The main reasons why these cracks occur are due to the high stresses that the mill end castings are subjected to and therefore metal fatigue. Most suppliers have taken steps to reduce metallurgical or casting failures as much as possible, but pre-delivery inspection and testing would be one step. Many problems are caused insufficient care by the maintenance department when welding, grinding the mill shell, or cutting bolts/drilling holes for fixing bolts. Of course there are steps you can take to during operation to mitigate against these cracks developing, (i) broken wear protection plates must be replaced immediately, (ii) temperature monitoring. Ultimately there is no alternative to frequent and rigorous inspection.