Abstract: Machine tool guide rail is the basis of machine tool motion accuracy, and each component is installed or moved based on it. However, wear often occurs due to improper maintenance and other reasons during long-term use. At this time, it needs to be repaired to maintain the movement accuracy of the machine tool and the integrity of the equipment, so as to ensure that the machine can meet the teaching standards. This paper discusses the classification, wear reasons and repair measures of machine tool guide rail, hoping to play a reference role in the follow-up repair of guide rail.
Keywords: machine tool guide rail; Wear; repair
0 Introduction
Industry is the lifeblood of a country. Only a strong industrial level can support the country's infrastructure needs. Machine tools are the most important and commonly used equipment in industry, known as "machine tools". High precision machine tools have become one of the goals pursued by industrial development in the new era. The accuracy of machine tools directly reflects the national industrial standard. The most important part of the machine tool is the guide rail. The guide rail is the basis of the machine tool motion accuracy. All parts are installed or moved based on it. The wear of the guide rail will reduce the machine tool processing accuracy and affect the processing quality. The machine tool in teaching is difficult to process due to the wear of the guide rail, which can not meet the teaching requirements. Therefore, it is necessary to discuss the methods of repairing the wear of machine tool guide rails, whether it is for the guarantee of enterprise benefits or for the improvement of school teaching quality.
1 Classification and Characteristics of Machine Tool Guideway
The guide rail pair is also called the guide rail, which guides and supports the moving components and guides them to move back and forth in a certain direction. The guide rail acts as a balancing force on the movement of moving components, constraining their movement direction and ensuring their stable movement. The fixed part of the guide rail connected with the machine tool is the static guide rail or the support guide rail. The part that is relatively short connected with the moving parts is usually called the dynamic guide rail. The former supports the latter, and the static guide rail surface is called the bearing guide surface. According to the working principle and characteristics of the machine tool guideway, we divide it into the following categories: sliding guideway, hydrostatic guideway, magnetic levitation guideway, rolling guideway and composite guideway, among which the sliding guideway and rolling guideway are the most common and commonly used.
Sliding guideway
The sliding guideway is the most commonly used guideway structure and is widely used in machine tool products because of its simple, reliable, strong anti-seismic performance, economical and applicable, and low manufacturing cost. Thanks to its structure, the sliding guide rail has the advantages of large bearing capacity and strong rigidity. It is usually used for cutting with large load. The damping performance is good, which can well restrain the vibration generated when the machine starts and stops. At present, various countries pay more and more attention to the research of sliding guide rail. With the development of science and technology, sliding guide rail still has its place to be excavated .
Rolling guideway
High speed CNC machine tools often use rolling guideway, which is different from sliding guideway. It uses cylinder, sphere, etc. as rolling carrier, replacing sliding friction with rolling friction to reduce friction. As a result, there is no vibration when the rolling guide operates, and even at low speed, there is no "creeping" phenomenon. During operation, the guide rail has less heat and longer service life. Based on the above advantages, the rolling guide can be professionally produced at a relatively low cost. At present, the research and development trend of the rolling guide is also towards modularization, integration and low noise.
Hydrostatic guide rail
The working principle of the hydrostatic guide rail is to flow lubricating oil into the guide rail surface through the restrictor to form an oil film, which separates the moving guide rail from the static guide rail. When the moving guide rail is loaded, the oil film becomes thinner, and the oil is continuously squeezed back to the oil tank through the sealing edge. At the same time, the gap between the oil returns becomes thinner, the resistance becomes larger, and the mailbox pressure becomes larger, which can balance the internal and external pressure differences. The hydrostatic guide rail can avoid the wear between the dynamic and static guide rails, At the same time, the machining accuracy is far higher than that of rolling guide rail and sliding guide rail.
Magnetic levitation guideway
The magnetic levitation guideway is a technology that uses magnetic force to lift the guideway. Compared with the first three kinds, the magnetic levitation guideway can meet the requirements of ultra precision and ultra-high speed machining, and is an ideal state for future machine tools.
2 Common failure forms and cause analysis of machine tool guide surface
In the process of machine tool processing, in addition to supporting the moving parts, the guide rail must also ensure that the moving parts can move accurately and linearly in the given direction under the action of external forces. Therefore, the guide rail needs to meet the requirements of certain guiding accuracy, portability and stability in the movement process, sufficient stiffness, good wear resistance, little influence by temperature change and structural manufacturability.
The guide rail is an essential part of the machine tool. If the guide rail is worn, the accuracy of the machine tool will be greatly affected and cannot work. This state is called failure. Common failures of guide rail include wear, surface indentation, scratch, fatigue, rust, fracture and burn. The surface indentation is caused by excessive processing load, and the indentation damage will be caused when the processing load is greater than the bearing capacity of the guide rail; Scratches are caused by particles falling into the guide rail, resulting in the damage caused by rolling hard particles during the working process; Fatigue damage is the phenomenon that the service life of the machine tool guide exceeds the limit, and the staff is careless in maintenance, resulting in damage in heavy load work, resulting in the peeling of the guide surface; Rust refers to the damage caused by corrosive substances generated on the metal guide rail surface due to the debris, oil stain and other corrosive liquids generated during the processing or the high air humidity when the metal guide rail surface does not work for a long time and the maintenance interval is too long. The fracture is generally caused by the poor quality of the guide rail, the low wear resistance of the materials used, and the large load in the work, which is difficult to bear; Burns are generally caused by large processing load, substandard lubrication conditions, excessive friction and heat generation, which cause the color change of the guide rail surface.
The damage of the guide rail will seriously affect the machining quality of the machine tool. The machining accuracy is not up to standard or defective products appear, which will seriously affect the production efficiency, production quality and teaching quality of the enterprise. There are also potential safety hazards in the use process. Later maintenance will also consume a lot of manpower, material resources and financial resources. It always takes a certain time to completely repair, so it needs to be paid attention. The causes of wear can be subdivided into the following:
2.1 Poor lubrication
Boundary lubrication is the most commonly used lubrication method for the guide rail. It is to inject oil on the guide rail surface to form a dense oil film, which will be adsorbed on the friction surface, thus greatly reducing the friction coefficient of the friction surface. However, with the increase of the running time of the guide rail, the oil circuit will be blocked or cannot be supplied in time, which will lead to local dry friction. The resistance of dry friction is far greater than that in the case of oil film, and the track will be damaged soon.
2.2 Oxidative wear
Some machine tools or parts of the machine tools are not used frequently. If the maintenance is neglected, the surface of the guide rail will be exposed to the air for a long time, which will cause oxidation reaction and form oxide impurities. If the guide rail is used later, the oxide will fall off. The guide rail will constantly rub these impurities in the reciprocating motion, forming damage. This damage is common to the rarely used guide rail parts.
2.3 Sundries wear
During the operation of the machine tool, in addition to the daily maintenance, the oil filter must be replaced regularly. The working condition of the guide rail needs to ensure that the guide rail surface is clean and free of impurities. In addition to its own oxidation to form impurities, impurities are often mixed in the lubricating oil. If the filter is not clean, the guide rail surface will also be damaged. That is, it is equally important to ensure that the working environment of the machine tool is clean.
2.4 Insufficient rigidity of machine tool guide rail
Due to the working property, the guideway bears a variable load rather than a static load. If the guideway is not rigid enough and works for a long time, the guideway of the machine tool will be deformed. Once the guideway is deformed, it will greatly affect the machining accuracy.
To sum up, there are many reasons for the damage of the guide rail. At present, more and more attention has been paid to the repair research of the guide rail.
3 Common repair methods for failure of machine tool guide surface
In the working process of the machine tool, various factors will lead to the failure of the guide rail surface, affecting the normal production, so the guide rail failure should be repaired in time. The common methods of repairing the guide rail surface are scraping, planing/grinding, brazing and plastic bonding.
3.1 Scraping and grinding repair method
The type of rail that can be repaired by scraping method is the rail that has not been quenched. This repair method is the most common and basic repair method. Scraping R&D can repair and deal with rough, wear and bite relatively light guide rail. The tools used are scrapers and datum measuring tools to reduce the surface roughness of the guide rail, ensure the smoothness of the contact surface of the guide rail, and improve the accuracy of the guide rail of the machine and the service life of the guide rail. The scraping method can effectively remove various roughness conditions of the guide rail surface and reduce the residual tolerance. When scraping, the contact surface is even, the scraping force is small, the contact accuracy is high, and the guide rail is not easy to deform. Moreover, the scraping method is simple in operation, will not cause unnecessary damage to the track, has strong reliability, high operability, and is not limited by the location, shape and size of the damage. Although this method is simple and reliable, its processing efficiency is low, the cycle is long, and the working intensity is high.
3.2 Fine planing/grinding method
Precision planing and precision grinding can be used to repair hardened machine tool guides. This is also one of the common repair methods, belonging to the planing process. Precise surface grinding method can effectively improve the surface finish of the guide rail and make it reach the tolerance value required by the process. When using this method to repair the guide rail, it is necessary to remove the machine from the base, which will cause the fastening screws to become loose after reassembly, which will not reach the original torque, resulting in deformation, and ultimately reduce the processing accuracy. In addition, disassembly and reassembly are time-consuming and laborious, and indirectly increase the maintenance cost.
3.3 Spray welding
According to the spray welding process, the surface of the guide rail needs to be preheated first, and the preheating temperature is above 150 ℃. Then spray the welding powder on the rough surface and continue heating. When the temperature reaches above 900-1200 ℃, the welding powder will melt and cover the guide rail surface. The disadvantage of spray welding is that it takes a long time to heat the guide rail and is easy to form thermal stress. Compared with electric arc welding, cracks are more likely to occur, and there is a greater tendency for cracks to occur due to linear shrinkage. The generation of cracks is generally related to the heating time and spray welding thickness. The size of damage that can be repaired by spray welding is limited to a certain extent, and the welding parts need to be cleaned before welding. The color after welding is different from the color of the original guide rail, which is related to the content of iron and nickel in the welding powder.
3.4 Arc welding
Cast iron electrode Z248 shall be used for welding and repair according to the corresponding process. There are two processes. The first is preheating to 550~650 ℃ before welding for welding repair, and heat preservation for 5~8h after welding. The second type: the workpiece is not preheated before welding, and the heat preservation is 3~4h after welding. The welding repair quality of these two methods is not easy to guarantee, cracks and hard spots are easy to appear, and the welding repair is not easy to be machined. Electrodes are cheap. According to the corresponding welding repair process with nickel base cast iron electrode Z308, the heating state hammer technology, the welding and welding layers should be suspended for cooling below 60 ℃, so as to reduce the pores and cracks in the welding area, with good cutting performance, high bonding strength and no falling off. Because the processing of the machine tool guide rail is affected by oil absorption and electrode strike force, it is easy to form "welding trace" undercut. Nickel electrode contains a lot of elements, the color of weld area is very different from that of parent metal, and the electrode is expensive.
3.5 Brazing repair method
Brazing repair method is usually used for local damage to the machine tool guide surface, such as scratches, hair bundles and grooves. After welding and maintenance, the surface of the guide rail shall be machined to meet the required process requirements. The traditional welding and repair technology can not completely repair the defects of the machine tool guide rail surface. The secondary porosity is easy to produce thermal deformation of the workpiece during welding and maintenance, and the color difference between welding and maintenance is large. Traditional welding repair technology is usually used to repair non machined surfaces or invisible machined surfaces. Compared with the traditional welding and maintenance process, the use of cold welding machine can significantly improve the quality of welding? Repair quality.
3.6 Plastic bonding method
Plastic adhesive can be used to repair the machine tool guide rails with deep scratches and serious damage. Plastic sheets are usually made of polytetrafluoroethylene (PTFE). PTFE has low friction coefficient, good lubrication performance, stable machine operation, low vibration and noise, and good effect on low speed creep. In addition, PTFE is soft, and metal fragments can be embedded in the guide rail without scratching the surface of the guide rail. During the bonding process, level the worn part of the guide rail, clean the surface of the guide rail, trim the plastic sheet to make the surface smooth, scratch the bracket, and accurately measure the compensation size. Apply epoxy or polyurethane adhesive evenly on the surface and apply pressure to cure it. Finally, machining and scraping are required.
3.7 Thermal spraying method
The above methods are commonly used for rail repair at present, but each has its advantages and disadvantages. There is also a repair method called thermal spraying, which uses the high temperature generated by oxyacetylene combustion to melt and lay engineering plastics on the surface of the rail. The oxyacetylene will not cause thermal damage to the rail, and the rail is not easy to deform. While repairing the rail damage, it can avoid secondary damage to the rail, which can really play a role in extending the life of the rail.
4 Conclusion
This paper first introduces the working principle and mode of the track, and then analyzes the common faults and failure forms of the guide rail in daily work to a certain extent, and puts forward relevant repair methods. With the progress of technical means and the renewal of maintenance concept, more and more maintenance methods have been discovered. In the contemporary society with rapid industrial development, the progress of guide rail repair technology can help our country achieve sustainable development. It is of great significance to repair and regenerate waste machine tool guides, avoid waste of resources, and respond to the call of the country to reduce consumption and energy.
More about Marginal Plastic Self-lubricating Bearings:
Filament wound self-lubricating bearings,High strength glass fiber with epoxy resin baked with PTFE and special fiber as bearing lining, the back material provides high load capacity and bearing lining offers low friction under dry condition. Therefore, this special structure offered an outstanding anti-wear feature and anti-impact performance, suitable for high load with high corrosion resistance application like lifting machinery, logistic machinery, agricultural machinery, and port machinery etc. This special structure offered an outstanding anti-wear feature and anti-impact performance, suitable for high load with high corrosion resistance application like lifting machinery, logistic machinery, agricultural machinery, and port machinery etc,.
Self-lubricating bearings work by having lubricant impregnated within the sliding layer of the bearing. This lubricant can either be liquid (oil) or solid (graphite, MoS2, lead) based on the requirements of the application (such as operating temperature). As the bearing operates, the lubricant is released through pores in the sliding layer, lubricating the bearing surface. The lubricant is uniformly dispersed throughout the sliding layer and thus there is no reduction in low friction bearing performance, even if the sliding layer becomes worn. A “running-in” surface is also usually included at the top of the sliding layer to provide low friction bearing performance at start up before the impregnated lubricant reaches the bearing surface.
