CHALLENGES IN TOLERANCE TRANSFER FOR ADDITIVE MANUFACTURING
Gaurav Ameta, Shawn P. Moylan, Paul W. Witherell
Abstract: From the design point of view, datum features are used to imply design intent of particular function of the part or sequence of assembly of components in a product. Each feature in a part could potentially have different datum reference based on the design intent. In traditional manufacturing, datum references are used to identify positions of machining features. In order to save time and cost, the number of datum references used in creating features are reduced in manufacturing. As the datum references are changed, validity of the tolerance specification and design intent is verified through a process called tolerance transfer or conversion. In additive manufacturing, parts and assemblies are built layer by layer, implying that all the features in the process will have common a datum reference. Different datum references could still be specified for post processing steps. This paper identifies issues related to tolerance transfer in AM processes. In AM, layer by layer manufacturing of part may lead to features being completed before the features design datum reference is completed. Furthermore, based on the build direction, variations in feature and datum references can occur. When performing tolerance transfer, a process planner needs to consider (a) design intent, (b) build direction (c) process variation and (d) datum references. A case study in tolerance transfer with AM will demonstrate these issues. Suggestions for additional tools in design specification standards (ASME Y14.5, ISO 1101 and upcoming ASME Y14.46) that may alleviate some of these issues will be made.