Analysis and repair of common grinding damage of t

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Common grinding damage and analysis and repair of metal cutting machine tools

some parts and components of machine tools that slide relatively, such as sliding plate and guide rail, shaft and sliding bearing, worm and worm gear, often have scratches or grooves on their surfaces after running for a period of time. We call this phenomenon grinding damage. The grinding damage destroys the accuracy of the machine tool and affects the service life of the machine tool. If the repair is not timely, the particles generated by the grinding damage will also aggravate the grinding damage. However, the grooves generated by the grinding damage are easy to hide dirt, which will also aggravate the grinding damage. In serious cases, it can stop the sliding of the sliding parts and produce the bite phenomenon. Abrasion damage is essentially abnormal wear. The common abrasion damage on machine tools can be divided into two types according to the causes: one is adhesive abrasion type abrasion damage, and the other is abrasive abrasion type abrasion damage. This article briefly introduces the causes, preventive measures and repair methods of these two types of grinding injuries

I. adhesive wear type abrasion damage

this kind of abrasion damage refers to that when the friction pair moves relatively, due to mutual friction, the materials on the contact surface are transferred from one surface to another, resulting in scratches and grooves on the friction surface

1. The research on the generation mechanism of adhesive wear type grinding damage shows that there are uneven defects on the solid surface from the microscopic point of view, and the bumps and valleys can not be completely eliminated even after polishing. When two friction surfaces are in contact, in fact, the peaks of the two friction surfaces are in contact with each other. Due to the large contact stress, elastic and plastic deformation will occur, which will increase the contact area until it can bear all the load. In this case, there will be firm adhesion points on the metal contact surface, which is commonly referred to as cold welding adhesion. These adhesion points are generated when there is no surface facial mask. When there is a surface facial mask on the friction pair surface, it is only affected by the normal force, and its cold welding adhesion will not occur. If there is a tangential force at the same time, and both the normal force and the tangential force are large, and the temperature of the friction surface will rise during relative sliding. Under high temperature and high pressure, the oil film will be damaged, and the contact metal surface will soften or melt. The contact point will produce a cycle process of adhesive tearing, which will transfer the materials on the contact surface from one surface to another, Thus, scratches and grooves are formed on one (or both) of the surfaces, that is, adhesive wear type abrasion damage is formed

2. Classification of adhesive wear type abrasive damage

according to the damage degree of the surface abrasive damage of the friction pair, we can divide the common adhesive wear type abrasive damage on the machine tool into four categories:

(1) the coating abrasive damage only occurs on the shallow surface of the soft metal, and the thin layer of the soft metal is transferred to the hard metal surface by coating. For example, after the worm pair has operated for a period of time, the copper on the worm surface is coated on the worm surface

(2) the abrasion and grinding damage occurs in the shallow part below the surface layer of the soft metal surface. The damage mode is to produce small scratches along the moving direction. Sometimes, it is also possible to scratch on the hard metal surface. The most common is the wear of the shaft and sliding bearing during the initial operation of the machine tool

(3) the gluing (or tearing) abrasion damage occurs in the deep part of one or both substrates of the two parts that rub against each other. Due to the high local temperature and pressure on the surface, the adhesive bonding strength is the shear strength of any base metal. When the friction pair slides relatively, its surface is subject to tearing damage for one day and collision for one day, resulting in gluing abrasion damage. Such as cam pair, worm pair and gear pair, which are common; When the sliding plate and guide rail of the machine tool are in dry friction due to lack of lubricating oil, this kind of grinding damage often occurs

(4) when the friction pair surface is seized, the instantaneous flash temperature is quite high, the adhesion area is large, and the strength of the adhesion point is also quite high. The adhesion cannot be cut off from the substrate, resulting in the suspension of relative movement. Biting is the most serious kind of grinding injury. For example, the axle and sliding bearing are subject to this kind of grinding damage when the axle is locked due to poor lubrication; Large area grinding damage caused by lack of oil in the guide rail of large machine tools will also cause seizure. 3. influencing factors and preventive measures for adhesive wear type abrasion damage

there are many factors affecting this kind of abrasion damage. We can take corresponding measures against these factors to prevent its occurrence and development

(1) factors of lubricating grease the lubricating state has a great impact on adhesive wear type abrasion damage. As long as the friction surface always maintains a lubricating film of sufficient strength to avoid dry or semi dry friction caused by direct metal contact between the friction surfaces, the occurrence and development of adhesive wear type abrasion damage can be effectively prevented and controlled. Among the various factors that affect such abrasion damage, ensuring good lubrication of the friction surface is the most important factor. Adding oily or viscosity additives to the lubricating oil can improve the adsorption capacity and oil film strength of the lubricating oil film, which can double the superior control function and improve the anti abrasion ability

(2) pressure factor adhesive wear type abrasion damage generally increases with the increase of pressure. When the pressure load exceeds a certain value of the material hardness of the friction pair, the oxide film on the surface of the friction pair will be crushed, and the protruding points of the new surface between the two surfaces will be embedded into each other. When the two surfaces move relatively, such grinding damage will increase sharply, and in serious cases, the friction pair surface will be glued or even killed. Therefore, when using the machine tool, the worktable and sliding guide rail should avoid overload operation. For example, the workpieces should be properly distributed and not overweight when the Longmen planer, milling machine, boring machine, etc. are clamped on the worktable. When the machine tool is under major repair, its guide rail is quenched, which also has a great effect on preventing this kind of grinding damage

(3) temperature has a great influence on adhesive wear type abrasion damage. The heat generated in the friction process will raise the temperature of the friction surface to a certain extent. If it is light, it will destroy the lubricating film and make the metal surface directly contact to form dry friction or semi dry friction; If the material is heavy, it can make the material in tempering state and reduce the hardness of the material; More importantly, the local area can make the material on the friction surface melt. All of these will promote the occurrence of such injuries. Selecting materials with high thermal stability or strengthening cooling are effective methods to prevent adhesive wear type abrasion damage caused by temperature

(4) the factor of sliding speed when the pressure is constant, the small sliding speed reduces the effect of forming lubricating oil film, and the oil film thickness is small. The oil film is often partially damaged because it can not bear the pressure of moving parts, resulting in direct contact between two metals, which is easy to cause abrasion damage. However, when the sliding speed is large to a certain extent, the temperature of the friction pair will rise, which is easy to damage the lubricating oil film and lead to abrasion damage. Therefore, selecting an appropriate sliding speed can reduce the tendency of adhesive abrasion type abrasion damage

(5) generally speaking, the smaller the surface roughness of the friction pair, the greater the ability to resist adhesive wear type abrasion damage. Properly reducing the surface roughness can prevent this kind of abrasion damage. For new machine tools, gradually loading and running in operation are often used to reduce the surface roughness and reduce the early adhesive wear type abrasion damage. However, the surface roughness of the friction pair is reduced too low, and the lubricant cannot be stored between the friction surfaces, which is easy to cause grinding damage. Therefore, when the pattern on the machine tool guide rail is worn, when maintaining the machine tool, the pattern is often scraped again, so as to facilitate oil storage and prevent grinding damage

(6) material factor brittle materials have better adhesion wear resistance than plastic materials; The friction pairs composed of materials with high miscibility are more prone to wear type abrasion damage than those composed of materials with low miscibility; The friction pair composed of metal and nonmetal has less tendency to produce such abrasion damage than the friction pair composed of two metals. During the maintenance of the machine tool, the non-metallic coating can be used to repair the slide guide rail, the shaft and sleeve with different materials, or the shaft surface treatment process can be carried out to reduce the adhesive wear type abrasion damage

II. Abrasive wear type grinding damage

this kind of grinding damage refers to the significant cutting effect of the hard particles that contaminate the two friction surfaces when they move on the surface, resulting in scratches and grooves on the friction surface

1. The mechanism of abrasive wear damage

hard particles enter between two friction surfaces and are subjected to two forces, which are perpendicular to the surface and parallel to the surface. Under the action of the former, hard particles penetrate the surface, while under the action of the latter, they produce tangential movement, which leads to the surface being ploughed or cut, leaving scratches and grooves, that is, abrasion damage

2. Classification of abrasive wear type abrasion damage

(1) for example, when the guideway of a machine tool is bumped by objects such as workpieces or tools under high stress, the surface of the guideway will appear pits, and bulges or burrs will appear around the pits. If it is not repaired in time, when the sliding plate slides on the guide rail, the bulge or burr on the guide rail is bound to grind the sliding plate

(2) for example, if the chips or molding sand between the two friction surfaces of hard particles enter between the guide rail and sliding plate of the machine tool, a typical abrasive wear type abrasion damage will be formed: under the vertical pressure, the chips or molding sand will crush the surface of the guide rail or sliding plate, and the guide rail or sliding plate will be abraded when moving

(3) if dirt enters the friction surface, for example, dirt enters the friction surface through lubricating oil. Although it will not press the friction surface, it can change the contact condition of the friction surface. Due to the support of dirt, the compressive stress at the contact position with dirt increases. After long-term operation, the friction surface is bound to be scratched or slightly scratched

3. Prevention of abrasive wear type grinding damage

(1) firstly, prevent the surface of friction pair from being bruised, such as not stacking tools, measuring tools, workpieces and other sundries on the machine tool guide rail. Secondly, repair the surface of friction pair in time after being bruised. For example, the bulges and burrs generated shall be repaired in time with an oilstone or scraper, so as to prevent the surface of friction pair from being bruised. Do not place inflammables and explosives around the equipment; Wipe the surface; The pits shall be repaired as much as possible to prevent the hidden dirt in the pits, which will leave hidden dangers to the research damage

(2) set up defense and configure shield to prevent hard particles such as chips and molding sand from entering the friction surface. Once hard particles or sundries are found to enter the friction surface, they should be removed immediately to prevent the generation or expansion of grinding damage. (3) The machine tool shall be maintained on time, and the dirt and oil dirt on the guide rail, sliding plate, oil hole and dead corner shall be removed in time to prevent the dirt from entering between the friction surfaces and form abrasive wear damage

III. repair of grinding damage

once the friction pair of the machine tool has grinding damage, it must stop running immediately, find out the cause, the degree of grinding damage, and take different measures. Generally speaking, the abrasion damage caused by coating may not be repaired, but the lubrication shall be strengthened to prevent expansion; For scratches (including those caused by dirt), the dirt shall be removed and the scraped part shall be leveled with an oilstone or scraper before continuing to operate. For the abrasion caused by gluing, biting and hard particles, it must be repaired. Now I will give a brief introduction to the repair of grinding damage of several common friction pairs:

1 Repair of sliding bearing and shaft

(1) after the sliding bearing surface is abraded, the preferred repair scheme should be the stone scraper method. Generally speaking, the sliding bearing has a margin for adjustment and repair, which can meet a repair cycle of the machine tool. If the grinding damage is very serious and the repair allowance cannot be met, replacement should be considered. After the two half pad sliding bearings are abraded, they can be thinned

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