Towards an integrated strategy for effective machine tool probing

Spindle-mounted probing systems on machine tools have become commonplace within manufacturing industry over the last ten years. On-machine probing (OMP) is often used as an automatic way of finding workpiece offsets on machine tools to allow better repeatability and more automated procedures. In more advanced engineering OMP may be used for in-process gauging, error correction or even final pass-off. They therefore form part of the measurement process that defines the overall quality control chain.

When used in production the probing system, and the machine that carries them, is subjected to harsh and variable conditions that are not experienced in a metrology room. This includes undergoing tool change routines, being affected by changing thermal conditions and contaminated by cutting debris. Unlike a traceable quality control department there is also the possibility of comparatively poor calibration techniques as this is considered a machine tool function. As a result, many companies do not use their OMP effectively, either because of a lack of understanding of capability or mistrust in the results due to a lack of understanding of uncertainties.

This research has provided the building blocks for a “bottom up” strategy to analyse influencing factors for OMP to enable end users to get reliable results with the lowest uncertainty for the desired function. Crucially, the research assumes different tolerance requirements for various functions that can be performed with such a probing system in order to avoid recommending onerous pre-requisite procedures where they are not needed.

The influencing factors for OMP are identified through literature review and a failure mode analysis conducted with experts from industry and academia. The magnitudes of the key factors and effects are then analysed through stated system specifications or experimentation on example machines. However, the dissertation concluded that all machine tools behave differently, and have different accuracy requirements. The main outcome of this work is, therefore, to determine which tests and checks are pre-requisite for OMP on any machine, depending upon their intended use. Ultimately, though, the work concludes that the unpredictability of thermal behaviour is likely to cause the greatest problems for on-machine probing, with a full understanding being required for reproducibility of measurement results from a machine tool.