I. First Step: Determine Granite Base Material and Grade (Most Critical)
1) Stone Types (From High to Low)
Indian Black (M10) / Imported Black Granite: High density, lowest thermal expansion coefficient, best stability, used for high-precision / laboratory-grade CMMs (aerospace, medical, optics).
Jinan Green: Top domestic grade, good rigidity and vibration absorption, first choice for mid-to-high-end bridge machines; relatively expensive.
Zhangqiu Black (Sesame Black): King of cost-performance, most commonly used for ordinary molds, auto parts, and workshop-type CMMs.
Northeast Sesame White: Average stability, only used for low-precision, economical equipment.
❌ Pitfall Avoidance: Do not buy “dyed granite” or “synthetic marble”; they deform, fade, and lose precision significantly after 1–2 years of use.
2) Precision Grade (Flatness)
000 Grade (Ultra-high precision): ≤2μm/m, used in laboratories and metrology rooms.
00 Grade (Industrial standard): ≤4–5μm/m, standard configuration for the vast majority of CMMs.
0 Grade: ≤10μm/m, only used for rough measurements and economical machines.
3) Granite Base Requirements
Monolithic Structure (Non-assembled): Base Y-axis guide rail worktable must be from the same block of material, with no assembly stress.
Natural Aging ≥ 2 Years: Internal stress fully released to avoid later deformation.
Surface Grinding Ra≤0.4μm: Guide rail and worktable surfaces must be mirror-grade.
II. Select Structure: Moving Bridge vs. Fixed Bridge (Different Granite Applications)
1) Moving Bridge (Most Common, Recommended)
The worktable is a massive granite block, fixed in place; the bridge moves in the Y-direction.
Advantages: High load capacity, stable, good vibration resistance, convenient workpiece loading/unloading; first choice for molds, auto parts, and 3C products.
Granite: Integrated base worktable Y-axis guide rail, thickness 150–300mm.
Moving Bridge CMM
2) Fixed Bridge (High Precision)
The bridge is fixed, and the granite worktable moves in the Y-direction.
Advantages: Extremely high rigidity, highest precision (up to 0.5μm level); aerospace, medical, optics.
Disadvantages: Limited travel, slightly lower load capacity, high price.
Fixed Bridge CMM
3) Gantry Type (Large Workpieces)
Large monolithic granite base, equipped with cast iron or steel crossbeams; used for automotive body panels and large aerospace components.
Gantry CMM
III. Reverse Selection Based on “Precision Requirements” (Most Practical)
1) Ultra-high Precision (Within ±1μm)
Scenarios: Aircraft engines, optical lenses, medical implants
Configuration: Fixed bridge Indian Black / Jinan Green 000 grade air bearing guide rails constant temperature 20±0.5℃.
2) Mid-to-high Precision (±2–3μm)
Scenarios: Precision molds, automotive transmissions, 3C molds
Configuration: Moving bridge Jinan Green / Zhangqiu Black 00 grade air bearing guide rails workshop air conditioning.
3) Ordinary Precision (±5μm)
Scenarios: Machined parts, stamped parts, workshop spot checks
Configuration: Moving bridge Zhangqiu Black 00 grade linear guide rails ordinary environment.
Empirical Formula: CMM precision ≈ 1/3 to 1/5 of the workpiece tolerance.
IV. Travel and Load Capacity: Granite base must be “large and thick enough”
X/Y/Z Travel: Based on maximum workpiece size 100–200mm margin; e.g., for a 500×400×300 workpiece, select a 700×600×500 machine model.
Worktable Load Capacity: Granite base load capacity ≥ (workpiece fixture total weight) × 1.5; Common: 500kg, 1000kg, 2000kg.
Base Thickness:
Small machines (below 700): ≥150mm
Medium machines (around 1000): ≥200mm
Large machines: ≥250–300mm
V. Environmental Adaptation: Granite advantages must match the site
High vibration (workshop, with overhead cranes): Must use granite air-bearing guideways independent vibration-proof foundation; shock absorption is 3–5 times stronger than cast iron.
Large temperature fluctuations (no constant temperature): Select Jinan Blue / Indian Black (thermal expansion coefficient ≈ 6.5×10⁻⁶/℃), much more stable than cast iron.
Humid / with cutting fluid: Granite does not rust, is acid and alkali resistant, and is easier to maintain than cast iron.
VI. Budget Range Reference (2026 market price, domestic)
Economic type (±5μm): Moving bridge Zhangqiu Black Grade 00, 150,000–250,000
Mid-to-high end (±2–3μm): Moving bridge Jinan Blue Grade 00 air-bearing, 300,000–500,000
High precision (±1μm): Fixed bridge Indian Black Grade 000, 600,000–1,000,000
VII. Quick Selection Checklist (Directly check off)
✅ Granite: Monolithic, natural aging ≥ 2 years, Grade 00 and above
✅ Stone material: Precision → Indian Black / Jinan Blue; Standard → Zhangqiu Black
✅ Structure: Mold / Auto parts → Moving bridge; Aerospace / Medical → Fixed bridge
✅ Travel: Workpiece size 100–200mm
✅ Load capacity: ≥ (workpiece fixture) × 1.5
✅ Guideways: High precision → Air-bearing; Standard → Linear guideways
