In recent years, driven by national policy adjustments, the machine tool industry has experienced rapid development. A significant number of large-scale, high-precision, and advanced CNC machine tools have been successfully developed, breaking the long-standing technological blockade imposed by foreign countries. Our company specializes in several major types of large, high-end CNC machine tools, including vertical and horizontal machining centers. The key large-scale castings for these machines mainly comprise columns, worktables, crossrails, spindle housings, and tailstock bodies. The quality requirements for these machine tool castings are exceptionally stringent. They must satisfy demanding technical specifications such as high strength, high precision, and excellent wear resistance. Simultaneously, rigorous standards are imposed on dimensional accuracy, internal cleanliness, surface roughness, and metallurgical quality.
I. Large Column Castings for Machine Tools
1. Process Research
The column casting has a net weight of 66t, a rough casting weight of 78t, and a total pouring weight of 88t. Its contour dimensions are 10,040 mm × 3,315 mm × 1,930 mm, and the material is HT350. The internal ribs are 30mm thick, and the guideways demand particularly high strength and hardness. The complexity lies in achieving complete filling and sound solidification for such a massive, structurally intricate machine tool casting.
2. Process Design and Numerical Simulation
The column features 19 internal cavities with a double-wall structure. To ensure complete mold filling, the gating system was designed to feed the mold from three faces. To control shrinkage porosity in the guideways, they were positioned on the side of the mold. Given the enclosed sand core structure, ample venting was incorporated into each core assembly. Crucially, numerical simulation software was employed to validate the process. A 3D model of the casting, including the gating system, feeders, and chills, was constructed for analysis.
(1) Gating System: The system was set at both ends along the length and on the plane opposite the guideways. It consisted of 57 ceramic tubes (Ø40 mm) as ingates, with sprue sections of 2-Ø120 mm and 4-Ø120 mm ceramic tubes. The cross-sectional area ratio was: $$ \sum A_{\text{sprue}} : \sum A_{\text{runner}} : \sum A_{\text{ingate}} = 1 : 1.4 : 1.5 $$. The target pouring time was 180 seconds.
(2) Feeder and Vent Placement: For the guideway sections, 18 neck-down feeders (Ø60 mm × 1200 mm) were used. Eleven Ø70 mm neck-down feeders were placed at the two end faces, and 25 mm × 50 mm × 800 mm vent feeders were positioned on the large plane to aid both feeding and gas expulsion.
3. Molding and Core Assembly
A pit molding with assembled cores was adopted. The entire mold consisted of 140 individual cores, forming both the external shape and internal cavities (ceramic tubes for the gating system were also embedded directly into the cores). Vent channels between each layer of cores were pre-formed during core making to ensure the smooth escape of gases generated during pouring, promoting a stable filling process. After pouring, the casting was allowed to cool in the mold for 30 days to prevent distortion and cracking.
4. Chemical Composition Control
Precise control of chemistry is fundamental for the required properties in large machine tool castings. The target composition for the HT350 column was as follows:
| Element | Target Range (wt.%) |
|---|---|
| C | 2.9 – 3.0 |
| Si | 1.3 – 1.4 |
| Mn | 1.0 – 1.1 |
| P | ≤ 0.07 |
| S | ≤ 0.12 |
| Cu | 0.5 – 0.6 |
5. Numerical Simulation Results
The simulation (using Huazhu CAE software) indicated that during solidification, the thin internal ribs (30mm) solidified first. The guideways (270mm × 120mm × 8500mm), the top section of the casting, and the bolting areas on the drag side (210mm × 280mm × 1830mm) were the last to solidify. Graphite chills were placed against the guideway faces and bolting areas to promote directional cooling. The top sections, being the last to solidify, were fed by multiple ladle pours during casting. The simulation confirmed the feasibility of the overall process plan for this critical machine tool casting.
6. Production and Melting Process Control
(1) Raw Materials: High-quality pig iron (Z14/Z18), low-carbon steel scrap, and clean returns with minimal Cr content were used. Alloying elements included 75% ferrosilicon (5-20mm), ferromanganese, and high-purity carburizer.
(2) Charge Make-up: Pig Iron: 30%, Steel Scrap: 60%, Returns: 10%.
(3) Duplex Melting Practice: For large machine tool castings requiring 60-100t of molten metal, a duplex melting process combining cupola and medium-frequency induction furnace was utilized. Molten iron from a 9t cupola was transported to a 20t induction furnace for superheating and final composition adjustment.
(4) Process Sequence:
- Melt 20t in the induction furnace, analyze at 1450°C, and adjust composition (elemental deviations ≤ ±0.05%). Transfer to a holding furnace.
- Melt another 20t batch, adjust, and tap into a ladle.
- Receive 15t of cupola iron into the induction furnace, adjust the remaining 5t based on analysis, and superheat.
- All transferred iron was reheated in the induction furnace within 3.5 hours to prevent freezing in ladles.
- High-Temperature Refining: Each furnace charge was superheated to above 1500°C for a short period for degassing and impurity removal.
7. Intensive Inoculation Practice
To ensure a fine graphite structure and prevent chilling, a multi-stage inoculation strategy was implemented for these heavy-section machine tool castings.
| Stage | Inoculant | Size | Addition Rate | Purpose/Timing |
|---|---|---|---|---|
| Stream Inoculation | 75% FeSi | 8-10 mm | 0.3% | Added during the final 80-90% of tapping. |
| Late In-mold (Floating Silicon) | 75% FeSi | 60-80 mm | 0.2% | Added to the ladle just before pouring to combat fade. |
| Instant Inoculation | Ba-containing FeSi | 0.5-2 mm | 0.1% | Added into the stream during casting. |
8. Pouring Temperature Control
A meticulous schedule was calculated for each ladle, accounting for transfer, reheating, treatment, and pouring times. Based on estimated temperature drops, the treatment temperature for each ladle was set to ensure all pours entered the mold within a 20°C range, guaranteeing uniform solidification conditions for the large machine tool casting.
9. Production Outcome
Six columns were cast using this comprehensive methodology, and all passed inspection, meeting the stringent quality standards for high-end machine tool castings.
II. Quality Improvement for Spindle Housing and Tailstock Body Castings
For years, severe shrinkage porosity and cavities on the upper surfaces of large spindle housing and tailstock body castings posed a significant challenge, persisting despite various traditional risering techniques.
1. Analysis of Defect Causes
(1) Solidification Complexity: These castings are tall (often ~2m). Even with stepped gating, the metal reaching the top copes suffers significant heat loss, disrupting favorable directional solidification.
(2) Ineffective Riser Feeding: Conventional or even insulated risers often proved inadequate. Due to the height, the metal in the risers was near its solidification temperature when the mold was filled. With high pouring temperatures, severe thermal stratification occurred, causing the riser to solidify prematurely and lose its feeding capability. This highlighted a core problem in the manufacture of these box-type machine tool castings.
2. Initial Process Experiment
A trial was conducted on a spindle housing with contour dimensions 2900mm × 2650mm × 2680mm, a maximum wall thickness of 300mm, material HT250, total weight 62t, and a pouring weight of 70t.
Process Modification: The goal was to achieve balanced and directional solidification in the lower/mid sections and strict directional solidification in the upper section. Chills were used on heavy sections and thermal junctions. A novel two-tier gating system was introduced: a conventional stepped system for the lower part and a separate, independent stepped gating system feeding the upper portion of the casting.
Melting & Chemistry: To improve properties while allowing a higher carbon equivalent (CE ≈ 3.8%), 0.5% Cu was added.
| Element | Target Range (wt.%) |
|---|---|
| C | 3.2 – 3.3 |
| Si | 1.6 – 1.8 |
| Mn | 0.8 – 0.9 |
| P | < 0.15 |
| S | 0.05 – 0.12 |
| Cu | 0.5 |
Pouring temperature was 1360-1370°C for the main ladles, with the central ladle for the upper gating system ~20°C hotter. Multi-stage inoculation was used.
Result: Shrinkage depth was reduced to 10-20mm from the previous 200mm, indicating significant improvement but not complete elimination.
Analysis: The pouring time (313s) was too long, leading to excessive heat loss. The upper gating system had insufficient ingates, and the central ladle temperature was lower than specified. The carbon equivalent was also on the lower side.
3. Process Optimization and Final Solution
Based on the analysis, the gating system was completely redesigned into a large-capacity, two-tier system with a substantially increased number of ingates to reduce pouring time and improve thermal control.
(1) Gating System Design: A large-flow, partially choked rectangular system.
(2) Gating System Calculations:
Total metal weight: $$ G = 55,000 \text{ kg (casting)} + 7,000 \text{ kg (risers)} = 62,000 \text{ kg} $$
Calculated pouring time (using empirical formula): $$ t = S_1 \sqrt[3]{\delta G} $$ where $S_1$ is a coefficient (1.15), $\delta$ is the minimum wall thickness (20mm). Thus, $$ t = 1.15 \times \sqrt[3]{20 \times 62,000} \approx 178 \text{ s} $$. A target time of $t = 160$ s was selected for optimization.
Average effective metal head pressure: $$ H_p = H_0 – \frac{P^2}{2C} $$ where $H_0=298$ cm, $P=266$ cm, $C=268$ cm. Therefore, $$ H_p = 298 – \frac{266^2}{2 \times 268} \approx 166 \text{ cm} $$.
Minimum choke cross-sectional area: $$ \sum F_{\text{choke}} = \frac{G}{0.31 \cdot t \cdot \mu \cdot \sqrt{H_p}} $$ where the resistance coefficient $\mu = 0.4$. $$ \sum F_{\text{choke}} = \frac{62,000}{0.31 \times 160 \times 0.4 \times \sqrt{166}} \approx 242.6 \text{ cm}^2 $$
Accounting for the two-tier system, 23 choke tubes of Ø40 mm were selected, providing a total area of $23 \times \pi \times (2)^2 \approx 289 \text{ cm}^2$.
(3) Feeder Design: 32 insulated neck-down risers (Ø150/Ø100 mm × 700 mm) were designed to solidify after the casting’s hot spots.
(4) Chemistry and Pouring Practice: Remained as per the initial trial.
4. Final Production Outcome
The optimized process was implemented. The subsequent casting was completely free from shrinkage defects. This validated process was then successfully applied to produce three spindle housings (92.4t pouring weight) and two tailstock bodies (90.5t pouring weight), all meeting quality and performance specifications. This methodology has now become the standard for all large box-type machine tool castings in our production.
III. Conclusions and Key Learnings
The development and consistent production of high-quality large machine tool castings rely on a systems engineering approach that integrates advanced simulation, rigorous process design, and precise metallurgical control.
- Numerical Simulation: The use of casting solidification simulation software is an indispensable tool for predicting and eliminating defects in complex machine tool castings. It allows for virtual prototyping and optimization before costly production trials.
- Design for Manufacturability (DFM) Review: A critical review of casting designs from a foundry perspective is essential. Designs focused solely on mechanical function may incorporate features detrimental to sound casting production, potentially leading to defects like hot tearing in thin sections connected to heavy walls.
- Advanced Gating Strategy: For tall, box-like machine tool castings, a multi-tiered, independently fed gating system is highly effective. Separating the feed for the lower/mid sections from the upper section allows for precise control over the solidification sequence, effectively eliminating shrinkage in the top regions, which was historically problematic.
- Intensive and Late-Stage Inoculation: Given the extended processing times for large castings, inoculation fade is a significant risk. Implementing a multi-stage inoculation strategy, culminating in late ladle additions and instantaneous inoculation during pouring, is crucial for achieving the required fine graphite microstructure and mechanical properties throughout the machine tool casting.
- Precise Thermal Management: In multi-ladle pours, maintaining a tight temperature window (e.g., ±20°C) between all ladles is vital for uniform solidification behavior. Meticulous planning of the melting, transfer, treatment, and pouring schedule is necessary to achieve this control.
The continuous refinement of these integrated techniques—from simulation-driven design and innovative gating to controlled duplex melting and robust inoculation—forms the foundation for reliably manufacturing the large, high-integrity castings that underpin modern, high-performance machine tools. The pursuit of excellence in machine tool casting technology remains a critical endeavor for advancing the manufacturing capabilities of the industry.