Abstract

One of the most important fundamental forms for engineering components is the circular cross-section. Circular forms arise in many applications, particularly in bearing surfaces such as rotating shafts and ball bearings. In manufacturing environments, variations on circular features may occur due to imperfect rotation, erratic cutting action, inadequate lubrication, tool wear, defective machine parts, chatter, misalignment of chuck jaws, etc. The importance of studying roundness form deviations of circular and cylindrical features is to avoid the excessive lateral or radial deviations of the rotating and reciprocating parts during dynamic operations. Hence, whether roundness error can be evaluated accurately and efficiently or not will directly influence the part assembly, product’s function/performance and life. For example, it is inferred that a rotational bearing whose components are not accurately round will tend to be noisy and is likely to fail prematurely. However, the current definition of roundness or circularity per ISO or ASME standards considers the surface’s radial deviations only. If these radial deviations are constrained within two concentric circles with minimum radial separation, this radial separation is defined as the roundness/circularity form error. It is, however, difficult to understand how this radial deviation alone can assure functionality while completely overlooking the circumferential variations/deviations on the surface. This paper is an attempt to investigate the form of circularity deviations along the circumference and whether it plays a significant role in the functionality of round machined parts. The investigation focuses on an accurate spindle and turntable type measuring instrument. On this instrument, the component is rotated on a highly accurate spindle which provides an imaginary circular datum. The workpiece axis is required to be aligned with the axis of the spindle by means of a centering and tilt adjustment leveling table. The polar trace obtained off the surface on a section perpendicular to the spindle axis is analyzed through a Fast Fourier Transformation (FFT). This reveals any periodic circumferential deviations in the corresponding amplitude spectrum of the different harmonics. It is intriguing to note that often only one or two amplitudes in the spectrum are significantly larger than the rest and affect the functionality the most. This paper explores how these multitudes of parameters are to be understood and be incorporated into Geometric Dimensioning and Tolerancing (GD&T) drawings, and further be infused in the undergraduate and graduate engineering curriculum, and be taught as an improved toolkit to the aspiring engineers, design & process engineers and quality control professionals.

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