International Journal of Machine Tools & Manufacture 43 (2003) 973–985 Development of a tool wear-monitoring system for hard turning C. Scheffer a , H. Kratz b , P.S. Heyns a,* , F. Klocke b a Dynamic Systems Group (DSG), Department of Mechanical and Aeronautical Engineering, University of Pretoria, Pretoria, ZA-0002, South Africa b Laboratory for Machine Tools and Production Engineering (WZL), Chair of Manufacturing Technology, RWTH Aachen University of Technology, Aachen, Germany Received 5 March 2003; received in revised form 3 April 2003; accepted 8 April 2003 Abstract This paper describes an in-depth study on the development of a system for monitoring tool wear in hard turning. Hard turning is used in the manufacturing industry as an economic alternative to grinding, but the reliability of hard turning processes is often unpredictable. One of the main factors affecting the reliability of hard turning is tool wear. Conventional wear-monitoring systems for turning operations cannot be used for monitoring tools used in hard turning because a conglomeration of phenomena, such as chip formation, tool wear and surface finish during hard turning, exhibits unique behavior not found in regular turning operations. In this study, various aspects associated with hard turning were investigated with the aim of designing an accurate tool wear- monitoring system for hard turning. The findings of the investigation showed that the best method to monitor tool wear during hard turning would be by means of force-based monitoring with an Artificial Intelligence (AI) model. The novel formulation of the proposed AI model enables it to provide an accurate solution for monitoring crater and flank wear during hard turning. The suggested wear-monitoring system is simple and flexible enough for online implementation, which will allow more reliable hard turning in industry.  2003 Elsevier Science Ltd. All rights reserved. Keywords: Process monitoring; Tool wear; Hard turning; Cutting forces; Neural networks 1. Introduction 1.1. An introduction to hard turning In the metal cutting industry, turning heat-treated pro- ducts with hardness above HRC 55 with a single-point tool is referred to as hard turning. The customary method for machining such parts has always been grinding. The development of new tool materials, such as Cubic Boron Nitride (CBN), has made it possible to use turning instead of grinding. Hard turning has proved effective in reducing costs and lead-times. The reduction in costs is due to the fact that turning can incorporate more oper- ations into a single machine. Turning also uses less type- dependent tools and has a shorter setup time. Further- more, the energy necessary to remove a certain quantity * Corresponding author. Tel.: +27-12-420-2432; fax: +27-12-362- 5087. E-mail address: sheyns@postino.up.ac.za (P.S. Heyns). 0890-6955/03/$ - see front matter  2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0890-6955(03)00110-X of metal with hard turning is 5–10% of the value required for grinding [1]. However, in terms of the pure metal removal rate, grinding can still outperform turning. The most significant cost reductions are found in small batch size production where a variety of products are manufactured. Hard turning can dramatically reduce these production lead-times by reducing the number of required production steps significantly. The high heat resistance of CBN tools allows for a dry cutting oper- ation without negative influences on the tool life or the part subsurface properties [2]. Dry cutting is a key bene- fit for the environmental impact of cutting operations. A major drawback of hard turning is the occurrence of wear at the cutting edge. Tool wear governs the geometri- cal, surface and subsurface qualities of the workpiece. These workpiece characteristics influence important part properties, such as residual stresses [3]. The thermal effects resulting from tool wear accelerate the shape devi- ation of the part, which is why compensation strategies become necessary. Two process features that determine the reliability of hard turning are as follows: