Selection of cemented carbide turning tools using EMF and optimization criteria

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Abstract

Tool life and quality of surface finish of the workpiece influence production rate and production cost. Cemented carbide tools have found wide application in the production arena of Bangladesh. In the absence of any testing facility in the country for the selection of cost-effective tools, poor quality carbide tools are frequently imported. A testing method and a selection criteria were developed for testing carbide tools of different shapes and sizes and for selecting the most cost-effective tool. For each tool cutting tests were performed to generate tool life data under different cutting conditions. Mild steel served as the work material. The relationship between tool life and cutting speed was developed using a curve-fitting software on a PC. From these relationships the values of the constants of the Taylor's equation were determined. The cost equation was then derived for each tool for a given amount of work in a cutting speed range. Finally, the most cost-effective tool was selected on the basis of the relative location of the cost curves of the tools tested.

Introduction

Bangladesh is a developing country and as with many other developing countries, it is dependent on imported technology for its heavy industries such as fertilizer factories, steel mills, jute mills, oil refineries, etc. However, the country has to perform a substantial amount of metal cutting and processing operations for the supply of these industries with spares, and for manufacturing machinery for use in various sectors, such as agriculture, transport, textile and others.

Cutting tools play a vital role in metal-cutting operations. For cutting cast iron, stainless steel, heat-resistant alloys etc., especially at high speeds, the cutting tool has to be heat- and wear-resistant. Until very recently, high speed steel (HSS) tools dominated the small scale production sector of Bangladesh. However, due to the clear competitive advantage of cemented carbide tools over HSS tools and also as a result of growing awareness amongst the local entrepreneurs, cemented carbide tools have replaced HSS tools, especially for turning, boring, milling, shaping, planning and similar other operations.

In the local market, a variety of single-poing cemented carbide turning tools of various shapes and sizes are available. In most of the cases the composition and cutting properties of these inserts are unknown. As a result it is difficult to choose the correct tool for a particular operation. The inserts are known in the local market by the brand–manufacturers' name. Inserts of China, India, Poland, Sweden, etc. are very popular in the local market. The local users select the inserts depending on their past experiences. In the absence of any testing facility in the country for checking the quality of cemented carbide tools poor quality tools were imported on various occasions by different organizations of the country. Keeping this in mind, a testing set-up for checking the quality of carbide inserts has been developed as part of the present work. Since the available carbide tools have variation not only in their properties but also in their purchase costs and dimensions, so cost effectiveness should be the criteria for tool selection rather than the properties of the tool alone.

Section snippets

Literature survey and aims and objectives

It is reported that the wear of a cemented carbide tool is mainly due to diffusion [1]especially above a limit of a cutting speed limit beyond which there exists no built-up edge. The process of diffusion is intensified by rise of cutting temperature, which bears a direct relationship with the cutting speed, thickness of cut, material hardness, etc. If other parameters are unchanged, the intensity of tool wear increases with the cutting speed. Markov [2]has successfully established a

Methodology and experimental setup

In this study, 12 categories of carbide inserts from different origins were tested. Of these inserts, three groups were manufactured in the former USSR, one in Poland, one in China, one in India, and the other six by a Local Government Organization (coded as BOF). The material of inserts USSR-1 was BK-8 having a composition of 92% WC and 8% Co; the material of inserts USSR-2 was T5K10 having a composition of 5%, TiC, 85% WC and 10% Co; and that of USSR-3 was T15K6 having a composition of 15%

Formulation for cost analysis

Economic analysis [9]was performed to compare the tested groups of cemented carbide tools. The cost of a tool between two regrinds (Ct) can have three components, as follows: (i) tool changing costs, C1=t1×Rc; (ii) grinding–sharpening costs, C2=t2×Rs; (iii) proportion of the original cost of the tool, C3=CTNt, in which: Ct=C1+C2+C3, where: t1=time to change a tool (min); t2=time to grind or sharpen a tool (min); Rc=labor and overhead rate applied to the metal cutting operation (Taka–min); Rs

Results and discussion

The relationships between the EMF and the cutting speed are shown (up to a limited value of cutting speed) in Fig. 2. The intensities of tool wear versus cutting speed are shown in Fig. 3. It may be observed from Fig. 2, Fig. 3 that above a cutting speed of 80 m min−1 the intensity of tool wear is approximately proportional to the EMF developed by the different tests inserts. Thus, an insert which develops lower a EMF during cutting in this cutting speed range is likely to show a lower

Conclusions

A method of testing the quality of carbide inserts has been established using electromotive-force (EMF) as the criterion. It has been observed that the EMFs developed during turning with different carbide inserts are almost proportional to the intensities of wear of these inserts under identical cutting conditions.

An alternative method is proposed to choose the most cost-effective tool from a number of different carbide tools using a minimum-cost criterion. This method appears to be appropriate

References (9)

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