Surface roughness is typically caused by several factors, including the processing method, friction between the tool and the workpiece surface during processing, plastic deformation of the surface metal during chip separation, and high-frequency vibration in the processing system .
The depth, density, shape and texture of marks on the machined surface may vary depending on the specific processing method and material used.
In this article we will discuss the causes of surface roughness defects and the preventative measures that can be taken to avoid them.
Analysis of common roughness defects
1. Knife mark roughness analysis
The presence of rough marks on the tool is usually a result of increasing the cutting feed rate. This is because during the cutting process, the shape of the tool can cause some metal on the processed surface to not be fully cut, leaving behind what are known as tool marks.
2. Scale sting phenomenon
The appearance of cracks and burrs on the surface when cutting metallic plastic materials is a common occurrence when the cutting speed is low and high-speed steel or carbide tools are used. This is known as the “scale bite phenomenon”. This is often seen in machining processes such as broaching, grooving and milling.
When cutting plastic materials at low speeds with a small rake angle, chips are often compressed and cracked, causing a periodic change in force between the tool and the chip, leading to metal accumulation and resulting in fractures and flaking on the machined surface.
3. Scratches and abrasions
Scratches and abrasions are also common forms of roughness defects. Examples of these defects include gnawed teeth in gear processing and abrasions in grinding. By analyzing the evidence left behind by scratches and abrasions, it is possible to determine the causes and develop measures to resolve them.
4. Irregular knife pattern
The main cause of an uneven knife pattern is the machine tool, which results in uneven cut marks on the surface of the processed metal.
5. High frequency vibration pattern
During metal processing, the entire process system may experience vibrations, which can significantly affect the surface roughness of metal parts. This includes the machine tool, cutting tool and workpiece. Low-frequency vibrations in the process system tend to produce ripples on the surface of the part, while high-frequency vibrations result in roughness.
Process system vibrations can be divided into two types: forced vibration and self-excited vibration. Forced vibration is caused by periodic external forces, while self-excited vibration is produced by the system itself. The most common form of self-excited vibration is cutting-induced vibration.
Troubleshooting Analysis of Common Roughness Defects
1. Knife Mark Elimination Analysis
To improve cutting roughness, it is important to choose a suitable feed. It is recommended to select a smaller feed within the permitted range, however, the feed should not be too small, as it may negatively impact roughness. Furthermore, when grinding the tool, increasing the arc radius of the tool tip within the allowable range can positively affect the roughness.
2. Elimination and analysis of the scale bite phenomenon
First, control the cutting speed. The occurrence of scale bites can be attributed, to some extent, to the cutting speed. If the speed exceeds or falls below the designated range, scale bites will occur.
Second, adjust the cutting thickness. It is recommended to minimize the cutting thickness as much as possible. An increase in cutting thickness will result in greater pressure between the chip and the front of the tool, leading to more frequent and severe scale chipping.
Furthermore, the use of high-quality cutting fluid can effectively prevent the formation of scale bites. Carefully selecting the angle of the cutting tool is also an effective solution.
Finally, improve the machinability of the workpiece material. For example, heating the material before cutting can reduce the scale pitting phenomenon in some cases.
3. Scratch and abrasion exclusion analysis
If the distribution of scratches and wear marks is consistent, it is usually the result of a problem with the machine tool. In traditional systems such as spindle box, feed box and kick box, regular scratches and abrasions can occur due to shaft bending, poor gear engagement or damage.
To solve this problem, it is important to regularly inspect the machine tool and carry out frequent maintenance and repairs.
However, if the scratch marks and abrasions are inconsistent, it could be related to chips, tools, or cutting fluid. For example, when machining deep holes, poor chip removal can result in scratch marks on the inner surface.
Workpiece surface roughness is often caused by abrasive particles and debris falling during the grinding process or by the use of an unsuitable grinding wheel or unclean cutting fluid. To avoid this, it is important to choose the appropriate grinding wheel and keep the cutting fluid clean.
4. Elimination and analysis of irregularities in the knife pattern
There are many reasons for the uneven knife pattern, but it is common to see spiral linear strokes appearing on the surface of parts when grinding an outer circle. This is usually due to a large straightness error in the grinding wheel shaft, which emphasizes the importance of carefully selecting and regularly maintaining the grinding wheel.
Another example of an uneven knife pattern is caused by the machine tool bed or tool holder slipping, which leads to uneven tool movement. To prevent this from happening, it is crucial to properly maintain and repair the machine tool.
5. High frequency vibration ripple elimination analysis
The main way to eliminate high-frequency vibration marks is to identify the source of vibration, eliminate it or reduce it to an acceptable level. For example, vibration caused by unbalanced rotation of parts or the machine tool transmission system is considered forced vibration.
By locating the source of vibration and repairing the machine tool, the vibration can be eliminated and the vibration marks will disappear.
If chatter is the result of natural vibration induced by cutting, it occurs throughout the cutting process and requires adjustments to the entire machine tool, tool, and part system. This may involve changing cutting parameters, selecting appropriate tool geometry, properly clamping the tool and workpiece, adjusting machine clearances, and improving the vibration resistance of the machine tool.
Concluding
By researching and analyzing common roughness defects in lathe machining, the factors that affect the surface roughness during cutting can be identified and the corresponding measures and elimination methods can be determined.
This approach not only helps prevent problems before machining, but also allows for timely and accurate identification of the root cause of any issues that may arise, leading to immediate resolution. This is of practical importance for improving product quality and promoting interchangeable production.
