Effects of Tool Wear on Surface Roughness and Cutting Force in Thermoplastics Turning

Effects of Tool Wear on Surface Roughness and Cutting Force in Thermoplastics Turning

János Farkas (Simonyi Károly Faculty of Engineering, Wood Sciences and Applied Arts, University of West Hungary, Sopron, Hungary), Etele Csanády (Simonyi Károly Faculty of Engineering, Wood Sciences and Applied Arts, Institute of Woodworking Machinery, University of West Hungary, Sopron, Hungary) and Levente Csóka (Simonyi Károly Faculty of Engineering, Wood Sciences and Applied Arts, Institute of Wood and Paper Technology, University of West Hungary, Sopron, Hungary)
DOI: 10.4018/IJMFMP.2018010101

Abstract

This paper presents a study of the effects of tool wear on cutting force and surface roughness. The cutting force was measured using a piezoelectric force meter which was attached to the cutting machine's revolving head. The surface roughness was measured after the cutting process was complete using a mechanical touch method. A range of thermoplastic materials and cutting layouts were used to give a broader understanding of the topic. After the measurements were taken, the data were evaluated statistically and the effects of tool wear are illustrated graphically. Furthermore, to understand all of the types of wear which can occur during thermoplastics turning, worn turning inserts taken from industrial machines were examined under a microscope. The aim of the study was to define a method for monitoring tool wear during the turning process to avoid tool breakage and/or reduce the number of scrapped parts.
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Introduction

In thermoplastics manufacturing, the most commonly used manufacturing technologies are based on thermoforming, where the material is in its molten state during the forming process. Additive technologies, for example rapid prototyping, are not widely used for series production, and subtractive technologies are also rarely employed, and in most cases only in follow-up processes. Machining can be an appropriate manufacturing technique for prototyping and low-quantity production, as injection moulding becomes expensive due to the high cost of producing the mould. In addition, certain materials such as polytetrafluorethylene cannot be manufactured using the more commonly used methods. Due to the nature of the machining process, the time requirement for high material removal rates must be balanced with the need for continuous production. Downtime is thus very important from both a process and financial point of view. The aim is to minimise the chance of an unexpected stoppage in production, and this is only possible for a well-understood manufacturing process. To increase the reliability of a production process, it must be carefully monitored. Avoiding tool fracture and reducing the scrap rate from machining with a worn tool is an area which can be improved. In general, tools fail in three ways: fracture, plastic deformation and wear. Fracture and plastic deformation are not considered is this paper; instead, the focus of this paper is tool wear where failure can be prevented through maintenance and appropriate monitoring of the process. The purpose of this research is to identify the changes in parameters, features, etc. that become evident as a tool wears. This is important for monitoring and can help to prevent unexpected downtime and improve production rates.

Tool wear is a complex mechanical-thermal process; it occurs when the surfaces of a tool and a workpiece are pressed together under higher pressure at high temperature and in most cases move in opposite directions at different speeds. One way to characterise wear type is based on the location at which it occurs. The most common type of wear is flank wear, which occurs on the surface of the tool which comes into contact with the part. Another common type is crater wear, which is wear of the rake surface. A third type is called nose wear; this type of wear is important for surface finish. The surface of the part is determined by a combination of tool profile and feed rate. As a result, a change in nose radius will lead to a change in the surface roughness of the turned part. In this paper, the body of knowledge regarding tool wear during metal and composite machining will be extended to include thermoplastics machining.

There are two main types of technology employed in tool wear monitoring. Direct technologies are based on visual investigation of tools and/or workpieces. Indirect technologies are based on signals collected from the machining process, for example acoustic emission. Both direct and indirect methods were examined as part of this research. Dutta et al. (2013) presented a detailed overview of direct tool condition monitoring (TCM) technologies. A number of methods for digital image processing (DIP) are available, which can be subdivided into direct (visual inspection of the tool) and indirect (visual inspection of the machined surface) methods. DIP techniques are very useful for fast and easy tool wear detection, and can be used for any type of wear, even those difficult to recognise using other methods.

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