一、 Common causes affecting casting surface finish
1. The shape of raw materials, such as molding sand, is divided into round, square and triangular. The worst is triangular, with particularly large gaps (if it is resin sand modeling, the amount of resin added will also increase, and of course the amount of gas will also increase at the same time. If If the exhaust is not good, it is easy to form pores), the best is round sand. If it is coal powder sand, the sand ratio (the strength and humidity of the sand) also has a great 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 become loose and the surface material will be rough.
3. Casting system. If the casting system is unreasonable, it will easily lead to loose castings. In serious cases, the castings may not be poured, or even complete castings may not be made.
An unreasonable slag holding system will cause slag to enter the mold cavity and create slag holes.
4. Slag making. If the slag in the molten iron is not cleaned or the slag is not blocked during casting, causing the slag to run into the mold cavity, slag holes will inevitably appear.
5. Man-made, due to carelessness, the sand is not cleaned or falls into the box when closing the box, the sand is not compacted into the shape, or the sand ratio is unreasonable, the sand strength is not enough, and the casting will produce trachoma.
6. Exceeding the standard of sulfur and phosphorus will cause cracks in castings. When producing or guiding production, these are matters that must be paid attention to in order to ensure the quality of castings.
The reasons mentioned above are only a small part of them. Due to the ever-changing and profound nature of casting production, problems encountered during production will often occur. Sometimes a problem occurs and the cause cannot be found for a long time.
二. Three major factors affecting the roughness of gray cast iron
As an important measure of the surface quality of gray cast iron, surface roughness not only directly determines the exquisite appearance of gray cast iron parts, but also has a great impact on the equipment quality of the machine 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 the machine tool, poor spindle accuracy, weak fixation of the machine tool, and large gaps between the various parts of the machine tool will affect the surface roughness of gray cast iron parts.
For example: if the runout accuracy of the machine tool spindle is 0.002mm, which is 2 micron runout, then it is theoretically impossible to machine a workpiece with a roughness lower than 0.002mm. Generally, workpieces with a surface roughness of Ra1.0 are okay. Process it out. Moreover, gray cast iron itself is a casting, so it cannot be processed with high surface roughness as easily as steel parts. In addition, the conditions of the machine tool itself are poor, making it more difficult to ensure surface roughness.
The rigidity of the machine tool is generally set at the factory and cannot be modified. In addition to the rigidity of the machine tool, the spindle clearance can also be adjusted, the bearing accuracy can be improved, etc. to make the machine tool clearance smaller, thereby obtaining higher surface roughness during processing of gray cast iron parts. degree is guaranteed to a certain extent.
2.Effect of cutting tools on surface roughness of gray cast iron parts
Tool material selection
When the affinity between the metal molecules of the tool material and the material to be processed is high, the material to be processed is easy to bond with the tool to form built-up edge and scaly. Therefore, if the adhesion is serious or the friction is serious, the surface roughness will be large, and vice versa. . When processing gray cast iron parts, it is difficult for carbide inserts to reach the surface roughness of Ra1.6. Even if it can be achieved, its tool life will be greatly reduced. However, CBN tools made of BNK30 have low friction coefficient of tool materials and excellent high-temperature heat resistance. Stability and wear resistance, the surface roughness of Ra1.6 can be easily processed at a cutting speed several times higher than that of carbide. At the same time, the tool life is dozens of times that of carbide tools, and the surface brightness is improved by one Magnitude.
Selection of tool geometry parameters
Among the tool geometric parameters that have a greater impact on surface roughness are the main declination angle Kr, the secondary declination angle Kr’ and the tool tip arc radius re. When the main and secondary declination angles are small, the height of the residual area of the processed surface is also small, thus reducing the surface roughness; the smaller the secondary declination angle, the lower the surface roughness, but reducing the secondary declination angle will easily cause vibration, so the reduction The secondary deflection angle should be determined according to the rigidity of the machine tool. The influence of tool tip arc radius re on surface roughness: When re increases when stiffness allows, the surface roughness will decrease. Increasing re is a good way to reduce surface roughness. Therefore, reducing the main declination angle Kr, the secondary declination angle Kr’ and increasing the tool tip arc radius r can reduce the height of the residual area, thereby reducing the surface roughness.
Tool engineers have said, “It is recommended to choose the arc angle of the tool tip based on the rigidity and roughness requirements of the workpiece to be processed. If the rigidity is good, try to choose a larger arc angle, which will not only improve the processing efficiency, but also improve the surface finish. “But when boring or cutting slender shafts or thin-walled parts, a smaller tool tip arc radius is often used due to poor system rigidity.”
Tool wear
The wear of cutting tools is divided into three stages: initial wear, normal wear and severe wear. When the tool enters the severe wear stage, the tool flank wear rate increases sharply, the system tends to become unstable, the vibration increases, and the change range of surface roughness also increases sharply.
In the field of gray cast iron, many parts are produced in batches, which require high product quality consistency and production efficiency. Therefore, many machining companies choose to change tools without waiting for the tools to reach the third stage of severe wear, which is also called mandatory. When changing tools, machining companies will repeatedly test the tools to determine a critical point, which can ensure the surface roughness requirements and dimensional accuracy of gray cast iron without affecting the overall production efficiency.
3.The influence of cutting parameters on the surface roughness of gray cast iron parts.
The different selection of cutting parameters has a greater impact on the surface roughness and should be paid enough attention to. Finishing is an important process to ensure the surface roughness of gray cast iron parts. Therefore, during finishing, the cutting parameters should be mainly to ensure the surface roughness of gray cast iron parts, taking into account productivity and necessary tool life. The cutting depth of finishing is determined by the margin left after rough machining based on the machining accuracy and surface roughness requirements. Generally, the cutting depth is controlled within 0.5mm. At the same time, as long as the rigidity of the machine tool permits, 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.
4. The influence of other factors on the surface roughness of gray cast iron parts
For example, gray cast iron parts themselves have some casting defects, unreasonable cutting fluid selection, and different processing methods will affect the roughness of gray cast iron parts.
Tool engineers have said, “In addition to the three major factors of machine tools, cutting tools, and cutting parameters, factors such as cutting fluid, gray cast iron parts themselves, and processing methods also have a certain impact on the surface roughness of gray cast iron parts, such as turning, milling, When boring gray cast iron parts, CBN tools can also machine a surface roughness of Ra0.8 if the machine tool, cutting parameters and other factors allow it, but it will have an impact on the tool life. The specifics need to be judged according to the actual processing conditions. “.
5. Summary
In view of the fact 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 many aspects, it is necessary to take all factors into consideration and make more economical adjustments to surface roughness as needed applicable requirements.
三、 How to improve the surface finish of castings (ductile iron castings)
Sandblasting
Craftsmanship:
Wash with gasoline (120#) and blow dry with compressed air → Sand blasting → Blow the sand with compressed air → Install and hang → Weak corrosion → Rinse with flowing cold water → Electro-galvanize or hard chrome.
Weak corrosion process: w (sulfuric acid) = 5% ~ 10%, room temperature, 5 ~ 10s.
Etching and scrubbing methods
When the workpiece is not allowed to be sandblasted due to special requirements for accuracy or surface finish, only etching and scrubbing methods can be used to purify the surface.
step:
①Petrol scrubbing (120#). When oily workpieces or dirty gasoline are used, wash them again with clean 120# gasoline.
② Blow dry with compressed air.
③Erosion. w (hydrochloric acid) = 15%, w (hydrofluoric acid) = 5%, room temperature, until the rust is removed. If there is too much rust and the oxide scale is too thick, it should be scraped off mechanically first. The etching time should not be too long, otherwise it will easily cause hydrogenation of the substrate and expose too much free carbon on the surface, resulting in partial or complete failure to coat the coating.
④ Brushing with lime slurry can fully expose the crystal lattice on the surface of the workpiece and obtain a coating with good bonding force.
⑤ Rinse and wipe. Remove lime adhering to the surface.
⑥ Installation and hanging. Cast iron parts have poor electrical conductivity, so they should be in firm contact when installed and hung. There should be as many contact points as possible. The distance between workpieces should be slightly 0.3 times larger than that of electroplated parts made of other materials.
⑦Activation. The purpose of activation is to remove the oxide film formed during scrubbing, mounting and other processes. Formula and process conditions: w (sulfuric acid) = 5% ~ 10%, w (hydrofluoric acid) = 5% ~ 7%, room temperature, 5 ~ 10s.
⑧Rinse with running water.
⑨Electro-zinc plating or hard chromium.
Post time: May-26-2024
