Lecture 3 theory of metal cutting

Theory of Metal Machining

Content

Common features of machining processes, geometry of single point tool and tool signature, concept of speed, feed and depth of cut applicable to various machining processes.

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Tool must be sharp (what do you mean by sharp?) Relative velocity InterferenceTool material shall be harder than the work piece material

Physical Phenomenon in Machining Plastic flow Fracture Friction Heat Molecular diffusion Chatter

Mechanics of Metal Cutting

At extreme condition• Sticking friction at tip• Deformation at high strain

and strain rate• Nascent surface exposed

after deformation is veryactive

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Objectives During Machining

High Material Removal Rate (MRR)

Good accuracy and Surface finish

Long tool life

Contradicting

Cost

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Cutter RelatedMaterial

Geometry Mounting

Workpiece RelatedMaterial (composition, homogeneity)

Geometry (bar, block, casting etc.)Depth of cut

Spindle speed Feed rate

Machine RelatedCutting fluid type andapplication method

Depth and Width of cutSpindle speed

Feed rate

Others – Cutting fluid type and application

method Depth and Width of cut

Spindle speedFeed rate

Processing Parameters in Machining

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Cutting forces and

Torques and power

Tool temperature

Frictional effects

on tool face

Built up edge

Formation

Chatter, noise and

Vibrations

Effects of Processing Parameters

Work hardening

Thermal softening

Hot spots on the

machined surface

Deflection and

diameter variations

Tool life

Surface finish

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Theories of Chip Formation

Chip formation studies helps in understanding

mechanics of metal cutting or physics of machining

They lead to equations that describe the

interdependence of the process parameters such as

depth of cut, relative velocity, tool geometry etc.

These relations help us in selecting optimal process

parameters.

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Theories of Chip Formation – Theory of Tear

A crack propagates ahead of the tool tip causing tearing similar to

splitting wood [Reuleaux in 1900]

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Against the traditional wisdom, the tool was observed to

wear, not at the tip, but a little distance away from it.

Therefore, this theory was subscribed by many researchers

for a long time.

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Further studies attributed the wear away from the tip to the

following:

Chip velocity w.r.t. the tool is zero at the tip.

The tip is protected by BUE.

Temp is also high a little away from the tip due to the

frictional heat.

Subsequent studies proved the chip formation as shear and

not tear. Thus the theory of tear was rejected.

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Theories of Chip Formation – Theory of Compression

The tool compresses the material during machining.

This was based on the observation that the chip length

was shorter than the uncut chip length.

Later it was established that this shortage in length

corresponds to the increase chip thickness.

Thus this theory too was wrong

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Theories of Chip Formation – Theory of Shear

The excessive compressive stress causes shear of the chip

at an angle to the cutting direction [Mallock in 1881].

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Theories of Chip Formation – Theory of Shear

Emphasis on the influence of friction at chip-tool interface

Studied the effect of cutting fluids

Studied the influence of tool sharpness

Studied chatter

His observations on the above studies still hold good although

he could not explain all of them at that time.

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Mallock’s other contributions

Difficulties in Machining Mechanics studies

Several physical phenomenon such as plastic flow,

fracture, friction, heat, molecular diffusion and chatter are

involved. Some of them occur in extrême conditions

Friction – sticking; deformation – high strain and strain

rate; nascent surface exposed after deformation is very

active causing diffusion

The cutting zone is covered by chips and coolant.

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Typical machining is oblique, i.e., forces, torques and

deflections exist in all 3 directions.

Difficulties in Machining Mechanics studies

The typical machining operations are too short and the

stock (depth and width of cut) keeps changing.

Furthermore, velocity also may change along the cutting

edge as well as over time. These changes further

compound the difficulties to observe the process carefully.

Orthogonal cutting experiments were developed to

overcome these difficulties.

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Facing of thin pipe on a lathe with the cutting edge radial to the pipe.

Orthogonal Cutting

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A wedge shaped tool is used

Cutting edge is perpendicular to the direction of cut. In

other words, cutting edge angle and cutting edge

inclination angle

Uncut chip thickness is constant along the cutting edge

and w.r.t. time.

Cutting edge is longer than the width of the blank and it

extends on its both sides.

Cutting velocity v is constant along the cutting edge and

w.r.t. time

Characteristics of Orthogonal Cutting

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Quick stopping devices to freeze the chip formation

Cutting wax manually slowly so as to observe it

Marking grids on the side of the work piece and study their

deformation.

Microscopic studies

Photoelastic studies (tools made of transparent material such

as persbex or resin (araldite); work piece is wax. Resulting

fringe patterns are observed under polarized glasses.

Observation using high speed cameras

Force, torque and power measurements using dynamometers.

Temp measurements

Orthogonal Cutting - Experiments

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• Plastic Deformation• The outer surface is usually smooth

due to the burnishing effect of the tool

• Shear Plane • The angle formed by the shear

plane and the direction of the tool travel is called the shear angle


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The type of chip produced depends upon workpiece material, tool geometry, and operating conditions.

Discontinuous chips Individual segments Fracture of the metal

Brittle materials (cast irons)No plastic deformation

Continuous chips Machining ductile materials like steel and Al

Continuous deformation without fracture Chip breakers are required Tool wear increases with sliding


Compressive deformation will cause it to be thicker and shorter than

the layer of workpiece material removed

The work required to deform this material usually accounts for the

largest portion of forces and power involved in a metal removal

operation

The ratio of chip thickness, to the un-deformed chip thickness (effective

feed rate) is called the chip thickness ratio. The lower the chip thickness

ratio, the lower the force and heat, and the higher the efficiency of the

operation


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Method based on the MRR

Unit Horse Power The unit horsepower factor (P) is the approximate power required at the spindle to remove 1 in3/min of a certain material.

Mechanics of Metal Cutting –Power Consumption

Lecture 3 theory of metal cutting

Education

Transcript of Lecture 3 theory of metal cutting