# Positioning Accuracy vs. Repeatability in CNC Machines

Positional accuracy and repeatability are two distinct concepts often used to describe the performance of mechanical systems in the industrial sector.

Repeatability is generally higher than positional accuracy.

## Positional Accuracy

Positional accuracy is a specific value, not a range, indicating the degree of deviation between the target position a mechanical system aims to reach in a single motion and its actual position.

It reflects the system’s accuracy in achieving the target position in a single movement.

In simple terms, it refers to the deviation between the actual position a piece of equipment stops at and the position it was supposed to reach.

Positional accuracy is a definite value (e.g., 0.01mm) rather than a range (e.g., ±0.02mm).

The error in positional accuracy depends on the manufacturing inaccuracies of the transmission components.

For example, if a shaft is meant to move 100mm but actually moves 100.01mm, the 0.01mm difference represents the positional accuracy. If a lead screw is set to move 100mm and the actual movement ranges from 99.99mm to 100.01mm, its positional accuracy is 0.02mm (100.01 – 99.99 = 0.02).

## Repeatability

Repeatability refers to the precision with which a mechanical system can repeatedly reach the same target position over multiple movements. It demonstrates the system’s consistency and repeatability in achieving the same target position across multiple movements, which is crucial for applications requiring repetitive motion.

In layman’s terms, it is the error produced when positioning to the same location multiple times. Repeatability is expressed as a range (e.g., ±0.02).

The error in repeatability is related to the clearance in transmission components. Additionally, repeatability is influenced not only by the accuracy and resolution of the grating scale (displacement sensor) but also by errors in the mechanical system, such as the guidance system, transmission system, and structural rigidity.

These factors can reduce positional accuracy and, to some extent, affect repeatability.

For instance, if a shaft is required to move 100 mm, it might actually move 100.01mm on the first attempt and 99.99mm on a repeated attempt. The error between these two measurements, 100.01 – 99.99 = 0.02mm, represents the repeatability. Repeatability is expressed as ±x, with the above error represented as ±0.01mm, or ±(0.02/2).

For example, if a lead screw is set to move 100mm and over 100 movements the actual distance ranges from 99.988mm to 99.992mm, its repeatability is ±0.002mm.

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Author

#### Shane

##### Founder of MachineMFG

As the founder of MachineMFG, I have dedicated over a decade of my career to the metalworking industry. My extensive experience has allowed me to become an expert in the fields of sheet metal fabrication, machining, mechanical engineering, and machine tools for metals. I am constantly thinking, reading, and writing about these subjects, constantly striving to stay at the forefront of my field. Let my knowledge and expertise be an asset to your business.

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