In the realm of heavy industry, the production of large-scale castings for machine tools presents significant challenges, particularly when tight deadlines and limited resources are involved. This article details our firsthand experience in manufacturing an ultra-large machine tool casting, specifically a crossbeam for a vertical turning mill, using an innovative approach that combines flask molding with pit molding. The machine tool casting in question had design dimensions of approximately 15,000 mm in length, 1,800 mm in width, and 1,650 mm in height, with a net weight of around 18 metric tons and a material specification of HT300 gray iron. Faced with a short delivery window of two months and a lack of dedicated tooling such as large flasks, we devised a unconventional yet effective工艺 strategy to ensure timely production without compromising quality. This process underscores the versatility and adaptability required in modern foundry practices for machine tool casting.
The core challenge was the absence of suitably sized flasks for such a massive machine tool casting. Constructing new flasks would have consumed nearly a month, escalating costs and jeopardizing the schedule. Therefore, we leveraged existing flasks for the upper and lower sections while utilizing a pit for the middle segment. This hybrid method not only saved time and resources but also demonstrated the feasibility of producing oversized castings with limited工装. The pit available measured 12 m × 6 m × 3 m, which provided ample space for the middle mold section. Our approach involved using wooden patterns for the flask sections and expanded polystyrene (EPS) boards for the pit section, balancing cost-efficiency and precision for this single-piece machine tool casting.
The success of any machine tool casting project hinges on a meticulously designed casting process. We began by determining the parting planes, which are critical for mold assembly and ejection. Given the casting’s geometry, a three-part mold was deemed necessary. The upper and lower sections were created in standard flasks using resin sand, while the middle section was formed directly in the pit. After leveling a resin sand base in the pit, the lower flask was positioned on it. The EPS pattern for the middle was then placed, surrounded by sturdy boards on two sides (with the pit walls serving as the other two boundaries) to contain the sand. The middle and upper sections were sequentially rammed with resin sand. After hardening, the mold was opened to remove the patterns. The entire mold assembly was then secured within the pit, with additional sand packed around the flasks to prevent metal runoff during pouring. This setup is visualized below, illustrating the integration of flask and pit molding for this machine tool casting.
Another crucial aspect was the pouring position. To achieve a balanced temperature field and minimize turbulence, we designed a gating system based on the principle of “high flow rate, low velocity, and smooth, clean filling.” For this machine tool casting, four identical gating sets were employed: two at each end along the length and two on either side at the mid-section. Each set consisted of a sprue, runner, branch sprue, and ingates. This configuration aimed to promote uniform metal distribution and reduce the risk of defects such as cold shuts or slag entrapment in the machine tool casting.
Key process parameters were established based on empirical knowledge and calculations. The machining allowances were set at 12 mm for the top surfaces and 10 mm for the bottom and sides. A distortion allowance of 10 mm was incorporated to compensate for potential warping during cooling. The core assembly was complex, involving 36 cores fixed via core prints, core covers, or chills. Proper core positioning was vital for achieving the internal geometry of the machine tool casting.
The design of the gating system is paramount in ensuring the integrity of a machine tool casting. We applied the large-orifice outflow theory to size the gating components. For gray iron castings like this, a choke-controlled (closed) system is preferred to aid in slag separation. The cross-sectional area ratio for the sprue, runner, branch sprue, and ingate was set as:
$$ A_{\text{sprue}} : A_{\text{runner}} : A_{\text{branch sprue}} : A_{\text{ingate}} = 1.2 : 1.4 : 1 : 1 $$
Thus, by calculating the minimum choke area at the branch sprue, all other areas could be determined. The formula derived from the large-orifice theory is:
$$ A_{\text{min}} = \frac{Q}{\rho t \mu \sqrt{2g h_p}} $$
Where:
- $Q$ is the total poured weight of the iron (in kg),
- $\rho$ is the density of the iron (approximately 7,000 kg/m³ for gray iron),
- $t$ is the pouring time (in seconds),
- $\mu$ is the flow loss coefficient (taken as 0.48, accounting for mold type and resistance),
- $g$ is the acceleration due to gravity (9.81 m/s²),
- $h_p$ is the average effective static pressure head (in meters).
The pouring time was estimated using an empirical formula:
$$ t = S \sqrt{Q} $$
For large castings, the coefficient $S$ ranges from 2.0 to 1.8; we used $S = 1.8$. Given $Q = 18,000$ kg, the pouring time calculated to:
$$ t = 1.8 \times \sqrt{18000} \approx 1.8 \times 134.16 \approx 241.5 \text{ seconds} $$
For a bottom-gating system, the average static pressure head is given by:
$$ h_p = \frac{k_2^2}{k_1^2 + k_2^2} \left( h_0 – \frac{h_c}{2} \right) $$
Where:
- $k_1 = A_{\text{sprue}} / A_{\text{runner}} = 1.2 / 1.4 \approx 0.857$,
- $k_2 = A_{\text{sprue}} / A_{\text{ingate}} = 1.2 / 1 = 1.2$,
- $h_0$ is the height from the ingate to the pouring basin liquid level (2.2 m),
- $h_c$ is the height of the casting (1.65 m).
Substituting the values:
$$ h_p = \frac{1.2^2}{0.857^2 + 1.2^2} \left( 2.2 – \frac{1.65}{2} \right) = \frac{1.44}{0.734 + 1.44} \left( 2.2 – 0.825 \right) = \frac{1.44}{2.174} \times 1.375 \approx 0.662 \times 1.375 \approx 0.91 \text{ m} $$
With these parameters, the minimum choke area was computed:
$$ A_{\text{min}} = \frac{18000}{7000 \times 241.5 \times 0.48 \times \sqrt{2 \times 9.81 \times 0.91}} $$
First, calculate the denominator stepwise:
- $\sqrt{2 \times 9.81 \times 0.91} = \sqrt{17.8542} \approx 4.225$
- $7000 \times 241.5 = 1,690,500$
- $1,690,500 \times 0.48 = 811,440$
- $811,440 \times 4.225 \approx 3,428,000$
Thus:
$$ A_{\text{min}} \approx \frac{18000}{3,428,000} \approx 0.00525 \text{ m}^2 = 5250 \text{ mm}^2 $$
In practice, we used standard firebrick tubes for the gating components. Each gating set had 6 ingates, so with 4 sets, there were 24 ingates total. Selecting a branch sprue tube with a diameter of 120 mm (area approximately 11,310 mm² per tube) would exceed the requirement, but to match standard inventory, we chose tubes with a cross-sectional area of 1,256 mm² each (equivalent to 40 mm diameter). However, based on our ratio, the actual design was finalized as per the table below, which summarizes the gating system dimensions for this machine tool casting.
| Component | Cross-Sectional Area Ratio | Calculated Area (mm²) | Selected Standard Tube Size (mm) | Actual Area per Tube (mm²) | Quantity per Set | Total Area (mm²) |
|---|---|---|---|---|---|---|
| Sprue | 1.2 | 6,300 | 90 | 6,362 | 1 | 6,362 |
| Runner | 1.4 | 7,350 | 100 | 7,854 | 1 | 7,854 |
| Branch Sprue | 1.0 | 5,250 | 80 | 5,027 | 1 | 5,027 |
| Ingate | 1.0 | 5,250 | 40 | 1,256 | 6 | 7,536 |
Note: The total area values are approximate and based on practical selections to ensure adequate flow for the machine tool casting. The actual total choke area at the branch sprue was 24 × 1,256 mm² = 30,144 mm², which provided a safety margin.
Operational control was critical during the production of this machine tool casting. With numerous cores, many lacking prints and relying on chills for fixation, we fabricated several gauging plates to ensure dimensional accuracy during core setting. These plates allowed for precise measurement and alignment, mitigating the risk of core shift that could compromise the machine tool casting’s integrity. Since the middle mold section was flapless, conventional clamping between the upper and lower flasks was impossible. Instead, we placed heavy weights, including additional flasks and counterweights, on the upper flask. The total weight exceeded 5 times the casting’s weight (over 90 metric tons) to prevent mold lifting during pouring, a common issue in large machine tool casting.
The pouring operation required meticulous coordination. Three ladles, each carried by a separate crane, were used to pour simultaneously into three pouring basins: one at each end and a long basin serving the two mid-section gating sets. Prior to the actual pour, we conducted rehearsals to synchronize the crane movements and ladle operations. Each basin was equipped with a stopper; only after all basins were filled with molten iron were the stoppers raised simultaneously to initiate filling. This synchronization minimized turbulence and ensured a steady, controlled fill for the machine tool casting.
The entire process, from pattern making and mold preparation to pouring and shakeout, was completed within six weeks, ahead of schedule. The resulting machine tool casting met all dimensional and quality specifications, with no major defects such as shrinkage cavities or cold shuts. This success validated our hybrid molding approach for producing oversized machine tool castings under constraints.
In reflection, this project highlights several key lessons for foundries engaged in machine tool casting. First, adaptability in process design is essential when traditional tooling is unavailable. Combining flask and pit molding can be a viable solution for single-piece or low-volume large castings. Second, rigorous application of gating design principles, such as the large-orifice theory, is crucial for ensuring proper metal flow and solidification in machine tool casting. The formulas used here can be generalized for other large castings, with adjustments based on specific geometry and material properties. Third, operational discipline, including core positioning, mold weighting, and synchronized pouring, is as important as theoretical design in the success of a machine tool casting project.
To further elucidate the工艺 parameters, the table below consolidates the key data involved in producing this machine tool casting.
| Parameter | Value | Remarks |
|---|---|---|
| Casting Dimensions (L × W × H) | 15,000 mm × 1,800 mm × 1,650 mm | Approximate net shape |
| Net Weight | 18,000 kg | Material: HT300 gray iron |
| Molding Method | Combined Flask and Pit Molding | Upper/lower: flasks; Middle: pit |
| Pattern Materials | Wood (flask sections), EPS (pit section) | Cost-effective for single piece |
| Parting Planes | Three-part mold | Facilitates mold assembly |
| Machining Allowances | Top: 12 mm; Sides/Bottom: 10 mm | Based on industry standards |
| Distortion Allowance | 10 mm | Empirical for long castings |
| Number of Cores | 36 | Various fixation methods |
| Gating System Type | Closed (choke-controlled) | 4 identical sets |
| Pouring Time | ~241 seconds | Calculated via $t = S \sqrt{Q}$ |
| Average Static Pressure Head | 0.91 m | For bottom-gating design |
| Minimum Choke Area | 5,250 mm² | Theoretical from large-orifice formula |
| Actual Choke Area | 30,144 mm² | Using standard tubes (24 ingates) |
| Mold Weighting | >90,000 kg | To prevent lifting during pour |
| Pouring Ladles | 3 | Synchronized operation |
The浇注 system design can be further analyzed using fluid dynamics principles. The continuity equation and Bernoulli’s equation underpin the large-orifice theory. For incompressible flow, the volume flow rate $Q_v$ is constant:
$$ Q_v = A_1 v_1 = A_2 v_2 $$
Where $A$ and $v$ are cross-sectional area and velocity at different points. Considering energy losses, the velocity at the ingate can be expressed as:
$$ v = \mu \sqrt{2g h_p} $$
Thus, the mass flow rate is $\dot{m} = \rho A v$. Integrating over the pouring time $t$ gives the total mass $Q = \rho A v t$, leading to the formula for $A_{\text{min}}$. This theoretical foundation is vital for optimizing gating in machine tool casting to reduce turbulence and oxidation.
Another aspect is solidification control. For a thick-section machine tool casting like this, ensuring directional solidification is key to avoiding shrinkage defects. The gating design aimed to create a thermal gradient, with hotter metal in the risers or upper sections. Although not detailed in the initial process, we monitored cooling rates and could have employed insulating sleeves or chills if needed. The use of resin sand, which offers good collapsibility and dimensional stability, also contributed to the quality of the machine tool casting.
In terms of metallurgical quality, the HT300 iron required a specific carbon equivalent (CE) to achieve the desired strength and machinability. While not covered in the original account, controlling melt chemistry is fundamental in machine tool casting. Typically, CE is calculated as:
$$ \text{CE} = \%C + \frac{\%Si + \%P}{3} $$
For HT300, CE should be around 3.9–4.1. Proper inoculation and pouring temperature (approximately 1,350–1,400°C) are also critical to ensure graphite formation and minimize chilling in this machine tool casting.
The hybrid molding approach has broader implications. It reduces capital investment for occasional large castings, making it suitable for jobbing foundries. However, it requires skilled labor and careful planning. The pit must be properly prepared with a firm base and adequate drainage to handle moisture and gases. For this machine tool casting, the pit’s size and depth were sufficient, but for even larger castings, deeper pits or reinforced walls might be necessary.
Looking ahead, the lessons from this machine tool casting project can be applied to other oversized components, such as frames for presses or beds for lathes. Digital tools like simulation software could further refine the process by predicting flow patterns and solidification, reducing trial and error. Nonetheless, the hands-on experience gained here underscores the importance of foundational foundry principles in producing reliable machine tool castings.
In conclusion, the production of this ultra-large machine tool casting via combined flask and pit molding was a testament to innovative problem-solving in foundry engineering. By integrating traditional techniques with adaptive design, we overcame tooling limitations and delivered a high-quality casting on time. The detailed工艺 design, particularly the gating system based on fluid dynamics, ensured optimal metal flow and solidification. This experience reinforces that with careful planning, theoretical rigor, and operational precision, even the most challenging machine tool casting projects can be successfully executed. As industries demand larger and more complex castings, such hybrid methods will continue to play a crucial role in advancing manufacturing capabilities.