools, the shape of abrasive tools, the ratio of an outer diameter of the abrasive wheel to aperture, etc.

Among them, after the characteristics and specifications of abrasives are given, the performance of binder, mixing molding process and curing process conditions are the most important. In addition, the grinding ratio of the grinding wheel is also an important parameter.

5. Matrix combination of bonded abrasives

For the above factors involved, let's talk about the abrasive, abrasive thickness and particle gradation in the abrasive matrix combination one by one. The coarseness of abrasives refers to the overall coarseness of abrasives with different particle sizes after mixing together; Particle gradation refers to the combination or combination of abrasives of different types, sizes and quantity proportions.

Abrasive is an important factor for grinding tools. The selection of abrasives is mainly determined according to the properties of workpiece materials, such as hardness, tensile strength, toughness and so on.

The basic principle of selecting abrasives is: when grinding workpiece materials with high hardness, abrasives with higher hardness should be selected; When grinding workpiece materials with high tensile strength, abrasive with high toughness should be selected; When grinding materials with low tensile strength, silicon carbide abrasive with relatively brittle or high strength shall be selected. When selecting abrasive particle size, it should be mainly selected according to the requirements of machining accuracy, surface roughness and grinding efficiency.

The general principles are as follows: it is required that the workpiece to be ground has high roughness, and coarse particle size shall be selected; The surface roughness is required to be low, and fine grain size shall be selected. When the workpiece requires high geometric accuracy and low surface roughness, the mixed particle size should be selected. The geometric accuracy of the workpiece is required to be high. 

When the contact area between the abrasive wheel and the workpiece is small, the fine particle size should be selected; When the contact area is large, choose coarse grain size. The material of the workpiece is hard and brittle, and fine grain size shall be selected; The workpiece material is soft and tough, and coarse particle size shall be selected. The thermal conductivity of the workpiece is poor, easy to heat, deform and burn, so the coarse particle size should be selected. 

These problems also involve the thermal conductivity, heat capacity, density, pores and other factors affecting grinding. Abrasives are usually divided into coarse abrasives, medium abrasives, fine abrasives and micro powder abrasives. Under the same abrasive conditions, the total surface area of micro powder abrasive is larger and that of coarse abrasive is smaller.

In the production of abrasive tools, the surface needs to be wrapped with a binder to give bonding strength. The larger the total surface area of the abrasive, the more inorganic polymer binders need to wrap the abrasive surface. Generally, the binding dose of inorganic polymer used in coarse abrasive is less than that used in fine abrasive.

Abrasive particle gradation, in order to improve the grinding efficiency, we should not only consider the strength of abrasive tools and processing materials, but also determine the amount of abrasive, that is, the gradation of abrasive. When the grinding tool is grinding, on the one hand, the grinding tool is impacted by the grinding body, on the other hand, it is also damaged by the grinding body, so as to ensure the sharpness of the grinding tool and complete the whole grinding process.

Obviously, in unit time, the more contact points of the grinding body participating in grinding, that is, the more abrasive particles participating in grinding in unit time, the higher grinding efficiency. When the abrasive is fixed, to increase the contact between the abrasive and the grinding body, the smaller the size of the grinding abrasive, the better. But on the other hand, if you want to finish grinding, the grinding tool must have enough impact capacity.

The task of abrasives is to ensure not only sufficient ability to grind the materials of the workpiece, but also to ensure that the abrasives grind the workpiece to a certain fineness. Therefore, under certain other conditions (such as abrasive strength, abrasive speed, etc.), this task can be completed only by selecting abrasives of appropriate size and reasonably proportioning them.

6. Manufacture of Bonded abrasives

In the manufacture of abrasive tools, the gap between abrasives is theoretically produced after the abrasive is coated by the binder. In order to reduce the amount of inorganic polymer binder to meet the strength of abrasive tools and improve grinding efficiency, the gap between abrasive particles should be reduced as much as possible. 

Another idea that should be clarified is that people are used to burning the workpiece during grinding, which is due to the dense group value of the abrasive wheel and the lack of sufficient chip holding pores. The air hole plays the role of chip holding and chip removal during grinding, and can hold the coolant, which is conducive to the dissipation of grinding heat.

In order to meet some special processing requirements, the pores of traditional abrasives can also be impregnated with some fillers, such as sulfur and paraffin, so as to improve the service performance of abrasives. This filler is also traditionally known as the fourth element of abrasives. This is not always true, because the amount and volume of wear debris produced during high-speed grinding are by no means large enough to be accommodated by small pores.

Although the superhard abrasive wheel has no pores at all, it can work well as long as its surface is properly trimmed. If the pore volume ratio of the resin abrasive wheel is increased, it is bound to reduce its strength and make it wear prematurely, which is not worth the loss. In addition, the influence of the working linear speed of the grinding wheel on the amount of debris is complex. When the grinding wheel is increased from 28.8m/s to 33.6m/s, the speed is increased by 16%, and the blockage is increased by three times.

Because the increase of the linear speed of the grinding wheel reduces the maximum cutting depth of the abrasive particles, the chip cross-sectional area, and the cutting times and grinding heat increase, these two factors increase the amount of blockage. However, when the linear speed of the grinding wheel reaches a certain degree (such as more than 50m / s), the blockage of the grinding wheel is greatly reduced. For all kinds of workpiece materials, each has a certain critical grinding wheel speed with the smallest blockage.