Evaluation of thermal models on a machining center

Machine tool positioning accuracy varies with the thermal state of the machine. As electrical energy is input into the servomotors, hydraulic pumps, and other machine systems, energy is transferred into the part, atmosphere, and most importantly the machine's structure. This transfer of energy throughout the machine results in temperature changes and thus structural deformations that change the machine's accuracy. In order to mitigate these detrimental accuracy variations, models have been employed that try to predict and correct for these variations based on discrete temperature readings from the machine. In this study three thermal models and one geometric model are evaluated by comparing their accuracy improvement on a Cincinnati Milacron Maxim 500 machining center. One thermal model is the simple geometric model with first order correction of the scale errors. The two remaining thermal models utilize a new implementation of a neural network. One of the neural network models is trained from error measurements neural network. One of the neural network models is trained from error measurements taken as the thermal state is varied with non-machining actuation, while the other utilizes actual machining. The laser ball bar is utilized to collect the training data for the models in a timely manner and to allow machining between measurements. The models are evaluated by measuring body diagonals with the laser ball bar and by comparing the accuracy of machined parts at different thermal states of the machine.; The body diagonal and part machining tests reveal that the thermal models are capable of 2-4X error reduction at several thermal states. The completely deterministic first order thermal model performed as well or better than the neural network models. Durability tests showed that the models were capable of error reduction over a 9 month period. No clear preference was found for training with or without machining, rather the use of coolant appeared to be a more important factor. Thermal compensation is a viable technique that should be embraced by industry.