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Error Motions of a Non-orthogonal Rotary Axis



Michael L. McGlauflin, Shawn P. Moylan


Error-motions of rotating axes are one of many sources of imprecision in machining. Accurate determination of axis of rotation error motions of the rotary axes has a direct influence on the accuracy of the finished part. International and U.S. national standards on machine tool performance contain parameters and procedures that allow end users to quantify and verify these error sources. The National Institute of Standards and Technology employs a commercially available high-speed, 5-axis machining center that utilizes a “B-axis” which is oriented at a -45° angle to the Y-axis. This unusual configuration creates a difficult challenge when attempting to determine axis of rotation error motions. A non-orthogonal orientation, such as the one investigated in this paper, introduces multiple considerations regarding location, alignment, and coverage area of the measurement artifact as relates to standoff distances and fixture interference. Subsequent consideration must also be given to the proper analysis of the resultant data to correctly orient the error-motions to the machine coordinate system. This paper investigates a novel method for determining radial and tilt error-motions in a rotary axis whose axis of rotation is -45° to the X/Y plane. The method is based on a spindle-mounted touch-trigger probe and simple table-mounted spherical artifact. The target axis moves to equally spaced angular positions throughout its range of motion and the probe touches the sphere surface at multiple points at each angular position. A data log is created that is subsequently reduced through a three step process of non-linear least squares regression, sphere fitting algorithm, matrix translation of the coordinate system, and a non-linear best-fit circle. The relative changes in sphere center locations are used to reveal tilt and radial error-motions. Repeatability testing and comparison to data obtained with high-sensitivity capacitance probes verify the performance. The paper describes the design of the test, analysis procedures and an evaluation of the measurement uncertainty.
Proceedings Title
Proceedings ASPE 2008 Annual Meeting
Conference Dates
October 19-24, 2008
Conference Location
Portland, OR


Rotary axis, Radial Error-motions, Tilt Error-motions, Probing, Non-orthogonal


McGlauflin, M. and Moylan, S. (2008), Error Motions of a Non-orthogonal Rotary Axis, Proceedings ASPE 2008 Annual Meeting, Portland, OR, [online], (Accessed May 20, 2024)


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Created October 19, 2008, Updated February 19, 2017