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Manufacturing Science 2

L1 : Introduction Manufacturing Science 2


Introduction

Machining : Removal of excess material from work piece with harder and pointed object (tool) due to relative motion between work-piece and tool

Workpiece

Tool Depth of Cut

Chip

Cut surface

Speed

Relative Motion: Motion responsible for cutting action Primary Motion - Cutting Motion(CM) Motion responsible for feeding the uncut portion Secondary Motion - Feed Motion (FM)


Commonly used machining Processes

Turning

Drilling



Turning

Drilling

Primary Motion - Cutting Motion(CM)

Rotation of work-piece Rotation of drill bit

Secondary Motion - Feed Motion (FM)

Linear movement of tool Linear motion of drill bit



Turning

Drilling


Linear movement of tool (Intermittent) Linear movement of tool


Linear movement of workpiece (Intermittent)

Linear movement of workpiece

Shaper Planner



Milling

Boring


Rotation of tool Rotation of workpiece


Linear movement of workpiece (continues)

Linear movement of tool


Concept of Directrix and Generatrix

Generatrix (G) : The line generated by the cutting motion Directrix (D) : The line generated by the feed motion Eg.: Generation of cylindrical surface Eg.: Interaction between G and D


Generatrix (G) : The line generated by the cutting motion Directrix (D) : The line generated by the feed motion Ways to generate Directrix and Generatrix (a) Tracing (Tr) – G and/or D is attained as a trace of path of a moving point (b) Forming (F) – G is simply the profile of the cutting edge



Ways to generate Directrix and Generatrix (contd.)

(c) Tangent Tracing (TTr) : Directrix due to tangent to the series of paths traced by the cutting edges (d) Generation (G) : G or D is obtained as an envelope being tangent to the instantaneous positions of a line or surface which is rolling on another surface.



Shaping

Planning

G-CM-W-Tr D-FM-T-Tr

G-CM-T-Tr D-FM-W-Tr

Tool-workpiece interaction


Q. Show the tool-work motions and the Generatrix and Directrix in drilling

G-CM-T-Tr D-FM-T-Tr



Milling

Boring

G-CM-W-Tr D-FM-T-Tr

G-x-T-F D-(FM+CM)-(W+T)-TTr



Milling

G-x-T-F

D-(FM+CM)-(W+T)-TTr




How is Cutting and feed Motion expressed in turning, facing, shaping, drilling and milling

Cutting Motion Feed motion

mm/min (rpm) mm/rev

mm/min (rpm)

mm/min

mm/min

mm/stroke

rpm mm/rev

rpm

mm/min



Mechanics of Machining Operation

Basic model of cutting operation (2D, Orthogonal)



Orthogonal : Relative velocity is perpendicular to the cutting edge Oblique : Relative velocity is not perpendicular to the cutting edge



Chip Formation : Different types of chips of various shape, size, colour etc. are produced by machining depending upon • type of cut, i.e., continuous (turning, boring etc.) or intermittent cut (milling) • work material (brittle or ductile etc.) • cutting tool geometry (rake, cutting angles etc.) • levels of the cutting velocity and feed (low, medium or high) • cutting fluid (type of fluid and method of application) Mechanisms involved in chip formation are • Yielding – generally for ductile materials • Brittle fracture – generally for brittle material



Chip Formation in ductile materials: Chips are produced by shearing ( Primary and Secondary ) Primary Shearing : Shearing of workpiece marital along the plane of maximum shear stress

Micrograph showing primary and secondary shearing



Chip Formation in ductile materials: Chips are produced by shearing ( Primary and Secondary ) Secondary Shearing : at the interface of tool and chip



Chip Formation in ductile materials: Chips are produced by shearing ( Primary and Secondary ) Secondary Shearing : at the interface of tool and chip Workpiece material adheres to tool surface, sliding motion of chip on rack phase dose not occur material first sticks and then shears


Shear Angle ø Rake angle ɑ Chip thickness ratio r=t1/t2

Chip velocity Vc

Cutting speed V Shearing velocity Vs


Shear strain model (Piispanen model – Applicable to ductile material)


Shear strain


Shear strain model (Piispanen model – Applicable to ductile material)



Chip Formation in brittle materials


Mechanics of Machining Operation Chip Formation :



Types of Chip Continuous Built-up edge (BUE) Segmented Discontinuous Continuous chip : • Usually formed with ductile material, small uncut thickness, high cutting speed,

large rake angle, suitable cutting fluid • Good surface finish; steady cutting forces; undesirable in automated machinery

(CNC) Built-up edge (BUE) : • Layers of Material from workpiece deposited on cutting edge of the tool • BUE are unstable, breaks when become larger • Part of BUE goes with chip and reaming deposited at the workpiece resulting in

rough surface • Higher the affinity of tool and workpiece material, the grater the tendency of

BUE formation • BUE occurs in alloys and not in pure metal ???? • BUE is some time desirable ?????



BUE hardness increases significantly due to hardening

BUE

Strain hardening (work hardening) – strengthening by cold-work (cold plastic deformation). Cold plastic deformation causes increase of concentration of dislocations, which mutually entangle one another, making further dislocation motion difficult and therefore resisting the deformation or increasing the metal strength. Grain size strengthening (hardening) – strengthening by grain refining. Grain boundaries serve as barriers to dislocations, raising the stress required to cause plastic deformation.

http://www.substech.com/dokuwiki/doku.php?id=plastic_deformation



http://www.substech.com/dokuwiki/doku.php?id=imperfections_of_crystal_structure

http://www.substech.com/dokuwiki/doku.php?id=grain_structure



Built-up edge (BUE) :

Continuous Chip • Without BUE work material – ductile Cutting velocity – high Feed – low Rake angle – positive and large Cutting fluid – both cooling and lubricating • With BUE work material – ductile cutting velocity – medium feed – medium or large cutting fluid – inadequate or absent

BUE



Jointed or segmented type or Serrated Chips – Chips with saw-tooth like appearance Generally appears with metal with low thermal conductivity and strength that decreases sharply with temperature eg Ti Conditions: - work material – semi-ductile - cutting velocity – low to medium - feed – medium to large - tool rake – negative - cutting fluid – absent

Low temperature, higher strength

lesser strain

High temperature, low strength more

strain



Discontinuous type • of irregular size and shape : - work material – brittle like grey cast iron • of regular size and shape : - work material ductile but hard and work hardenable - feed – large - tool rake – negative - cutting fluid – absent or inadequate

Segmented Chip

Discontinuous Chip

References:

Kalpakjian Chapter 20

Ghosh Mallik Chapter 4

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