Analysis on the quality of the hottest grinding wh

2022-08-05
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Quality analysis of grinding wheel products

grinding wheel belongs to the category of tools, which is compared to "industrial teeth", and is widely used in various industries

generally speaking, the passing rate of random inspection of grinding wheel products in recent years is still low (see Table 1), and there are various problems that can not be ignored, some of which have seriously threatened the safety of people and property. Table 1 Average qualified rate and minimum qualified rate of random inspection of grinding wheel product quality in recent 6 years

average qualified rate of product name% minimum qualified rate% fiber reinforced resin sheet grinding wheel 59.736.4 cymbal grinding wheel 59.550 It greatly reduces the user's manual operation of 0 flat grinding wheel 82.842.8 rubber flake grinding wheel 94.366.7 note: only one spot check is conducted for resin flake grinding wheel, and only two spot checks are conducted for diamond grinding wheel, which is not included in the statistics for the time being

flat grinding wheel, sheet grinding wheel (resin sheet, rubber sheet) and cymbal grinding wheel are all products with large quantity and wide range, and they are also the main varieties for quality spot check. There are also many quality problems found in the spot check, mainly focusing on the quality characteristics such as rotary strength, hole diameter, hardness, static balance, form and position tolerance, product mark, etc

1 main quality problems and their harm

1) unqualified rotation strength

rotation strength refers to the ability of the grinding wheel to resist cracking under the action of centrifugal force during rotation, which reflects the tensile stress of the grinding wheel. It is a very important index in the manufacture and use of grinding wheels. In order to ensure the safe use of the grinding wheel, the standard lists the unqualified rotation strength as a "fatal defect", that is, "a defect that is dangerous or unsafe to the user or the equipment". The sampling inspection scheme is that the sample number n=10, the received number ac=0, and the rejection number rc=1 when we developed this software; That is to say, the rotation strength of 10 grinding wheels must be qualified. If one of them is unqualified, it will be judged as unqualified. It has been found in the quality spot check for many years that the rotation strength of fiber reinforced resin flake wheel and cymbal wheel is not ideal, some are lower than the maximum working speed, and some break before reaching the specified rotation test speed; Some of them reach the test speed but fail to maintain for 30s. The percentage of unqualified rotary strength is 7.1%~18.7% (see Table 2 for detailed data)

fiber reinforced resin flake grinding wheel and cymbal grinding wheel are mostly hand-held cutting and grinding, and the operator is close to the grinding wheel. Therefore, once the rotation strength is poor, the grinding wheel is broken in the process of cutting or grinding, which is very easy to cause personal injury and death accidents

2) unqualified static balance

the grinding wheel is an uneven object. When the grinding wheel rotates, its mass center does not coincide with the rotation center, causing vibration. This state is called the imbalance of the grinding wheel. The unbalance of grinding wheel will bring many hazards:

acts on the grinder bearing, causing the spindle to vibrate and accelerating the wear of the grinder spindle

additionally increases the rotation stress borne by the grinding wheel - accelerate the construction of the industry wide innovation system, reduce its rotation strength, and even cause its cracking during rotation

the vibration of grinding wheel and workpiece system is intensified, which is manifested by the appearance of vibration marks on the surface of the workpiece to be machined, and the deterioration of machining accuracy and surface roughness

affects the uniformity of the self sharpening of the grinding wheel and causes uneven wear of the grinding wheel

the static balance of the grinding wheel is also one of the main quality indicators reflecting the internal characteristics and service performance of the grinding wheel. The unqualified static balance is listed as "heavy defect" in the standard, that is, "the defect that seriously reduces the service performance of the product and has a serious impact on the performance of the machined parts". In the spot check, the situation of unqualified static balance is shown in Table 2. For a fiber reinforced resin sheet grinding wheel with a diameter of 400mm and a linear speed of 70m/s, the standard stipulates that the static imbalance value is ≤ 13g, but the measured value is more than 18G. It is conceivable that such a grinding wheel vibrates during use

3) hardness unqualified

hardness of grinding wheel refers to the resistance of the binder to resist the separation of abrasive particles from the surface of the grinding wheel under the action of external force, or is understood as the difficulty of the separation of abrasive particles from the surface of the grinding wheel. Hardness can accurately reflect the grinding performance of grinding wheel, and is one of the important indicators to measure the quality of grinding wheel. There are two requirements for the evaluation of the hardness of the grinding wheel:

the uniformity of the hardness (the difference between the maximum and minimum hardness values of the measured points) shall comply with the provisions of the standard

the compliance requirements of hardness, that is, the average value of hardness (arithmetic mean value of hardness measured at each measuring point) shall be within the allowable range specified in the standard. The proportion of unqualified hardness is also quite high. For example, the unqualified hardness of flat grinding wheel accounts for more than 20% of the total unqualified hardness (see Table 2). Table 2 summary of unqualified items in unqualified products

product name unqualified

number of pieces unqualified items unqualified

total hole diameter thickness shape position

tolerance hardness unbalanced rotation

strength mark

incomplete mark

error fiber reinforced resin sheet grinding wheel cymbal shaped grinding wheel flat grinding wheel rubber sheet grinding wheel 44--------4

4) the inner hole of unqualified

grinding wheel is the installation basis, The unqualified diameter of grinding wheel is listed as "heavy defect" in the standard. If the aperture is large, the grinding wheel will be eccentric after installation, which will make the grinding wheel unbalanced. When rotating at high speed, it will aggravate the vibration, affect the grinding quality, and even lead to the fracture of the grinding wheel and the damage of the equipment. If the aperture is too small, the grinding wheel cannot be installed and cannot be used. If it is installed forcibly, the spindle will be scratched, and the grinding wheel may also be damaged, resulting in the fracture of the grinding wheel during operation, which is easy to cause accidents. As shown in Table 2, the unqualified hole diameter is the most serious in the spot check. Accounting for 16. 5% of the total unqualified flat grinding wheels 7%, accounting for 58 2%。

5) mark error

the mark of the grinding wheel indicates the basic performance of the grinding wheel and plays an important role in guiding users to use it, which cannot be ignored. Mark errors are listed as "major defects" in the standard. There are many problems in the signs, and some signs are incomplete; Some signs are wrong; Even the trademark is deliberately shoddy, false as true, and falsely used, causing confusion to users, affecting their use, damaging their interests, and may lead to major accidents

6) unqualified end face runout and radial runout

unqualified end face runout and radial runout will cause deflection and affect the balance performance of the grinding wheel during the rotation of the fair, and have adverse effects on the machine tool and workpiece, so it should be controlled within the allowable range

2 analysis on the causes of quality problems of grinding wheel products

1) raw material problems

grinding wheel is a complex system composed of a variety of main and auxiliary raw materials, the main materials are abrasives and binders, and the auxiliary materials include fillers, reinforcing materials, etc

a. abrasive problem abrasive is the main material in the grinding wheel. At present, the abrasive quality in the market is mixed with good and bad, which is mainly manifested as follows: 1) the chemical composition of the abrasive is often qualified, but the physical properties are poor, such as hardness, strength, self sharpening, crushing resistance, thermal stability, hydrophilicity, etc; 2) The grain size composition of abrasive is disordered, which is quite different from the standard

b. bond problem in the grinding wheel, the abrasive particles are bonded together by the bond, and the performance of the bond directly affects the performance and quality of the grinding wheel. 1) The resin quality is unstable, and the content of free phenol in the resin varies from high to low. The so-called free phenol refers to the phenol remaining in the resin. The molecular formula is C6H5OH. Long term heating can crack the hardened resin and affect the strength of the grinding wheel; 2) The resin powder and Urotropine are mixed unevenly. The scientific name of urotropine is hexamethylene tetramine ((CH2) 6n4). As a resin hardener, the amount of urotropine is insufficient, and the resin hardening is incomplete, which affects the strength and hardness of the grinding wheel; If the content is too high, the excessive Urotropine will not combine with the resin, and will decompose and volatilize during the hardening process, which will increase the porosity of the grinding wheel and reduce its strength and hardness. The resin powder and Urotropine are mixed unevenly, and the consequence is the harm caused by too high or too low content of urotropine; 3) The binder grain is too coarse. It is generally believed that the fine grain size of the binder is appropriate, which is conducive to the uniform distribution of the binder and the full contact of the hardener. If the grain is too coarse, the formed material is not easy to mix evenly, which affects the hardness and strength of the grinding wheel. Even for the resin flake cutting wheel and cymbal wheel with coarse particle size, the particle size of the selected binder (resin powder) should also be finer than 320 °. For manufacturing fine-grained grinding wheel products, the particle size of the binder shall be several microns

2) manufacturing process problem

one of the characteristics of grinding wheel manufacturing is that it has strong processability. Any omission and deviation in the manufacturing process may have a negative impact on the quality of grinding wheel products. During the rotary test, it was observed in the broken grinding wheel that some of the broken parts have coarse abrasive grains, loose structure, and lack of binder, and the cloth will be separated with a little pulling. These defects are related to the improper manufacturing process. There are many problems in mixing, forming, firing or curing, shaping and other processes in manufacturing

a. the batching process is the preliminary preparation for mixing. Mixing means that the abrasives, wetting agents, binders, fillers, etc. are uniformly adhered to the abrasive particles according to certain procedures and methods to make them evenly distributed among each other

the key to mixing is uniformity. The formed material shall be evenly distributed, loose, easy to spread and have good forming performance. In some factories, the time for mechanical mixing or manual mixing is not enough, and the mixing and screening operations are sloppy, which can not meet the above requirements

b. the correct operation of the forming process is to pour the forming material in the middle of the mold, and then spread the material towards the radius with a special scraper to make it evenly distributed in the mold, so that the green body reaches the specified size and shape without cracks or delamination. In some enterprises, the operation of employees is not essential, resulting in uneven quality distribution and out of tolerance of form and position and static balance

c. firing or curing is the most critical production process. There are two factors that can easily cause quality problems. One is that the temperature difference between curing oven and firing kiln is too large; Second, the way of charging is improper

the temperature difference in the oven or kiln is generally measured when the grinding wheel is not installed. At this time, the temperature difference is small. When the oven or kiln is filled with grinding wheel, improper installation will hinder the flow of hot air in the oven or kiln, and the temperature difference will increase, which deviates from the set hardening specification, so that the solidification (hardening) quality of the grinding wheel can not be fully guaranteed

d. truing some grinding wheels need to be trued after firing or curing to ensure the geometric dimension and form and position tolerance of the grinding wheel. Some enterprises have missed this process, and some plastic processing is careless and the process is not in place

e. process documents and process parameters are copied mechanically. Some small enterprises have incomplete process documents and will not adjust process parameters according to the fluctuations of raw material composition, changes in climatic and environmental conditions or different requirements of users. The quality of grinding wheels produced by these enterprises is often unstable

3) equipment and tooling problems

a. equipment problems production equipment is an important material condition for grinding wheel production. Some enterprises have insufficient equipment, for example, there is no "flaker", there is a lack of mixer, and the tonnage specification of hydraulic press is small. Secondly, the equipment maintenance is poor, and the equipment cannot be in good condition. Therefore, not only the production efficiency is low, the labor intensity is high, but also the product quality is not easy to guarantee

b. tooling problems tooling management in some enterprises is weak. There is no complete account for molds, no acceptance and use records, no good storage place, and they are piled up at the production site<

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