Unified functional tolerancing approach for precision cylindrical components

Current geometric dimensioning and tolerancing (GD&T) standards dictate that the geometry of a cylindrical manufactured part should be characterized in terms of its roundness, straightness, cylindricity and diameter. However, standards de. ne the form errors using maximum peak-to-valley values - a very simplistic geometric description. As a result, measurements based on GD&T definitions for manufactured parts are ineffective for identifying and diagnosing error sources in a manufacturing process. This paper introduces a new tolerancing scheme for cylindrical parts and its application to functional tolerancing for diesel engine components. The new tolerance definition method is based on Legendre and Fourier polynomials for modelling and characterizing typical geometric errors found in machined cylindrical parts. As the tolerance parameters in the new tolerancing scheme represents the whole pro. le characteristics of the part geometry, they provide a direct link to part function and error sources of manufacturing processes. This paper describes the Legendre and Fourier polynomials-based tolerancing method. Typical machining errors of cylindrical parts in precision turning and grinding processes are analysed and modelled using the new tolerancing method. Application of the new tolerancing method is illustrated with an example of functional tolerancing a production diesel engine component, which shows that the new tolerancing method is effective for the specification and control of fuel leakage variation in diesel engine component design and manufacturing.