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Analysis of the occurrence and elimination of surface roughness defects in metal processing
Add Time:2020-10-22
  In metal processing, surface roughness defects seriously affect the quality of metal processing, and the mechanism of surface roughness defects and the elimination methods of roughness defects in various aspects are worthy of our in-depth study and continuous improvement. Surface roughness is a standard for measuring the surface quality of parts, and it has varying degrees of influence on the appearance of parts, friction of contact surfaces, sealing of contact surfaces, fatigue strength of rotating parts, etc. In the process of part design and manufacturing, the surface roughness grade of the part must be determined and guaranteed. However, in the actual metal processing process, the surface roughness of the parts will be defective due to various reasons such as tools, technology, lubrication, etc. The roughness of the parts may not meet the requirements, and the parts may be scrapped. Therefore, the roughness defects of the parts must be dealt with accordingly. Research on the elimination method.
  1. Analysis of common roughness defects during metal processing
  1. Rough analysis of knife marks.
  The roughness of tool marks is generally reflected when increasing the cutting feed rate, mainly due to the fact that part of the metal processing surface is not removed due to the shape of the tool during the cutting process and remains on the processed surface, which is called tool marks.
  2. Scaly thorn phenomenon.
   Generally, when the cutting speed is low, and when high-speed steel or cemented carbide tools are used to cut plastic metal materials, scaly cracks and burrs are prone to appear on the surface. This phenomenon is called scaly. This situation is generally prone to occur during broaching, slotting, and gear hobbing. When cutting plastic materials with low speed and small rake angle tools, chipping will be formed, which will cause the force between the tool and the chips to change periodically, which makes the metal accumulate and the machined surface breaks. And scale thorn phenomenon.
  3. Scratching and napping.
  Scratches and napping are also common in the roughness defect category. Teeth nibbling during gear processing, napping during grinding, etc. are all representative phenomena of scratches and napping. And we can analyze the causes of scratches and napped marks in order to formulate elimination measures.
  4. The knife is uneven.
  For the unevenness of the knife, the main reason is the machine tool, which is mainly manifested by the uneven cutting marks of the tool on the metal surface.
  5. High-frequency chatter marks.
  In the process of metal processing, the entire process system will vibrate. Machine tools, cutting tools, and workpieces will all have a great influence on the surface roughness of metal parts. Among them, the low-frequency vibration of the process system generally produces waviness on the surface of the workpiece, and the vibration lines generated by the high-frequency vibration of the process system belong to the category of roughness. The vibration of the process system mainly includes forced vibration and self-excited vibration. Forced vibration is the vibration produced by periodic external forces. Self-excited vibration is the vibration excited by the system movement itself, and the most common self-excited vibration is cutting self-vibration.
   2. Common roughness defect elimination analysis in metal processing
  1. Knife mark elimination analysis.
  When cutting first, select a smaller feed rate within the allowable range, but the feed rate cannot be too small, otherwise the cutting roughness will also be affected. Secondly, when sharpening the tool, it is necessary to appropriately increase the radius of the tool tip arc within the allowable range, which will be beneficial to the roughness.
  2. Analysis of the elimination of scales and spines.
  First, the cutting speed must be controlled. The scaly phenomenon is due to the cutting speed to a certain extent, and the scaly phenomenon will occur if the speed exceeds or falls below a certain speed range. The second is the cutting thickness. The cutting thickness should be reduced as much as possible. As the cutting thickness increases, the pressure between the chip and the front of the tool will become greater and greater. If cracked chips or unit chips are formed, the scaly phenomenon will appear even more. Frequent and severe. In addition, the use of better quality cutting fluids can also effectively suppress the occurrence of scaly. A reasonable choice of the angle of the cutter is also an effective method to eliminate the phenomenon of scales. Finally, the machinability of the workpiece material should be continuously improved. For example, in some cases, the material can be heated before cutting, so that the scaly phenomenon will also be reduced.
  3. Scratch and rough removal analysis.
Scratches and napping if the traces are distributed regularly, it is generally due to problems with the machine tool. In the machine tool, such as spindle box, slide box, feed box and other traditional systems, the shaft will be bent, gears are bad or damaged, and regular scratches will appear. It is necessary to check the machine tool regularly and perform frequent maintenance.
  If the distribution of scratches and napping marks is irregular, it may be related to chips, tools, and cutting fluid. For example, in the process of deep hole machining, poor chip removal will cause scratches on the inner surface. The surface roughness of the workpiece is generally caused by the shedding of abrasive particles and debris during the grinding process. It may also be caused by the improper grinding wheel and the unclean cutting fluid. Therefore, it is necessary to choose a suitable grinding wheel and clean cutting fluid.
  4. Analysis of the elimination of uneven cutting.
  There are many reasons for uneven cutting, but the common one is that when grinding the outer circle, the surface of the part will appear spiral linear marks. This is caused by the large straightness error of the grinding wheel bus, which requires us to do a good job in the selection and maintenance of the grinding wheel. In addition, for example, the crawling of the machine tool table or tool post will also cause uneven cutting. Therefore, the inspection and maintenance of the machine tool must be in place to prevent and avoid it to the greatest extent.
  5. High-frequency chattering elimination analysis.
  The main method to eliminate high-frequency chatter marks is to find the source of the vibration, eliminate the vibration, or reduce the vibration to the allowable range.
For example: vibration caused by unbalanced rotation of parts, vibration caused by the machine tool transmission system, etc., are all forced vibrations. By finding the vibration source, adjusting and repairing the machine tool, the effect of vibration can generally be eliminated, and vibration marks Will also disappear. If it is cutting self-vibration, this kind of movement exists in the entire cutting process, which requires adjustments to the entire process system of the machine tool, tool, and workpiece, such as changing the cutting amount, reasonably selecting the geometric parameters of the tool, and reasonable clamping Cutting tools and workpieces, adjusting the clearance of the machine tool, and improving the vibration resistance of the machine tool.
  Three, summary
Through the research and analysis of the common roughness defects in lathe processing, find out the factors that affect the surface roughness in cutting, and find out the corresponding measures and elimination methods. On the one hand, it can prevent problems before processing, on the other hand, When there is a problem, the cause of the problem can be found in time and accurately, and the problem can be solved in time, which has practical significance for improving product quality and promoting interchangeability.