We have developed a precision wire electrical discharge machining (WEDM) machine that utilizes advanced mineral castings for its structural components. This approach addresses significant drawbacks of traditional cast iron, such as high energy consumption, environmental pollution, and inherent instability in precision applications. Our focus on machine tool casting innovations has led to a design that enhances insulation, reduces vibrations, and improves overall machining accuracy. In this article, we detail the material properties, structural design, and rigorous testing of this machine, highlighting its superiority in high-current and narrow-pulse-width cutting scenarios.
The use of mineral castings in machine tool casting represents a shift toward sustainable manufacturing. Unlike conventional cast iron, which requires high-temperature processing and generates substantial pollutants, mineral castings are formed at room temperature with minimal environmental impact. We have integrated these castings into key parts like the bed, saddle, and column to leverage their high damping capacity and excellent insulation. Our experiments demonstrate that this machine tool casting approach not only meets but exceeds precision standards, making it ideal for demanding applications in mold and die production.
Material Properties of Mineral Castings
Mineral castings, also known as artificial granite or polymer concrete, consist of a composite material where epoxy resins act as binders mixed with granite aggregates and reinforcing agents. This composition results in a machine tool casting material with exceptional mechanical and thermal properties. We selected mineral castings for their ability to be cast into complex shapes at ambient temperatures, eliminating residual stresses common in cast iron. The following table compares the properties of mineral castings with traditional materials like steel, cast iron, and natural granite, underscoring the advantages of machine tool castings in precision applications.
| Property | Steel | Cast Iron | Natural Granite | Mineral Castings |
|---|---|---|---|---|
| Compressive Strength (N/mm²) | 250-1200 | 600-1000 | 70-300 | 110-170 |
| Tensile Strength (N/mm²) | 400-1600 | 150-400 | <30 | 25-40 |
| Elastic Modulus (N/mm²) | 210 | 80-120 | 20-40 | 30-40 |
| Thermal Conductivity (W/mK) | 50 | 50 | 2.4 | 1.3-2.0 |
| Thermal Expansion Coefficient (×10⁻⁶/K) | 12 | 10 | 6.8-8.5 | 12-20 |
| Density (g/cm³) | 7.85 | 7.15 | 3.0 | 2.4-2.6 |
| Damping Coefficient | 0.002 | 0.003 | 0.015 | 0.02 |
The low thermal conductivity of mineral castings, at 1.3-2.0 W/mK, ensures minimal sensitivity to thermal variations during machining, which is critical for maintaining precision. Additionally, the high damping coefficient of 0.02, compared to 0.003 for cast iron, allows for superior vibration absorption. This property is vital in machine tool casting for reducing dynamic errors in high-speed operations. The insulation resistance of mineral castings is exceptionally high, often exceeding 100 MΩ, which prevents energy leakage and enhances discharge stability in WEDM processes. We modeled the electrical behavior using a simplified circuit where the insulation resistance \( R_2 \) approaches infinity in mineral castings, whereas in cast iron, it decreases due to contamination, leading to leakage currents. The discharge model can be represented as:
$$ V = I \cdot R_0 + U_Z $$
where \( V \) is the voltage, \( I \) is the current, \( R_0 \) is the discharge channel resistance, and \( U_Z \) is the maintaining voltage. For mineral castings, \( R_2 \to \infty \), eliminating parallel leakage paths and ensuring efficient energy transfer to the workpiece.
Structural Design of the Machine
We designed the WEDM machine with a T-shaped bed made from mineral castings to provide symmetry and stability along the Y-axis. This machine tool casting approach allows for integrated pre-embedded components, such as mounting holes and coolant channels, reducing assembly errors and residual stresses. The bed supports the X and Y slides, worktable, wire transport system, and column, all constructed from mineral castings to maximize insulation and damping. The low thermal expansion coefficient of mineral castings, similar to that of steel, ensures dimensional stability under varying temperatures, a key advantage in precision machine tool casting.
The worktable and slides employ a cross-slide configuration driven by servo motors and ball screws, with linear guides for accurate motion. We thickened the guide mounting surfaces to over 100 mm to minimize deformation, leveraging the high compressive strength of mineral castings. The wire transport system uses a moving wire spool on linear guides to maintain consistent wire tension and speed, typically 8-12 m/s. The mineral casting material’s inherent vibration damping reduces oscillations from the spool’s reciprocating motion, enhancing cut quality. The column and wire guide assembly are also made from mineral castings, forming a C-shaped structure that improves rigidity and facilitates easy access. This holistic use of machine tool castings ensures that the entire machine benefits from reduced vibration and improved insulation.
In terms of mathematical modeling, the static deflection \( \delta \) of the bed under load can be approximated by:
$$ \delta = \frac{F L^3}{3 E I} $$
where \( F \) is the applied force, \( L \) is the length, \( E \) is the elastic modulus, and \( I \) is the moment of inertia. For mineral castings with \( E \approx 35 \) N/mm², deflection is minimized due to the optimized geometry and material properties. Additionally, the natural frequency \( f_n \) of the structure is given by:
$$ f_n = \frac{1}{2\pi} \sqrt{\frac{k}{m}} $$
where \( k \) is the stiffness and \( m \) is the mass. The high damping of mineral castings shifts \( f_n \) away from operational frequencies, reducing resonance risks.
Cutting Performance and Accuracy Testing
We conducted extensive cutting tests to evaluate the precision of our mineral casting-based WEDM machine. Using a continuous wire of 300 m length and a water-based dielectric fluid (JR3C at 1:90 ratio), we machined ten octagonal prisms from Cr12 steel with a thickness of 40 mm. The results showed dimensional deviations within ±7 μm, meeting the group standard TCMTBA 1010-2021 requirements. The following table summarizes the key metrics from these tests, demonstrating the consistency achieved with machine tool castings.
| Workpiece Number | Positive Deviation (μm) | Negative Deviation (μm) | Longitudinal-Transverse Size Difference (μm) | Surface Roughness Ra (μm) |
|---|---|---|---|---|
| 1 | 5 | -3 | 6 | 1.18 |
| 2 | 6 | -4 | 7 | 1.19 |
| 3 | 4 | -5 | 8 | 1.17 |
| 4 | 7 | -2 | 5 | 1.20 |
| 5 | 5 | -4 | 6 | 1.18 |
| 6 | 6 | -3 | 7 | 1.19 |
| 7 | 4 | -5 | 8 | 1.17 |
| 8 | 7 | -2 | 5 | 1.20 |
| 9 | 5 | -4 | 6 | 1.18 |
| 10 | 6 | -3 | 7 | 1.19 |
Additionally, we performed drumness tests on a 15 mm × 15 mm × 200 mm square prism, resulting in a drumness error of 0.022 mm, well below the standard limit of 0.025 mm. The surface roughness at the center was Ra 2.00 μm, satisfying the requirement of less than Ra 2.5 μm. For load testing, we achieved a cutting speed of 200.8 mm²/min on a 60 mm thick “Great Wall” pattern without burn marks, using JR3A dielectric at a 1:20 ratio. A 24-hour continuous cutting test at 155.4 mm²/min showed no wire breakage and a minimal electrode wear of 0.01 mm. These results underscore the reliability of machine tool castings in sustaining high-performance operations.
Electrical Performance Comparison
We compared the electrical insulation and discharge characteristics of our mineral casting machine with a traditional cast iron machine. Using an insulation resistance tester, we measured the resistance between the worktable (anode) and the power feed (cathode). The mineral casting machine exhibited insulation resistance over 100 MΩ, effectively infinite, while the cast iron machine showed near-zero resistance due to leakage paths. This difference is critical in machine tool casting for maintaining energy efficiency. We quantified the leakage current \( I_{\text{leak}} \) in cast iron using:
$$ I_{\text{leak}} = \frac{V}{R_2} $$
where \( V \) is the voltage and \( R_2 \) is the insulation resistance. In cast iron, \( R_2 \) decreases with contamination, increasing \( I_{\text{leak}} \) and reducing cutting efficiency.
For high-current cutting tests on 100 mm thick 45 steel workpieces, we set parameters to a pulse width of 40 μs, duty cycle of 1:4, and six power tubes, achieving an average current of 8 A. The mineral casting machine maintained stable wire speed and showed more ignition delay waveforms, indicating better inter-electrode conditions. The cutting efficiency was 235 mm²/min, compared to 220 mm²/min for cast iron, due to minimal energy leakage. The discharge energy \( E_d \) per pulse can be expressed as:
$$ E_d = V \cdot I \cdot t_p $$
where \( t_p \) is the pulse width. In mineral castings, higher insulation ensures that \( E_d \) is fully utilized, whereas in cast iron, leakage dissipates part of the energy.
In narrow-pulse-width trimming tests on 40 mm thick Cr12 steel, with a duty cycle of 1:4 and two power tubes, the mineral casting machine stable trimmed at 0.5 μs pulse width, while the cast iron machine exhibited distorted waveforms at 1 μs and failed at 0.5 μs. The pulse waveform fidelity \( F \) can be modeled as:
$$ F = \frac{1}{1 + \frac{R_2}{Z_c}} $$
where \( Z_c \) is the characteristic impedance. For mineral castings, \( R_2 \to \infty \), so \( F \approx 1 \), preserving pulse integrity. This capability allows our machine tool casting design to excel in fine finishing operations.
Conclusion
Our development of a precision WEDM machine using mineral castings demonstrates significant advancements in machine tool casting technology. The material’s excellent insulation, damping, and thermal stability enable superior performance in high-current and narrow-pulse-width cutting, outperforming traditional cast iron. We have validated through rigorous testing that this machine tool casting approach meets precision standards while offering environmental and economic benefits. Future work will focus on optimizing the composite mix for even higher performance and expanding applications to other precision machining domains. The success of this project highlights the potential of mineral castings to revolutionize the machine tool industry, providing a sustainable path forward for high-accuracy manufacturing.