Dimnesional Fidelity of Replica Casting Compounds

Abstract

Difficulty arises when wanting to measure features on a part or assembly that is inaccessible with measuring equipment. Sometimes, these features are small and in a recess, such as an O-ring groove; other times, the feature is on a complete assembly which is large, such as a valve seat on an engine. A common practice is to use a replica casting compound to make a negative impression of the feature of interest and measure the replica. Advantages of making replicas include being able to (1) measure a small imperfection on a large piece without having to disassemble or destroy the original, (2)provide archiving of such imperfections, and (3) eliminate light scattering which might cause difficulty with some optical techniques for dimensional metrology. Reprorubber®, a metrology-grade casting material, is a commonly used silicone-based replica casting compound made from polyvinyl siloxane (PVS). We investigated the fidelity to which this material replicates surface texture and geometry, using step height reference specimens and parameter reference specimens (Type A per ASME B46.1:2009), along with the fidelity for longer spatial wavelength features using optical flats (and Type C specimens per ASME B46.1:2009), and other surfaces with known form geometry. Results of step heights are presented here, but all results will be presented at the conference. Our results are used to assign a Type B uncertainty associated with the replication process when replicating a feature which is inaccessible to dimensional instruments.