Spindle Error Analysis FAQ

What is Synchronous Error Motion?

Error motions that are “synchronized” with the rotational speed. These include integer multiples of the rotational frequency. Previously this characteristic was referred to as “average” error motion.

What is Asynchronous Error Motion?

Error motions that are not “synchronized” with the rotational speed of the spindle. These include all frequencies that are not integer multiples of the rotational frequency. Bearing elements (i.e. balls and races) will generate error motions that are related to speed, but typically are not integer multiples. Bearing “defect” frequencies generally cause asynchronous error motion.

When measuring error motion, how much target eccentricity is necessary?

If an encoder is being used with the system, no eccentricity is necessary.

Otherwise, the eccentricity has to be greater than the error motion. For an accurate smooth running spindle, 0.5 µmeters (20 µinches) might be adequate. For a general machining spindle, eccentricity is generally between 25 and 100 µmeters (0.001 and 0.004 inches).

What is the difference between runout and error motion?

When a probe measures the movement of a rotating target, the “raw” measurement is the runout. The raw measurement includes the off-center mounting of the ball, out off roundness of the ball, and thermal drift effects.

Error motion removes these other sources leaving only motion measurements that are relative to the spindle performance itself.

What is typically the largest source of error in a machine tool?

For most machine tools, the largest errors are caused by thermal distortion of the machine structure. As the temperature distribution changes, the machine structure will change length and shape. Causes of thermal problems include sunlight, room temperature changes, drive motors, and heat buildup in bearings.

What is the maximum RPM at which SEA can measure?

180,000 RPM. At higher speeds, the number of samples per revolution must be decreased.