CNC Machining Tolerance: The Core to Guarantee Precision and Quality

What is CNC Machining Tolerance

Classification and Application of Standard Tolerances

1. Dimensional Tolerance

• ISO 2768 (General metal machining)ISO 2768-f (Fine grade): ±0.05 mmISO 2768-m (Medium grade): ±0.1 mmISO 2768-c (Coarse grade): ±0.2 mmISO 2768-v (Very coarse grade): ±0.5 mm
• ANSI B4.1 (American standard)Precision grade: ±0.025 mmMedium grade: ±0.1 mmCoarse grade: ±0.2 mm

2. Geometric Tolerance

• Straightness: Ensures straightness of part edges and axes
• Flatness: Controls surface flatness, widely used for gaskets and seals
• Roundness: Maintains uniformity of circular parts, usual tolerance ±0.01 mm
• Concentricity: Guarantees identical central axis of multiple cylindrical surfaces, applied to shaft parts
• Position Tolerance: Secures positional accuracy of key features like holes, typical tolerance ±0.02 mm

3. Surface Roughness

• Ra 3.2 μm: Ordinary machined surfaces from milling and turning
• Ra 1.6 μm: Precision surfaces for sliding bearings and sealing components
• Ra 0.8 μm: High-precision aerospace parts
• Ra 0.2-0.4 μm: Ultra-precision ground and polished parts such as optical components

Factors Affecting Tolerance

• Material Property: Hardness and thermal expansion coefficient impact tolerance control. Aluminum alloy is easier to machine than stainless steel yet features larger thermal expansion, requiring temperature variation consideration.
• Machining Process
  Milling: Suits complex shapes, common tolerance ±0.01 mm
  Turning: Applies to rotary parts, usual tolerance ±0.005 mm
  Grinding: Used for ultra-precision machining, tolerance up to ±0.001 mm
• Machine Tool Accuracy: High-precision 5-axis CNC machines meet stricter tolerance requirements.
• Tool Wear: Regular replacement and grinding are required to avoid precision loss.
• Fixtures & Tooling: Stable fixtures reduce workpiece vibration and displacement to improve tolerance accuracy.

Methods to Optimize Tolerance Control

1. Set Reasonable Tolerance Range

   Overly tight tolerance raises cost and processing difficulty, while excessive tolerance impairs part performance. Set specifications according to practical functional demands.
2. Adopt High-precision Equipment

   Deploy 5-axis CNC machines and CNC grinding machines for rigorous tolerance management.
3. Optimize Cutting Parameters

   Adjust cutting speed, feed rate and tool path to minimize thermal deformation and machining errors.
4. Measurement & Quality Inspection

• Coordinate Measuring Machine (CMM): Detects critical dimensions and geometric tolerances
• Laser Measuring System: Non-contact inspection for high-precision parts
• Real-time Monitoring: Dimension testing via measuring probes during processing

Tolerance Application at BRICS

• Milling tolerance: ±0.01 mm
• Turning tolerance: ±0.005 mm
• Grinding tolerance: ±0.001 mm
• Minimum surface roughness: Ra 0.2 μm