The grinding process is the cutting process of the abrasive grains in the grinding tool to the workpiece. The choice of abrasive tools is to make full use of the cutting ability of the abrasive particles to overcome the resistance generated by the physical and mechanical properties of the workpiece material. Due to the variety of specifications of the abrasive tools, each type of abrasive tool is not a universal cutting tool, only a certain scope of application. Therefore, for each type of grinding work, the characteristics of the grinding tool must be properly selected to achieve a good grinding effect. Abrasive properties mainly include abrasive particles, particle size, hardness, binder, structure, shape and size. Here, the general principle of selecting abrasive tools is described in terms of the characteristics of the abrasive tools.
There are many types of abrasives, and the selection principle is directly related to the physical and mechanical properties of the materials being processed.
In general, when grinding workpiece materials with high tensile strength, it is advisable to select corundum abrasives with high toughness, and to grind workpiece materials with low tensile strength, to select silicon carbide with high brittleness and high hardness. Abrasives are preferred.
When selecting abrasives, consider the chemical reaction between the workpiece material and the abrasive, the chemical affinity between the abrasive and the workpiece material, and the thermal stability of the abrasive (ie, red hardness).
Abrasive particle size selection
The choice of abrasive grain size primarily considers the requirements for grinding efficiency and surface roughness of the workpiece. Generally, it can be selected according to the following points.
(1) The workpiece processing precision is high and the surface roughness value is low. The abrasive material with fine abrasive grain size should be selected.
(2) When the contact area between the abrasive tool and the workpiece surface is relatively large, or the grinding depth is also large, the abrasive with coarse grain size should be selected.
(3) Rough grinding is that the machining allowance and the grinding depth used are relatively large, and the grinding tool should be coarser than the fine grinding, so as to improve the coarse grinding tool.
(4) For the cutting and trenching process, a grinding wheel with coarse grain size, loose structure and high hardness should be used.
(5) When grinding tough metals and soft metals, such as brass, copper, soft bronze, etc., the surface of the abrasive is easily blocked by the chips, so a coarse-grained abrasive should be used.
(6) Grinding materials with high hardness, such as hardened steel, alloy steel, etc., should use abrasive tools with coarser grain size.
(7) For workpieces with small cutting allowance or small contact surface between the grinding tool and the workpiece, a grinding tool with a finer grain size may be selected. Abrasives for wet grinding than dry grinding can be finer in particle size.
(8) When machining on a rigid grinding machine, a grinding wheel with a coarser grain size may be selected.
(9) When forming and grinding, it is desirable that the shape of the surface of the grinding wheel is kept good, so that a finer particle size is preferred.
(10) In the case of high-speed grinding, in order to improve the grinding efficiency, the grain size of the abrasive is rather finer than that of the ordinary grinding.
Abrasive tool selection
Reasonable selection of the hardness of the abrasive tool is the key to good grinding results.
The most basic principle for selecting the hardness of the grinding tool is to ensure that the grinding tool has proper self-sharpness during the grinding process to avoid excessive wear of the grinding tool and to ensure that the grinding temperature is not excessively high during grinding.
The most basic method for selecting the hardness of the grinding tool is: the hardness of the workpiece is high, the hardness of the grinding tool is low; the hardness of the workpiece is low, and the hardness of the grinding tool is high.
To choose the hardness of the abrasive tool, generally consider the following situations:
(1) The contact area between the grinding tool and the workpiece is large. The hardness of the grinding tool should be selected to be soft to avoid excessive heating of the workpiece and affect the grinding quality.
(2) When grinding the surface and casting the burr, the hard or super hard grinding wheel should be selected; when the heavy steel slab is heavy, the hard or super hard grinding wheel should be selected to prevent the grinding wheel from wearing too fast.
(3) The diamond grinding tool (grinding wheel or oil stone) used for dressing requires a high hardness due to the high pressure during dressing, so super-hard grinding tools are often used.
(4) Heavy-duty grinding machines and grinding machines with better rigidity, because of the small vibration during grinding, the abrasive grains are not easily damaged, so a grinding wheel with a softer hardness can be used.
(5) When the outer circle is cut into the grinding, in order to avoid the burn of the workpiece, the hardness of the grinding wheel should be softer than that during the axial feeding.
(6) The automatic cutter grinding machine can use a softer grinding wheel than the manual cutter grinding machine.
(7) The smoother the surface requirements and the more precise the workpiece size requirements, the softer the grinding wheel should be selected to avoid excessive heat during grinding and deterioration of the surface layer of the workpiece.
(8) Scratches on the surface of the workpiece are often related to improper selection of the hardness of the grinding tool.
(9) The workpiece is prone to heat when dry grinding, and the grinding wheel of soft 1~2 should be used when wet grinding.
(10) When the production efficiency requirement is high, a soft grinding wheel can be selected to facilitate the self-sharpening of the grinding wheel and reduce the number of dressings.
(11) During high-speed grinding, when the feed rate is constant, the chips cut by the abrasive grains become thinner, and the cutting force of the abrasive grains is correspondingly reduced. The wear of the grinding wheel is also increased in order to improve the self-sharpness of the grinding wheel. Its hardness is 1 to 2 grades softer than ordinary grinding, which is the case when grinding at high speed. Similarly, for some unbalanced workpieces (such as crankshafts), since the workpiece speed during grinding cannot be too high, the hardness of the grinding wheel should be selected to be soft to avoid burning the workpiece.
(12) When grinding steel balls (balls), the super-hard grinding wheel should be selected; in general, the hardness of the grinding wheel should be selected to the intermediate hardness.
(13) When sharpening hard alloy and high speed steel tools, the grinding wheel of J to G hardness should be selected.
(14) In the case of profile grinding, in order to maintain the correct geometry of the workpiece, the wear of the grinding wheel should not be too large, so the hardness of the grinding wheel should be higher.
Binding agent selection
The properties of the binder in the tool affect its ability to react with the abrasive particles and its strength. The reaction ability between the binder and the abrasive grains is good, and the binding force of the binder to the abrasive grains is stronger, and the abrasive grains are not easily broken or peeled off. The higher the strength of the binder, the higher the grinding force of the particles, and the higher the rotational strength of the grinding wheel without breaking.
The choice of bonding agent is mainly related to the grinding method, the speed of use and the surface processing requirements of the workpiece. Each binder has its own advantages and disadvantages, and the type of abrasive binder should be selected in combination with the conditions under which the grinding is performed. The types of binders are as follows:
1. Ceramic bond (V)
2. Resin bonding agent (B)
3. Rubber bonding agent (R)
4. Diamond bituminous binder
5. Other binders
Abrasive tissue selection
The microstructure of the abrasive tool has a great influence on the grinding performance. Grinding wheels of different organizations have different pores and different density of particles. Therefore, the hardness of the grinding wheel is the same in the same time, and the grinding performance of the grinding wheel is different when the structure is different. When the structure of the abrasive tool is loose, the grinding efficiency is high, but the wear of the abrasive tool is fast and the life is short; when the tissue is too tight, it is easy to burn the workpiece because it is difficult to accommodate the chips.
When grinding materials with low hardness and high toughness, the abrasive tools are easily blocked by the wear debris, and it is necessary to use a loose abrasive material. The loosely ground abrasives allow the abrasive particles to cut into the workpiece to the maximum extent and carry away the thicker abrasive debris.
In the case of profile grinding and fine grinding, the structure of the grinding wheel should be selected to be tighter in order to maintain the formability of the working surface of the grinding wheel and obtain higher precision. When the contact area between the grinding wheel and the workpiece is large, or when the viscous and difficult-to-grind material is processed, the chip removal is difficult and the cooling condition is not good. To avoid overheating of the grinding area, a loose grinding wheel should be used.
In the case of heavy cutting load grinding, in order to ensure sufficient strength and long life of the grinding wheel, the most precise grinding wheel is generally used.
Abrasive tool shape and size selection
The shape of the tool should be based on the conditions of the grinding machine and the shape of the workpiece.
The shape of the grinding wheel is many, and the grinding wheel of various shapes and sizes is used as follows
The diameter of the parallel series of grinding wheels is in the range of 3 to 1600 mm, and grinding machines such as inner circle, outer circle, plane, centerless, tools and threads can be used. Generally, parallel grinding wheels with an outer diameter of less than 150 mm and a thickness of 10 to 100 mm are used more on a horizontal axis grinder. Parallel grinding wheels with an outer diameter of 250 to 500 mm and a thickness of 32 to 50 mm are often used in medium-sized cylindrical grinding machines; ～900mm, thickness 63～75mm parallel grinding wheel is often used for grinding rolls or for other cylindrical grinding machines; grinding wheel with outer diameter of 750～1100mm and thickness of 28～100mm is used for grinding crankshaft; grinding wheel with outer diameter of 1200～1600mm and thickness of 80～120mm It is mainly used to grind large crankshafts; grinding wheels with outer diameter of 350-750mm and thickness of 125-550mm, most of which are used for grinding wheels of centerless grinding machines, which are used to grind the outer circle, cone and cylindrical roller outer circle of inner and outer rings of bearing. The spindle of the textile machine, the piston pin on the engine and the outer circumference of the workpiece produced by mass production; the parallel grinding wheel with the outer diameter of 250-500mm and the thickness of 8~63mm, after the edge is trimmed into a sharp corner, it is used for grinding on the thread grinder. Single or multi-thread externally threaded workpieces; parallel grinding wheels with an outer diameter of 80 to 250 mm and a thickness of 6 to 10 mm, which are used to grind internal threaded workpieces after being trimmed into sharp corners; parallel grinding wheels with an outer diameter of 150 to 600 mm and a thickness of 13 to 75 mm On the table, floor or suspension The grinder; outer diameter of 50 ~ 250mm, a thickness of 10 ~ 25mm in parallel with the grindstone hand grinder.
The curved grinding wheel is used to grind the channel of the bearing.
Select ordinary grinding tools according to various grinding conditions
Select the abrasive tool according to the adaptability of the abrasive to the material being ground
Safe use of abrasive tools
Most of the grinding work is carried out by a high-speed rotating grinding wheel, and the grinding wheel (abrasive tool) is a brittle object. There are many factors affecting its strength in all aspects of its manufacture, transportation and use. , thus affecting the security of use. The various factors that affect the safe use of abrasive tools are listed below.
Transportation and storage of abrasive tools
1 When transporting long distances, pack them in wooden or wicker baskets and separate the abrasives with soft materials such as straw or sawdust. To prevent collisions and impacts during handling, stacking should be stable.
2 Do not place the ceramic abrasive in a wet or frozen place; the rubber abrasive should not be in contact with the oil; the resin abrasive should not be in contact with the alkali. Failure to do so will reduce the strength of the tool and its ability to grind.
3 Abrasives should be placed separately according to specifications, and signs should be provided at the storage to avoid confusion and errors.
4 The abrasive tools should be stored in a dry place at room temperature not lower than 5 °C.
5 Placement and setting methods should be based on the shape and size of the tool. Larger or thicker grinding wheels are placed upright or slightly inclined. Thinner and smaller grinding wheels should be placed flat, but the stacking height should not exceed 500-600mm. The stacking height of rubber or resin sheet grinding wheel It is necessary to place a flat iron plate on the top of 200mm and prevent the grinding wheel from being deformed or broken. Small diameter grinding wheels (under 50mm) can be stored in a string. Shaped, cup-shaped and dish-shaped grinding wheels should be placed one on top of the other, but not too high.
(6) Both rubber and resin have an "aging" phenomenon, so the storage period of these two types of bonding abrasives can not exceed one year. Abrasives that exceed the shelf life must be re-examined before use.
(7) The modified grinding wheel must be re-tested before it can be used.
Safe speed of the grinding wheel
In order not to cause errors, the wheel is marked with a safe working speed, especially for high-speed grinding wheels. Must be used at the working speed of the mark on the grinding wheel.
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