1 fundamentals of machining

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.


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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.


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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)


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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


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1995

Cutting Processes

Remove material from the surface of a workpiece by producing chips


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Cutting Features

Schematic illustration of turning process

Feed rate: the distance the tool travels horizontally per unit revolution of the workpiece (mm/rev)


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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


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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


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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..


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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


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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


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1995


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


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1995


3. Velocities in the cutting zone

Vc : chips velocity

V : cutting speed

Vs: shearing velocity

From the velocity diagram:

cos( )=

=

=

=


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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


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1995


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


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1995


c. Serrated/Segmented/Nonhomogeneous

Semicontinuous chips

Sawtooth appearance

Metals with low thermal conductivity


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1995


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


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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


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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


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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( )

=

= +

=

= +


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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


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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??


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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%


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1995

Temperatures in Cutting

•Excessive Tlowers tool strength, soften, alters shape

• Uneven cut difficult to control accuracy & tolerances

• Induce thermal and metallurgical damage


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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.


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Examples of Wear and Tool Failures

(a) Schematic illustrations od types of wear observed on various types of cutting tools


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Examples of Wear and Tool Failures

(b) Schematic illustrations of catastrophic tool failures


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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


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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

1 fundamentals of machining

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