QE 107: Workshop Technology Section 4: Metal Cutting

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QE 107 Workshop TechnologySection 4 Metal
Cutting

• Asanga Ratnaweera
• Department of Mechanical Engineering
• University of Peradeniya
• Peradeniya

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Material Removal Processes
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Basic Cutting Principle

• The basic principle is the use of a cutting tool
  to form a chip removed from the part (by shear).
• It requires the relative motion between the tool
  and part.
• The primary motion is called speed, v, and the
  secondary motion is called feed, f.
• The cutting tool needs to cut into the part,
  called the depth of cut, d.

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Cutting Mechanism

• During machining, the material is removed in form
  of chips, which are generated by shear
  deformation along a plane called the shear plane.

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Chip Types

• Continuous chips
• Ductile material
• Ex Mild steel, Al
• High speed
• Low feed
• Small depth of cut

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Chip Types

• Discontinuous chips
• Brittle material
• Ex Cast iron
• Low speed
• High feed
• Large depth of cut

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Chip Types
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Chip Types

• Built Up Edge (BUE)
• Some of the cut material will attach to the
  cutting point.
• This tends to cause the cut to be deeper than the
  tip of the cutting tool and degrades surface
  finish.
• Also, periodically the built up edge will break
  off and remove some of the cutting tool. Thus,
  tool life is reduced.

BUE
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Chip Types

• Built Up Edge (BUE)
• built up edge can be reduced by
• Increasing cutting speed
• Decreasing feed rate
• Increasing rake angle
• Reducing friction (by applying cutting fluid)

BUE
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Tool Geometry

• The shape and orientation of the cutting tool
  greatly affects the chip formation mechanics

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Rake Angle

• Of particular importance is the rake angle that
  the tool makes with the workpiece normal
• Effects on the chip formation

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Clearance angle

• Clearance angle is that the tool makes with the
  newly machined work surface
• Avoids the work-surface being rubbed by the tool

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Basic Cutting Geometry

• Orthogonal cutting the cutting edge of the tool
  is straight and perpendicular to the direction of
  motion.
• Oblique cutting the tool edge is set at angle.

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Cutting Tools

• There are basically two types of cutting tools
• Single point (e.g. turning tools).
• Multiple point (e.g. milling tools).

turning tools
drilling tools
milling tools
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Cutting Tools
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Geometry of turning tool
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Turning operations
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Milling

• Work is fed into the rotating cutter
• Typically uses a multi-tooth cutter.
• Cutting tools for this process are called milling
  cutters.
• Capable of high material removal rate
• Two basic milling operations
• Peripheral milling
• Face milling

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Peripheral Milling

• Peripheral milling is also called plain milling
• axis of the tool parallel to the surface,
• cutting operation is performed by cutting edges
  on the outside periphery of the cutter.

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Face milling

• Axis of rotation perpendicular to workpiece
  surface
• The milled surface is flat and has no
  relationship to the contour of the cutter
• Combined cutting action of the side and face of
  the milling cutter

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Feed Direction

• Conventional milling (up milling)
• Most common method of feed
• Feed work against the rotation of the cutter
• Maximum chip thickness at end of cut
• Climb milling (down milling)
• Maximum chip thickness at beginning of cut
• Suited for machining of thin and hard to hold
  parts
• 20 less Hp than conventional milling
• Machine must be very rigid to safely

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Geometry of milling tools
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Chip Thickness

• f - is the feed per tooth
• d - is depth of cut
• N cutter speed
• D diameter of the cutter
• tc - undeformed chip
• thickness
• v - linear speed of the workpiece
• n number of teeth in the cutter

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Chip Thickness

• f v / Nn

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Drilling Operation

• Basic hole making processes
• account for approximately 50-70
• of all the metal removal processes
• utilized today.
• Hole making methods
• Casting
• Punching
• Flame cutting
• Machining including
• Drilling, milling, EDM, etc.

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Geometry of drilling tools
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Rake angle
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Rake angle
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Chip thickness
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The cutting force

• Externally applied forces

Fc Cutting force, Ft Thrust force
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The cutting force

• Forces on the tool

F Friction force, Fn Normal force
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The cutting force

• Forces on the chip

Fs
Fco
Fs Shear force, Fco Compressive force
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The cutting force

• Merchants theory

a
? -a
Fc
?
a
? friction angle