ENB331 Materials and Manufacturing 2 Week 2.docx

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Prasad Yarlagadda

ENB331 Lecture Materials and Manufacturing 2 Mechanics of Metal Cutting Material Removal Processes A family of shaping operations wherein material is removed from a starting work piece to impart the final desired geometry Machining Involves material removal by a sharp cutting tool (turning, milling drilling) Abrasive Processes Involve material removal by hard, abrasive particles (grinding) Nontraditional Processes Various energy forms other than sharp cutting tools to remove material Cutting actions involves shear deformation of work material to form a chip which is subsequently separated from the work. As chip is removed, new surface is exposed. Advantages Disadvantages Variety of part shapes and special geometric features Wasteful of material (scrap) possible such as  Chips generated in machining are wasted  Very straight edges and surfaces material, at least in the unit operation.  Accurate generation of other geometric features Time consuming (e.g. round holes and chamfers etc)  A machining operation generally takes more  Screw threads time to shape a given part than alternative Good dimensional accuracy and surface finish shaping processes such as casting, rolling or forming.  Roughness value (i.e. Ra) less than 0.4 micron) Variety of work materials can be machined  Most frequently used to cut metals and alloys. Machining operations Primary Machining Operations Secondary Machining Operations Turning Shaping and planning Drilling Broaching Milling Sawing Reaming Tapping Honing Trepanning Boring 1 Week 2 Tuesday, 27 August 2013 ENB331 Lecture Materials and Manufacturing 2 Turning – accomplished by a single point cutting tool removes material from a rotating workpiece to form a cylinder shape with one cutting edge Drilling – a rotating tool (drill bit) with two cutting edges is used to create round holes in the work material with progressive feed, feed motion collinear with axis of rotation. Milling – Rotating multiple-cutting-edge tool is moved across work to cut a plane or straight surface, feed motion perpendicular to axis of rotation 1. Peripheral milling (new surface parallel with rotation axis) and 2. Face Milling (new surface perpendicular with rotation axis) Machine Tools A power-driven machine that performs a machining operation 1. Holds workpart including grinding. Term is also applied to machines in metal 2. Controls feed and depth of cut forming operations, plays role in accuracy and finish quality 3. Provides power at speed Cutting Tools Single-point Tools  One dominant cutting edge  Point is usually rounded to form a nose radius  Turning uses single point tools Multiple Cutting Edge tools  More than one cutting edge  Motion relative to work achieved by rotating  Drilling and milling use rotating multiple cutting edge tools Cutting conditions in machining Primary Motion Cutting speed (v) For certain operations, material Secondary Motion Feed (f) removal rate can be computed as Penetration Depth of cut (d) 3 MRR = (vfd) mm /s 2 Week 2 Tuesday, 27 August 2013 ENB331 Lecture Materials and Manufacturing 2 Cutting conditions for turning Machining in manufacturing sequence Machining usually succeeds other manufacturing processes such as casting, forging and bar drawing.  These other processes create the general shape of the starting workpiece  Machining provides the final shape, dimensions, finish and special geometric details that other processes cannot create Roughing vs Finishing Roughing – removes large amounts of material from Finshing – completes part geometry starting workpiece.  Creates shape close to desired geometry, but  Final dimensions, tolerances and finish leaves some material for finish cutting  Low feeds and depths, high cutting speeds  High feeds and depths, low speeds Orthogonal cutting model  Simplified 2-D model of machining that describes the mechaics of machining fairly accurately  Wedge shaped tool whose cutting edge is perpendicular to the direction of cutting speed  Tool force into the material forms a chip due to shear deformation along a shear plane  Shear plane is oriented at an angle φ with the work surface  Material deforms plastically along a shear plane where the bulk of the cutting energy is consumed. Geometric parameters in Orthogonal cutting Two dimensional model  Chip thickness ratio (r0tc/t )  Rake angle (α)  Shear angle (φ) 3 Week 2 Tuesday, 27 August 2013 ENB331 Lecture Materials and Manufacturing 2 Orthogonal Model Actual Shear Zones Chip Thickness Ratio (r) At the beginning of a cut, the cutting edge of the tool is The ratio of 0 to c is called the Chip thickness ratio (r). positioned at a depth below the original work surface. Since tcis always greater than t0, <1.0 This depth is defined as the chip thickness before chip formation t 0 As the chip forms along the shear plane, its thickness increases to tc Determining Shear Plane Angle t l sin 0 s t l cos( ) c s t l sin sin r  0  s  c lscos( ) cos( )  tan  rcos 1rsin 4 Week 2 Tuesday, 27 August 2013 ENB331 Lecture Materials and Manufacturing 2 Shear Strain in Chip Formation Actual Chip formation 3 important mechanisms must be pointed out to 1. Size of the deformation zone distinguish the actual chip formation from the 2. Secondary Shear zone orthogonal cutting model 3. Type of chip formation Size of Shear Deformation  Shear deformation occurs within a zone as opposed to a plane  For shearing to occur across a plane of zero thickness, it must be instantaneous  A realistic model would predict shearing to occur in a very thin zone Experiments indicate that the shear zone is in fact only
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