Due to its high hardness, excellent wear resistance, and low coefficient of thermal expansion, granite is widely used in precision equipment fields such as precision equipment bases, inspection platforms, and machine tool bases. However, natural granite is not absolutely zero-deformation. Affected by residual internal stress, processing technology, assembly environment, and operating conditions, it will experience slight warping and dimensional drift during long-term service, directly affecting equipment accuracy. To achieve dimensional stability for decades, full-chain control is required—from raw material selection, aging treatment, structural design, precision machining, installation protection, to operation and maintenance—to keep the deformation within the precision-allowable range.
I. Raw Material Selection: Reducing Basic Deformation Risks from the Source
- Stone Grade Screening
Prioritize high-quality granite with dense grains and uniform mineral composition, such as diabase-type granite like Jinan Green. Its mineral structure is dense, residual stress is small, and the coefficient of thermal expansion is low. Avoid raw blocks with many cracks, high mica content, vastly different mineral particle sizes, or weathering layers. Excessive mica content makes it extremely prone to slow creep deformation under long-term alternating temperature and humidity. - Raw Block Selection Requirements
Select intact large raw blocks, avoiding mineral layer boundaries, dark cracks, and inclusion areas. Raw blocks must not be selected from surface minerals; surface stone has undergone surface weathering and contains latent stress, the release of which will cause component warping later. Prioritize materials mined from deep within the mineral core, where the mineral tissue is uniform and stable. - Initial Inspection of Raw Blocks
After cutting the raw block into a blank, perform an initial flat measurement to observe whether warping has already occurred when the blank is placed freely. Stone that shows deformation at the blank stage is unsuitable for making high-precision 00 or 000 grade precision components.
II. Multi-stage Aging Treatment: Releasing Internal Residual Stress of the Stone
During geological formation, mining, and sawing/cutting processes, natural granite accumulates a large amount of internal residual stress. The slow release of stress is the main cause of long-term deformation, which cannot be eliminated by grinding and machining alone; graded aging must be performed.
- Естественное старение
Raw blocks or rough-machined blanks should be placed at room temperature in the open air or workshop. The placement period should be adjusted according to the component size; for large-sized bases, it is recommended to place them for more than 6-12 months to allow internal stress to release slowly. The blanks should be placed freely without stacking or heavy pressure to avoid manually applying new stress.
Note: Natural aging is affected by seasonal temperature and humidity and has a long cycle, making it suitable for early batch stock preparation, but one cannot rely entirely on natural aging.
- Thermal Cycle Artificial Aging (Industrial Aging)
Precision components must undergo artificial aging to simulate the temperature alternation of the four seasons. Perform gradient heating and cooling cycles on the blank to accelerate the release of internal residual stress.
- Heat up slowly, hold the temperature, and then cool down slowly. Rapid thermal shock is strictly prohibited, as rapid temperature differences will cause micro-cracks in the stone and introduce new stress.
- The aging process should be placed after rough milling and rough grinding, but before semi-precision grinding. Subsequent precision machining should be performed only after artificial aging is completed.
- Prohibition of Large-scale Cutting After Aging
For blanks after aging is completed, try to only perform grinding and precision machining. Do not perform large-area sawing or large-margin milling again. Cutting will re-disrupt the stress balance of the stone and induce later deformation again.
III. Structural Design to Avoid Stress-Induced Deformation
The later-stage deformation of many granite bases is not entirely due to the stone itself, but rather unreasonable structural design.
- Reasonable Thickness and Aspect Ratio
Thin plates with large aspect ratios are most prone to warping. Precision bases must ensure sufficient thickness and stiffness. For example, large platforms should not be made too thin solely for weight reduction; weight-reduction grooves and hollow holes should not be too deep or dense. Excessive reduction of the stone cross-section will lower overall rigidity, making deformation more likely during stress release. - Layout of Openings and Threaded Holes
Bolt holes and mounting holes should be far from the edges of the component, and hole layouts should be as symmetrical as possible to avoid local stress concentration. The pre-embedding process of threaded sleeves must be standardized. Since the expansion coefficient of metal sleeves differs from that of granite, tightening torque must be strictly controlled. Over-tightening is prohibited, as excessive locking force will apply continuous compressive stress to the stone, causing deformation over time. - Special Attention to Joined Components
For large platforms assembled from multiple granite pieces, each individual blank must complete a full aging treatment before assembly. Use low-shrinkage, high-stability specialized stone structural adhesive for joining; the curing shrinkage of the adhesive can cause local deformation. After assembly, the entire structure should undergo another round of static aging before the surface is precision-machined. Do not grind and ship immediately after assembly.
IV. Processing Control to Avoid Introducing New Stress
- Управление процессом шлифования
Gradually reduce the grinding allowance through rough grinding, semi-fine grinding, and fine grinding. Single-pass deep cutting is strictly prohibited. High-speed grinding generates local high temperatures, causing thermal stress on the stone surface. Residual internal stress remains after cooling, leading to warping during slow later-stage release. Ensure proper cooling during the grinding process to control grinding heat. - Multiple Cycles of Grinding and Resting
High-precision components should adopt a “grinding-resting for stress release-re-grinding” process. After each grinding pass, the workpiece should be placed freely and left to rest for several days to allow processing-induced stress to release before the next grinding pass. Do not grind directly to the finished dimensions in one go. - Constant Temperature Environment for Final Machining
Final fine grinding and inspection must be completed in a constant-temperature metrology workshop, with temperature fluctuations controlled within ±2°C, to eliminate temporary dimensional deviations caused by processing temperatures.
V. Installation and Foundation Conditions to Prevent External Force Deformation
Even if the stone itself is stable, improper installation can cause artificial deformation, which may be mistaken for stone deformation.
- Support Point Design
Granite bases should use multi-point uniform support, following the three-point positioning principle. Other support points should only provide auxiliary support; forcibly jacking and pushing for leveling is strictly prohibited. Forcibly jacking or prying to level will apply continuous external force to the massive stone body, causing the stone to undergo creep bending under long-term stress. - Stable Foundation
The foundation for equipment installation must not settle. Foundation settlement will cause the base to twist under stress, indirectly leading to surface deformation. The foundation must be moisture-proof, and no local water accumulation or rising damp should occur on the ground. - Environmental Corrosion Isolation
Provide adequate protection to avoid long-term erosion of the stone surface by oil, acids, and alkalis. Although granite is corrosion-resistant, long-term chemical erosion can alter the surface mineral structure, causing local dimensional changes. The surface can be treated with a penetrative protective coating; thick coatings are not recommended, as aging and delamination of thick paint films can interfere with precision.
VI. Operating Environment and Maintenance, Ensuring Decades of Dimensional Stability
- Control the Temperature Difference of the Working Environment
Although the thermal expansion coefficient of granite is low, large components will still undergo thermal expansion and contraction under long-term, significant diurnal and seasonal temperature differences. Precision equipment should be placed in a constant-temperature workshop as much as possible, avoid direct sunlight on the base, and stay away from local heat sources such as welding machines and ovens, as local heating will cause uneven thermal deformation. - Avoid Impact, Heavy Pressure, and Unbalanced Loading
Do not apply concentrated single-point overload pressure for a long time, as long-term local high pressure will cause stone creep; avoid impacts that generate internal micro-cracks, as cracks will gradually expand and change the stress state. - Regular Re-inspection and Release of Assembly Stress
For large precision bases, all fastening bolts can be loosened annually and left to rest freely for a period of time, then re-tightened according to the standard torque to release accumulated assembly stress, and re-measure the flatness and parallelism.
