Factors affecting tool life in machining processes

Factors Affecting Tool Life In Machining Processes


What are these??

Factors Affecting Tool Life

In Machining Processes

ME 572 Analysis of Manufacturing Processes

Mohammad AlJuhani


CONTENTS

Introduction

Tool Life and Machinability

Tool Damage

Causes of tool damageFactors Affecting Tool Life in Machining Processes


INTRODUCTION

•Cutting tool life is important consideration in metal cutting processes.

• In machining operations, cutting conditions such as tool angles, cutting speeds and feed rates are usually selected to give an economical tool life.

•Conditions giving a short tool life are uneconomical because tool grinding and tool replacement costs are high.

• Factors affecting tool life should be carefully monitored to minimize their consequences.


• A tool that doesn’t perform the desired function can be declared as failed, or reached the end of its useful life.

• At this point of time, the tool is not necessarily unable to cut the workpiece but is merely unsatisfactory (may not give the desired surface finish or dimensional tolerances) for the purpose required

• The tool life is defined as the length of cutting time that the tool can be used.

Tool Life and Machinability


• Tool damage can be classified into two groups, wear and fracture by means of its scale and how it progresses.

• Wear is loss of material on an asperity or micro-contact, or smaller scale, down to molecular or atomic removal mechanisms. It usually progresses continuously.

• There are two main types of tool wear

• Flank wear

• Crater wear

• Fracture, on the other hand, is damage at a larger scale than wear and it occurs suddenly.

Tool Damage


Tool damage. Flank wear vs. cutting time


Mechanical Damage

Thermal Damage

Adhesion

Causes of tool damage


Mechanical Damage

Mechanical Damage

FractureChippingAbrasion Fatigue

It is independent of temperature.




Mechanical Damage


Thermal Damage

Therma

l Da

mage

Plastic deformation

Diffusion

Chemical

reaction

It increases drastically with increasing temperature



Thermal Damage



Thermal Damage



Occurs when work or chip material welds to the tool surface.

Adhesion Damage





Factors Affecting Tool Life

Cutting Tool

Geometry

Cutting Tool

Material

Cutting Tool

Characteristics

Cutting Condition

s

Workpiece

Material.

Cutting Fluid

Geometry of

cutting tool

varies with

Type of operation

being performed,

Surface finish requirements.

Power and speed of machineWorkpiece

material,


Cutting Tool

Geometry

Cutting tool

geometry is

directly affects

Machined surface quality.

Productivity of machining.

Chip control.

Magnitude and direction of the cutting forcesTool life.Distribution of

the thermal energyTemperature distribution in

the cutting wedge


Cutting Tool

Geometry


Cutting Tool

Geometry


Rake angleRake angles are three types, positive, zero and negative.

For ductile materials, the rake inclination is a positive angle

Since their shearing is low.

For brittle materials, the rake inclination is a negative angle.

The advantages of increasing rake angle

• Reduces the horsepower consumption.

• The cutting force and tool-chip contact temperature change in approximately the same way.

The disadvantages of increasing rake angle

• The strength of the cutting wedge decreases when the rake angle increases.

• The region of the maximum contact temperature at the tool-chip interface shifts toward the cutting edge.

Cutting Tool

Geometry


Flank angleThe flank angle affects the performance of the cutting tool mainly by decreasing the rubbing between the tool surface and workpiece surface.

With increasing the flank angle the following advantages are obtained:

• Cutting edge radius decreases which leads to decrease in the frictional and deformation components of the flank force.

• The strength of the region adjacent to the cutting edge decreases.

• Heat dissipates through the tool decreases.

Cutting Tool

Geometry


Nose RadiusNose radius is very critical part of the cutting edge.

If the radius is made of sharp angle, the produced finished surface will be rough and tool life will be short.

A large nose radius benefits

• Better surface finish.

• Faster feed rate.

• Stronger tool.

• Allow faster cutting speed

However, large radii have negative sequences such as tool chatter.

Cutting Tool

Geometry


Cutting Tool

Material

Factors affecting selection cutting tool material

The cutting operation involved

Workpiece material

Production requirements

Surface finish and accuracy requirements.

Machine to be used


Cutting Tool

Material

Major qualities required in a cutting tool

Resistance to heat (hot hardness)

Resistance to mechanical impact thermal shock.

Resistance to wear (hardness)

Resistance to fracture (toughness)

Chemical stability to the workpiece material


Cutting Tool

Material


Today, 50% of HSS, 85% of carbide and 40% of super-hard tools used in industry are coated

Cutting Tool

Material

Cutting Tool Coati

ng

Improve wear

resistance

Reduce thermal conducti

vity.

Reduce frictional contact.


Cutting Tool

Material

Uncoated cutting tool

Coated cutting tool


Cutting Tool CharacteristicsTo stand against high stress

Pressure resistance.

Bending strength.

Edge strength.

Inner bonding strength.

High temperature strength.

Small propensity to diffusion and

adhesion (Chemically stable).

Abrasion resistance.

Reproducible wear behavior.

Cutting Tool Characteristi

cs


Depth of cut

Feed rate

Cutting Speed

Cutting Conditions


Cutting speed has the greatest influence on tool wear and tool life.

Most materials can be machined over a wide range of speeds.

When a shorter tool life is allowable the speed can be increased

Relationship between tool life and cutting speed is expressed by Taylor equation for tool life

Cutting Conditions

Depth of cut

Feed rate

Cutting Speed


Cutting Conditions

Depth of cut

Feed rate

Cutting Speed


Feed rate changes have a less effect on tool life than cutting speed does.

Relationship between tool life and cutting conditions is expressed by modified Taylor equation

Limitations to maximizing feed rate• Horsepower available on the machine tool.• Chip groove geometry.• Surface finish required.• Part configuration.

Depth of cut

Feed rate

Cutting Speed

Cutting Conditions


Cutting Conditions

Depth of cut

Feed rate

Cutting Speed


Depth of cut has least effect upon tool life, so the heaviest (10 times the feed rate) possible depth of cut should always be used.

Depth of cut is limited by • Amount of metal to be machined from the workpiece, • Power available on machine tool, • Rigidity of the work piece and cutting tool • Rigidity of the setup. • Workpiece configuration• Insert size & thickness

Cutting Conditions

Depth of cut

Feed rate

Cutting Speed


Cutting Conditions

Depth of cut

Feed rate

Cutting Speed


Machinability of

workpiece criteria

Tool life

Surface roughne

ss

Surface integrity

Magnitude of cutting

forces

Energy (power)

consumption

Workpiece Material.


Factors determin

e conditions of the

work material

Microstructure.

Grain size.

Heat treatment.

Chemical composition.Fabrication

Yield and tensile

strength.

Hardness

Workpiece Material.


Main Cutting Fluid Functions

Lubrication effect

Cooling effect

Removing chips away from the cutting zone

Cutting Fluid


Other functions of cutting fluids

Reduction of cutting forces/ energy consumptions.

Improvement of surface finish and dimensional accuracy.

Extend tool life.

Facilitate chip removal.

Corrosion prevention on workpiece and machines.

Lowering workpiece temperature.

Lubrication of machine tool sliding guides

Cutting Fluid


Cutting fluids properties

Good lubricating properties

High cooling capacity

Low viscosity to provide free flow of cutting fluid.

Chemically stable.

Non-corrosive.

High flash point to reduce fire risks.

Allergy free.

Less evaporative.

Low cost.

Cutting Fluid

Cutting Fluids Selection

Type of machining processes.

Type of machined workpiece material.

Type of cutting tool material.


Cutting Fluid


Cutting FluidCutting Fluid Effects

EnvironmentalBiodegradability

Life cycle assessment

Bioaccumulability and biomagnifications

Energy saving and fuel economy

Renewability

HealthRespiratory problems.

Dermatological problems


Environmental machining

Dry cutting Minimum quantity lubricant

Vegetable based cutting fluids

Cutting Fluid



Thank you


Cutting Tool

Geometry

Education

Factors affecting tool life in machining processes