CNC Machining: Positioning Accuracy and Repeatability

Positioning accuracy and repeatability-what are they exactly?

I define the accuracy of a CNC machine tool as how accurately its axis reaches the end of the command along a predetermined path under load. I define its repeatability as its ability to accurately replicate command actions in multiple cycles of a day (again, under load).

Positioning accuracy

This is the definition of dynamic accuracy and repeatability. They may differ from the specifications of your machine manufacturer. Builders’ specifications usually indicate static accuracy and repeatability; that is, the machine is not in a cyclical machining operation when the relevant measurements are taken.

Repeatability

Improved accuracy by reducing errors caused by load, thermal changes, calibration, calculation, and motion errors (motion error-speed, acceleration, deceleration). Improved repeatability by reducing static friction, gaps and thermal changes.

Accuracy related factors

Certain accuracy-related factors are beyond the control of CNC users. Once a machine is installed, these measures include: machine structure, machine structure, machine calibration, and environment.

Ensure that the machine installation can provide enough dynamic accuracy for your application-and keep it properly maintained, half of the acceptable component problems. You must also confirm that the machine can accurately repeat from the first workpiece to the last hour, hour after day-even if the machine parts warm up after being idle. Some of the most serious repetitive problems have nothing to do with machine design. Instead, they are affected by machine applications. Any type of change-during the production run or from one job run to the next-will affect repeatability. As the cycle changes, a time-consuming adjustment is required. If the change is large enough, waste products will be generated.

Summarize

Achieving accuracy is much more expensive than achieving repeatability. If the accuracy is not good, you won't get any good parts, even if the repeatability is okay. If accuracy and repeatability are both possible, then you will get a whole batch of high-quality CNC machined parts.