Precision Engineering 36 (2012) 104–120 Contents lists available at ScienceDirect Precision Engineering j o ur nal homep age: www.elsevier.com/locate/precision A Technique for measuring radial error motions of ultra-high-speed miniature spindles used for micromachining K.Prashanth Anandan a , Abhinandan S. Tulsian a , Alkan Donmez b , O.Burak Ozdoganlar a,∗ a Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA b Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA a r t i c l e i n f o Article history: Received 2 February 2011 Received in revised form 20 June 2011 Accepted 29 July 2011 Available online 5 August 2011 Keywords: Ultra-high-speed spindles Air bearing spindles Spindle metrology Micro-manufacturing Runout Micromachining Error motion Axis of rotation Uncertainty a b s t r a c t Ultra-high-speed (UHS) miniature spindles are widely used for mechanical micromachining processes, such as micromilling and microdrilling, as well as for precision machining processes. The accuracy of features created in those processes depends on the trajectory of the tool tip as the spindle rotates. The tool- tip trajectory can be obtained by measuring the speed-dependent radial motions (which are sometimes referred to as the dynamic runout) at the tool tip. The main contributors to the tool-tip speed-dependent radial motions are the error motions of the spindle, the form accuracy of the cutting tool, the alignment of the tool with respect to the axis of rotation, and the vibrations resulting from the rotating eccentricity. This paper describes a methodology that uses two laser Doppler vibrometer (LDV) systems to measure the radial motions at two axial locations of a precision cylindrical artifact attached to the spindle, while the spindle is rotated at its operational speeds. Measured radial motions are then processed to obtain radial and tilt error motions of the UHS spindle in the rotating sensitive direction. An alignment procedure is developed to ensure the mutual perpendicularity of the two (X and Y) laser beams. The methodology is demonstrated on an UHS air-turbine driven spindle with aerostatic-bearings. Subsequently, an analysis is performed to determine the measurement uncertainty associated with the presented methodology. It is concluded that the presented methodology can be used to effectively measure radial and tilt error motions of UHS spindles. Furthermore, it is shown that the average radial motion, synchronous radial error motion value and the standard deviation of the asynchronous radial error motion vary significantly with the spindle speed due to dynamic effects. © 2011 Elsevier Inc. All rights reserved. 1. Introduction Mechanical micromachining is one of the emerging micro- manufacturing techniques for fabricating micro-scale features and components with three-dimensional complex geometries on metals, polymers, ceramics and composites [1,2]. Miniature com- ponents made from a broad range of materials are increasingly utilized in the medical, aerospace, automobile and consumer prod- ucts industries due to their small size, low weight and improved efficiency [3]. To attain effective material removal rates while using micro-scale tooling, ultra-high-speed (>80 000 revolutions per minute (r/min)) miniature spindles are utilized during micro- milling, micro-drilling and micro-grinding processes [4,1]. The attainable dimensional accuracy, form accuracy, and surface roughness during micromachining operations are critically affected by the undesired motions of the tool tip. Such undesired motion of the cutting tool tip results from the error motion of the spindle (axis ∗ Corresponding author. Tel.: +1 412 268 9890; fax: +1 412 268 3348. E-mail address: ozdoganlar@cmu.edu (O.Burak Ozdoganlar). of rotation error motion), on which the cutting tool is mounted, as well as the form errors and the misalignment of the cutting tool with respect to the spindle. In addition, relative vibrations within the structural loop and the unbalance of the spindle/tool combina- tion contribute to the undesired motion of the tool tip with respect to the workpiece during cutting. For mechanical micromachining processes, the trajectory that the tool tip follows as the tool rotates is correlated to the dimen- sional form, and surface quality that can be obtained. The difference between the ideal and actual tool-tip trajectory is sometimes incor- rectly referred to as runout. Runout is defined in the Axis of Rotation Standards [5,6] as the total displacement measured by a stationary indicator sensing against a moving surface, or by a moving indicator with respect to a fixed surface. According to this definition, runout is a single value computed over one or more revolutions (for a rotat- ing structure), representing the range of displacement sensed by an indicator, which is equivalent to the total indicator reading (TIR). However, the actual value of interest is the trajectory of the tool tip as the tool rotates (e.g., as represented in a polar plot with the rotation angle). Thus, the current definition of the runout is insuffi- cient to fully describe the tool-tip trajectory. The tool-tip trajectory 0141-6359/$ – see front matter © 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.precisioneng.2011.07.014