PROJECT BO

Introduction

As technology progresses and manufacturing require-ments become more stringent, dimensional measure-ment becomes increasingly vital to providing qualityproducts. At all scales of dimensional measurement, alarge part of the associated uncertainty is attributableto thermal effects. Thermal variation is considered tobe the largest source of dimensional measurementuncertainty and must be monitored and modelled togauge their probable effects.This particular review extends and further detailskey temperature measurement technologies that havebeen identified for use in the Light ControlledFactory.1These are mature technologies that are suit-able for measurement in the range of 0 °C–50 °C. Thisrange is designed to capture assembly, integration andtest (AIT) environments around room temperaturewith an extension for more extreme scenarios.

Temperatures appreciably higher or lower than this canbe found within assembly processes from dowel pinscooled with liquid nitrogen up to the welding of prod-ucts. Most of the technologies presented here are suit-able for use over wide ranges spanning hundreds ofdegree Celsius.In 2013, Muelaner et al.2recognised thermal varia-tion models as one of the enabling technologies formeasurement-assisted assembly (MAA). Work onMAA carries on as part of the Light Controlle

Factory project, which aims to deliver significant pro-cess improvements through the use of optical metrol-ogy.3A review of MAA applications in the assembly ofaerospace products was carried out by Mei andMaropoulos4in 2014.Modelling thermal variation has been the goal of anumber of studies with the primary focus being onmachining processes such as thermal modelling of chipformation for machining5and the estimation ofmachining characteristics in die sinking electro-discharge machining.6Finite element analysis has beenused recently in combination with machine learning algorithms to model thermomechanical loads in turn-ing processes.7Thermal variation has been studied for mechanical assemblies and the thermal impact on tolerances.8,9A recent study included temperature as one of several operating parameters in tolerance analysis todetermine the ‘operating window’ of an assembly process.10Temperature is one of the most commonly measuredquantities in industry so there is a well-established traceability path. National standards of temperature are derived from the International Temperature Scale of1990 (ITS-90).11National measurement institutes suchas the National Physical Laboratory (UK) calibrate devices to the ITS-90 and provide accreditation to local calibration facilities. The ITS-90 supersedes the previous scales: ITS-68 and ITS-48. Consensus in the field is that the ITS-90 is a good scale for current needs for some years to come. Studies are continually being carried out on the ITS-90 and some articles in the literature have picked up on sub-range inconsistencies, which will be useful in the development of the next scale.12–14Research priorities for temperature measurement are discussed in two domains: technology-focused priorities and practical temperature measurement.