Elimination of Settling Error Due to Clamping Forces in On-Machine Measurement

When using OMM (on machine measurement), accuracy in the measurement of finished, machined parts is important; these parts are commonly measured by CMMs (coordinate measuring machines). This paper reports on new OMM hardware (and its operating software) developed to enhance accuracy in measurement. In assessing the value of the system, measurement accuracy was evaluated by comparing results of both CMM and OMM. Factors that can affect accuracy (sensor installation error, repeatability of the CNC machine tool, and settling error in the set-up of the workpiece) were then analysed quantitatively. The results show that the major source of error in OMM accuracy is settling error due to clamping forces, specifically in the workpiece clamping system (for example, a pallet). Therefore, the greatest improvement in measurement accuracy can be achieved by estimating and compensating for settling error beforehand.