What Is The Accuracy Of A CNC Machine, And How Is It Calculated?

Researchers, engineers, and others with extensive technical knowledge will find the best CNC machine precision by considering the various factors that determine this specific measurement.

This article explores those factors in detail to answer the question What is the accuracy of a CNC machine? This article will explain how to calculate the accuracy of your own CNC machine in layman’s terms so that you can understand this measurement more thoroughly than ever before.

The accuracy of a CNC machine can be broken down into two factors

Repeatability and total allowable error. Repeatability is a measure of how well an axis follows its intended path. This can be calculated by dividing TAE by full scale. For example, a machine with 5/64 repeatability will follow its intended path within 1/16 every time it moves in that direction. For a high-performance machining center designed for complex parts, repeatability should be 1/10 or less. Total allowable error refers to how far an axis can deviate from its programmed coordinates before it becomes unusable for cutting or other machining operations.

To calculate TAE, divide full scale by half of your maximum tool length. Then multiply that result by 0.5 percent. So, if you are using a 3/4 tool on a 1 diameter part, your maximum tool length is 0.75.

Your maximum deviation would then be 0.75 x .005 = .0038. If you were able to maintain ±0.0038, you could still use your machine even though it has exceeded its tolerance limit (0). As stated earlier, many advanced machines have tolerances as low as ±0.0005.

These machines are typically used for aerospace applications where accuracy is absolutely critical to ensure proper function and performance of any component that requires precision machining techniques such as turning or milling (1).

Offset Compensation

There are many variables in creating high-accuracy parts with a milling machine. Three of these variables include spindle error, tool length and cutter deflection. Once you’ve completed a part run and determined you have an error greater than 0.0005 (with a 10-tooth endmill), it is important to determine whether or not your error is due to one of these variables or if your error is due to wander – also known as offset compensation.

To accurately determine if offset compensation is present during an axis cut, change spindle speeds slightly (10 RPM) and take multiple measurements from both sides of your part run in order to verify that no outside forces are influencing your measurement data. If there is still too much discrepancy between your two sets of readings, then you may be experiencing offset compensation. In some cases, changing your chip load can help reduce or eliminate error caused by offset compensation.

However, if you continue to experience high levels of offset after switching chip loads on a particular material, then it might be best to switch machines entirely until you find one that better suits your needs.

Resolution (i.e. bit size)

We typically use a metric called microns for measuring tool path resolution. As with most other units of measurement in machining, however, there is no official standard regarding what constitutes one micron.