Traceable onboard metrology for machine tools and large-scale systems

Author: Unai Mutilba Larrea Thesis director Dr. José Antonio Yagüe, Universidad de Zaragoza y Dr. Eneko Gómez Acedo, Tekniker Date2019

The machine tool (MT) industry is characterised by its family-owned small and medium-sized enterprises (SME) dominated landscape, strong concentration on flexible and small-batch production of custom-built and high precision machines, and the export orientation of companies.

Product features such as precision and accuracy are far more important than unit labour costs in a technology-intensive sector such as MTs, as a source of competitive advantage. High-tech MT builders seek for differentiation and sources of uniqueness by creating value for their customers through the development of new processes, functionalities and services which help achieve high productivity levels, meet the precision needs of their customers and help lower their costs, looking for a positive impact on the overall equipment effectiveness (OEE) indicator of their customers, the MT users.

In this context, this PhD study aims to improve the accuracy of MTs and also to develop knowledge for traceable coordinate measurement machine (CMM) measurements on MTs. The technology to run a dimensional measurement on an MT already exists but the knowledge to do traceable measurements is under research, as it is reported in this thesis. Several factors affect the measurement accuracy in shop-floor conditions, so the traceability of the measurement process on an MT is not ensured yet and therefore the measurement is not sufficiently reliable for self-adapting manufacturing processes. The starting point of this PhD study is to fully present a qualitative approach of the error sources that contribute to the uncertainty budget for on-MT measurement. An error budget-type classification is proposed to predict the accuracy and repeatability of an MT working as a CMM. Thus, the error budget for traceable on-MT measurement is comprised of the measurement system (the MT with the touch-trigger probe and the measuring software), the component under measurement and the interaction between both of them.

The next milestone presents a quantitive approach to the proposed uncertainty budget. Here, a medium-size on-MT measurement experimental test is performed in shop-floor conditions according to the ISO 15530-3 technical specification. The obtained results demonstrate that traceable CMM measurements are realisable in MTs. Test results highlight the significance and the error source of each uncertainty contributor.

In this way, the measurement procedure uncertainty is the main contributor to the uncertainty budget and the geometric error of the MT is the main error source for the systematic error contributor. However, the ISO 15530-3 technical specification presents a strong limitation, it depends on a calibrated workpiece to understand how the systematic error contributor performs. Therefore, the scalability of the solution is limited to a medium-size MT. To deal with this limitation, an alternative on-MT measurement methodology is presented here based on a volumetric error mapping of the MT prior to the measurement process execution, which allows understanding how the systematic error contributor performs and it is a gateway to large on-MT traceable measurements.

Next, an integrated MT volumetric error mapping procedure that enables the scalability of traceable on-MT measurements to large MTs is developed. The integration of a tracking interferometer measurement device on the MT spindle breaks with the typical multilateration approach, based on sequential measurement scheme, and permits to measure the geometric error of an MT automatically within the complete working volume.

This PhD study focuses on making a special effort towards large scale manufacturing scenarios, where high-value components require fast and reliable feedback on the manufacturing scenario. Thus, either the measurement procedure for on-MT measurement without a calibrated workpiece or the integrated measurement procedure for automatic MT geometric verification are focused on large MTs and therefore both measurement procedures are developed within the large scale metrology (LSM) field. It means that when it comes to large-scale manufacturing scenarios, the traceability of on-MT measurement faces similar challenges to what the LSM does and therefore the measurement procedures developed within this PhD study consider the current state of the art to select the suitable technologies and measurement sequences.

Finally, a new LSM survey procedure is developed for the pointing accuracy assessment of the cutting-edge large synoptic survey telescope (LSST) project. From the methodology design and previous simulation to the in-situ measurement execution, the automatic measurement procedure shows how the LSM can help to execute major scientific projects.