The advancement of modern manufacturing places increasingly stringent demands on the dimensional accuracy and long-term stability of machine tool castings. As the foundational components for installation, such as beds, and other critical parts requiring precise fits—like worktables, columns, and saddles—the deformation of these machine tool castings directly impacts the machining precision of the entire equipment. Furthermore, deformation occurring during subsequent processing, transportation, and assembly introduces significant production challenges. To improve the dimensional stability of castings, a fundamental analysis of the influencing factors is essential. Based on investigation, the primary factors are as follows.
Key Factors Affecting Dimensional Stability
The stability of machine tool castings is governed by a complex interplay of structural, thermal, and material factors. The following table summarizes the primary contributors to dimensional instability:
| Factor | Description & Mechanism | Typical Impact / Example |
|---|---|---|
| 1. Structural Rigidity | Inadequate stiffness leads to elastic deflection under operational loads. | A long bed (3400 mm) supported at two ends sagged 23.5 µm under load. Adding a central support reduced sag to 0.7 µm. |
| 2. Thermal Gradients & Deformation | Caused by: a) Non-uniform microstructure with different thermal expansion coefficients; b) External temperature gradients creating part warpage. | For a 12m long, 0.8m high bed, a top-bottom temperature difference of 0.1°C can cause 25 µm of sag/crown. Microstructural expansion: Pearlite (α ≈ 10–11×10⁻⁶/°C), Ferrite (α ≈ 12–12.5×10⁻⁶/°C), Cementite (α ≈ 6–8.3×10⁻⁶/°C). |
| 3. Foundation Deformation | Variations in concrete composition and curing age affect foundation stiffness, transferring deflection to the machine. | Walking 10 people 2 meters near machines on different foundations changed leveling by 0.01–0.14 mm. |
| 4. Resistance to Micro-Plastic Deformation (MPD) | Creep and dimensional instability under sustained low stress due to micro-yielding at stress concentrators (e.g., graphite flakes, grain boundaries). | Graphite (strength ~2 kg/mm²) facilitates micro-yielding via dislocation glide. Increasing dislocation slip resistance improves MPD resistance. |
| 5. Residual Stress & Its Stability | Internal stresses from casting, machining, and aging itself. These stresses relax and redistribute over time, causing distortion. | Comprises: Casting Stress (uneven cooling), Machining-Induced Stress (material removal & localized deformation), Secondary Aging Stress (non-uniform cooling during aging). |
While factors 1-3 are addressed through design, environmental control, and proper installation, factors 4 and 5—often overlooked—are frequently the root cause of instability in machine tool castings. This discussion focuses on mitigating residual stresses through artificial aging to enhance stability.
Residual Stress in Machine Tool Castings: Composition and Stability
The total residual stress ($\sigma_{total}$) in a finished casting can be expressed as a superposition of stresses from different stages:
$$ \sigma_{total} = \sigma_{casting} + \sigma_{machining} + \sigma_{aging\_secondary} + \Delta\sigma_{relaxation}(t) $$
Where:
- $\sigma_{casting}$: Stress from uneven cooling within the elastic-plastic temperature range.
- $\sigma_{machining}$: Stress from cutting (stress redistribution and local plastic deformation).
- $\sigma_{aging\_secondary}$: Stress induced during the cooling phase of the aging treatment.
- $\Delta\sigma_{relaxation}(t)$: Time-dependent stress relaxation/redistribution.
The goal of aging is to minimize $\sigma_{total}$ and stabilize $\Delta\sigma_{relaxation}(t)$ to near zero. Compressive stresses are more desirable as they offer approximately three times greater resistance to deformation under tensile loads than tensile stresses do under compressive loads.
Thermal (Artificial) Aging Process Design
Thermal aging aims to relieve and stabilize residual stresses by heating the machine tool castings into the elastic-plastic range, allowing stress relaxation via creep mechanisms, followed by controlled cooling. The process is defined by the material’s temperature-dependent behavior. Short-term creep tests (e.g., 1.5 hours) determine this critical range.
For typical gray iron like HT200-400 (or similar grades), the behavior can be summarized:
- Below ~350°C: Predominantly elastic region. Minimal stress relief occurs.
- ~350°C to ~450°C: Elastic-plastic transition region. Significant stress relaxation can occur.
- Above ~450°C: Active plastic deformation region. High stress relief but risk of excessive softening.
Based on this, a recommended thermal aging cycle for medium/small precision machine tool castings is established, focusing on key parameters:
| Process Stage | Key Parameter & Rationale | Recommended Practice for HT200-400 |
|---|---|---|
| 1. Charging Temperature | Prevents thermal shock and minimizes initial stress. Castings should be loaded to avoid distortion under own weight. | Below 200°C. Use even support, proper spacing for stacked loads. |
| 2. Heating Rate ($\dot{T}_{heat}$) | Balances stress relief during heating with risk of cracking in complex shapes. | For castings <250 kg: ≤ 80°C/hour. Slower rates can reduce required soaking time. |
| 3. Soaking Temperature ($T_{soak}$) & Time ($t_{soak}$) | Maximizes stress relaxation without degrading hardness/mechanical properties. | $T_{soak} = 530-550°C$. $t_{soak} = 4-6$ hours for medium/small castings, accounting for furnace uniformity. |
| 4. Cooling Rate ($\dot{T}_{cool}$) | Critical to prevent secondary stresses. Must be slow through the elastic-plastic region. | Slow cooling above 350°C (e.g., ≤ 50°C/hr). Faster cooling permissible below 350°C. |
| 5. Furnace Temperature Uniformity ($\Delta T_{furnace}$) | Directly impacts aging effectiveness. Non-uniformity can induce new stresses. | Maintain $\Delta T_{furnace} \leq \pm 25°C$ during soaking and the initial cooling stage. |
The effectiveness of the aging cycle in stress relief ($\eta$) can be expressed as:
$$ \eta = \left(1 – \frac{\sigma_{post}}{\sigma_{pre}}\right) \times 100\% $$
Where $\sigma_{pre}$ and $\sigma_{post}$ are residual stress magnitudes before and after aging. Following the above规范, stress relief of 54-87% for HT200-400 and 45-90% for phosphor-copper-titanium wear-resistant iron has been achieved. Conversely, excessive cooling rates (e.g., 130°C/hr) or large furnace temperature differences (e.g., 160-190°C) can reduce relief to below 50% or even increase stress.
Strategic Sequencing of Aging in the Manufacturing Process
The placement of the aging operation within the manufacturing sequence significantly impacts the final stress state. Measurements on beds for coordinate boring machines and lathes reveal the stress evolution:
| Manufacturing Stage | Symbol | Stress Change Calculation | Approximate % Change vs. Initial Cast Stress* |
|---|---|---|---|
| As-Cast (Early Shakeout >200°C) | $\sigma_1$ | Baseline | +84% to +241% (vs. low-temp shakeout) |
| As-Cast (Low-Temp Shakeout) | $\sigma_1’$ | Baseline for comparison | 0% (Reference) |
| After Rough Machining | $\sigma_2$ | $\sigma_2 = \sigma_1 + \Delta\sigma_{machining}$ | +250% (Machining adds substantial stress) |
| After First (Post-Rough) Aging | $\sigma_3$ | $\sigma_3 = \eta \cdot \sigma_2$ | -54% to -87% (Relief from $\sigma_2$) |
| After Second Aging | $\sigma_4$ | $\sigma_4 = \eta’ \cdot \sigma_3$ | Further reduction (see next section) |
*Note: Percentages illustrate trend magnitude from specific trials; actual values depend on process specifics.
The key insights are:
- Avoid Early Shakeout: Shakeout at temperatures above 200°C can drastically increase initial casting stress.
- Machining is a Major Stressor: Rough machining, especially with heavy cuts, can more than double the existing residual stress.
- Optimal Aging Placement: Conducting the primary thermal aging operation after rough machining is most effective. This allows for the relief of the cumulative stress ($\sigma_{casting} + \sigma_{machining}$), which is often the largest stress state the part will see. Aging before machining addresses only the casting stress, leaving the significant machining-induced stress unrelieved.
This strategy leads to a lower, more stable final stress state, which directly translates to improved dimensional stability of the machine tool castings.
Number of Aging Cycles: Single vs. Double Aging
A critical question for precision machine tool castings is whether one aging cycle is sufficient or if multiple cycles are warranted. Comparative experiments on precision lathe and boring machine beds provide clear data. The effectiveness is judged by both final residual stress levels and long-term geometric stability under load and temperature variation.
Long-Term Dimensional Stability
| Material | Aging Process | Final Residual Stress (kg/mm²) | Maximum Guideway Deformation Over Time |
|---|---|---|---|
| Phosphor-Copper-Titanium Iron | Double Thermal Aging | 1.7 – 2.0 | < 3 µm / year |
| HT200-400 | Double Thermal Aging | 1.3 – 1.5 | 3 – 4 µm / 15 months |
| HT200-400 | Composite Aging* | ~2.4 | ~4 µm / year |
| HT200-400 | Non-Strict Double Aging** | ~5.5 | ~7 µm / year |
*Composite: First thermal aging + six months natural aging.
**Non-Strict: Poor furnace uniformity (>50°C), lower soak temperature, uneven cooling.
Resistance to Load and Thermal Disturbance
Tests on lathe beds subjected to a 400 kg load for 12 days and a 5°C ambient temperature change show the difference in performance:
| Aging Method | Residual Stress (kg/mm²) | Guideway Deformation (µm) | |
|---|---|---|---|
| Under Load | Under ΔT=5°C | ||
| Single Thermal Aging | >5.0 | 6.7 – 12.1 | 1.59 – 1.81 |
| Double Thermal Aging | <2.0 | 0.4 – 1.04 | 0.81 – 1.0 |
The results conclusively demonstrate that double thermal aging yields superior outcomes for precision machine tool castings:
- Lower Stable Stress: Final residual stresses are consistently reduced below 2 kg/mm², a threshold associated with high stability.
- Enhanced Dimensional Stability: Deformation over time is minimized, achieving levels (3-4 µm/year) comparable to high-end international machines.
- Improved Resistance to MPD: The castings exhibit significantly lower susceptibility to deformation from operational loads and ambient temperature fluctuations, indicating a heightened resistance to micro-plastic deformation.
The mechanism is twofold: the first aging relieves the bulk of the machining-induced stress, while the second aging further reduces and, crucially, stabilizes the remaining stress field, making it less prone to time-dependent relaxation (minimizing $\Delta\sigma_{relaxation}(t)$). A third thermal aging cycle is generally not justified from a stress-relief perspective, as the major stress sources have already been addressed.
Summary and Conclusions
Based on systematic experimentation with various machine tool castings, the following principles are established for optimizing dimensional stability through artificial aging:
- Control Casting Process: Avoid premature shakeout (above 200°C) to minimize the initial casting stress ($\sigma_{casting}$), which is the foundation for all subsequent stress accumulation.
- Adhere to a Defined Aging Cycle: Implement a thermal aging规范 with controlled heating (~80°C/hr), adequate soaking (530-550°C for 4-6 hrs), and, most critically, slow cooling (≤50°C/hr) through the elastic-plastic region (>350°C) to prevent secondary stresses.
- Ensure Furnace Uniformity: Maintain furnace temperature uniformity within ±25°C during soaking and the initial cooling phase. This is often as critical as the time-temperature profile itself.
- Sequence Aging After Rough Machining: Schedule the primary aging operation after rough machining to enable the relief of the maximum combined stress state (casting + machining).
- Employ Double Aging for Precision Castings: For critical, precision machine tool castings such as beds for jig borers and high-accuracy lathes, two cycles of thermal aging are recommended. This practice effectively reduces residual stress to a low, stable level (<2 kg/mm²), ensuring excellent long-term dimensional stability and enhanced resistance to operational loads and thermal gradients. For general-purpose machine castings, a single, well-conducted aging after rough machining provides substantial benefit (＞50% stress relief), with the decision for double aging based on specific accuracy requirements.
In essence, a scientifically designed and meticulously executed artificial aging process is a cornerstone for achieving the dimensional precision stability required by modern, high-performance machine tool castings. It directly addresses the core material factors of residual stress and micro-plastic deformation resistance that underpin long-term accuracy.
Note on Vibration Aging: Recent international developments show growing application of vibration aging (stress relief), where the workpiece is resonated at its natural frequency for 10-45 minutes. This method offers advantages such as very short processing time, low energy consumption, portability, and applicability to non-metallic materials or parts sensitive to thermal distortion. It presents a promising alternative or complement to thermal aging for certain classes of machine tool castings and fabricated components.