This paper discusses a practical example of machining crank shafts, which are the central part of compressors made in mass production, in order to point out the importance of selecting proper machining parameters and presenting their further relation to product quality. As the shaft's dimensions varied significantly after machining in comparison to the prescribed one, an analysis of the relationship between the machining parameters and data obtained from quality assurance tools was performed. These tools are frequently used in everyday process controls, however, they are often incorrectly used, or else not all the information is turned to advantage. All the possible causes that may influence machining accuracy are presented with the aim of determining the magnitude of each factor. Special attention is focused on the influence of variations caused by machine inaccuracies. This paper concludes with a proposal for the proper use and interpretation of statistical tools in quality assurance procedures in order to detect disturbances in the process caused by the excessive wear of the cutting tools.