Surface roughness is generally caused by the processing method used and other factors, such as the friction between the tool and the part surface during the processing, the plastic deformation of the surface metal during chip separation, and high-frequency vibration in the process system. Due to different processing methods and workpiece materials, the depth, density, shape and texture of the marks left on the processed surface are different.
Today we share the causes of surface roughness defects and corresponding preventive measures.
1, analysis of common roughness defects
(1) Tool mark roughness analysis
The roughness defect of tool marks usually occurs when the cutting feed is increased. This is mainly due to the fact that 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 processed surface, which is called tool marks.
(2) Scaling phenomenon
Generally, when the cutting speed is low and high-speed steel or carbide cutting tools are used to cut plastic metal materials, scaly cracks and burrs tend to appear on the surface. This phenomenon is called scaly phenomenon.
This situation usually occurs during broaching, slotting, gear hobbing and other processing processes. When cutting plastic materials at low speed and small rake angle, the chip will be squeezed and cracked, which will cause the force between the knife and the chip to change periodically, which will cause metal to accumulate and cause fractures and scaly phenomena on the machined surface.
(3) Scratches and naps
Scratches and naps are also common in the roughness defect category. Teeth gnawing during gear processing, naps during grinding, etc. are all representative phenomena of scratches and naps. And we can analyze the causes of scratches and naps based on the traces of scratches and naps, in order to formulate elimination measures.
(4) Uneven cutting
As for the phenomenon of uneven tool cutting, the main reason lies in the machine tool. The main manifestation is the uneven cutting marks of the tool on the metal processing surface.
(5) High frequency vibration pattern
During the metal processing process, the entire process system will vibrate, and machine tools, cutting tools, and workpieces 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 patterns produced by the high-frequency vibration of the process system fall into the category of roughness.
The vibration of the process system mainly includes forced vibration and self-excited vibration. Forced vibration is vibration caused by periodic external forces. Self-excited vibration is the vibration excited by the system movement itself. The most common self-excited vibration is cutting self-vibration.
2, common roughness defect elimination analysis
(1) Tool mark exclusion analysis
First of all, when cutting, you should choose a feed rate that is smaller within the allowable range, but the feed rate cannot be too small, otherwise it will also affect the roughness of the cutting. Secondly, when sharpening the tool, the tool tip arc radius should be appropriately increased within the allowable range, which will be beneficial to the roughness.
(2) Troubleshooting Analysis of Scaling Phenomenon
First of all, we need to control the cutting speed. The scaly phenomenon is caused to a certain extent by the cutting speed. If the cutting speed exceeds or falls below a certain speed range, the scaly phenomenon will occur.
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 chips and the front of the tool will become greater and greater. If cracked chips or unit chips are formed, the scaly phenomenon will occur more frequently and seriously.
In addition, the use of better quality cutting fluid can also effectively inhibit the occurrence of scaly phenomena. Reasonably choosing the angle of the tool is also an effective way to eliminate the phenomenon of scales and thorns.
Finally, the machinability of the workpiece material must 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 lint exclusion analysis
If scratches and naps are distributed regularly, it is generally because there is a problem with the machine tool. In machine tools such as spindle boxes, slide boxes, feed boxes and other traditional systems with bent shafts, poor gear meshing or damage, regular scratches and naps will occur. Therefore, machine tools must be checked regularly and maintained frequently.
If scratches and roughening marks are distributed irregularly, it may be related to chips, tools, and cutting fluid. For example, during deep hole machining, poor chip removal will cause scratches on the inner surface. Roughing on the surface of the workpiece is generally caused by the shedding of abrasive grains and grinding chips during the grinding process. It may also be caused by an inappropriate grinding wheel or unclean cutting fluid. Therefore, a suitable grinding wheel and clean cutting fluid must be selected.
(4) Exclusion analysis of uneven cutting
There are many reasons for uneven cutting, but a 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 busbar, which requires us to do a good job regularly in the selection and maintenance of the grinding wheel. In addition, for example, if the machine tool table or tool holder crawls, it will also cause uneven tool movement. Therefore, the inspection and maintenance of the machine tool must be done in place to prevent and avoid it to the greatest extent.
(5) High frequency vibration mark elimination analysis
The main method to eliminate high-frequency vibration marks is to find the source of vibration, eliminate the vibration, or reduce the vibration to the allowable range.
Vibration caused by rotational imbalance or a machine tool's transmission system is forced vibration. Identifying the source and adjusting or repairing the machine will generally eliminate its effects and the resulting vibration marks. Self-excited cutting vibration, however, occurs throughout the cutting process. The complete machine-tool-workpiece process system must then be adjusted: change cutting conditions, choose suitable tool geometry, clamp tools and workpieces correctly, adjust machine clearances and improve vibration resistance.
Through the research and analysis of common roughness defects in lathe processing, we can find out the factors that affect surface roughness in cutting, and find out the corresponding measures and elimination methods. On the one hand, we can nip problems in the bud before processing. On the other hand, when problems occur, we can promptly and accurately find the cause of the problem and solve it in time. This has practical significance for improving product quality and promoting interchangeable production.
(Source: Internet)