CHIP FORMATION MECHANISM

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1 LEARNING OBJECTIVES CHIP FORMATION MECHANISM Different type of chips formation Mechanism of chip formation Problems on chip formation. Introduction to orthogonal and oblique cutting History of Metal cutting research Early research in metal cutting started with in Cocquihat his work mainly directed towards measuring the work required to remove a given volume of material in drilling Hartig presented work required in cutting metal 1870 Time made first attempt to explain how chips are formed Tresca explained how chips are formed 1881 Mallock suggested correctly that cutting process is basically one of shearing of Workpiece to form the chip. He emphasized the importance of effect of friction He also explained reasons for instability of cutting process- chatter 1900 Finnie reports a step backward in understanding of metal cutting process- He and Reuleaux believed that a crack occurred ahead of tool ( like splitting of wood) and is a misconception that found popular support for many years Taylor reported the results of 26 years of research - he was interested in application of piece work system in machine shop, where a time allowance was set for a particular job and bonus was given to workman performing his task in allotted time. To assist in the application of such system Taylor investigated the effect of tool material and cutting condition on tool life during machining operation. He was able to increase production of Bethlehem steel company by He also made fundamental discovery regarding temperature of tool cutting edge controlled by tool wear rate Ernest and Merchant published most of the fundamental work on metal cutting Compiled by: Jagadeesha T, Assistant Professor, MED, National Institute of Technology, Calicut 1

2 a) b) Fig.1: Models of cutting process- a) present day model b) earlier misconception CHIP FORMATION Machining is a process of gradual removal of excess material from the preformed blanks in the form of chips. The form of machined chips depend mainly upon Work material Material and geometry of the cutting tool Levels of cutting velocity and feed and also to some extent on depth of cut Temperature and friction at the chip-tool and work-tool interfaces. Cutting fluid flow, type, velocity Knowledge of basic mechanism(s) of chip formation helps to understand the characteristics of chips and to attain favourable chip forms. Mechanism of chip formation in machining ductile materials During continuous machining the uncut layer of the work material just ahead of the cutting tool is subjected to compression. Due to such compression, shear stress develops, If shear stress reaches or exceeds the shear strength of that work material in the deformation region, yielding or slip takes place resulting shear deformation in that region Fig.2: Mechanism of chip formation in ductile material. Compiled by: Jagadeesha T, Assistant Professor, MED, National Institute of Technology, Calicut 2

3 Continuous chips are formed in machining ductile material such as mild steel, wrought iron, copper and aluminum. Basically this operation is one of shearing the work material to form the chip and sliding of the chip along the face of the cutting tool. The formation of chip takes place in zone extending from the tool cutting edge to the junction between the surfaces of the chip and workpiece: This zone is known as primary deformation zone. To deform the material in this manner the forces must be transmitted to chip across the interface between the chip and tool are sufficient to deform the lower layers of chip as it slides along the tool face (secondary deformation zone) Mechanism of chip formation in machining brittle materials The basic two mechanisms involved in chip formation are Yielding generally for ductile materials Brittle fracture generally for brittle materials During machining, first a small crack develops at the tool tip as shown in Fig. 3 due to wedging action of the cutting edge. At the sharp crack-tip stress concentration takes place. In case of ductile materials immediately yielding takes place at the crack-tip and reduces the effect of stress concentration and prevents its propagation as crack. But in case of brittle materials the initiated crack quickly propagates, under stressing action, and total separation takes place from the parent workpiece through the minimum resistance path Fig.3: Mechanism of chip formation in brittle material During the formation of a chip the material undergoes severe strain and fracture will occur in primary deformation zone when the chip is only partly formed. Under these conditions the chip is segmented and the condition is referred to as discontinuous chip formation. Discontinuous chips are produced when machining such materials as cast Compiled by: Jagadeesha T, Assistant Professor, MED, National Institute of Technology, Calicut 3

4 iron or cast brass but may also be produced when machining ductile material at very low speeds and high feeds. Mechanism of Built-up-Edge (BUE) In machining ductile metals like steels with long chip-tool contact length, lot of stress and temperature develops in the secondary deformation zone at the chip-tool interface. Under such high stress and temperature in between two clean surfaces of metals, strong bonding may locally take place due to adhesion similar to welding. The presence of this welded material increase the friction and temperature between chip and tool interface and leads to building of layer upon layer of chip material. This resulting pile of material is referred to as build up edge. (BUE). Often the build up edge continues to grow and then breaks down when it becomes unstable, the broken pieces being carried away by the underside of the chip an new workpiece surface. A study of build up edge formation in metal cutting is most important as it is one of the main factor affecting surface finish and can have a considerable influence on cutting tool wear. Effects of BUE formation Formation of BUE causes several harmful effects, such as: changes the rake angle at the tool tip causing increase in cutting forces and power consumption Repeated formation and dislodgement of the BUE causes fluctuation in cutting forces and thus induces vibration which is harmful for the tool, job and the machine tool. Surface finish gets deteriorated May reduce tool life by accelerating tool-wear at its rake surface by adhesion and flaking The basic major types of chips and conditions under which such chips are form are given in Table 1 and Figure 4.0 Compiled by: Jagadeesha T, Assistant Professor, MED, National Institute of Technology, Calicut 4

5 Table 1. The basic major types of chips & conditions of their formation. Figure 4 Three basic types of chips. Compiled by: Jagadeesha T, Assistant Professor, MED, National Institute of Technology, Calicut 5

6 ORTHOGONAL AND OBLIQUE CUTTING. All metal cutting operations involve a wedge shaped tool with a straight cutting edge, having a relative velocity with reference to the workpiece, contributing to chip formation. For analytical purpose, cutting operations are classified as Oblique cutting and orthogonal cutting. Oblique cutting is the most common type, normally found in several machining operations. The cutting edge is inclined to the normal to cutting velocity vector by an angle called the inclination angle. This type of cutting is also called three dimensional cutting due to forces raised in three directions perpendicular to one another. Orthogonal cutting is a special kind of cutting where the cutting edge is normal to cutting velocity vector. Being a two dimensional problem in view of forces in two perpendicular directions, where the number independent variables are reduced, this type of cutting is used for research purposes. ORTHOGONAL OBLIQUE CUTTING. The cutting edge of the tool remains normal to the direction of tool feed or work feed The direction of the chip flow velocity is normal to the cutting edge of the tool Here only two components of the forces are acting: cuttting force and thrust force. So the metal cutting may be considered as two dimensional cutting. The cutting edge of the tool remains inclined at an acute anle to the direction of tool feed or work feed The direction of the chip flow velocity is at angle with the normal to cutting edge of the tool. The angle is known as chip flow angle. Here three components of forces are acting : cutting force, radial for ce and thrust force. So metal cutting may be considered as three dimensional cutting. The cutting edge being oblique, the shear force acts on a larger area and tool life is increased. Compiled by: Jagadeesha T, Assistant Professor, MED, National Institute of Technology, Calicut 6

7 The role of inclination angle, λ on the direction of chip flow Figure 5 The role of inclination angle on the direction of chip flow. Figure 6 Pure orthogonal cutting. Compiled by: Jagadeesha T, Assistant Professor, MED, National Institute of Technology, Calicut 7

8 GEOMETRY AND CHARACTERISTICS OF CHIP FORMS The geometry of the chips being formed at the cutting zone follow a particular pattern especially in machining ductile materials. The major section of the engineering materials being machined are ductile in nature, even some semi-ductile or semi-brittle materials behave ductile under the compressive forces at the cutting zone during machining. The pattern and degree of deformation during chip formation are quantitatively assessed and expressed by some factors, the values of which indicate about the forces and energy required for a particular machining work. Assumption : No contact at flank, i.e. the tool is perfectly sharp No side flow of chips, i.e., width of the chips remains constant Uniform cutting velocity A continuous chip is produced with no BUE The chip is considered to be held in equilibrium by the action of the two equal and opposite resultant forces and assume that resultant is collinear. Figure 7 Pure orthogonal cutting. Chip reduction coefficient or cutting ratio The usual geometrical features of formation of continuous chips are schematically shown in Fig. 7. The chip thickness usually becomes larger than the uncut chip thickness The reason can be attributed to compression of the chip ahead of the tool frictional resistance to chip flow lamellar sliding according to Piispannen Compiled by: Jagadeesha T, Assistant Professor, MED, National Institute of Technology, Calicut 8

9 Mathematical expressions for Dynamic shear strain. Problems on chip formation. Compiled by: Jagadeesha T, Assistant Professor, MED, National Institute of Technology, Calicut 9