The Difference Between Precision and Accuracy, and Tolerances in CNC Machining

When discussing CNC machining services, terms like "high precision," "high accuracy," and "tight tolerances" are frequently used. While computer numerical control (CNC) machines undoubtedly offer these advantages, many manufacturers often overuse these technical terms without clearly explaining their meanings, leaving clients confused and uncertain.

This article aims to demystify the concepts of precision, accuracy, and tolerances in CNC milling, providing clear explanations to help you confidently discuss your manufacturing projects with any service provider.

The Difference Between Precision and Accuracy

Accuracy describes how close a measurement is to the true value, while precision refers to the ability of a measurement system to consistently return the same result. In simpler terms, accuracy is about "correctness," and precision is about "consistency."

To illustrate these concepts, imagine a shooting target. Accuracy refers to how close your shots are to the bullseye (the red center of the target). If your first shot hits the spot marked "x" in Figure 1, earning you 9 out of 10 points, your accuracy is 90%.

Dissatisfied with 90% accuracy, you try again but perform worse, hitting the spots marked "x" in Figure 2 three times in a row. Here, your accuracy is lower (50%), but your precision is higher. Precision is simply the repeatability of the measurement system.

For example, if you need a 100 mm × 50 mm × 10 mm groove machined into a metal workpiece, a highly accurate CNC service will cut the groove with minimal deviation from your specifications. Conversely, a highly precise CNC machine can repeatedly produce identical grooves across 100 workpieces.

What Are Tolerances?

Tolerances represent the allowable deviation in a part's dimensions, typically expressed as "±" followed by a value (e.g., ±0.05 mm).

For instance, if a CNC shop claims a tolerance of ±0.05 mm, their machining may deviate by up to 0.05 mm above or below your specified value. If you request a 10 mm diameter hole, the actual hole could range between 9.95 mm and 10.05 mm.

Tolerances are critical—even minor errors can render a design nonfunctional. Suppose you hire another manufacturer to produce a cylindrical rod to fit into the 10 ±0.05 mm hole. If this manufacturer works with ±0.07 mm tolerances, the rod's diameter could vary between 9.93 mm and 10.07 mm. At these extremes, the rod and hole may become incompatible, highlighting why clearly defined tolerances are essential.

CNC Machining Performance

CNC machining is one of today's most popular manufacturing processes due to its high accuracy, precision, and ability to maintain tight tolerances. With CNC machining, you can produce thousands of identical parts with deviations as small as ±0.04 mm.

However, these benefits are only achievable when working with top-tier CNC service providers. Selecting the right partner is crucial—they must possess advanced equipment, extensive experience, and a skilled technical team to ensure your parts meet design specifications and functional requirements.

Factors Affecting CNC Machining Precision, Accuracy, and Tolerances

• Machine Tool Precision: The CNC machine's inherent accuracy forms the foundation for part quality. Factors like structural design, manufacturing quality, and maintenance affect this.
• Tool Selection: Choosing the right cutting tools for specific materials and processes enhances efficiency and precision.
• Machining Parameters: Optimal settings for cutting speed, feed rate, and depth of cut improve results while minimizing tool wear.
• Material Properties: Hardness, strength, and thermal expansion coefficients impact machining outcomes, sometimes requiring specialized processes.
• Environmental Conditions: Temperature, humidity, and vibrations can affect precision, necessitating controlled environments like temperature-regulated workshops.
• Operator Skill: Technicians must understand machine operation, tool selection, and parameter adjustments to maintain quality.

How to Choose a CNC Machining Service Provider

• Verify Credentials and Experience: Prioritize providers with relevant certifications and a proven track record.
• Assess Equipment and Expertise: Ensure they use advanced machinery and employ skilled engineers.
• Evaluate Quality Control Systems: Robust inspection and testing protocols are vital for consistency.
• Communicate Requirements Clearly: Align expectations upfront to avoid misunderstandings.
• Review Client Feedback: Testimonials and case studies offer insights into reliability.

Conclusion

CNC machining plays a pivotal role in modern manufacturing due to its precision, accuracy, and tolerance control. Understanding these concepts and their influencing factors empowers better communication with service providers and ensures parts meet design intent. When selecting a CNC partner, thorough evaluation of their capabilities and quality systems is key to project success.