In the production of grinding machines, hydraulic components such as disc covers, flat plates, and square and short cylindrical parts are common. Their simple shapes but large cross-sectional thicknesses pose challenges in casting, with high technical requirements and relatively high scrap rates. This article explores the casting technologies for these hydraulic components, including the application of centrifugal slag collecting ladles in disc cover casting, the “horizontal casting and vertical casting” process for flat plates, and the one-piece multi-piece continuous casting and related processes for square and short cylindrical parts. The concept of modulus is introduced to control the cooling rate of castings, aiming to improve the quality and yield of hydraulic component casting and provide a reference for the manufacturing of grinding machine hydraulic components.
1. Introduction
The main products of Zhengzhou Second Machine Tool Factory are various types and specifications of internal grinding machines. Many movements of the internal grinding machine during operation, such as the reciprocating motion of the worktable, the transverse feed motion of the grinding wheel (fast forward, working feed, fast retreat), the automatic clamping of the workpiece, and the actions of the dresser falling and lifting, are all realized through hydraulics, which naturally requires many hydraulic components (collectively referred to as machine tool hydraulic components here). Among them, disc cover type, flat plate type, and square and short cylindrical type are the most common. Although these hydraulic components have simple shapes, they have large cross-sectional thicknesses and mainly need to control the internal quality, requiring high strength, high density, and a certain hardness. At the same time, there are many machined surfaces, and any casting defects such as pores and slag inclusions are not allowed. In the past, when organized production according to the conventional process, the scrap rate was as high as 30% – 40% (including internal and external scraps), causing great losses to the enterprise. In the long-term casting production practice, continuous exploration and attempts have been made according to the geometric characteristics and technical requirements of the castings, and the process plan has been further optimized with remarkable results.
1.1 Hydraulic Components in Grinding Machine Production
- Types and Functions: The hydraulic components in grinding machines play a crucial role in realizing various mechanical movements. For example, the disc cover type components are often used in the end covers of hydraulic cylinders and valve bodies, providing sealing and connection functions. The flat plate type components, like the distribution plates in the oil supply and distribution system, are responsible for ensuring the normal operation of the oil circuit. Square and short cylindrical components, such as the valve bodies of various control valves, control the flow and pressure of the hydraulic oil to achieve precise control of the grinding machine’s actions.
- Quality Requirements: These hydraulic components have strict quality requirements. They need to have high strength to withstand the pressure and force during operation. High density is required to prevent oil leakage and ensure the stability of the hydraulic system. A certain hardness is necessary to resist wear and prolong the service life. Moreover, the machined surfaces must be free of any casting defects to ensure the accuracy and reliability of the grinding machine.
1.2 Challenges in Casting Hydraulic Components
- Large Cross-Sectional Thickness: The thick cross-sections of these components lead to slow cooling rates during casting, which is prone to cause problems such as shrinkage porosity and cracks. It is difficult to ensure the uniformity of the internal structure and mechanical properties.
- High Technical Requirements: The need for high strength, high density, and no casting defects means that strict control is required in the casting process, including raw material selection, melting, pouring, and post-treatment. Any small mistakes may lead to product failure.
- High Scrap Rates: Due to the above challenges, the scrap rates in the production of these hydraulic components are relatively high, reaching 30% – 40%. This not only increases production costs but also affects production efficiency and product delivery time.
2. Casting Technology for Disc Cover Type Hydraulic Components
2.1 Characteristics and Problems of Disc Cover Type Castings
- Geometric Characteristics: Disc cover type castings are generally small in size but relatively thick. They have a circular or disc-like shape with features such as mounting holes and oil passages.
- Common Problems: In the casting process, due to the small size of the parts, the molten metal cools quickly after pouring. The slag has a large resistance when floating to the top surface of the cavity and is often entrapped in the casting, resulting in “slag inclusions” and scrapping of the casting.
2.2 Application of Centrifugal Slag Collecting Ladle
- Principle and Structure: The centrifugal slag collecting ladle utilizes the centrifugal force generated by the tangential inflow of the molten metal to separate the slag from the metal. It is usually designed with a certain shape and size, and is connected to the pouring system through a runner.
- Effect on Casting Quality: By using the centrifugal slag collecting ladle, the slag in the molten metal can be effectively removed, reducing the occurrence of slag inclusions in the casting. At the same time, it can also play a role in feeding, ensuring the density and integrity of the casting.
2.3 Case Analysis: Connecting Disc Casting
- Original Process and Problems: The original process of the connecting disc casting adopted clay sand wet molding, with the process points of “pouring instead of risering”, two pieces in one mold, and sharing a sprue (also serving as a riser). The sprue size was φ70mm × 150mm, and the cross-sectional area of the ingate (also serving as the riser neck) was 2.24 cm², with a length of about 20mm. The castings produced by this process often had a large number of scrappings due to slag inclusions, with a scrap rate of 35%.
- Process Improvement and Results: The improved process replaced the original sprue with a centrifugal slag collecting ladle. The size of the slag collecting ladle was φ72mm × 85mm. An additional sprue was set at an appropriate position in the upper mold and was tangentially connected to the slag collecting ladle through a runner. The distance between the sprue and the center of the slag collecting ladle was 90mm. The ingate also had the function of a riser neck, and its minimum cross-sectional size could ensure that the feeding channel was not prematurely cut off to prevent shrinkage porosity in the casting. The ingate was also approximately tangentially connected to the slag collecting ladle, but the rotation direction was controlled to be opposite to that of the metal liquid in the slag collecting ladle. The length of the ingate was 20mm. A large proportion of the gate was selected to make the slag collecting ladle fill up quickly and reduce the risk of slag being sucked into the cavity. The production verification showed that the improved process was applied well, and the scrap rate was less than 4%.
2.4 Case Analysis: Clamping Hydraulic Cylinder Casting
- Original Process and Problems: The original process of the clamping hydraulic cylinder casting adopted clay sand dry molding, with an edge gate, “pouring instead of risering”, two pieces in one mold, and sharing a sprue (also serving as a riser). The sprue size was φ80mm × 150mm, and the width of the edge gate was 5 – 6mm. In this process application, the metal liquid in the sprue flowed turbulently, and the slag was easily 卷入 the cavity, resulting in “slag inclusions” in the casting. Moreover, the metal liquid was also prone to splash and form “iron beans”, with a high comprehensive scrap rate (about 30%).
- Process Optimization and Results: Based on the theory of balanced solidification and combined with the design of the centrifugal slag collecting ladle, the process was further optimized. The casting was located in the lower mold, with two pieces in one mold. A slag collecting ladle (also serving as a safety riser) was set in the middle of the two castings, with a size of φ65mm × 90mm. An additional sprue was set and was tangentially connected to the slag collecting ladle through a runner. A flash gate was used to introduce the casting, and the size of the flash gate was 65mm × 6mm, with a length of about 10mm. After the optimization of the process, the metal liquid flowed smoothly during pouring, the slag collecting ladle had good slag removal performance, and the comprehensive scrap rate was reduced to 3%.
3. Casting Technology for Plate Type Hydraulic Components
3.1 Characteristics and Quality Requirements of Plate Type Castings
- Geometric Characteristics: Plate type castings are usually flat and rectangular, with a relatively large surface area and a certain thickness.
- Quality Requirements: The two large surfaces of the plate type castings are the most critical parts, and many oil grooves, oil holes, and mounting holes need to be machined on them. The casting is required to have a dense structure, and any casting defects such as pores and slag inclusions are not allowed.
3.2 “Horizontal Casting and Vertical Casting” Process
- Process Principle: In the “horizontal casting and vertical casting” process, the casting is made horizontally in the mold and then erected for pouring. This makes the two large surfaces of the casting in a vertical position during pouring, which is beneficial to the floating and removal of slag and gas, and can improve the quality of the casting surface.
- Advantages and Disadvantages: The advantage of this process is that it can effectively improve the quality of the two large surfaces of the casting, reduce the occurrence of defects such as pores and slag inclusions, and improve the yield of qualified products. The disadvantage is that the process is relatively complex and requires higher requirements for mold design and production.
3.3 Case Analysis: Distribution Plate Casting
- Part Characteristics and Requirements: The distribution plate of the M224 semi-automatic internal grinding machine is a key part of the oil supply and distribution system. All six surfaces are machined surfaces, and the two large surfaces have the highest requirements, with a surface roughness requirement of Ra = 0.8μm. Many oil grooves, oil holes, and mounting holes are designed on it. If there are casting defects such as loose structure, pores, and slag inclusions between any two oil grooves, between oil holes, or between oil grooves and oil holes, it will cause the two oil circuits to communicate, resulting in abnormal machine tool operation. The casting material is HT200, the blank weight is 48kg, and the contour size is 596mm × 312mm × 34mm. The casting is required to have a dense structure, and any casting defects such as pores, slag inclusions, and shrinkage porosity are not allowed.
- Casting Process and Results: The casting process adopted clay sand, dry molding, two pieces in one mold, and the “horizontal casting and vertical casting” process. The machining allowances were 6mm on the side and bottom surfaces and 10mm on the top surface (pouring position). The pouring system was set in the middle of the two castings, with a stepped gate form and shared by the two castings. The cross-sectional area of the ingate was 3.6cm², and the cross-sectional area of the runner was 8.6cm². A bright riser was set in the middle of the top surface of the cavity, with a size of 200mm × 70mm × 70mm, which was beneficial to feeding, venting, and slag collection. The chemical composition was wC = 3.35%, wsi = 1.7%, wMn = 0.85%, wS = 0.1%, and wP = 0.1%. Since the application of this process, the effect has been remarkable, and the scrap rate has been basically controlled within 3%.
4. Casting Technology for Square and Short Cylindrical Hydraulic Components
4.1 Characteristics and Technical Requirements of Square and Short Cylindrical Castings
- Geometric Characteristics: These castings are mainly square or short cylindrical in shape, with relatively simple outer shapes.
- Technical Requirements: The outer surfaces of these castings need to be machined, and valve holes, oil holes, oil grooves, and mounting holes are usually designed. The valve holes have high technical requirements, including wear resistance, dense structure, uniform hardness, and no casting defects.
4.2 One-Piece Multi-Piece Continuous Casting and “Horizontal Casting and Vertical Casting” Process
- Process Introduction: The one-piece multi-piece continuous casting process combines multiple castings into one blank for casting, which can improve production efficiency and reduce production costs. The “horizontal casting and vertical casting” process is also used to ensure the quality of the casting.
- Benefits and Significance: This process can effectively solve the problems of low process yield and high scrap rate in the casting of thick-section hydraulic components. It can also ensure the uniformity of the internal structure and mechanical properties of the casting, and improve the quality and performance of the hydraulic components.
4.3 Case Analysis: Reciprocating Manipulator Box Casting
- Part Details and Requirements: The reciprocating manipulator box casting of the M224 semi-automatic internal grinding machine is made of HT300. It has 5 valve holes, dozens of oil holes and mounting holes, and several oil grooves. The valve holes have the highest requirements, with a surface roughness of Ra = 0.4μm, a cylindricity error of 0.005mm, and a hardness requirement of > 170HBW. The blank outer dimension is 148mm × 150mm × 108mm, and the weight is 19kg. There should be no slag inclusions, sand holes, and pores in the casting.
- Casting Process and Results: The continuous casting vertical pouring process was adopted. One blank was cast with four pieces, and the blank size was 108mm × 810mm × 150mm. The 604mm part was the casting, and the upper 206mm part was the riser (after the casting was formed, the 604mm part was cut into 4 pieces with a length of 148mm on a sawing machine, and the remaining part was the riser and used as return material). The single blank weight was 99.7kg (including the riser part). Two blanks were cast in one mold, and the “horizontal casting and vertical casting” process was used. After molding, drying, assembling, and tightening, the sand mold was erected for pouring. A gate was set in the middle of the two cavities, and a stepped gate form was adopted, which could ensure the smooth filling of the molten metal and make the temperature of the metal liquid in the riser part the highest, with good feeding effect. At the same time, by using the “horizontal casting and vertical casting” process, the slag and pores in the molten metal would float to the top of the cavity under the action of buoyancy, and the top part was the riser. After the casting was formed, it was cut off to ensure the good quality of the casting. In addition, the pressure of the metal liquid in the cavity was relatively large during vertical pouring, and the formed structure was denser after solidification. The material was HT300, and the composition of the molten iron was wC = 3.10%, wsi = 1.4%, wMn = 1.2%, wS = 0.1%, and wP = 0.1%. The control before furnace: the white mouth of the triangular test piece was 10 – 12mm. The process effect was good, the process yield reached 80.5%, and the scrap rate was within 4%.
5. Assurance of Material Properties
5.1 Chemical Composition Control
- Importance of Chemical Composition: The chemical composition of the casting directly affects its mechanical properties, such as strength, hardness, and toughness. Appropriate chemical composition selection is the key to ensuring the quality of the hydraulic component.
- Control Methods: According to the performance requirements and wall thickness of the casting, select the appropriate chemical composition, accurately calculate the charge ratio, and strictly control the quality of raw materials (stable composition). For example, for HT200 and HT300 materials, the content of carbon, silicon, manganese, sulfur, and phosphorus needs to be strictly controlled within a certain range.
5.2 Cooling Rate Control
- Influence of Cooling Rate: The cooling rate of the casting during solidification affects its microstructure and mechanical properties. Too fast or too slow cooling may lead to defects such as shrinkage porosity and cracks.
- Control Measures: Introduce the concept of modulus to calculate and compare the modulus of various geometries. When casting the same brand of castings with the same or similar modulus, they can be arranged in the same sand box or the sand molds can be 集中 placed together to ensure the uniformity of the cooling rate and the quality of the casting. The modulus calculation formulas of several common geometries are shown in the following table:
| Geometry Name | Graphic | Modulus Calculation Formula |
|---|---|---|
| Plate or Circular Plate | [Image of plate or circular plate] | For a plate or circular plate with a ≥ 57, M = T/2 |
| Rectangular Rod | [Image of rectangular rod] | For a rectangular cross-section rod-shaped part, M = ab/[2(a + b)] |
| Solid Cylinder | [Image of solid cylinder] | For a short cylinder with h ≤ 2.5D, M = rh/[2(r + h)]; for a long cylinder with h > 2.5D, M = D/4 |
| Hollow Cylinder (Sleeve Type) | [Image of hollow cylinder] | For a hollow ring with b < 5a, M = ab/[2(a + b)]; for a hollow tube with b > 5a, M = a/2 |
6. Conclusion
In the production of hydraulic components for grinding machines, different casting technologies are adopted for different types of components to solve the problems of slag removal, feeding, and ensuring the quality of large surfaces and valve holes. The application of the centrifugal slag collecting ladle in disc cover casting, the “horizontal casting and vertical casting” process for flat plates, and the one-piece multi-piece continuous casting and “horizontal casting and vertical casting” process for square and short cylindrical parts have all achieved good results. At the same time, by controlling the chemical composition and cooling rate, the quality and performance of the hydraulic components can be further ensured. These casting technologies and control methods provide a reference for improving the production quality and efficiency of grinding machine hydraulic components。
