Transcript QE 107 Workshop Technology Section 4: Metal cutting

QE 107: Workshop
Technology
Section 4: Metal Cutting
Asanga Ratnaweera
Department of Mechanical Engineering
University of Peradeniya
Peradeniya
7/17/2015
ACR/QE107/SEC4
Material Removal Processes
7/17/2015
ACR/QE107/SEC4
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.

7/17/2015
ACR/QE107/SEC4
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.
7/17/2015
ACR/QE107/SEC4
Chip Types

Continuous chips

Ductile material




7/17/2015
Ex: Mild steel, Al
High speed
Low feed
Small depth of cut
ACR/QE107/SEC4
Chip Types

Discontinuous chips
 Brittle material

Ex: Cast iron
Low speed
 High feed
 Large depth of cut

7/17/2015
ACR/QE107/SEC4
Chip Types
7/17/2015
ACR/QE107/SEC4
Chip Types

Built Up Edge (BUE)



7/17/2015
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.
ACR/QE107/SEC4
BUE
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)

7/17/2015
ACR/QE107/SEC4
BUE
Tool Geometry

The shape and orientation of the cutting tool greatly
affects the chip formation mechanics
Rake
Angle 
tc
2
to
Clearance Angle
1

3
Shear Angle
7/17/2015
ACR/QE107/SEC4

Rake Angle

Of particular importance is the rake angle that the tool
makes with the workpiece normal
Positive Rake
Cutter
Velocity
Neutral Rake
Cutter
Velocity
Negative Rake
Cutter
Velocity
-
+
0
Workpiece
Normal

Workpiece
Normal
Effects on the chip formation
7/17/2015
ACR/QE107/SEC4
Workpiece
Normal
Clearance angle

Clearance angle is that the tool makes with the newly machined
work surface
Rake
Angle 
tc
2
to
Clearance Angle
1

3

Shear Angle

Avoids the work-surface being rubbed by the tool
7/17/2015
ACR/QE107/SEC4
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.
7/17/2015
ACR/QE107/SEC4
Cutting Tools

There are basically two types of
cutting tools:


Single point (e.g. turning tools).
Multiple point (e.g. milling tools).
turning tools
7/17/2015
milling tools
ACR/QE107/SEC4
drilling tools
Cutting Tools
7/17/2015
ACR/QE107/SEC4
Geometry of turning tool
7/17/2015
ACR/QE107/SEC4
Turning operations
7/17/2015
ACR/QE107/SEC4
Milling

Work is fed into the rotating
cutter


Cutting tools for this process
are called milling cutters.


Typically uses a multi-tooth
cutter.
Capable of high material
removal rate
Two basic milling operations:


7/17/2015
Peripheral milling
Face milling
ACR/QE107/SEC4
Peripheral Milling

Peripheral milling is also called
plain milling


7/17/2015
axis of the tool parallel to the
surface,
cutting operation is performed
by cutting edges on the outside
periphery of the cutter.
ACR/QE107/SEC4
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
7/17/2015
ACR/QE107/SEC4
7/17/2015
ACR/QE107/SEC4
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
7/17/2015
ACR/QE107/SEC4
Geometry of milling tools
7/17/2015
ACR/QE107/SEC4
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

7/17/2015
ACR/QE107/SEC4
Chip Thickness
f = v / Nn
7/17/2015
ACR/QE107/SEC4
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:

7/17/2015
Drilling, milling, EDM, etc.
ACR/QE107/SEC4
Geometry of drilling tools
Cutting edges
7/17/2015
ACR/QE107/SEC4
Rake angle
Drill sectioned along the cutting edge showing
change of rake due to flute form. Rake angle
changes from negative to positive when radius
increases
7/17/2015
ACR/QE107/SEC4
Rake angle
7/17/2015
ACR/QE107/SEC4
Chip thickness
7/17/2015
ACR/QE107/SEC4
The cutting force
 Externally applied forces
P
Fc
Fc – Cutting force, Ft – Thrust force
7/17/2015
Ft
ACR/QE107/SEC4
The cutting force
 Forces on the tool
Fn
F
F – Friction force, Fn– Normal force
7/17/2015
ACR/QE107/SEC4
The cutting force
 Forces on the chip
Fs
Fco
Fs – Shear force, Fco– Compressive force
7/17/2015
ACR/QE107/SEC4
The cutting force
 Merchant’s theory
Fs
Fco
R
λ – friction angle
7/17/2015
λ
α
λ -α F
c
α Ft
F
Fn
Merchant’s Circle
ACR/QE107/SEC4