1. What is grinding? Try to cite several forms of grinding.
Answer: Grinding is a processing method that removes the excess layer on the surface of the workpiece by the cutting action of the abrasive tool, so that the surface quality of the workpiece meets the predetermined requirements. Common grinding forms usually include: cylindrical grinding, internal grinding, centerless grinding, thread grinding, grinding of flat surfaces of workpieces, and grinding of forming surfaces.
2. What is an abrasive tool? What is the composition of the grinding wheel? What factors determine its performance?
Answer: All tools used for grinding, grinding and polishing are collectively referred to as abrasive tools, most of which are made of abrasives and binders.
Grinding wheels are composed of abrasive grains, binders and pores (sometimes without), and their performance is mainly determined by factors such as abrasives, particle size, binders, hardness and organization.
3. What are the types of abrasives? List several commonly used abrasives.
Answer: The abrasive is directly responsible for the cutting work, and should have high hardness, heat resistance and certain toughness, and should be able to form sharp edges and corners when broken. At present, there are three types of abrasives commonly used in production: oxide series, carbide series and high-hard abrasive series. Commonly used abrasives are white corundum, zirconium corundum, cubic boron carbide, synthetic diamond, cubic boron nitride, etc.
4. What are the forms of grinding wheel wear? What is the meaning of grinding wheel dressing?
Answer: The wear of the grinding wheel mainly includes two levels: abrasive loss and grinding wheel failure. The loss of abrasive grains on the surface of the grinding wheel can be divided into three different forms: passivation of abrasive grains, crushing of abrasive grains, and shedding of abrasive grains. With the prolongation of the working time of the grinding wheel, its cutting ability gradually decreases, and eventually it cannot be ground normally, and the specified machining accuracy and surface quality cannot be achieved. At this time, the grinding wheel fails. There are three forms: dulling of the working surface of the grinding wheel, blockage of the working surface of the grinding wheel, and distortion of the contour of the grinding wheel.
When the grinding wheel is worn out, it is required to re-dress the grinding wheel. Dressing is a general term for shaping and sharpening. Shaping is to make the grinding wheel have a geometric shape with certain precision requirements; sharpening is to remove the bonding agent between the abrasive grains, so that the abrasive grains protrude from the bonding agent to a certain height (about 1/3 of the size of the general abrasive grains), forming a good cutting edge and sufficient crumb space. The shaping and sharpening of ordinary grinding wheels are generally carried out in one; the shaping and sharpening of superabrasive grinding wheels are generally separated. The former is to obtain the ideal grinding wheel geometry and the latter is to improve the sharpness of the grinding.
5. What are the forms of grinding motion in cylindrical and surface grinding?
Answer: When grinding the outer circle and the plane, the grinding motion includes four forms: main motion, radial feed motion, axial feed motion and workpiece rotation or linear motion.
6. Briefly describe the grinding process of a single abrasive particle.
Answer: The grinding process of a single abrasive grain is roughly divided into three stages: sliding, scoring and cutting.
(1) Sliding stage: During the grinding process, the cutting thickness gradually increases from zero. In the sliding stage, due to the extremely small cutting thickness acg when the abrasive cutting edge and the workpiece begin to contact, when the blunt circle radius rn>acg at the top corner of the abrasive grains, the abrasive grains only slip on the surface of the workpiece, and only produce Elastic deformation, no chips.
(2) Scribing stage: with the increase of the intrusion depth of the abrasive particles, the pressure between the abrasive particles and the surface of the workpiece gradually increases, and the surface layer also transitions from elastic deformation to plastic deformation. At this time, the extrusion friction is severe, and a large amount of heat is generated. When the metal is heated to the critical point, the normal thermal stress exceeds the critical yield strength of the material, and the cutting edge begins to cut into the surface of the material. The slippage pushes the material surface to the front and sides of the abrasive grains, causing the abrasive grains to carve grooves on the surface of the workpiece, and bulges on both sides of the grooves. The characteristics of this stage are: plastic flow and bulge occurs on the surface of the material, and chips cannot be formed because the cutting thickness of the abrasive particles does not reach the critical value of chip formation.
(3) Cutting stage: When the intrusion depth increases to a critical value, the cut layer obviously slips along the shear surface under the extrusion of abrasive particles, forming chips to flow out along the rake face, which is called the cutting stage.
7. Use the J.C.Jaeger solution to theoretically analyze the temperature of the grinding zone during dry grinding.
Answer: When grinding, the contact arc length is also small due to the small depth of cut. So it can be considered as a band-shaped heat source moving on the surface of a semi-infinite body. This is the premise of J.C.Jaeger’s solution. (a) Surface heat source in the grinding zone (b) Coordinate system of the surface heat source in motion.
The grinding contact arc area AA￠B￠B is a belt heat source, and its heating intensity is qm; its width w is related to the diameter of the grinding wheel and the grinding depth. Heat source AA￠B￠B can be regarded as the synthesis of innumerable linear heat sources dxi, take a certain linear heat source dxi for investigation, its heat source intensity is qmBdxi, and moves along the X direction with the speed Vw.
8. What are the types of grinding burns and their control measures?
Answer: Depending on the appearance of burns, there are general burns, spot burns, and line burns (line burns on the entire surface of the part). According to the nature of surface microstructure changes, there are: tempering burns, quenching burns, and annealing burns.
In the grinding process, the main reason for burns is that the temperature of the grinding zone is too high. In order to reduce the temperature of the grinding zone, two approaches can be taken to reduce the generation of grinding heat and accelerate the transfer of grinding heat.
The control measures taken are often:
(1) Reasonable selection of grinding amount;
(2) Correctly select the grinding wheel;
(3) Reasonable use of cooling methods
9. What is high-speed grinding? Compared with ordinary grinding, what are the characteristics of high-speed grinding?
Answer: High-speed grinding is a process method to improve grinding efficiency and grinding quality by increasing the linear speed of the grinding wheel. The difference between it and ordinary grinding lies in the high grinding speed and feed rate, and the definition of high-speed grinding is advancing with time. Before the 1960s, when the grinding speed was 50m/s, it was called high-speed grinding. In the 1990s, the maximum grinding speed reached 500m/s. In practical applications, the grinding speed above 100m/s is called high-speed grinding.
Compared with ordinary grinding, high-speed grinding has the following characteristics:
(1) Under the condition that all other parameters are kept constant, only increasing the grinding wheel speed will lead to the reduction of the cutting thickness and the corresponding reduction of the cutting force acting on each abrasive particle.
(2) If the workpiece speed is increased in proportion to the grinding wheel speed, the cutting thickness can remain unchanged. In this case, the cutting force acting on each abrasive grain and the resultant grinding force do not change. The biggest advantage of this is that the material removal rate increases proportionally with the same grinding force.
10. Briefly describe the requirements of high-speed grinding for grinding wheels and machine tools.
Answer: High-speed grinding wheels must meet the following requirements:
(1) The mechanical strength of the grinding wheel must be able to withstand the cutting force during high-speed grinding;
(2) Safety and reliability during high-speed grinding;
(3) sharp appearance;
(4) The binder must have high wear resistance to reduce the wear of the grinding wheel.
Requirements for high-speed grinding on machine tools:
(1) High-speed spindle and its bearings: The bearings of high-speed spindles generally use angular contact ball bearings. In order to reduce the heating of the spindle and increase the maximum speed of the spindle, most of the new generation of high-speed electric spindles are lubricated by oil and gas.
(2) In addition to the functions of ordinary grinders, high-speed grinders also need to meet the following special requirements: high dynamic accuracy, high damping, high vibration resistance and thermal stability; highly automated and reliable grinding process.
(3) After the speed of the grinding wheel increases, its kinetic energy also increases. If the grinding wheel breaks, it will obviously cause more harm to people and equipment than ordinary grinding. For this reason, in addition to improving the strength of the grinding wheel itself, special The wheel guard for high-speed grinding is an important measure to ensure safety.
Post time: Jul-23-2022