Metal Cutting.pptx

Chapter :1 Mechanics of Metal Cutting

Introduction -Metal Cutting •

Metals are shaped in to usable forms through various processes. • No-cutting shaping: No chip formation takes place, and the metal is shaped under the action of heat, pressure or both. Ex: Forging, drawing , Spinning, Rolling, Extruding,etc. • Cutting shaping: The components are brought to the desired shape and size by removing the unwanted material from the parent metal in the form of chips through machining. Ex: Turning, Boring, Milling, Drilling, Shaping, Planning, Broaching, etc.

Mechanics of chip formation  A typical metal cutting process can be schematically represented as shown in fig (a), wedge shaped tool is made to move relative to the work piece.  As the tool makes contact with the metal it exerts a pressure on it resulting the compression of the metal near the tool tip. This induces shear type deformation with in the metal and starts moving upward along the top face of the tool.  As the tool advances the material ahead of it sheared continuously along a plane called the shear plane.  The cutting edge of the tool is formed by two interesting surfaces ,the surface along which the chip moves upwards is called “Rake surface” And the other surface which is relieved to avoid rubbing with the machined surface called “Flank Surface”

Basic Elements of Machining • The basic elements of machining operations are: 1. Work piece 2. Tool 3. Chip

Objectives of Metal Cutting • The ever increasing importance of machining operations is gaining new dimensions in the present industrial age. • The growing completion calls for all the efforts to be directed towards the economical manufacture of machined parts. • Basic objectives of the economical and efficient machining practice : 1. Quick Metal Removal (or MRR) 2. High class surface finish 3. Economy in tool cost 4. Less power consumption 5. Economy in the cost of replacement and sharpening of tools. 6. Minimum deal time of machine tools.

Basic Elements of Machining  For providing cutting action, a relative motion between the tool and work piece is necessary.  This relative motion can be provided by: 1. Either keeping the work piece stationary and moving the tool. Or 2. By keeping the tool stationary and moving the work. Or 3. By moving both in relation to one another.

Influence of Parameters on Machining  The work piece provides the parent metal, from which the unwanted metal is removed by the cutting action of the tool to obtain the predetermined shape and size of the component.  The chemical composition and physical properties of the metal of the workpiece have a significant effect on the machining operation.  The tool material and its geometry are equally significant for successful machining.  The type and geometry of the chip formed are greatly effected by the metal of the work piece, geometry of the cutting tool and the method of cutting.  The chemical composition and the rate of flow of the cutting fluid also provide considerable influence over the machining operation.

ORTHOGONAL AND OBLIQUE CUTTING

ORTHOGONAL AND OBLIQUE CUTTING Oblique Cutting

ORTHOGONAL AND OBLIQUE CUTTING

Cutting Models

Tool

workpiece

ORTHOGONAL GEOMETRY

Tool

workpiece

OBLIQUE GEOMETRY

Orthogonal Vs Oblique Orthogonal Cutting   The cutting edge of the tool remains normal to the direction of tool feed or work feed.  The direction of chip flow velocity is normal to the cutting  edge of the tool. (chip flow angle)  The angle of inclination ‘i’ of the cutting edge of the tool with the normal to the velocity Vc is  zero.  The angle between the direction of chip flow and the  normal to the cutting edge of the tool , measured in the plane of the tool face is zero.  The cutting edge is longer than  the width of the cut.

Oblique Cutting The cutting edge of the tool always remains inclined at an acute angle to the direction of tool feed or work feed. The direction of chip flow velocity is at an angle β with the normal to the cutting edge of the tool. (chip flow angle) The cutting edge of the tool is inclined at an ‘i’ with the normal to the direction of work feed or tool feed Vc . Three mutually perpendicular components of cutting forces act at the cutting edge of the tool. The cutting edge may or may not be is longer than the width of the cut.

Classification of Cutting Tools  The cutting tools used in metal cutting can be broadly classified as: 1. Single point tools : Those having only one cutting edge. Ex: Lathe tools, shaper tools, planer tools, boring tools, etc. 2. Multi-point tools: Those having more than one cutting edge. Ex: milling cutters, drills, broaches, grinding wheels, etc.  The cutting tools can be classified according to the motion as: 1. Linear motion tools: Ex: Lathe, boring, broaching, planing, shaping tools, etc. 2. Rotary Motion tools: Ex: milling cutters, grinding wheels, etc. 3. Linear and Rotary Motion tools: Ex: drills, honing tools, boring heads, etc.

Principal Angles of Single Point Tools

Principal Angles of Single Point Tools

Designation of cutting tools By designation or nomenclature of a cutting tool is meant the designstion of the shape of the cutting part of the tool. The systems to desgignate the tool shape ,which are widely used are:1. American Standards Association System(ASA) or American National Standards Institute(ANSI) 2. Orthogonal rake System(ORS) 3. BIS 4. DIN

Reference Planes  The following two systems of reference planes are used to describe the geometry and locate the different parameters of single point cutting tool.  The Coordinate System (ASA)  The Orthogonal System (ORS)

Reference Planes  The Coordinate System:  The tool being held in hand against a stationary work piece (Tool in Hand System).  Base plane : The horizontal plane which contains the base of the shank of the cutting tool.  Longitudinal plane : Vertical plane normal to the base plane and parallel to the direction of feed (f ) .  Transverse Plane : Plane perpendicular to the both the above reference planes and is parallel to the depth of cut (d ).  This combination of reference planes are known as Coordinate System of Reference Planes.

Tool Geometry in Coordinate System The Coordinate System:  Also called as ASA System of tool signature.  Because of the nomenclature of the reference planes X,Y,Z it also called as X-Y-Z Plane System.  Various tool angles shown in figure The order of representation of various parameters as: are : αy = Top Rake / Back Rake angle αx = Side Rake angle βy = End Relief / Clearance angle βx = Side Relief / Clearance angle Φe = End Cutting Edge Angle Φs = Side Cutting Edge Angle θ= Nose Angle

Back Rake , Side Rake, End Relief, Side Relief, End Cutting Edge, Side Cutting Edge, Nose Radius.  The values of Nose Radius θ will depends on the values of Φe and Φs  For Example: 8,14,6,6,6,15,1/8’’

 ASA

( American Standards Association) system

Tool Signature 

American System

 It defines the principal angles like side rake, back rake, nose, etc. with regarding to the cutting edge and with out any reference to their locations.  This system of nomenclature does not give any indication of the tool behavior with regard to the flow of chip during the cutting operation.  The three reference planes adopted for designating different tool angles are similar to conventional machine drawing.

Tool Signature American System For example a tool may designated in the following sequence: 8-14-6-6-6-15-1

1. Bake rake angle is 8 2. Side rake angle is 14 3. End relief angle is 6 4. Side relief angle is 6 5. End cutting Edge angle is 6 6. Side cutting Edge angle is 15 7. Nose radius is 1 mm

Reference Planes  The Orthogonal System:  This system assumed as the cutting tool is operating against the work piece.  Base Plane : Horizontal Plane contains the base of the cutting tool.

 Cutting plane : Plane which is perpendicular to the base plane contains the principal cutting edge (c).  Orthogonal Plane: third plane which is perpendicular to the both of the above planes.  This set of reference planes is known as Orthogonal System of reference planes.

 Planes

and axes of reference

Tool Geometry in Orthogonal System  The Orthogonal System:  Also called as Orthogonal Rake System (ORS) or International System.  Because of the nomenclature of the reference planes L,M,N it also called as LM-N Plane System.  Due to the cutting tool operating on the work piece, many tool parameters are variables in this system.  Their actual values are effected by the tool position with regarding to the work piece in actual operation.

Tool Geometry in Orthogonal System The Orthogonal System:  Various tool angles shown in figure are : Φ0 = Plane Approach angle Φ1 = Auxiliary cutting edge angle λ = Angle of Inclination α = Orthogonal Rake Angle The order of representation of only main γ = Side Relif Angle parameters as: β = Wedge Angle δ = Cutting Angle ( = γ + β ) Inclination , Orthogonal Rake , Side α1 = Side Rake Angle Relief, End Relief, Auxiliary Cutting, γ1 = End Relief Angle Approach , Nose Radius. β1 = Side Wedge Angle  For Example: 0,10,5,5,8,90,1

 πR

= Refernce plane perpendicular to the cutting velocity vector, CV  πC = cutting plane; plane perpendicular to πR and taken along the principal cutting edge  πO = Orthogonal plane; plane perpendicular to both πR and πC and the axes;  Xo = along the line of intersection of πR and πO  Yo = along the line of intersection of πR and πC  Zo = along the velocity vector, i.e., normal to both Xo and Yo axes.

Inter-Relationship Between ASA and ORS System

Chip Formation The fig. represents the shaping operation, where the work piece remains stationary and the tool advances in to the work piece towards left.  Thus the metal gets compressed very severely, causing shear stress.  This stress is maximum along the plane is called shear plane.  If the material of the workpiece is ductile, the material flows plastically along the shear plane, forming chip, which flows upwards along the face of the tool.

Chip Formation  The complete plastic deformation of the metal does not take place entirely along the shear plane only.  It actually occurs over a definite area PQRS.  The metal structure starts getting elongated along the line PQ below the shear plane and continues above the shear plane and continues up to the line RS where its deformation is completed.  The complete area PQRS is known as shear zone.  The shape of the shear zone is a wedge shape, with its thicker portion near the tool and the thinner one opposite to it.  This shape of shear zone is one of the reasons to curl the chip.  The produced chip is very hot and its safe disposal is very necessary.

Chip Formation The tool will cut or shear off the metal, provided by

(i) The tool is harder than the work metal, (ii) The tool is properly shaped so that its edge can be effective in cutting the metal, (iii) The tool is strong enough to resist cutting pressures but keen enough to sever the metal, and (iv) Provided there is movement of tool relative to the material or vice versa, so as to make cutting action possible.

Types of Chips The chips produced during machining can be broadly classified as three types. 1. Discontinuous or Segmental Chips 2. Continuous Chips 3. Continuous Chip with built-up edge

Types of Chips 1. Discontinuous or Segmental Chips  This type of chips produced during machining of brittle materials like cast iron and bronze.  These chips are produced in the form of small segments.  As the tool advances forward, the shear plane angle gradually reduces until the value of compressive stresses acting on the shear plane becomes too low to prevent rupture.  At this stage, any further advancement of the tool results in the fracture of the metal ahead of it, thus producing a segment of the chip.  With further advancement of the tool, the processes of metal fracture and production of chips segments go on being repeated, and this is how the discontinuous chips are produced.

Types of Chips 1. Discontinuous or Segmental Chips  These are also produced in machining of ductile materials when low cutting speeds are used adequate lubricant is not provided.  This causes excessive friction between the chip and tool face, leading to the fracture of the chip in to small segments.  This will also result in excessive wear on the tool and the poor surface finish on the work piece.  Other factors responsible : smaller rake angle, too much depth of cut.

Types of Chips 2. Continuous Chips  The basis of the production of the continuous chip is the continuous plastic deformation of the metal ahead of the tool, the chip moving smoothly up the tool face.  This type chip is produced while machining a ductile material, like mild steel, under favorable conditions, such as high cutting speeds and minimum friction between the chip and the tool face.  The friction between the chip-tool interface can be minimized by polishing the tool face and adequate use of coolant.  Other factors responsible : bigger rake angle, finer feed and keen cutting edge.

Types of Chips 3. Continuous Chips with built-up edge  While machining ductile material when high friction exists at the chip-tool interface results the continuous chips with built-up edge.  The normal reaction of the chip on the tool face is quite high.  It is maximum at the cutting edge or nose of the tool.  This gives rise to an extensively high temperature and compressed metal adjacent to the tool nose gets welded to it.  The chip is also sufficiently hot and gets oxidized as it comes off the tool and turns blue in colour.  The extra metal welded to the nose of the tool is called built-up edge.

Types of Chips 3. Continuous Chips with built-up edge  Metal in built-up edge is highly strain hardened and brittle.  During the chip flow up the tool, the builtup edge is broken and carried away with chip, rest of it bonded to the work piece and make it rough.  Due to the built-up edge the rake angle also altered and so is the cutting force.  Other factors responsible : low cutting speed, excessive feed, small rake angle, lack of lubricant.

Types of Chips 3. Continuous Chips with built-up edge  Adverse effects of built-up edge formation: • Rough surface finish. • Fluctuating cutting force, causing, vibrations in cutting tool. • Chances of carrying away some material from the tool by the built-up surface, producing crater on the tool face and causing tool wear.  Precautions to avoid built-up edge formation: • The coefficient of friction at the chiptool interface should be minimized by means of polishing the tool face. • Adequate supply of coolant. • Large rake angle. • High cutting speeds and low feeds.