introduction
The spindle headstock is the core part of various machine tools that carries the cutting head and operates. It is equipped with a spindle and its complex transmission components, and carries a large load during operation, directly affecting the processing accuracy and stability of the machine tool. Therefore, it is required that the spindle headstock must have high quality, especially to ensure that the shaft cylinder supporting the rotation of the spindle does not have any casting defects. Based on the simulation of the process plan of the spindle headstock using ProCAST software, analysis of the metal flow field and temperature field, optimization and improvement measures are proposed to further obtain qualified spindle headstock castings.
Part Analysis
The casting of the machine tool spindle box is shown in the figure. Its maximum external dimensions are 475mm×369 mm ×350 mm, with a total weight of 101.16 kg, belonging to medium-sized castings. The wall thickness difference at the shaft cylinder part of the casting is large, with a maximum wall thickness exceeding 60mm and a minimum wall thickness of only 10.5mm. The process plan should avoid the occurrence of shrinkage cavities, cracks, etc. The material of the spindle box is HT300, with specific chemical composition; the carbon equivalent of HT300 is about 3.558%, belonging to hypoeutectic cast iron, with a eutecticity of about 0.81, and poor fluidity. Due to CE<3.6%, only shrinkage occurs during casting solidification, which increases the tendency for shrinkage cavities and porosity in the casting. Therefore, in order to obtain high-quality castings, measures should be taken to improve the fluidity of the iron melt and design a feeding system.
| component | C | Si | Mn | P | S | Fe |
| content | 2.90~3.20 | 1.40~1.70 | 0.80~1.00 | <0.15 | <0.12 | the rest |
Due to the large difference in wall thickness of the spindle box, it is difficult to achieve simultaneous solidification. Therefore, the formation of sequential solidification should be considered first. Therefore, based on the structural characteristics of the spindle box, it is better to select the pouring position shown in the figure; the parting surface is selected on the upper surface of the shaft cylinder square hole, which is the largest cross-section of the casting and uses the least amount of live blocks, making it convenient for molding.
Numerical simulation and optimization
ProCAST numerical simulation technology was used to analyze the two options and select the best one. The process option was established using UG to create a 3D model, which was then imported into ProCAST in .stp file format for meshing. The material selection was EN-GJL-300 (i.e., HT300), and the mold was selected as resin sand. The remaining parameter settings are shown in the table.
flow field simulation
The flow field simulation of the process plan is shown in Figure 4. From the figure, it can be seen that the side runner created a rotation phenomenon during the initial filling stage, and the molten metal flowed in a The rotating state is full of the bottom, which is prone to defects such as gas bubbles and slag inclusion; and as can be seen from the figure, the two streams of molten metal flow forward along the sides of the mold cavity and meet at the bottom of the shaft cylinder. The slender flow channel can cause the temperature of the molten metal to drop too quickly, and it is easy to form weld marks at the intersection of the shaft cylinder bottom. In subsequent flows, the molten metal fills smoothly. The filling diagram of the bottom runner shows that due to the vertical placement of the runner, a large pressure head causes the molten metal to produce a fountain phenomenon, as shown in the figure, which is prone to defects such as air holes. As can be seen from the figure, the equal-section runner allows the molten metal in the support seat to fill faster and more, which can maintain the liquid level at both the distal and proximal ends roughly level, avoiding severe disturbances caused by liquid level differences; subsequent filling is relatively stable. Although there are certain defects in the initial filling of both schemes, scheme 1 inevitably causes rotation of the molten metal due to the structure of the casting, which further deteriorates the quality of the casting and is difficult to solve; scheme 2 causes a fountain phenomenon due to excessive pressure head, so it is only necessary to take measures to slow down the flow rate of the molten metal to improve the fountain phenomenon, which can eliminate defects. Selecting scheme 2 for optimization.
Sprue optimization
The fundamental reason for the excessive flow rate is that the pressure head of the molten metal when it first enters the mold cavity is too large, resulting in a height difference of nearly During the injection process, the metal liquid can obtain a large velocity under the action of gravity, resulting in the fountain phenomenon. Therefore, a buffer runner is introduced into the sprue. In order to achieve a good buffer effect, the cross-sectional area of the buffer runner should be larger than that of the sprue, forming an open structure. Therefore, the cross-sectional area of the buffer runner is selected to be 1.3 times that of the sprue. In order to facilitate molding and slag retention, the buffer runner is located at the parting surface, as shown in the figure. This method divides the sprue into two parts. After entering the pouring system through the first sprue, the metal liquid is first buffered by the sprue nest and buffer runner, and then buffered by the runner and ingate. This can effectively reduce the velocity of the metal liquid entering the cavity, thus avoiding the fountain phenomenon. Numerical simulation analysis shows that the velocity of the metal liquid entering the cavity is about 0.49 m/s, which is about 50% slower than before optimization. The height of the metal liquid spray is also greatly reduced, showing a flat state, proving that the flow rate is appropriate and can ensure smooth initial filling.
Temperature field simulation and optimization
The simulated temperature field and defect distribution are shown in the figure. It can be seen from the figure that the scheme before optimization had a collapse at the top of the shaft, proving that self-feeding was not effective.Feet; in addition, the top and four ends of the shaft cylinder have high temperatures, and the difference in wall thickness causes it to solidify slowly. From the distribution of defects in the figure, shrinkage cavities and porosity It also basically appears in these positions and needs to be eliminated through the use of risers and chillers. The feeding distance of its risers can reach 8 times the diameter of the riser, so according to the structural dimensions of the spindle box, four risers can be set, including two on the top of the side wall and two at the shaft tube. The size of the risers is calculated using the modular method, and the specific values are shown in Table 4. In addition, due to the large size of the hot spot areas of the spindle box, it is necessary to use chillers to achieve better feeding effects, so a total of 25 iron external chillers are arranged at the hot spot areas such as the shaft tube and side wall.
As shown in the comparison diagram, in the optimization scheme, the temperature distribution of the casting shows a decreasing trend from top to bottom, with the highest temperature occurring in the risers. From the distribution of defects in the diagram, shrinkage cavities and porosity are concentrated in the gating and riser system, and there are almost no shrinkage cavities or porosity found on the casting, proving the rationality of the optimization scheme. Using this scheme, high-quality qualified castings can be obtained.
Epilogue
The wall thickness of the machine tool spindle box varies greatly, and the formation of sequential solidification should be considered first. In order to ensure the filling effect of HT300, it is best to choose a semi-closed bottom gating system. Numerical simulation found that the opening of side internal runners will cause the rotation of the metal liquid, which is prone to defects such as gas entrainment and slag inclusion, while the bottom internal runner will produce a fountain phenomenon, which is prone to defects such as porosity. Adding a buffer runner in the sprue can effectively reduce the speed of the metal liquid entering the cavity, thus avoiding the fountain phenomenon of the bottom internal runner and ensuring smooth filling. The simulation temperature field found that there are many hot spots in the spindle box, and
HT300 has insufficient self-shrinkage, but the use of risers and chills can enable the casting to form a good sequential solidification, avoiding the formation of shrinkage cavity-type solidification defects.