Knife wear

To deal with the problem of cutter wear, a thorough and detailed examination of the wear and corresponding dynamic characteristics between the cutter and the cutter or between the cutter and the material being cut is required. For the user of the cutting system, the wear of the blade is a discussion topic that will never end. To understand this problem, we must first distinguish between the two basic types of blade wear, namely: 1. The wear that occurs between two cutters; 2. The wear that occurs between the material being cut and the cutter. Abrasion between cutter and cutter A characteristic signal of wear between the cutters can be found at the intersection between the upper and lower blades. It is difficult to avoid the occurrence of wear on hard metal surfaces that are in contact with two rotating states, usually without lubricant. By examining the signs of wear on each surface, we can guide us to find the most likely cutter wear mechanism. Did the blade hit debris, burr, sink, or become dull? Each signal indicates a unique pattern of knife wear. Effect of tilt angle: The tilt angle may cause debris to form between the two blades, and metal burrs form on the upper and lower blades. In some cases, an excessively large angle tilt can also cause the metal to curl up and peel off the newly installed cutter. Harder steels are more likely to grind debris, and softer steels tend to lift up to form burrs. An angle of more than 1 degree may cause the upper blade to be particularly prone to chippings and burrs, and the lower blade will soon become rounded. Unnecessarily large angles (greater than 1 degree) cause the lateral forces to concentrate on particularly small marks around the blade, and the resulting pressure can be so high that it exceeds the maximum resistance of the metal, so that debris is generated. The influence of the intersection: As the intersecting portion of the blade increases, the angle at which the cutting edge intersects with the other blade becomes steeper and steeper, and wear between the blade and the blade is accelerated. This angular intersection still occurs even if the two blades are driven synchronously. The smaller the blade, the larger the overlapping area and the more obvious the result. The typical band wear visible on the cutter contact surface is the slowly formed, worn band. Strictly control the intersection range within 0.8 mm (0.03 inch). Using the largest cutter as possible is very important for delaying the formation of wear bands. Since the quality of the cut decreases as the width of the wear strip increases, each application has a maximum tolerable wear band width standard. Radial yaw (eccentricity) and the effects of vibration: Even if the yaw or vibration of the blade is zero, some wear still occurs due to the angular overlap between the two blades. This is an inevitable problem with the cutting system. In reality, all the cutters will have a little vibration, which will speed up the longitudinal movement between the two blades. At lower speeds, the resulting wear is acceptable, but if the speed is too high, even a small amount of vibration can cause trouble and cost. The wear caused by the yaw is accelerated and can be seen from the concave wear band; or in the case where the yaw is severe, the wear band may also be elliptical, and the size of the ellipse indicates the size of the eccentricity. Vibration in the cutting system is very destructive. In a vibratory cutting system, all the swinging forces concentrate on the delicate intersection between the two cutters. This adds unnecessary wear on the basis of the above-mentioned wear caused by the blade overlap. If the vibration is a low-frequency, high-energy method, severe abrasion marks can be seen on the wear band of the blade; if the vibration is high-frequency and low-energy vibration, obvious marks may not be visible on the blade, but the wear band can be very bright. And it's probably concave. The effect of the cutter's grinding angle (the cross section of the cutter): Due to the touching of the lower knife, the sharper blades are very susceptible to chipping and impact damage. Sometimes, depending on the alloy composition of the cutter, some may even turn over, form small "burrs", or form ridges on the cutting edge. This happens because the metal on the tip of the blade is too small, because there is no metal yield force support, so it is difficult to resist the contact pressure between the two blades. For friction-driven cutting systems, the pressure of the contact surface must be high enough to ensure the rotation of the cutter. In this case too thin a knife can easily be destroyed. For a two-shaft driven dual-axis cutting system, the contact pressure between the two blades can be lower, and the ultra-thin cutting knife becomes more practical. The effect of the alloy composition of the cutter: The hardness of the metal and the alloy composition affect the rate of wear between the cutter and the knife. As the cutter hardness and metal strength increase, the wear resistance improves. However, in practice, the upper limit of the hardness of steel cutters is around 62-64 Rockwell hardness C. To improve wear resistance, high hardness metals such as chromium, tungsten, or vanadium must be added to the cutter steel. As the content of these alloy components increases, the wear resistance is improved. The 12% chromium alloy steel (D-2) is a typical example of a steel that is economical and has enhanced wear resistance. Even so, it is still necessary to use a higher alloy content, such as high tungsten or high vanadium content. Alloy steel for higher wear resistance. These "tool grade" steel grades are better able to withstand the grinding pressure between rotating cutters. In addition to high-alloy steels, it is also possible to use a variety of hard coatings and strips of chromium carbide or ceramic inlaid on the lower blade. The use of chromium carbide on the lower blade has been proven in practice, and if this material is properly installed, it also helps to improve the wear resistance of the upper blade. The use of a hard material coating on the upper knife is good for controlling the wear between the cutters, but it is less effective for improving the wear between the knife and the material being cut. When the alloy content or hardness is low, the wear band on the cutting blade forms a relatively faster speed, while the shape of the wear band tends to be concave. In severe cases, the edges of the cutter roll over, forming ridges or "burrs," indicating that the metal's yield strength is low. Effect of grinding and polishing: When grinding, the grinding wheel causes a series of tiny wrinkles (spikes) and depressions on the surface of the cutter (see Figure 1). In a typical cutter assembly, these spikes are the pressure points at the intersection of the surfaces of the two knives. Therefore, the contact pressure is concentrated on these ridges, causing accelerated wear until the wrinkles are flattened and the contact surface is enlarged. becoming steady. When the polishing is particularly smooth, the initial contact pressure can be distributed over a wide area due to the inconspicuous change of the surface, reducing the initial wear rate and the subsequent change of the knife profile. In general, the maximum allowable roughness is converted to a micro finish of 16. As the tool hardness and alloy content increase, the surface finish should be increased to 8 or better. Carbide and ceramic cutters require a glass-like smooth surface that would otherwise erode with their metal cutters at an alarming rate. The most noticeable sign of wear between the cutting knife and the material being cut and the material being cut is that the blade edge of the upper knife gradually becomes circular and blunt. The steel on the blade is worn by the material being cut. This form of wear is particularly noticeable on the upper knife because it protrudes into the material being cut and is subjected to the impact of grinding. Even very small round blunt surfaces can be seen through the flashlight. The lower knife does not protrude into the material like the upper knife, so the wear rate is lower. Influence of the characteristics of the material being cut: Grinding components in the material, such as titanium, silicates, oxides and a large number of other components, will cause severe erosion of the edges of the upper and lower knife edges. As the material density increases, the pressing force on the cutter increases, and the wear increases. Even the oscillation of the plate accelerates wear. Some of the material being cut produces abrasive dust that acts as a "grind polish" to further abrade the cutter. Aluminum cutting is an example: Alumina dust is produced - the same as the composition of the grinding wheel. The degree of wear depends not only on the abrasive properties of the material being cut but also on the wear characteristics of the cutting system. The knife's grinding angle or cross section, overlap area, alloy content, and finish are the four main factors to consider. Effect of the grinding angle of the cutter (cross-section of the cutter): The cross-section of the upper cutter has a very important significance in the wear between the material being cut and the cutter. The grinding angle of the cutting edge and the width (thickness) of the part where the cutting blade actually extends into the moving plate must be taken into consideration. Grinding angles of 45 degrees or more sharper than that, due to too little metal on the blade, are easily affected by the wear of the material being cut. In this wear, the amount of metal on the blade has a direct effect on the durability of the cutter. The thin, razor-like cutting edge is quickly worn away by the cutting material, and the crisp, sharp edge becomes smooth and rounded. If the "flash test" described above is used, a bright, reflective blade is seen instead of a dark, non-reflective interface, which indicates that this wear has occurred with the material being cut. If the grinding angle of the cutter cannot be reduced, high alloy materials or hard coatings are required. The effects of overlap: The larger the overlap, the deeper the upper knife penetrates into the material being cut and the more severe the wear is. As the cross-section (cross-sectional area) of the knife into the material increases, the "wedge" effect increases, and the grinding pressure from the material on the insert gradually increases. When the cutter enters the material being cut, the linear speed of the cutting edge is lower than the speed of the material, resulting in increased wear. This situation is particularly noticeable in friction-tracting cutting systems, where the speed of the cutter is related to the size of the cutter's only overlap. In the two-axis system, because the speed of the upper and lower cutters can be matched with the speed of the material, it is very useful to compensate the vector speed loss of the cutter relative to the speed of the plate by increasing the speed of the upper cutter. Smaller cutters require more compensation than larger cutters. Effect of cutter alloys: High-carbon steel (52-100, etc.) moderate wear resistance, suitable for cutting low wear materials, such as polyethylene film, thin paper and so on. With the increase of mineral fillers and other abrasive ingredients, the cutter needs to increase the content of hard metal. Chromium is an economical additive. In addition, 12% of chromium-containing steel, ie, D-2, is suitable for general grinding. In the case of high grinding, steels with a high content of tungsten or vanadium are required. The advantage of this steel is that it allows the cutter to run for a longer period of time under grinding conditions with a sharp grinding angle and a minimal cross section. . Prior to the re-grinding of the cutter, a hard coating material such as titanium nitride, ceramic or DLC (Diamond-Like Coatings) on the cutter has a positive effect on preventing wear between the cutter and the cutting material. Re-grinding removes the coating in the most critical area that prevents wear between the cutter and the cutting material. At this point, unless the matrix metal is a sufficient alloy, grinding will continue to occur on the exposed metal. As described above, embedding tungsten carbide or ceramic on the edge of the lower cutter has a positive effect on extending the life of the upper and lower cutters from the viewpoint of wear between the cutter and the cutter. However, from the viewpoint of abrasion between the cutting blade and the cutting material, the use of carbide embedded in the lower cutting edge is not very useful because the lower cutting blade does not penetrate into the cutting material like the upper cutting blade, and is not affected by the grinding of the cutting material. Effect of surface finish: This is related to the part of the cutter that is in contact with the cutting material: Due to the same reason as “wear between the cutter and the cutter”, the rough surface is very fast compared to a smooth polished surface. It will be worn away, and the sharp points and bulges formed by the polishing will be easily eroded by the abrasive. The sharp edge of the texture (less than or equal to 45 degrees) will be quickly ground to a smooth round blunt shape. For general cuts, there is a common clumsy way to test the texture of the cutter, which is not felt by the "nail test" - the finish is 8 or better. If the angle of the blade is 45 degrees or more sharp, the finish should be higher. Summary To determine the cause of cutter wear, look for evidence indicating the wear pattern on the cutter. This will help you determine whether the above-mentioned wear between the cutter and the cutter, or between the cutter and the cutting material . The accompanying diagrams are specifically designed to determine these patterns and possible causes. Once the patterns of wear and tear are discerned, it is easy to find problems and take solutions, rather than relying on “groping in the dark” to spend a lot of money to solve a relatively simple problem.