It’s tough to pinpoint spindle error. It may be caused by positioning error, and it may be a consistent or an inconsistent error Sometimes it’s linked to spindle speed. When manufacturing engineers at Cummins Engine Co. (Columbus, IN) wanted to find out how good a job was done on recently rebuilt machine tools, they decided to use an SEA 2.30 Spindle Error Analyzer from Federal Products Co. (Providence, RI).

This analyzer, which works on spindles operating at speeds up to 120,000 rpm, uses capacitance gages to measure the volume of air between the gage and a precision ball or mandrel mounted on the machine’s toolholder. As the spindle rotates, position variations are detected, amplified, and fed to the computer for analysis. This type of system is specified for use in CNC machine tool calibration under the ANSI/ASME B5.54 standard.

In the case of the rebuilt machine tool, they were able to detect a serious error caused by improperly loaded spindle bearings. After analysis and repair, asynchronous radial error at 2600 rpm was reduced 45% from 469 to 247 mu-in. (12.0 to. 6.3 mu-m) and radial error motion 43% from 150 to 86 pin. (3.8 to 2.2 mu-m).

Cummins now uses the Federal system to monitor all 50 CNC machine tools in its facility. In addition to detecting the need for immediate repair, the gage will also track performance changes and help schedule maintenance. Data obtained will also help managers assign jobs to machines based on required accuracy. For more information Circle 229.