In the rapidly advancing manufacturing industry, the demand for high-precision and high-performance large CNC machine tools has significantly increased. As a critical component, the ram plays a vital role in ensuring machine stability, accuracy, and durability under complex loads such as impact and vibration. The production of high-quality machine tool castings, particularly for large rams, presents challenges due to their substantial size, intricate structures, and stringent requirements for mechanical properties and microstructural integrity. This study focuses on optimizing the casting process for a large CNC machine tool ram made of HT300 gray iron, aiming to achieve superior performance and minimize defects. Through a combination of empirical experience, simulation analysis, and experimental validation, we have developed an efficient casting methodology that enhances the quality of machine tool castings and provides valuable insights for similar large-scale applications.
The ram casting investigated in this research is designed for a high-end CNC machine tool, with a total weight of 2,400 kg and a casting rough weight of approximately 3,200 kg. Its overall dimensions are 520 mm in width, 580 mm in height, and 3,895 mm in length, featuring multiple precision-machined holes and complex geometries that necessitate careful casting design. The material of choice, HT300 gray iron, is widely used in machine tool castings due to its excellent wear resistance, damping capacity, and strength. However, achieving the desired properties requires precise control over chemical composition and casting parameters to avoid common issues like shrinkage porosity, gas defects, and poor microstructure.
The chemical composition of HT300 is critical for obtaining the required mechanical properties and microstructure. Typically, it consists of carbon, silicon, manganese, phosphorus, and sulfur within specific ranges to promote the formation of type A graphite and a pearlitic matrix with minimal free carbides. The target composition for this machine tool casting is summarized in Table 1, which ensures high strength and good machinability. Proper control of these elements during melting, often using induction furnaces, is essential to prevent defects and achieve consistent quality in machine tool castings.
| Element | Content (wt.%) |
|---|---|
| C | 2.9–3.2 |
| Si | 1.0–2.5 |
| Mn | 0.5–1.4 |
| P | ≤0.15 |
| S | ≤0.12 |
In terms of microstructure, HT300 machine tool castings should exhibit a pearlite content exceeding 95%, with type A graphite distributed uniformly at lengths corresponding to grades 4–5. The presence of primary austenite dendrites is desirable, while intergranular and free cementite must be avoided to prevent brittleness and ensure dimensional stability. This microstructure directly influences the casting’s ability to withstand operational stresses and maintain precision over time. Therefore, our approach integrated rigorous compositional checks and thermal analysis to optimize the solidification behavior of the machine tool casting.
The gating system design is a cornerstone of successful casting processes for large machine tool components. For this ram casting, we selected a semi-open gating system with bottom-filled rain-gate injection. This configuration reduces metal oxidation, promotes smooth filling, minimizes slag inclusion, and enhances feeding efficiency. The gating system comprises a sprue, runner, ingates, and a pouring basin, with dimensions calculated based on the casting weight and geometry. The cross-sectional areas were optimized to ensure balanced flow and reduce turbulence, which is crucial for preventing defects in machine tool castings.
To determine the optimal riser design for feeding the solidifying casting and preventing shrinkage defects, we evaluated two schemes using ProCAST simulation software. Riser placement at hot spots is vital for compensating volumetric shrinkage during solidification. In Scheme 1, four risers were positioned at the top hot spots of the casting, with specific dimensions: a central section of radius 50 mm and length 300 mm, and end sections of radius 20 mm and lengths 200 mm and 315 mm, respectively. Scheme 2 involved five risers with adjusted radii (R33 mm and R15 mm) to assess feeding efficiency. The simulation analyzed filling patterns at key stages: initial runner filling, filling of complex internal cavities, near-complete filling, and full mold filling.
The results indicated that Scheme 1 provided a more stable filling sequence, with metal entering the farthest ingate first and progressing smoothly, reducing the risk of turbulence and shrinkage. In contrast, Scheme 2 showed earlier filling of nearer ingates and increased turbulence at the top, leading to a higher likelihood of defects. Thus, Scheme 1 was adopted for its superior performance in producing sound machine tool castings. This iterative design process underscores the importance of simulation in optimizing riser configurations for large machine tool castings.
Other critical casting parameters were meticulously defined to complement the gating and riser design. The pouring temperature was set at 1,430 °C, based on multiple trials that balanced fluidity and defect formation. Higher temperatures can cause mold expansion and increased scrap rates, while lower temperatures may result in misruns and shrinkage cavities. The pouring time was calculated using the empirical formula common in casting practice for large components:
$$ t = S_1 \cdot \delta \cdot G_L^{1/3} $$
where \( t \) is the pouring time in seconds, \( S_1 \) is a coefficient typically ranging from 1.7 to 1.9 for bottom gating (we used 1.7), \( \delta \) is the average wall thickness in mm (40 mm for this ram), and \( G_L \) is the pouring weight in kg (3,200 kg). Substituting the values, we obtained:
$$ t = 1.7 \cdot 40 \cdot (3200)^{1/3} \approx 79 \text{ seconds} $$
This calculated pouring time ensures adequate mold filling without excessive turbulence. Additionally, the cooling time in the sand mold was established as 20 hours, allowing for complete solidification and stress relief, which is essential for achieving the desired mechanical properties and minimizing distortions in machine tool castings. Premature cooling can lead to cracks and residual stresses, compromising the component’s performance.
The detailed gating system design included 10 rectangular ingates with dimensions of 40 mm by 15 mm each, providing a total cross-sectional area that facilitates uniform metal distribution. The runner was designed with a trapezoidal cross-section of 72 cm², using dimensions of (80 mm + 100 mm) × 80 mm / 2. The sprue featured a circular cross-section of 85 cm², resulting in a diameter of 104 mm, and a pouring basin with a diameter of 130 mm and height of 160 mm was incorporated to cushion the metal flow. The ingates were arranged at 36-degree intervals around the runner’s center to promote symmetrical filling. This comprehensive gating system layout is tailored to the demands of large machine tool castings, ensuring efficiency and quality.
To validate the optimized process, we conducted actual casting trials and produced a ram casting that met all design specifications. Visual inspection revealed no surface defects such as burrs, sand adhesion, shrinkage cavities, or porosity, confirming the effectiveness of the gating and riser design. Microstructural analysis further demonstrated a pearlite content over 98%, type A graphite at lengths of grade 4–5, and no undesirable carbides, aligning with the requirements for high-performance machine tool castings. The mechanical properties were evaluated through tensile and hardness tests, as summarized in Table 2, showing tensile strength (Rm) values between 309 and 316 MPa, exceeding the standard 300 MPa for HT300, and Brinell hardness (HBW) values in the range of 224–231, which favor subsequent machining operations.
| Sample No. | Tensile Strength, Rm (MPa) | Hardness, HBW |
|---|---|---|
| 1 | 316 | 231 |
| 2 | 309 | 224 |
| 3 | 312 | 227 |
The success of this study highlights the importance of integrated process design for machine tool castings. By optimizing parameters such as pouring temperature, gating system geometry, and riser configuration, we achieved a casting with excellent mechanical and microstructural properties. The low hardness facilitates easy machining, while the high strength ensures durability in demanding applications. This approach can be adapted to other large machine tool castings, providing a reference for improving quality and reducing scrap rates in industrial settings.
In conclusion, the research demonstrates that a systematic approach to casting process design is crucial for producing high-quality machine tool castings. Key findings include the optimal pouring temperature of 1,430 °C, the use of four risers in a specific configuration, and a semi-open gating system with bottom-filled rain-gate injection. The calculated pouring time and cooling period further contribute to defect-free production. The resulting machine tool casting exhibits superior performance, underscoring the value of simulation and experimentation in advancing casting technologies for large-scale components. Future work could explore the effects of varying alloy compositions or cooling rates on the properties of machine tool castings to further enhance their application in precision engineering.