Knowledge of machining technology is critical for the selection of the right equipment for the job. That said, there appears to be a lot of misunderstanding of three common machining terms: precision repeatability, and accuracy. This misunderstanding of terms causes people to use them incorrectly, and when the customer and the CNC machining company are not on the same page, miscommunication can ensue. Knowing the proper definition of these terms permits all parties involved to have a common frame of reference.
Precision is defined as the number of significant digits to which a value has been measured. To a CNC user, precision is basically the smallest movement a machine can make. For instance, a CNC machine that moves 0.0005 inches is more precise than one that moves 0.005 inches. While CNC in general offers high precision, this is usually related to the cost of the machine.
Repeatability refers to the sameness in measurements taken by an individual or instrument on similar items under the same conditions. A measurement is said to be repeatable when any variation is similar or equal to a set limit. In CNC machining, this term relates to the difference in the measurements of multiple parts based on single computer-aided manufacturing (CAM) file. More specifically, if repeatability is good, the cut parts will be extremely close in size with each other.
The accuracy of a measurement system refers to the degree or closeness of measurements of a number to its true or actual value. Put simply, accuracy is the result of the combination of precision and repeatability (precision + repeatability = accuracy). To a CNC machining service this is the ability to continuously produce perfectly identical or near-identical parts.
When the terms are misused
If you cut a part on a CNC machine that’s precise, but the results are not repeatable or accurate, occasionally you may get parts that don’t fit well together. Likewise, if the results are repeatable, but the machine is neither precise nor accurate, you may get parts that fit well together, but the finished product is not in the size that you want.
As a customer, you want to work with a CNC machining service that gets the precision + repeatability = accuracy equation right. Bates Machine and Manufacturing is one of the most trusted names in the industry because they deliver the expected results consistently.
Is CNC Machining Really Better Than RP?, MoldMakingTechnology.com
How Accurate Is Your Machining Center?, ModernMachineShop.com