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 various roughness defects are worthy of our in-depth study and continuous improvement.
Surface roughness is a standard for measuring the surface quality of parts, which has different degrees of influence on the beauty of parts, contact surface friction, fitting surface sealing, fatigue strength of rotating parts, etc. In the process of part design and manufacturing, the surface roughness level of parts must be determined and guaranteed.
However, in the actual metal processing process, the surface roughness of parts will be defective due to various reasons such as tools, processes, and lubrication. If the roughness of parts does not meet the requirements, it may cause the parts to be scrapped, so it is necessary to study the corresponding elimination methods of part roughness defects.
- Analysis of common roughness defects
1) Analysis of rough cutter marks
Rough cutter marks are generally reflected when the cutting feed is increased. It is mainly because during the cutting process, due to the shape of the tool, part of the metal on the metal processing surface is not removed and remains on the processing surface, which is called a cutter mark.
2) Scaling phenomenon
Generally, when the cutting speed is low and high-speed steel or carbide tools are used to cut plastic metal materials, scale-like cracks and burrs are likely to appear on the surface. This phenomenon is called scaling phenomenon. This phenomenon is generally prone to occur during processing such as broaching, slotting, and gear hobbing. When a low-speed, small-front-angle tool is used to cut plastic materials, the chips will be squeezed and cracked, which will cause the force between the tool and the chips to act and change periodically, which will cause metal accumulation, fractures and scalping on the processed surface.
3) Scratches and burrs
Scratches and burrs are also common in the category of roughness defects. Gnawing in gear processing, burrs in grinding processing, etc. are representative phenomena of scratches and burrs. We can analyze the causes of scratches and burrs based on the traces of scratches and burrs in order to formulate elimination measures.
4) Uneven tool marks
For the uneven tool marks, the main reason is still the machine tool, which is mainly manifested by uneven tool cutting marks on the metal handover surface.
5) High-frequency vibration marks
During the metal processing process, the entire process system will vibrate, and the machine tool, tool, and workpiece will have a great impact 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, while the vibration marks produced 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 caused by the action of periodic external forces. Self-excited vibration is the vibration excited by the system movement itself. The common self-excited vibration is the cutting self-vibration.
- Analysis of common roughness defect elimination
1) Analysis of tool mark elimination
First, when cutting, a smaller feed rate within the allowable range should be selected, but the feed rate should not be too small, otherwise it will also affect the roughness of the cutting. Secondly, when sharpening the tool, the radius of the tool tip arc should be appropriately increased within the allowable range, which will have a certain benefit to the roughness.
2) Analysis of burr phenomenon elimination
First, the cutting speed should be controlled. The occurrence of burr phenomenon is to a certain extent due to the cutting speed. Exceeding or falling below a specific speed range will cause burr phenomenon. Secondly, the cutting thickness should be minimized. The increase of cutting thickness will increase the pressure between the chip and the front of the tool. If the chip is squeezed or the chip is unit, the scale phenomenon will occur more frequently and seriously. In addition, the use of high-quality cutting fluid can effectively prevent the occurrence of scale phenomenon. Reasonable selection of the angle of the tool is also an effective way to eliminate the scale phenomenon. Finally, the cutting processability of the workpiece material should be continuously improved. For example, in some cases, the material can be heated before cutting, so that the scale phenomenon will be reduced.
3) Analysis of scratch and burr elimination
If the traces of scratch and burr are distributed regularly, it is generally because of problems with the machine tool. In the traditional system of the machine tool, such as the spindle box, slide box, feed box, etc., the central axis is bent, the gear mesh is poor or damaged, and regular scratch and burr phenomena will occur. Therefore, the machine tool should be checked regularly and repaired and maintained frequently.
If the traces of scratch and burr are distributed irregularly, it may be related to the chip, tool, and cutting fluid. For example, in the process of deep hole processing, poor chip removal will cause scratches on the inner surface. The roughening of the workpiece surface is generally caused by the shedding of abrasive particles and grinding chips during the grinding process. It may also be caused by an inappropriate grinding wheel and unclean cutting fluid, so it is necessary to choose a suitable grinding wheel and clean cutting fluid.
4) Analysis of the elimination of uneven tool marks
There are many reasons for uneven tool marks, but the common one is that when grinding the outer circle, spiral linear marks will appear on the surface of the part. This is caused by the large straightness error of the grinding wheel generatrix, which requires us to do a good job in the selection and maintenance of the grinding wheel regularly. In addition, for example, the crawling of the machine tool workbench or the tool holder will also cause uneven tool feeding, so the inspection and maintenance of the machine tool must be done in place to prevent and avoid it to the greatest extent.
5) Analysis of the elimination of high-frequency vibration marks
The main method to eliminate high-frequency vibration marks is to find the source of vibration, eliminate vibration, or reduce 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 source of vibration, adjusting and repairing the machine tool, the vibration influence can be generally eliminated, and the vibration marks will disappear. If it is cutting self-vibration, this kind of movement exists in the whole cutting process, which requires the adjustment of the whole process system of machine tool, tool and workpiece, such as changing the cutting amount, reasonably selecting the geometric parameters of the tool, reasonably clamping the tool and workpiece, adjusting the machine tool gap, improving the vibration resistance of the machine tool, etc.
Through the research and analysis of common roughness defects in lathe processing, the factors affecting the surface roughness in cutting are found, and the corresponding measures and elimination methods are found. On the one hand, it can prevent problems before processing, and on the other hand, the cause of the problem can be found and solved in time when problems occur, which has practical significance for improving product quality and promoting interchangeable production.
