In recent years, with the adjustment of national policies, the development of machine tool products has progressed rapidly. Numerous large-scale, high-precision CNC machine tools have been developed, breaking the long-term technological blockade imposed by foreign countries. Our company specializes in the production of several types of large high-precision CNC machine tools, including vertical and horizontal models. The key large castings for these machine tools include columns, worktables, crossbeams, spindle boxes, and tailstock bodies. These machine tool castings must meet stringent quality requirements, including high strength, high precision, wear resistance, and strict dimensional accuracy, internal cleanliness, surface roughness, and metallurgical quality.
The production of large machine tool castings involves complex processes to ensure structural integrity and performance. We have conducted extensive research and development to optimize the casting processes for various components, focusing on aspects such as gating system design, riser placement, mold assembly, chemical composition control, and numerical simulation. This article details our approaches to manufacturing large column-type castings and improving the quality of spindle box and tailstock body castings, emphasizing the use of advanced techniques to achieve superior results in machine tool castings.
Large Column-Type Machine Tool Castings
Column castings are critical components in machine tools, requiring high rigidity and stability. We developed a column casting with a net weight of 66 tons, gross weight of 78 tons, and total pouring weight of 88 tons. The overall dimensions are 10,040 mm × 3,315 mm × 1,930 mm, made of HT350 material. The internal rib thickness is 30 mm, and the guideways demand high strength and hardness.
Process Research and Design
The column structure consists of 19 chambers with double-walled construction. To ensure complete filling, the gating system was designed to pour from three sides of the mold. The guideways were positioned on the side of the mold to minimize shrinkage porosity. Given the enclosed sand structure, vent holes were incorporated to facilitate gas escape from the cores. Numerical simulation using casting simulation software was employed to validate the process.
The three-dimensional model included the casting, gating system, risers, and chills. The gating system comprised ceramic tubes: 57 inner gates of Ø40 mm, with main downspouts of 2-Ø120 mm and 4-Ø120 mm. The cross-sectional area ratio was: ∑Adownspout : ∑Arunner : ∑Aingate = 1 : 1.4 : 1.5. Pouring was completed in 180 seconds from three sides.
Risers included 18 neck-down risers of Ø60 mm × 1,200 mm for the guideways, 11 neck-down risers of Ø70 mm on the end faces, and 25 mm × 50 mm × 800 mm vent risers on the large plane to aid feeding and venting.
| Element | Control 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 |
Mold Assembly and Production Conditions
We used pit assembly core molding, where the entire mold was composed of 140 sand cores to form the external shape and internal cavities. Ceramic tubes for the gating system were embedded directly into the cores. Vent channels between core layers ensured smooth gas escape during pouring, promoting a stable process. After pouring, the casting was cooled for 30 days to prevent deformation and cracking.
Our production facilities include a German-made spectrometer, four PY45 digital thermometers, a 20-ton medium-frequency induction furnace, a 20-ton holding furnace, and various ladles (e.g., two 30-ton, four 20-ton, two 10-ton). Additionally, we have two 9-ton cupola furnaces with 15-ton forehearths.
Melting Process Control
Raw materials include Z14 or Z18 pig iron, ordinary carbon steel scrap, and returns with low Cr content. Alloy additions include 75SiFe (5–20 mm), ferromanganese, and carbon enhancer. The charge ratio is 30% pig iron, 60% steel scrap, and 10% returns.
For large castings with pouring weights of 60–100 tons, we employ duplex melting using both electric and cupola furnaces. The process involves:
- Melting 20 tons in the induction furnace, heating to 1450°C, and analyzing C, Si, Mn, P, S with deviations ≤±0.05%. Carbon enhancer is added if needed, with 80–90% absorption.
- Transferring the adjusted iron to the holding furnace.
- Melting another 20 tons in the induction furnace and transferring to a ladle.
- Transporting 15 tons of iron from the cupola to the induction furnace for composition adjustment. Subsequent batches are handled similarly.
- Reheating ladle iron in the induction furnace within 3.5 hours to prevent solidification.
- High-temperature refining at >1500°C for degassing and impurity removal.
Enhanced Inoculation
To improve microstructure, we use multi-stage inoculation:
- Stream inoculation: 0.3% 75SiFe (8–10 mm) added during tapping over 80–90% of the tapping time.
- Floating silicon inoculation: 0.2% 75SiFe (60–80 mm) added per ladle after treatment.
- Instant inoculation: 0.1% barium-bearing silicon iron alloy (0.5–2 mm) added during pouring.
Temperature Control and Numerical Simulation
Temperature is meticulously controlled, with pouring temperature variations within 20°C. Numerical simulation indicated that internal ribs (30 mm thick) solidified first, while guideways (270 mm × 120 mm × 8,500 mm) and upper sections solidified last. Graphite chills were applied to promote cooling, and the upper section was repeatedly fed during pouring. Simulations confirmed the feasibility of the process, and all six castings produced met specifications.
The solidification process can be modeled using the Fourier heat equation:
$$ \frac{\partial T}{\partial t} = \alpha \nabla^2 T $$
where \( T \) is temperature, \( t \) is time, and \( \alpha \) is thermal diffusivity. For the column casting, the solidification time \( t_s \) for a section of thickness \( d \) can be estimated as:
$$ t_s = k \cdot d^2 $$
where \( k \) is a constant dependent on material properties.
Quality Improvement of Spindle Box and Tailstock Body Castings
Spindle box and tailstock body castings have historically exhibited severe shrinkage porosity and cavities on the upper surfaces. Despite various adjustments, results were unsatisfactory. We analyzed a spindle box with dimensions 2,900 mm × 2,650 mm × 2,680 mm, maximum wall thickness 300 mm, weight 62 tons, and total treated iron weight 70 tons, made of HT250.
Causes of Defects
The primary issues were:
- Complex solidification due to the height (~2 m), hindering directional solidification despite stepped gating.
- Ineffective riser feeding, as iron in risers neared solidification temperature upon arrival, reducing feeding efficiency.
Process Improvements
We modified the solidification pattern to achieve balanced and directional solidification in the lower and middle sections, and directional solidification in the upper section. Chills were used at thermal nodes for simultaneous solidification. A layered stepped gating system was implemented, with an additional separate gating system for the upper section.
The gating system included a pouring cup, 6× Ø80 mm downspouts, 3× 90 mm × 120 mm runners, 18× Ø40 mm branch downspouts, and 46× Ø40 mm ingates, forming two partially closed systems.
| Element | Control 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 |
Carbon equivalent (CE) was controlled at approximately 3.8% to enhance properties, calculated as:
$$ \text{CE} = \%\text{C} + \frac{\%\text{Si} + \%\text{P}}{3} $$
Pouring temperature was 1360–1370°C at the sprue cup, with three ladles used. The middle ladle was poured 20°C higher, and multi-stage inoculation was applied.
Initial Results and Analysis
One casting was produced, with shrinkage defects reduced to 10–20 mm depth compared to previous 200 mm. However, pouring time was 313 seconds, leading to excessive heat loss. The upper gating system had insufficient ingates, and low carbon equivalent affected solidification.
Process Revisions
The gating system was redesigned for higher flow rates: pouring cup, 2× Ø120 mm downspouts, 2× 130 mm × 150 mm runners, 23× Ø40 mm and 22× Ø40 mm branch downspouts, and 87× Ø40 mm ingates, forming two large-flow partially closed rectangular systems.
Pouring time was recalculated. Total iron weight \( G = 62,000 \) kg (casting + risers). Using the empirical formula:
$$ t = S_1 \cdot \sqrt[3]{\delta \cdot G} $$
where \( S_1 = 1.15 \), \( \delta = 20 \) mm (minimum wall thickness), we get:
$$ t = 1.15 \cdot \sqrt[3]{20 \cdot 62,000} \approx 178 \text{ seconds} $$
Adjusted to \( t = 160 \) seconds.
Average pressure head \( H_p \) was calculated as:
$$ H_p = H_0 – \frac{P^2}{2C} $$
where \( H_0 = 298 \) cm (sprue to ingate distance), \( P = 266 \) cm (ingate to highest point), \( C = 268 \) cm (total height). Thus:
$$ H_p = 298 – \frac{266^2}{2 \cdot 268} \approx 166 \text{ cm} $$
The minimum total choke area \( \Sigma F_{\text{branch}} \) was determined by:
$$ \Sigma F_{\text{branch}} = \frac{G}{0.31 \cdot t \cdot \mu \cdot \sqrt{H_p}} $$
where \( \mu = 0.4 \) (resistance coefficient). Substituting values:
$$ \Sigma F_{\text{branch}} = \frac{62,000}{0.31 \cdot 160 \cdot 0.4 \cdot \sqrt{166}} \approx 242.7 \text{ cm}^2 $$
We used 23× Ø40 mm branch downspouts, giving \( \Sigma F_{\text{branch}} = 282.6 \text{ cm}^2 \).
Risers were designed as 32× Ø150/Ø100 mm × 7,000 mm insulating risers to ensure they solidify after the casting hotspots. Chemical composition and pouring parameters remained unchanged.
Final Results
After revisions, one casting was produced with no defects. Subsequently, three spindle box castings (92.4 tons poured weight) and two tailstock body castings (90.5 tons poured weight) were manufactured using this process, all meeting specifications. This approach is now standard for all large spindle box and tailstock body machine tool castings.
Conclusion
The development of large machine tool castings requires integrated approaches combining advanced simulation, rigorous process design, and precise metallurgical control. Key insights from our work include:
- Numerical simulation is invaluable for predicting and mitigating defects in machine tool castings, enabling optimized gating and riser design.
- Design reviews from a casting perspective are essential to avoid structural issues that could lead to failures, such as cracking in thin sections.
- Multi-layer gating systems, particularly for tall castings, promote favorable solidification patterns and reduce shrinkage defects in box-type machine tool castings.
- Multi-stage inoculation compensates for fading during prolonged processing, ensuring consistent microstructure and mechanical properties.
- Precise temperature control during multi-ladle pouring minimizes thermal gradients, enhancing feeding efficiency.
Our experiences demonstrate that through systematic process optimization and adoption of advanced technologies, high-quality large machine tool castings can be reliably produced, supporting the advancement of precision manufacturing equipment.