1、 Common reasons affecting the surface smoothness of casting
1. The shape of raw materials, such as molding sand, can be divided into circular, square, and triangular shapes. The worst is the triangular shape, with particularly large gaps (if it is resin sand molding, it will increase the amount of resin added, and of course, it will also increase the gas generation. If the exhaust is not good, it is easy to form pores). The best is circular sand. If it is coal powder sand, the ratio of sand (strength and humidity of sand) also has a significant impact on the appearance. If it is carbon dioxide hardened sand, then it mainly depends on the coating
2. Material: If the chemical composition ratio of the casting is imbalanced, such as low manganese, it is easy to produce looseness and the surface material is coarse 3. Casting system. If the casting system is not reasonable, it can easily cause the casting to become loose, and in severe cases, the casting may not be poured properly, and even incomplete castings may not be made. Improper slag holding system can cause slag to enter the mold cavity and form slag holes
4. Slag making. If the slag in the molten iron is not cleaned up or not blocked during casting, causing the slag to run into the mold cavity, slag holes will inevitably appear 5. Due to carelessness and carelessness, the sand is not cleaned thoroughly or falls into the sand during the box closing process. The shape is that the sand is not solid enough, or the sand ratio is unreasonable, resulting in insufficient sand strength. As a result, sand holes will appear in the castings
6. Exceeding the standard of sulfur and phosphorus can cause cracks in castings. These are all necessary precautions to ensure the quality of castings during production or guidance The reasons mentioned above are only a small part of it. Due to the ever-changing and complex nature of casting production, problems often arise during production, and sometimes a problem cannot be found for a long time
II. Three factors affecting the roughness of gray cast iron
Surface roughness, as an important measure of the surface quality of gray cast iron, not only directly determines the appearance of gray cast iron parts, but also has a significant impact on the equipment quality of machines and the service life of gray cast iron parts.
. This article focuses on analyzing how to improve the surface roughness of gray cast iron parts from three aspects: machine tools, cutting tools, and cutting parameters
1. The influence of machine tools on the surface roughness of gray cast iron parts
Factors such as poor rigidity of machine tools, poor spindle accuracy, unstable fixation of machine tools, and large clearances between machine tool components can all affect the surface roughness of gray cast iron parts.
For example, if the spindle runout accuracy of a machine tool is 0.002mm, which is 2 micrometers of runout, it is theoretically impossible to process workpieces with a roughness lower than 0.002mm. Generally, workpieces with a surface roughness of Ra1.0 can still be processed. And gray cast iron itself is a casting part, so it will not be easy to process with high surface roughness like steel parts. In addition, the poor conditions of the machine tool itself make it even more difficult to ensure surface roughness
The rigidity of machine tools is usually set at the factory and cannot be modified. In addition to the rigidity of the machine tool, the spindle clearance can also be adjusted to improve bearing accuracy and reduce the machine clearance, thereby ensuring a higher surface roughness for gray cast iron parts during processing
2. The influence of cutting tools on the surface roughness of gray cast iron parts
Selection of tool materials
When the affinity between the tool material and the metal molecules of the processed material is high, the processed material is prone to bonding with the tool, resulting in the formation of chip deposits and scales. Therefore, for those with severe bonding and friction, the surface roughness will be high, and vice versa.
. When processing gray cast iron parts, it is difficult for hard alloy blades to achieve a surface roughness of Ra1.6. Even if they do, their tool life is greatly reduced. BNK30 CBN tools, on the other hand, due to their low friction coefficient, excellent high-temperature thermal stability, and wear resistance, can easily process Ra1.6 surface roughness at cutting speeds several times higher than hard alloy tools. At the same time, the tool life is tens of times that of hard alloy tools, and the surface brightness is increased by an order of magnitude
Selection of tool geometric parameters
The main angle Kr, the secondary angle Kr ', and the tool tip arc radius re have a significant impact on surface roughness in tool geometric parameters.
. When the main and auxiliary angles are small, the height of the residual area on the processed surface is also small, thus reducing the surface roughness; The smaller the secondary angle, the lower the surface roughness, but reducing the secondary angle can easily cause vibration, so reducing the secondary angle should be determined according to the rigidity of the machine tool. The influence of the radius of the cutting edge arc re on surface roughness: When re increases within the allowable stiffness, the surface roughness will decrease. Increasing re is a good method to reduce surface roughness. Therefore, reducing the main deflection angle Kr, the secondary deflection angle Kr ', and increasing the radius r of the tool tip arc can all reduce the height of the residual area, thereby reducing the surface roughness
The tool engineer once said, 'For the selection of the tool tip arc angle, it is recommended to choose based on the rigidity and roughness requirements of the processed workpiece. If the rigidity is good, try to choose a larger arc angle, which can not only improve processing efficiency but also improve surface smoothness. However, when boring or cutting slender shafts or thin-walled parts, due to poor system rigidity, smaller tool tip arc radii are often used.'
Tool wear
Tool wear is divided into three stages: initial wear, normal wear, and severe wear.
. When the tool enters the stage of severe wear, the wear rate of the tool back surface sharply increases, the system tends to be unstable again, the vibration increases accordingly, and the amplitude of the change in surface roughness also sharply increases
In the field of gray cast iron, many parts are mass-produced, requiring high consistency in product quality and production efficiency. Therefore, many machining companies choose to change the tool before it reaches the third stage of severe wear, also known as forced tool change. Machining companies will repeatedly test the tool to determine a critical point, ensuring the surface roughness and dimensional accuracy of gray cast iron without affecting overall production efficiency
4. The influence of cutting parameters on the surface roughness of gray cast iron parts. Different choices of cutting parameters have a significant impact on surface roughness and should be given sufficient attention. Precision machining is an important process to ensure the surface roughness of gray cast iron parts. Therefore, during precision machining, cutting parameters should be mainly focused on ensuring the surface roughness of gray cast iron parts, while also considering productivity and necessary tool life. The cutting depth of precision machining is determined by the machining accuracy and surface roughness requirements, and is determined by the margin left after rough machining. Generally, the cutting depth is controlled within 0.5mm. At the same time, under the condition of the rigidity of the machine tool, the cutting performance of the tool can be fully utilized, and high cutting speeds can be used for high-speed machining of gray cast iron parts
The influence of other factors on the surface roughness of gray cast iron parts
For example, the presence of some casting defects in gray cast iron parts, unreasonable selection of cutting fluid, and different processing methods can all affect the performance roughness of gray cast iron parts.
The tool engineer once said, "In addition to the three major factors of machine tool, cutting tool, and cutting parameters, cutting fluid, gray cast iron parts themselves, and processing methods also have a certain impact on the surface roughness of gray cast iron parts. For example, when turning, milling, and boring gray cast iron parts, CBN tools can also produce a surface roughness of Ra0.8 under the conditions of machine tool and cutting parameters, but it will have an impact on the tool life, which needs to be judged according to the actual processing situation
5、 Summary: Given that surface roughness has a direct impact on the performance of machine parts, and the factors that affect surface roughness in actual production come from multiple sources, it is necessary to comprehensively consider all factors and propose economically applicable requirements for surface roughness according to needs
