Surface Quality and Tool Wear in Interrupted Hard Turning of 1137 Steel Shafts

1. INTRODUCTION The machining of hardened steel components with PcBN inserts having a geometrically defined cutting edge has gained substantially in importance due to improvements in the performance of such modern cutting tool materials. Hard turning is increasingly a profitable alternative to finish grinding. The objective is to remove workpiece material in a single cut rather than a lengthy grinding operation, thus reducing processing time and production costs. Many studies have been carried out in the hard turning of continuous surfaces and the success of this method has encouraged in the recent years studies in the interrupted hard turning area [1], [2]. An interrupted cut occurs when the non-uniform shape of the workpiece disrupts the otherwise smooth removal of material. Consequently interrupted hard turning can be defined as a turning process of hardened parts (>45 HRC) having areas with interrupted surfaces. The studies in the interrupted hard turning field have concentrated mostly on tool wear evolution or mechanism [1], [4]. For continuous hard turning the maximum tool wear land width (VB max) shows a near linear increase with cutting distance after initial rapid wear [3]. The same effect has also been noticed for interrupted hard turning [1]. On the other hand, tool life has been found to be very sensitive to both cutting speed and frequency of interruption [4]. More recent studies [5] highlight that the low-content CBN tool is superior to a high–content CBN tool in terms of tool wear and surface integrity for intermittent hard turning. The cutting force decreases with the increase of the cutting speed while the thrust cutting force increases significantly with the increase of tool wear. In terms of surface finish the roughness parameters taken into consideration are generally the average roughness R a and average maximum height of the profile R z [6], [7], [8]. Results come mostly from studies of continuous hard turning. According to some authors [9] the main factors that have a direct influence on surface quality are the corner radius of the cutting edge and the feed rate. Generally speaking, the greater the feed, the greater the value of R a ; however, this is still influenced by the occurrence of tool nose chipping or surface cratering [4]. When analyzing surface quality in hard turning, special attention was paid to metallurgical alterations in the superficial layer. Most common for hard turning is the occurrence of a white layer. White …