The surface roughness is generally formed by the processing method and other factors, such as the friction between the tool and the part surface in the processing process, the plastic deformation of the surface metal when the chip is separated, and the high-frequency vibration in the process system.
Due to the different processing methods and workpiece materials, the depth, density, shape and texture of the marks left on the machined surface are different.
Today we share the causes of surface roughness defects and corresponding preventive measures.
Analysis of common roughness defects
1. Knife mark roughness analysis
The rough defects of tool marks are generally reflected in increasing the cutting feed rate, mainly because in the cutting process, due to the shape of the tool, some metal on the metal processing surface is not cut off and remains on the processing surface, which is called tool marks.
2. Scale prick phenomenon
Generally, when the cutting speed is low and high-speed steel or cemented carbide tools are used to cut plastic metal materials, scale cracks and burrs are easy to appear on the surface.
This phenomenon is called scale prick phenomenon.
Generally, this situation is easy to occur in broaching, slotting, hobbing and other machining 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 tool and the chip to change periodically, which makes the metal accumulate and the phenomenon of fracture and scale on the machined surface.
3. Scratches and galling
Scratch and galling are also common in the category of roughness defects.
The phenomenon of gnawing teeth in gear processing and galling in grinding are all representative phenomena of scratch and galling.
We can analyze the causes of scratches and galling according to their traces, so as to formulate troubleshooting measures.
4. Uneven knife pattern
For the unevenness of knife pattern, the main reason lies in the machine tool, which is mainly manifested in the unevenness of tool cutting marks on the metal processing surface.
5. High frequency vibration pattern
In the process of metal processing, the whole process system will vibrate, and machine tools, cutting tools and workpieces will have a great impact on the surface roughness of metal parts.
The low-frequency vibration of the process system generally produces waviness on the workpiece surface, while the vibration pattern produced by the high-frequency vibration of the process system belongs to the category of roughness.
The vibration of process system mainly includes forced vibration and self-excited vibration.
Forced vibration is caused by periodic external force.
Self excited vibration is the vibration excited by the system itself, and the most common self-excited vibration is cutting self vibration.
Troubleshooting Analysis of common roughness defects
1. Knife mark elimination analysis
First, when cutting, you should choose a smaller feed within the allowable range, but the feed should not be too small, otherwise it will also affect the cutting roughness.
Secondly, when grinding the tool, it is necessary to appropriately increase the arc radius of the tool tip within the allowable range, which will be beneficial to the roughness.
2. Elimination and analysis of scale prick phenomenon
First, control the cutting speed.
To some extent, the phenomenon of scale pricking is due to the cutting speed.
If it is beyond or below the specific speed range, the phenomenon of scale pricking will occur.
The second is the cutting thickness.
The cutting thickness should be reduced as much as possible.
With the increase of cutting thickness, the pressure between the chip and the front of the tool will be greater and greater.
If the extrusion chip or unit chip is formed, the scale and thorn phenomenon will appear more frequently and seriously.
In addition, the use of high-quality cutting fluid can also effectively inhibit the generation of scale pricking phenomenon.
Reasonably selecting the angle of the cutting tool is also an effective method to eliminate the phenomenon of scale stabbing.
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 scale and thorn phenomenon will be reduced.
3. Scratch and galling exclusion analysis
If the trace distribution of scratch and galling is relatively regular, it is generally due to the problems of the machine tool.
In the machine tool, such as the spindle box, chute box, feed box and other traditional systems, the shaft bending, poor gear engagement or damage will occur regular scratch and galling.
Therefore, the machine tool should be inspected regularly and repaired and maintained frequently.
If the marks of scratch and galling are distributed irregularly, it may be related to chips, tools and cutting fluid.
For example, in the process of deep hole machining, poor chip removal will cause scratch on the inner surface.
The roughening of workpiece surface is generally caused by the falling off of abrasive particles and debris in the grinding process, or the improper grinding wheel and unclean cutting fluid.
Therefore, it is necessary to select the appropriate grinding wheel and clean cutting fluid.
4. Elimination and analysis of unevenness of knife pattern
There are many reasons for unevenness of knife pattern, but it is common, for example, when grinding an outer circle, spiral linear traces will appear on the surface of parts.
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 regularly.
In addition, for example, the creeping of the machine tool workbench or tool holder will also cause uneven tool walking.
Therefore, the maintenance and repair of the machine tool must be in place to prevent and avoid it to the greatest extent.
5. Elimination analysis of high frequency vibration ripple
The main method to eliminate high-frequency vibration marks is to find out the vibration source, eliminate the vibration, or reduce the vibration to the allowable range.
For example, the vibration caused by the unbalanced rotation of parts and the vibration caused by the transmission system of the machine tool are all forced vibration.
By finding out the vibration source, adjusting and repairing the machine tool, the vibration influence can be eliminated, and the vibration pattern will disappear at the same time.
If it is cutting natural vibration, this movement exists in the whole cutting process, which requires the adjustment of the whole process system of the machine tool, tool and workpiece, such as changing the cutting parameters, reasonably selecting the geometric parameters of the tool, reasonably clamping the tool and workpiece, adjusting the machine gap, improving the vibration resistance of the machine tool, etc.
Wrap it up
Through the research and analysis of common roughness defects in lathe machining, find out the factors affecting surface roughness in cutting, and find out the corresponding measures and elimination methods.
On the one hand, it can prevent the trouble before machining, on the other hand, it can find the cause of the problem timely and accurately and solve it in time when there are problems, which is of practical significance to improve product quality and promote interchangeability production.