Predict and Assure the Matchable Degree in Selective Assembly via PCI-based Tolerance

Abstract

Selective assembly is a method to achieve high precision fit using low precision parts. The current practices in selective assembly lack a physical process model to guide the design and process of mating parts, consequently, selective assembly is often a nightmare for manufacturing people and historically an event to be avoided. Process capability indices (PCIs) are the widely accepted process parameters, functioning as a measure of process performance, such as process centering, process spread, process bias, etc. In this paper, the concept of PCI-based tolerance is proposed as an interface between quality requirements and statistical process control (SPC) parameters. An analytical model involving PCI-based tolerance is developed to predict and assure the matchable degree in selective assembly. A detailed case study of the fit of dry liners and cylinder blocks in a diesel engine is presented. As matchable degree and other process quality requirements can be assured at the stage of design by introducing PCI-based tolerance, the process of selective assembly can be improved significantly as an effective way to achieve precision assembly with economical manufacturing processes.