In our foundry operations, we have consistently focused on enhancing the quality of machine tool castings, particularly for critical components like the bed of the TC500 vertical machining center. The appearance quality of these machine tool castings is paramount, as it directly impacts the overall aesthetic and functional integrity of the final product. Through extensive analysis, we identified that the primary factors affecting the appearance quality are the design of the casting process and the application quality of surface coatings. This article details our first-hand experiences in optimizing these aspects to achieve significant improvements. We will explore the original process limitations, the systematic modifications implemented, and the resultant gains in efficiency and quality, all while emphasizing the importance of machine tool casting excellence.
The TC500 machine tool bed, made of HT250 with a rough weight of 713 kg, presented several challenges in its initial casting process. The original design employed a traditional gating system with bottom pouring and simultaneous pouring at the parting surface. The parting surface was located at the bed’s foot pads, with six foot pads placed in the upper mold and the remainder in the lower mold. This setup involved multiple internal cores, including three cavity cores and two bottom row cores, which complicated assembly and alignment. The core arrangement led to numerous matching surfaces between cores and between cores and the mold, increasing operational difficulty and variability in appearance quality. Although this process did not produce significant internal defects like gas holes or shrinkage porosity, the external appearance was inconsistent, with issues such as misaligned bridges, uneven motor pads, and irregular oil grooves. These problems underscored the need for a refined approach to machine tool casting processes.
To systematically address these issues, we first analyzed the original casting process in detail. The core assembly involved multiple interfaces, which were prone to misalignment due to operational complexities. For instance, the bridge pads were formed by the配合 of two bottom row cores, requiring precise alignment in both vertical and horizontal directions. Similarly, the motor pads at the lower part of the mold were produced by two cores配合, with additional challenges like hollow sections that increased the difficulty of achieving uniform matching. The oil grooves were formed by a thin, elongated core approximately 50 mm thick and 700 mm long, which was susceptible to deformation during handling, leading to mismatches and large gaps. Furthermore, the external surfaces, including planes and vertical faces, were affected by the gating system core, which required alignment with the mold side walls and large planes, often resulting in uneven surfaces due to variations in core head dimensions and刮砂 operations. These factors collectively contributed to a high rate of non-premium castings, with only 51.5% achieving excellent appearance quality initially.
We then proceeded to redesign the casting process, focusing on simplifying core structures and reducing the number of matching surfaces. The improvements were implemented in four key areas, each targeting specific problem zones. First, for the bridge pads, we consolidated the two separate cores into a single, integrated hollow core. This eliminated the need for配合 between cores, reducing operational complexity. However, this required careful attention during core making to prevent damage to the slender sand projections. The effectiveness of this change can be modeled using a simplicity index for core design, which we define as: $$ S = \frac{N_i}{N_f} $$ where \( S \) is the simplicity index, \( N_i \) is the number of integrated cores, and \( N_f \) is the number of former cores. A higher \( S \) value indicates reduced assembly complexity. For the bridge pads, \( S \) increased from 0.5 (two cores) to 1 (one core), directly improving alignment accuracy.
| Feature | Original Design | Improved Design | Complexity Reduction |
|---|---|---|---|
| Bridge Pads | Two cores配合 | Single integrated core | High |
| Motor Pads | Two cores配合 | Mold-integrated formation | High |
| Oil Grooves | Thin core配合 with mold | Mold-integrated formation | Medium |
| External Surfaces | Gating core配合 | Mold-integrated with ceramic tubes | High |
Second, the motor pads were redesigned to be formed entirely by the mold, rather than through core配合. This change avoided alignment issues but introduced new challenges in mold making, such as the creation of additional recesses that required careful coating application to prevent ash accumulation. The impact on quality can be quantified using a misalignment probability formula: $$ P_m = k \cdot A^{-1} $$ where \( P_m \) is the probability of misalignment, \( k \) is a process constant, and \( A \) is the number of alignment surfaces. By reducing \( A \) from multiple to zero for this feature, \( P_m \) decreased significantly, enhancing consistency in machine tool castings.
Third, the oil grooves were modified from a thin core to a mold-integrated design. This eliminated the deformation risks associated with long, slender cores and resolved matching inaccuracies. In this case, the core stability factor \( C_s \) can be expressed as: $$ C_s = \frac{L}{t} $$ where \( L \) is the core length and \( t \) is the thickness. For the original core, \( C_s = \frac{700}{50} = 14 \), indicating high instability, whereas the improved design has \( C_s = 0 \) (no core), ensuring better dimensional integrity. However, this required enhanced attention during core assembly to secure small side cores firmly.
Fourth, the external planes and vertical faces were improved by eliminating part of the gating system core and integrating the runners directly via ceramic tubes. This allowed the external surfaces to be formed entirely by the mold, resolving core displacement issues. The surface flatness improvement can be described by a flatness deviation metric: $$ \Delta F = \frac{\sum |d_i – d_{avg}|}{n} $$ where \( \Delta F \) is the average flatness deviation, \( d_i \) are individual surface measurements, and \( d_{avg} \) is the average surface height. Post-improvement, \( \Delta F \) decreased due to reduced core interference, leading to smoother machine tool castings.
In addition to process redesign, we overhauled the coating application procedures to address appearance inconsistencies. Previously, coating issues included uneven application, heavy brush marks, ash accumulation in corners, and inadequate coverage on critical surfaces like guide rail undersides. These problems were exacerbated by inconsistent brushing directions and failure to repair mold damages with coating paste. To systematize this, we classified casting surface quality into four categories based on assembly visibility and importance: A (highest, exposed after assembly), B (visible during machining/assembly), C (machined surfaces pre-processing), and D (internal cavities, least important). This classification guided targeted coating efforts.
| Surface Class | Coating Requirements | Tolerance for Defects |
|---|---|---|
| A | Uniform coverage, no leakage, minimal brush marks, no visible sand grains, no accumulation in corners, sharp edges | Zero tolerance |
| B | Uniform coverage, no leakage, light brush marks in consistent direction, no sand grains, no accumulation, sharp edges | Low tolerance |
| C | Uniform coverage, no leakage, acceptable heavier brush marks in consistent direction | Medium tolerance |
| D | Uniform coverage, no leakage | High tolerance |
The coating process was refined to include specific techniques: stirring coating thoroughly before application; using appropriately sized brushes for different areas (large brushes for simple planes, small brushes for complex features); following a sequence of top-to-bottom, high-to-low, and far-to-near brushing; and maintaining consistent brushing directions with overlaps of 60–80 mm between strokes and 20–30 mm between parallel passes. For hard-to-reach areas like guide rail undersides, we employed甩灰 techniques (flicking coating upward) followed by smoothing, ensuring alignment with planar brushing directions. Corners and recesses were meticulously cleaned with small brushes to prevent ash buildup. The coating thickness uniformity can be modeled using a brushing efficiency equation: $$ E_b = \frac{C_u}{C_t} \times 100\% $$ where \( E_b \) is brushing efficiency, \( C_u \) is the uniformly coated area, and \( C_t \) is the total area. Post-improvement, \( E_b \) increased significantly, reducing defects in machine tool castings.
The combined improvements in casting process and coating application yielded remarkable results. Prior to the changes, over a two-month period, 130 castings were inspected, with only 71 (51.5%) classified as premium quality. After implementation, in a subsequent two-month period, 204 castings were inspected, with 199 (97.6%) achieving premium status. This represents a 46 percentage point increase in excellence rate, demonstrating the effectiveness of our holistic approach. The overall quality enhancement can be summarized by a quality index formula: $$ Q = \frac{P_p}{T_p} \times 100\% $$ where \( Q \) is the quality index, \( P_p \) is the number of premium castings, and \( T_p \) is the total production. Post-improvement, \( Q \) rose from 51.5% to 97.6%, validating our strategies for machine tool casting optimization.
| Period | Total Inspected | Premium Castings | Quality Index (%) | Improvement |
|---|---|---|---|---|
| Before (Jun-Jul 2018) | 130 | 71 | 51.5 | Baseline |
| After (Jul-Sep 2018) | 204 | 199 | 97.6 | +46.1 points |
In conclusion, our first-hand experience in refining the TC500 machine tool bed casting process underscores the critical interplay between process design and coating quality in achieving high-quality machine tool castings. By simplifying core structures, reducing alignment surfaces, and standardizing coating procedures, we not only enhanced appearance quality but also boosted production efficiency. The significant rise in premium castings from 51.5% to 97.6% highlights the success of these measures. Future work will focus on further optimizing these parameters and extending similar approaches to other machine tool casting variants, ensuring consistent excellence across our product range. Through continuous innovation, we aim to set new benchmarks in the foundry industry for machine tool castings.