Machining
Introduction to Metal cutting
AMAN WASIM -B201094ME
Over view
• What is metal cutting
• Mechanism of metal cutting
• Types of chips
• Heating zones.
• Failure and wear
Metal cutting
What is metal cutting?
It is the removal of materials from a metal in the form of chip so as to attain the desired
shape and size
Broadly classified as convectional and non-conventional
Conventional -removal by means of mechanical forces by using sharp cutting tool(Eg
-Turning , milling and drilling)
Non-conventional-All other techniques (Eg-Laser metal cutting ,Electron beam machining
etc)
Basic terminologies
• Cutting tool - It is a sharp , wedge shaped thing which removes the material. It should be harder
than the workpiece. Eg - Single point cutting tool
• Machine tool-Setup for rigidly holding tool-work piece pair and providing a relative motion
between them. Eg-Lathe, Milling machine , Shaping machine.
• Primary motion -The relative motion between the tool and workpiece responsible for cutting.
Also known as cutting motion.
• Secondary motion-The gradual feed of the uncut portion .Also known as feed motion.
• Generatrix-Line generated by cutting motion.
• Directrix-Line generated by feed motion.
Cutting tool-terminologies
• Rake surface -Over which chip flows
• Flank surface -Faces the finished portion.
• Rake angle-b/w chip and rake surface
Important parameters effecting cutting
• Uncut thickness
• Chip thickness
• Rake angle-Angle b/w rake surface and chip velocity.
• Relative velocity b/w workpiece and tool.
Cutting can be divided into two
• Orthogonal cutting -Angle b/w the relative velocity of work and cutting edge is 90
• Oblique cutting-If the angle is not 90.
Mechanism of cutting
• The material breaks if the applied shear force exceeds ultimate shear force
• For example-The mechanism of scissors
Mechanism of chip formation
Shear zones
• The uncut layer deforms into chip after it goes through severe plastic deformation in primary
shear zone.
• As the chip follows through the rake plane it sticks with tool , despite this it flows over it
causing some plastic deformation . This zone is known as secondary shear zone.
Types of chip formation
Continuous chip
• These are common when ductile materials are machined .Eg-Al,Cu,mild steel.
• Also factors like high cutting speed , large rake angle ,sharp cutting edge , efficient
cutting fluid and low friction increases the continuity .
• Advantages-Smooth surface, High tool life, Low power consumption
• Disadvantages-Risk of formation of built-up edge, difficulty handling and disposal of the
chip.
Discontinuous Chip
• Separate , plastically deformed segments which either loosely adhere to each other or
remain completely unconnected.
• They are produced when brittle material such as cast brass, cast iron etc are machined.
• Factors favour-low cutting speed ,high feed.
Built-up edge
• At high speed of chip flow over rake surface the temperature increases , result in
formation of lump on rake surface, This known as BUE.
• Further it breaks and fragments adhere to finished surface causing poor surface finish.
• Factors favour the formation of BUE-Large uncut thickness, low rake angle, strong
adhesion b/w the chip and w/p , low cutting speed , insufficient cutting fluid.
• Avoided by reducing friction by increasing rake angle of cutting tool
• Also by using efficient cutting fluid.
Heat generation and cutting tool temperature
• As for high strength material , plastically deformation generates heat , which raises the
temperature leading to the decrease in tool strength causing faster wear and failure.
• Major deformation occur at primary shear zone and this heat source is known as primary heat
source(75-80%)
• Sliding motion of chip on rake surface also generate heat , this source is known as secondary
heat zone.(15-20%)
• Chips carry the major portion of these heat generated(70-80%)
• Workpiece-(10-15%)
Relations
• Rise in temperature due to primary heat
source is given by
• Rise in temperature due secondary heat
source
Failure of cutting tool and tool wear
Mainly three modes of failure
• Plastic deformation - Due to high temperature and large stress.
• Mechanical breakage-Due large forces and insufficient strength and toughness.
• Blunting -Caused due to a process of gradual wear.
The first two can be completely avoided in first instance by proper analysis of forces and
temperature and selecting suitable material and geometry
Blending
Mainly on two surface-where relative sliding occur
• On rake surface where chip flows, known as crater wear.
• On flank surface where rubbing of workpiece and tool occur known as flank wear.
Measuring the wear on these surfaces
•On flank surface (hf)
•On Rake surface it’s complex and given by
an non-dimensionless number
Graphs of wears
At various speed
