Define precision and accuracy

There are some terms and phrases that are very commonly used for CNC machining. The industry is known for its precision and speed (just like our name). Terms like tolerance, accuracy, and precision can be confusing. The development of smart technology and the robust machine tools now available make machining seem like a simple task in any application. There are still many factors that can affect the outcome of a finished part. Tolerances are often quoted as performance levels and are always specified in a machine part drawing or program. Tolerances define parameters for variability in the size and shape of a finished part and are often critical to its end use. This tolerance depends on several factors. Defining precision and accuracy can help us better understand how to handle machining tasks to maximize performance.

CNC machining


The term refers to the difference between the actual measurement and the measurement perceived by the CNC machine process. The Miriam Webster dictionary defines it as: free from error or error:

Correctness and conformity with truth or standard or model, or degree of agreement of measure with standard or true value:

Accuracy Accuracy is best defined in CNC manufacturing. Factors that affect accuracy beyond the programmed range include vibration, thermal expansion, cutting tool and workholding. As programming becomes more complex, it achieves greater accuracy by compensating for these factors.


A perfect example of why these terms are difficult to understand without specific reference to CNC machining, another quote from Miriam Webster. Accuracy is called precision and accuracy, but it is a defining adjective consistent with CNC machining.

1: For extremely precise measurements or manipulations

2: Maintain low tolerances in manufacturing

3: Marked by execution precision

Accuracy refers to the repeatability of an ensemble process that provides the same results. Both of these terms in CNC machining are combined with tolerances. Tolerance allows precision and accuracy calculations based on zero tolerance or perfect variable guides.


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