CHIP SEGMENTATION EFFECT ON CUTTING FORCES VARIATION S. Belhadi, M.A. Yallese, K. Bouacha and N. Kribes LMS Laboratory, Guelma university, B.P.401, Guelma 24000, Algeria belhadi23 @yahoo.fr L. Boulanouar LR3MI Laboratory, Badji Mokhtar University, BP 12, Annaba 23000, Algeria. Abstract This work is a contribution to the study of the physical phenomena which accompany the saw tooth chip formation, in the case of hard turning. For that we present an experimental study which will enable us to partly clarify chip genesis in hard turning. Indeed the study relates to the machining of hardened steel 35NCD16 with 52 HRC, by a CBN tool, under conditions close to the orthogonal cutting. The basic idea is to establish a relation between saw tooth chip morphology characterized by geometrical measurements and cutting forces signals, obtained from their high frequency measurements, these signals will be examined in the frequential field. 1. Introduction During many years, the progress of metals machining by cutting tool is accompanied by many new problems. The general aim of researchers is always the improvement of metal removal. However, there are many constraints other than machine power and torque limitations, such as qualification criteria like roughness and surface integrity that must be taken into account during cutting. This yields to a good choice of the working parameters. It is not always easy to find these parameters and consequently control the process. This is due to the complexity of coexisting different physical phenomena such as mechanical, thermal and metallurgical ones during tool and workpiece interaction. An example which illustrates the necessity to choose, in an optimal way, these parameters is that of hard turning (HT). Indeed, the latter process has been applied in many industrial cases and is up to now not yet controlled. Frequently, it is used in manufacturing bearings, shafts, gears, cams and other mechanical components. According to Guo & al. [1] HT has a significant difference when compared to his competitive process: grinding. This is due to that HT may induce an equivalent or better surface finish, form, and workpiece size accuracy than grinding. The present work is focused on genesis comprehension of saw-tooth chip resulting from turning of hardened 35NCD16. The methodology is based on acquisition of chip segmentation frequencies according to different cutting speeds and feed rates. 2. Saw-tooth chip formation For scientific reasons and in order to obtain a physical comprehension of saw-tooth chip formation, it seems to be essential to explain the manner in which this kind of chip is formed. For reasons of commodity, the authors proposed to study saw-tooth chip formation in a plane perpendicular to the cutting edge (Fig. 1.) The latter summarizes the T1 tooth formation cycle. The initial step is characterized by a compression and an upsetting of the new teeth (C0B0B1D1), where two zones are noted: A and B. Zone A is near the tool tip S and is subjected to high level stresses. Consequently, this location is wedged between the tool rake face and the limit (SC0). As the cutting time increases, wedged material has a tendency to escape to the free side (C0D1). According to Konig et al. [2], this stage of T1 deformation can be accompanied by an incipient crack beginning at C0. Moreover, Barry and Byrne [3] presented a photograph of a quick-stop which shows high-level deformations according to the C1D1 line. Metal will be pushed back towards zone B far from the tool tip, which causes a hump on the free face C0D1 and a slipping statement of T1 following narrower bands schematized by planes BiCi . The angle between this shearing plane and the cutting speed direction passes from φ0 to φ 1. In this cycle, a great amount of inelastic work is transformed into heat. This state will cause a thermal softening of material located in the primary deformation zone. The thermal softening phenomenon is pronounced as the thermal characteristics of hardened materials are poor [4–6]. Finally, the succession