Spindle Error Motions

In the world of spindle metrology, there are six different motions that are available in spindles. These six motions are as follows:

• Pure rotation around the spindle axis which is the Z axis.
• Motion in any of the other five directions
  • X axis
  • Y axis
  • Z axis
  • Tilt along the X axis
  • Tilt along the Y axis

Of all these motions, the first one is the desired motion while the other five are considered as spindle error motions. Tilt measurements help in calculating the error at some distance projected beyond the point at which another measurement is taken. Spindle errors are classified for the sole prupose of distinguishing the root cause of the error or its effect on part quality.

Drift
This is a type of slow change that occurs over time due to some external influence, like temperature, that makes a change in the tool position.

Shift
This is an abrupt change in an operating parameter due to some change in the system, like air-pressure change or speed change on an air-bearing spindle. Since high-speed machining involves a considerable level of centrifugal force on spindle components, speed changes can cause bearing elements which in turn affect tool position to be repositioned.

Synchronous error motions
Error motions that are "synchronized" with the rotational speed is known as synchronous error motion. Thse kind of motions repeat every revolution of the spindle. A plot of these errors displays lobes that can occur once, twice or at some multiple of times per revolution. This type of motion is also known as out-of roundness motion and it can predict the ability to bore a round hole or to turn a round part. The tool will cut a part with the same number of lobes as viewed in the synchronous error motion plot. Manufacturers can measure the synchronous error motions of spindles to find out which spindles they should use for critical jobs. Also, it helps in determining the effects of a crash on a spindle's precision. Spindles run true until they operate at a resonant speed that leads to excessive vibration. Synchronous error motion plots, viewed at various spindle speeds, can make resonant speeds apparent to allow operators to avoid them.

Asynchronous error motions
These motions are not synchronized with spindle rotation. In other words, error motions that are not "synchronized" with the rotational speed of the spindle is known as asynchronous error motions. Bearing "defect" frequencies usually cause asynchronous error motion. They do not repeat on successive spindle rotations. These error motions are depicted by the "fuzziness" of a polar plot, and are related to surface finish. Asynchronous error motions develop due to the interaction of spindle-bearing elements that are rotating at different speeds,like rolling elements, bearing races, rolling element retainers or cages. The errors can also develop from a machine tool's non-structural elements like as in hydraulic pumps or coolant pumps. They can develop from external factors as well like a forklift driving past the machine.

Spindle error analyzer
Spindle Error Analyzer is used for analyzing, measuring and monitoring machine tool spindle health and performance. Virtually all types of spindles in any machines and machine tools can be evaluated. It can also be used with precision spindles in disk drives and precision, high-speed drills. The analyzer enables manufacturers to predict and prevent part errors like as follow:

• Hole location
• Surface finish
• Roundness
  
  

by measuring and tracking error motions that indicates:

• Out of round bearing components
• Bearing wear
• Misaligned bearing seats
• Structural vibration
• Inadequate stiffness
• Improper preload
• Resonant machine frequencies etc.

The Spindle Error Analyzer performs tests by complying with some certifications standards like as follows:

• ANSI/ASME Standard B5.54-2005: "Methods for Performance Evaluation of CNC Machining Centers"
  
  
• ANSI/ASME B89.3.4: "Axes of Rotation, Methods for Specifying and Testing"
  
  
• ANSI/ASME B5.57-1998: "Methods for Performance Evaluation of CNC Turning Centers"
  
  
• ISO230-7: Test Code for Machine Tools Part 7, "Geometric Accuracy of Axes of Rotation "
  
  
• ISO230-3: Test Code for Machine Tools Part 3, "Determination of Thermal Effects"