8/19/2019 1 Fundamentals of Machining
1/29
1995
UMY
UniversitasMuhammadiyah
Yogyakarta
www.umy.ac.id
Teknik Pemesinan (MEEMN03)#1 Fundamentals of Machining
Tutik Sriani, PhD.
8/19/2019 1 Fundamentals of Machining
2/29
1995
Preface
Machining is a general term describing a group of processes that
consist of the removal of material and modification of the surfaces of
a workpiece after it has been produced by various methods. Thus,
machining involves secondary and finishing operations.
Material removal processes have the following disadvantages:
– Waste material
– Processes generally longer than other processes
– Require more energy than forming/shaping operations
– May have adverse effects on the surface quality and properties of the
product.
8/19/2019 1 Fundamentals of Machining
3/29
1995
Ex: Steps in the
machining of a
crankshaft
A. Preparation
• Cutting and centering
• Internal and external profile milling of the
pin of the bearing and connection rod
bearing pin
• Turning of the main bearing pins and theend parts
• Deburring
• Solid carbide deep hole drilling
B. Hardening (induction hardening)
C. Finishing
• Hard turning
• Threading
• Chain wheel broaching
• CBN grinding
• Fluorescent rupture test• Dynamic balancing
• Finishing (polishing and lapping)
D.Quality test
The raw crankshaft forging (right) and the finished
machined part (left)
8/19/2019 1 Fundamentals of Machining
4/29
1995
Objectives
At the end of the class, students understand about:
Basic concepts relevant to all machining operations
Mechanics of chip formation
Calculation of force and power in machining
Temperature rise during cutting
Mechanism of tool wear and other form of wear
Surface finish and surface integrity
8/19/2019 1 Fundamentals of Machining
5/29
1995
Cutting Processes
Remove material from the surface of a workpiece by producing chips
8/19/2019 1 Fundamentals of Machining
6/29
1995
Cutting Features
Schematic illustration of turning process
Feed rate: the distance the tool travels horizontally per unit revolution of the workpiece (mm/rev)
8/19/2019 1 Fundamentals of Machining
7/29
1995
Cutting Features
Schematic illustration of 2D cutting process (orthogonal cutting). Note that the
tool shape and its angles, depth of cut, and cutting speed are all independent
variables
8/19/2019 1 Fundamentals of Machining
8/29
1995
Factors influencing cutting processes:
Parameter Influence, Interrelationship
Cutting speed V, to,
feed n, cutting fluids
Forces, power, temp. rise, tool life, type of chip, surface finish
Tool angles As above, + chip flow direction, resistance to tool wear
Continuous chip Good surface finish, steady cut force, undesirable esp. inautomated machinery
Discontinuous chip Desirable for ease of chip disposal, fluctuating cutting forces,
can affect SF, vibration & chatter
Temperature rise Tool life
crater wear, dimensional accuracy of workpieceTool wear Surface finish, dimensional accuracy, temp. rise, forces and
power
Machinability Related to tool life, SF, forces, power, type of chip
8/19/2019 1 Fundamentals of Machining
9/29
1995
Got unacceptable results?
Typical investigation:
1. SF workpiece is poor/unacceptable?
2. Cutting tool wears rapidly/become dull?
3. The workpiece becomes very hot?
4. The tool begins to chatter/vibrate?
5. Etc..
8/19/2019 1 Fundamentals of Machining
10/29
1995
Mechanics of Chip Formation = 10− to 10− mm
Velocity diagram in the cutting zone
Schematic illustration of the basic mechanism of chip
formation in metal cutting
8/19/2019 1 Fundamentals of Machining
11/29
1995
Mechanics of Chip Formation
1. Cutting ratio : ratio of depth of cut to the chip thickness
=
=sin∅
cos (∅ ∝)
r = cutting ratio
to = depth of cut
tc = chip thickness
Φ = shear angle (at workpiece)α = rake angle (at tool)
r : parameter to evaluate
cutting conditions, r < 1
tan =cos
1 s i n
8/19/2019 1 Fundamentals of Machining
12/29
1995
Mechanics of Chip Formation
2. Shear strain
Hence:
Ф force & power requirements, t c & T
α« Ф« t c » » energy dissipation » T»
=
=
+
= + tan( )
Large shear strain:
low shear angle Ф
low/negative rake angle α
8/19/2019 1 Fundamentals of Machining
13/29
1995
Mechanics of Chip Formation
3. Velocities in the cutting zone
Vc : chips velocity
V : cutting speed
Vs: shearing velocity
From the velocity diagram:
cos( )=
=
=
=
8/19/2019 1 Fundamentals of Machining
14/29
1995
Types of Chips (Metal Cutting)
a. Continuous
- ductile materials
- V » and/or α »
- Good SF
- Not desirable for computer-controlled machine tools to clear away the chips
- Chip breaker, or change V, n, t o, use cutting fluids
8/19/2019 1 Fundamentals of Machining
15/29
1995
Types of Chips (Metal Cutting)
b. Built-up edge (BUE)
- Gradually deposited at tool-tip built-up
- Destructive change geometry
of the cutting edge and dulls it
- Happens to cold-worked metals
(non-hardened/annealed)
To reduce BUE:
• V » or t o« or α »• Use sharp tool
• Effective cutting fluid
8/19/2019 1 Fundamentals of Machining
16/29
1995
Types of Chips (Metal Cutting)
c. Serrated/Segmented/Nonhomogeneous
Semicontinuous chips
Sawtooth appearance
Metals with low thermal conductivity
8/19/2019 1 Fundamentals of Machining
17/29
1995
Types of Chips (Metal Cutting)
d. Discontinuous, formed due to:
• Brittle workpiece, or wp contains hard inclusions/impurities (ex:
graphite flakes in gray-cast iron)
•
Very low/very high V• t o» or α «
• Ineffective cutting fluid
• Low stiffness of toolholdervibration
8/19/2019 1 Fundamentals of Machining
18/29
1995
Cutting Forces and Power
Data on cutting forces is important
1. Design machine tool to minimize distortion of machinecomponents
2. Maintain the dimensional accuracy of the machined parts3. Help select appropriate toolholder/workholding devices
4. The workpiece to withstand these forces without excessivedistortion
Power must be known to select machine tool with adequateelectric power
8/19/2019 1 Fundamentals of Machining
19/29
1995
Cutting Forces
Fc: cutting force
Ft: thrust force
R: resultant force
F: friction forceN: normal friction force
Fs: shear force
Fn: normal force
β: friction angle
Ф: shear angle
8/19/2019 1 Fundamentals of Machining
20/29
1995
Cutting Forces
Fc in the direction of V , supplies the energy required for cutting. Always (+). Fc & Ft produces Resultant force R
Ft is important: to avoid tool deflections
Coefficient of friction:
Shear force:
Normal force:
= and =
= sin( ) and = tan( )
=
= +
=
= +
8/19/2019 1 Fundamentals of Machining
21/29
1995
Power
• Product of force and velocity
• Many factors involved based on experimental data
=
S
p e c i f i c E n e r g y o f C u t t i n
g
8/19/2019 1 Fundamentals of Machining
22/29
1995
Sample
Pada sebuah pemotongan orthogonal, diketahui dalam
pemakanan = 0.13 mm, cutting speed = 120m/min, sudut
antara tool dengan tatal = 10º, dan lebar pemakanan = 6 mm.
Setelah dilakukan pengukuran, ditemukan lebar tatal = 0.23mm, cutting force = 500N dan thrust force = 200N. Hitunglah
berapa persen dari total energi yang habis untuk mengatasi
friksi pada persinggungan tool dan tatal.
Diketahui??
8/19/2019 1 Fundamentals of Machining
23/29
1995
Answer
Given: t o = 0.13 mm, V = 120m/min, α =10º, t c = 0.23 mm, Fc =
500N, Ft = 200N
Asked: percentage of energy for friction!
Solution:
Percentage of energy =
=
×
× =
×
=
=0.13
0.23= 0.565
= →
= →
= + = 539
500 = 539 cos 10 → = 32°
= 53932° = 286
= ×
=
286 × 0.565
500
= 0.32 32%
8/19/2019 1 Fundamentals of Machining
24/29
1995
Temperatures in Cutting
•Excessive Tlowers tool strength, soften, alters shape
• Uneven cut difficult to control accuracy & tolerances
• Induce thermal and metallurgical damage
8/19/2019 1 Fundamentals of Machining
25/29
1995
Tool Life: Wear & Failure(a) Flank and crater wear in a cutting tool. Tool moves to the left. (b) View of the rake face of a turning tool,
showing nose radius R and crater wear pattern on the rake face of the tool. (c) View of the flank face of aturning tool, showing the average flank wear and VB and the depth-of-cut line (wear-notch). (d) Crater wear
on a carbide tool. (e) Flank wear on a carbide tool.
8/19/2019 1 Fundamentals of Machining
26/29
1995
Examples of Wear and Tool Failures
(a) Schematic illustrations od types of wear observed on various types of cutting tools
8/19/2019 1 Fundamentals of Machining
27/29
1995
Examples of Wear and Tool Failures
(b) Schematic illustrations of catastrophic tool failures
8/19/2019 1 Fundamentals of Machining
28/29
1995
Surface Finish & Integrity
• SF: geometric features of a surface
• Integrity: pertains to mat. prop., ex: fatigue, corrosion
resistance
Surfaces produced on steel by cutting, as observed with a scanning electron microscope:
(a) turned surface and (b) surface produced by shaping
8/19/2019 1 Fundamentals of Machining
29/29
1995
Machinability
Good machinability:
• Good SF and surface integrity of the machined part
• Long tool life
• Low force and power requirement
• The level of difficulty in chip control