Causes and elimination of common grinding injuries in metal cutting machine tools

Some of the machine tools do relatively sliding zero, parts, such as slides and guides, shaft and sliding bearings, worm and worm gear, etc. After running for a period of time, scratches or grooves often appear on the surface. We call this phenomenon grinding injury. . Grinding damages the accuracy of the machine tool and affects the service life of the machine tool. If the repair is not done in time, the particles produced by the research and intensification will intensify the research and injury, and the grooves produced by the research and injury will easily filth and intensify the research and injury. In severe cases, the sliders of each other can be stopped from sliding, resulting in seizing. Grinding injury is essentially the wear under abnormal conditions. The common injuries on the machine tool can be divided into two types according to the causes: One is the adhesive wear type grinding injury, and the other is the abrasive wear type grinding injury. . Here are some simple explanations of the causes, preventive measures, and repair methods for these two types of injury.

First, adhesive wear type grinding

This grinding injury means that when the friction pair rubs against each other during the relative movement, the contact surface material is transferred from one surface to the other, resulting in scratches and grooves on the friction surface.

1. Adhesion wear type grinding mechanism

Studies have shown that: solid surface conditions, from the microscopic point of view there is uneven defects, even after the polishing process can not completely eliminate the peaks and valleys. When the two friction surfaces are in contact, it is actually the convex peaks of the two friction surfaces that are in contact with each other. Due to the large contact stress, resulting in elastic and plastic deformation, the contact area is increased until it can withstand the full load. In this case, a firm sticking point will appear on the metal contact surface. This phenomenon is usually cold welding adhesion. These sticking points are produced without a surface film. When the surface of the friction pair has a surface film, it is only subjected to a normal force, and its cold welding adhesion does not occur. If there is a tangential force at the same time, and the normal force and tangential force are large, and the relative sliding, the friction surface temperature will increase, in the high temperature and pressure, resulting in damage to the oil film, contact with the metal surface It softens or melts, and the contact point creates a cycle of sticky-avulsion-cohesion-avulsion, which causes the material of the contact surface to transfer from one surface to the other so that one of the surfaces (or both surfaces) is The formation of scratches and grooves, which is the formation of adhesive wear type grinding injury.

2. Classification of Adhesive Wearable Grinding Injuries

According to the degree of destruction of the surface of the friction surface, we can classify the common adhesive wear and tear on the machine tool into four categories:

(1) Smearing damage only occurs on the surface of the soft metal surface. The thin layer of soft metal that has been scratched is transferred to the surface of the hard metal by smearing. For example, the copper coating on the surface of the worm after running for a period of time On the worm surface.

(2) Grinding and scratching occurs in the shallower part of the surface of the soft metal surface. The damage is caused by small scratches along the direction of movement, and scratches may sometimes be found on the hard metal surface. The most common is the wear of shafts and plain bearings during the initial period of machine operation.

(3) Bonding (or avulsion) Grinding occurs when one or both of the two components rub against each other in the deeper part of the substrate. Because the surface is locally hot and the pressure is high, any matrix metal of the adhesive bond strength is sheared. Intensity, when the friction pair does relative sliding, the surface of the monk hits the bell in one day to produce avulsion damage, and a glue-like scratch occurs. For example, cam pairs, worm gears, and gear pairs are common, and when the slides and guide rails of the machine tool cause dry friction in the absence of lubricating oil, they often produce such scratches.

(4) Scratching When the temperature of the flash surface of the secondary friction surface is extremely high, the adhesion area is large, and the adhesion point strength is also rather high, the adhesion cannot be cut off from the substrate, resulting in the phenomenon of relative motion suspension. Death is the most serious type of injury. For example, when shafts and sliding bearings are poorly lubricated, they will produce such grinding injuries. Large-scale machine tools suffer from large areas of grinding due to lack of oil on the guide rails.

3. Influencing factors and preventive measures of adhesive wear

There are many factors that affect this kind of injury. We can take corresponding measures to prevent these factors from occurring and developing.

(1) Lubricating grease factors Lubricating conditions have a great influence on the adhesive wear type grinding damage. As long as the friction surface always maintains a sufficient strength of the lubricating film and avoids the direct contact surface of the metal between the friction surfaces to form a dry friction or semi-dry friction, Effectively prevent and control the generation and development of adhesive wear-related injuries. Among the various factors affecting this type of injury, ensuring good lubrication of the friction surface is the most important factor. The addition of oily or viscosity additives to lubricating oils can increase the lubricating oil film adsorption capacity and oil film strength, and can double the resistance against scratching.

(2) Adhesion and abrasion of stress factors The injury is generally increased as the pressure increases. When the pressure load exceeds a certain value of the hardness of the friction pair material, the oxide film on the surface of the friction pair is crushed and the embossed points on the new surface between the two surfaces are embedded in each other. When the pressure is relatively moved, the amount of such injuries is increased sharply. Causes the friction surface to glue and even snap. Therefore, when using the machine tool, the worktable and sliding guide should avoid overload operation. For example, the planer, milling machine, boring machine, etc. are used to clamp the work piece on the worktable, and it is necessary to distribute it reasonably without overweight. When the machine tool is overhauled, the guide rails are quenched, which has a great effect on preventing such injuries.

(3) Temperature factors Temperature has a great influence on adhesion-induced wear. The heat generated during the friction process causes the temperature of the friction surface to rise to a certain degree, lighter, damage the lubricating film, and direct contact with the metal surface to form dry friction or semi-dry friction; in severe cases, the material can be brought back The state of fire reduces the hardness of the material; more severely, the local area enables the material of the friction surface to be in a molten state. All these will lead to the development of such research injuries. The use of materials with high thermal stability or enhanced cooling is an effective method to prevent adhesive wear due to temperature.

(4) Sliding speed factors Under certain pressure conditions, the sliding speed is small, the effect of forming a lubricating film is reduced, the thickness of the oil film is small, and the oil film is often partially damaged due to the pressure of the moving parts, causing direct contact between the two metals. , easily lead to injury. However, when the sliding speed is large enough, the temperature of the friction pair is increased, the lubricant film is easily damaged, and the abrasion is easily caused. Therefore, selecting an appropriate sliding speed can reduce the tendency of sticking abrasion type grinding damage.

(5) Surface roughness factors In general, the smaller the surface roughness of the friction pair, the greater the resistance to adhesive wear resistance. The appropriate reduction of the surface roughness can prevent such grinding, and is often adopted for new machine tools. The purpose of gradual loading run-in operation is to reduce the surface roughness to reduce the early generation of adhesive wear. However, the surface roughness of the friction pair is too low, lubricant can not be stored between the friction surface, but also easily lead to injury, so when the pattern on the machine tool rail is worn, often in the maintenance of the machine tool, scraping on the pattern The purpose is to facilitate oil storage and prevent research injuries.

(6) Material factors Brittle materials have stronger anti-adhesion and wear ability than plastic materials. Friction pairs composed of mutually-soluble materials are more likely to produce wear-type abrasions than friction pairs composed of mutually-miscible materials. Friction pairs made of non-metals have less tendency to produce such scratches than friction pairs made of two metals. When overhauling the machine tool, non-metallic coatings can be used to repair the slide rails, shafts and sleeves using different materials, or the surface treatment of the shaft, etc. to reduce the risk of adhesive wear.

Second, abrasive wear grinding injury

This grinding injury refers to the contaminating hard particles that enter between the two frictional surfaces and play a significant cutting action when the surface moves, resulting in scratches and grooves on the friction surface.

1. Mechanism of Abrasive Wear Grinding

After the hard particles enter between the two friction surfaces, they are subjected to two forces, perpendicular to the surface and parallel to the surface. Hard particles penetrate into the surface under the former, and tangential movement occurs under the latter, which results in the surface being ploughed or cut, leaving scratches and grooves, ie, scratches.

2. Classification of Abrasive Wear Grinding

(1) If the bumps on the machine tool such as those caused by bumps are bumped by high-stressed parts such as workpieces or tools, pits will appear on the surface of the rails, and bumps or burrs will appear around the pits. If it is not repaired in time, when the slide slides on the guide rail, the convex or burr on the guide rail will inevitably hurt the skateboard.

(2) Between the two friction surfaces of the hard particles, such as cuttings or molding sand, entering between the machine's guide rail and the slide, will form a typical abrasive wear type grinding injury: cuttings or molding sand under vertical pressure, crushed rails or skateboards On the surface, the track or skateboard is scratched when it is moved.

(3) If dirt enters between the friction surfaces, for example, dirt gets in between the friction surfaces through the lubricating oil. Although the friction surfaces are not pressed, the contact condition of the friction surfaces can be changed. Due to the dirt, the dirt can be held in contact with the dirt. The compressive stress increases. After a long period of operation, it is bound to produce scratches or minor scratches on the friction surface.

3. Prevention of abrasive abrasions

(1) First of all, it is necessary to prevent the surface of the frictional surface from being damaged. For example, do not stack tools, measuring tools, workpieces, and other debris on the machine tool guide rails, and secondly, repair the friction surface after a scratching occurs, such as protrusions and burrs. Use whetstone or squeegee to repair the surface in time to prevent scratches and scratches on the surface. Make repairs to the pits as much as possible to prevent filth in the pits and leave hidden dangers for research and injury.

(2) Arming and guarding shall be arranged to prevent hard particles such as chips and sand from entering the friction surface. Once hard particles or debris are found between the friction surfaces, they should be immediately removed to prevent the development or expansion of the scratch.

(3) Timely maintenance of the machine tool, timely removal of dirt from rails, slides, oil holes and dead spots, to prevent dirt from entering the friction surface, to form abrasive wear-resistant grinding injury.

Third, the repair of injury

Once the machine's friction pair has developed a scratch, it must immediately stop running, find out the reason, study the degree of injury, and take different measures. In general, for scratching a type of scratching, repairs may not be done, but lubrication must be strengthened to prevent expansion; for scratches (including scratches caused by dirt), dirt should be removed and rubbed with a whetstone or spatula After the part has been flattened, it can continue to work: For the scratches caused by glueing, seizing and hard particles, it must be repaired. Nowadays, several common friction pair repairs and repairs are briefly introduced:

1. Sliding bearing and shaft repair

(1) After the surface of the sliding bearing has been scratched, the preferred repair program should be a method of scraping stone. In general, the sliding bearings are left with a margin for adjustment and repair to meet a machine repair cycle. If the injury is very serious and the repair allowance cannot be met, you need to consider replacing it. After grinding the two half-waist sliding bearings, the thickness of the inner hole of the bearing bush can be scratched after the thinning shim is reassembled, and it can be used after being repaired.

(2) When the journal has been scratched, a small journal can be used to replace the sliding bearing. However, the dressing of the journal should not be too large. For example, the spindle of the machine tool, the amount of grinding should not exceed the quenching of the journal surface, or the thickness of carburizing, nitriding, and cyanidation, and the hardness of the journal surface after grinding should not be lower than the lower limit of the original designed hardness. For the drive shaft, the journal should not be smaller than the dimensions of the front end journal or boss in the assembly direction during assembly.

If the journal is to be repaired by welding, the effect of the welding heating process on the geometric accuracy, mechanical strength and surface hardness of the shaft must be considered. After the journal is repaired, it is generally necessary to reconfigure the journal bearing (or sleeve).

2. Repair of machine tool rails

For minor scratches and scratches on the machine's guide rails, it can be continued as long as it is repaired with a knives or oil stones. For gluing, it needs to be repaired by welding or filling and gluing. Even if it is a large-area grinding injury, it can be repaired by using such methods as long as it retains a large part of the intact rail surface. Until the machine tool can be used for requisition, then fine-planing, grinding or sticking, paneling and other methods to repair.

The welding repair and shimming of machine tool rails are relatively complex, especially the repair of large-area grinding. Before construction, consideration should be given to the details; the first consideration is the accuracy of the guide rails used in the repair process. Whether the mechanical performance has any effect, such as welding and gluing, most of which require heating, whether the temperature in the process will cause the deformation of the guide rail, and whether the mechanical strength and surface hardness of the guide rail will be changed. Second, consider the adaptability of the repair process used on the guide rail material. For example, cast iron rails generally do not use cast iron welding. Because the weldability of the casting is poor, cracks, air holes, and other defects are easily generated, and processing is also difficult. The process temperature is high (referring to gas welding), which can easily cause deformation of the guide rails. Third, we must consider the mechanical strength of the repair layer itself and the actual working environment of the guide rail. For example, we also used tin-antimony alloys to repair and repair the 4m planer guide rails and T68 bookbinding guide rails. Since the previous guide rails have good lubrication conditions, they have been used for 20 years and the situation is still good. The latter, due to poor lubrication conditions and poor working conditions, is only In three years it was lost. In short, it is necessary to make the repair layer meet a certain period of use as much as possible.

When the machine tool is overhauled, the guide rails are generally repaired in the following two ways: Firstly, the bed rails are fine-planed, fine-milled or fine-ground, the grinding is repaired and repaired, and the size of the bed rails is removed and the wear of the skateboards is reduced. , Or replace the slide, or glue other compensation materials on the slide rails, such as polytetrafluoroethylene soft tape, epoxy wear-resistant coating, insert nylon plate or metal plate, etc. to compensate the size chain Another method is to fine The machined bed rails are fitted with pre-heat treated steel rails or hardened steel strips.