1_Basic Metal Cutting

8/11/2019 1_Basic Metal Cutting.

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Material Removal Processes

A family of shaping operations, the common feature of whichis removal of material from a starting workpart so the

remaining part has the desired geometry

Machining material removal by a sharp cutting tool, e.g.,

turning, milling, drilling Abrasive processes material removal by hard, abrasive

particles, e.g., grinding

Nontraditional processes - various energy forms other than

sharp cutting tool to remove material


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Why is Material Removal Important

Significant proportion of all goods involve machined surfaces

The only way to achieve high precision

The only way to create sharp corners, flat surfaces and internal and

external profiles.

The only way to shape hardened or brittle material Economics for small part volumes (e.g. prototypes)

Can achieve special surface finishes

Indispensable for creating complex shapes with good dimensional

accuracy and surface finish


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Limitations of Material Removal

Generate lots of scrap.

Takes longer time to remove material than to form it

Can mess up the properties and surface finish if not done properly


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Basic Processes

1) Rotating part turning, drilling, etc

2) Stationary part- milling, drilling, boring, etc

Cutting process the same in both


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Factors Affecting Machining/Cutting Processes

Work piece Material, condition, temperature, (Machinability)

Temperature rise Tool (The cutting edge)

Material, condition/sharpness, coatings, shape, surface finish

Cutting parameters (How much material removed)

-feed, speed, depth of cut

Tool angles

Type of chip created

Tool wear

Temperature rise


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Machine design Force and power availability

Presence or absence of cutting fluid (How process is cooled

and lubricated)

Machine tool parameters (To achieve tolerances)

Stiffness, damping, backlash

Fixture design (How the work piece is held while shaped)

Also dependent on other variables


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Some are independent and some are dependent variables

Independent variables in cutting

Workpiece material - "machinebility"

Cutting tools

Cutting parameters

Presence or absence of fluid

Characteristics of the machine tool

Fixture design


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Some are independent and some are dependent variables

Dependent Variables

Material removal rate

Surface finish of the work piece

Force and energy dissipated

Type of chip produced

Temperature rise in work piece, tool and the chip

Wear and failure of the tool


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Machinability

Machinability depends on the strength, toughness and hardness of

the work piece materialMachinability can be improved by the addition of certain elements

Lead and sulfur added to steels gives free machining steels

Good Machinability indicates

Good surface finish and part integrity

no tearing

Long tool life

Low power and force requirementsGood chips (No long thin chips)


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Machinability of Materials

Steels: Depends on type

Stainless Steels: Generally difficult, Ferritic SS easy

Aluminum: Easy to machine but softer alloys give poor

surface finish

Magnesium: Easy but danger from fire

Grey Cast Iron: Machinable but abrasive

Wrought Copper: Difficult to machine because of ductility

Brass: Easy to machine

Cobalt based Alloys: difficult and abrasive, required low feeds andspeeds

Nickel-based Alloys: Difficult and abrasive

Titanium: Difficult because of poor thermal conductivity


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Cutting Tool

Tool Selection and Design:

Tool selection is a very complex process involving manyparameters:

Work piece Machinability

Type of cut - continuous, interrupted

Tool material typeProcess parameters (Feed,Speed,Depth of cut)

Shape

Cost

Tool life (critical to economics)


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Tool Design Parameters

Material Properties:

Hardness

Toughness (to resist impact forces)

Wear Resistance

Chemical Stability

Solid vs InsertsShape

Circles are stronger than triangles

Edge Strength

Edge design

Sharp vs rounded

Coating Material


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Year


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Cutting Process Parameters

Depend on the process (turning, milling, drilling, etc.)

Determines tool life for a specific tool material and

design


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Presence or absence of fluid

Functions of Cutting Fluids

Reduce friction and wear

Reduce forces and energy consumption

30% of total energy can go into friction and heat generated

Cooling the cutting zone

Wash away chips

Protect new surfaces from corrosion


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Cutting fluids Basically four types

Oils

Emulsions

Semi synthetic

Synthetics


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Characteristics of the Machine

The machine provides the power and ensures that the tool

is maintained in the chosen location relative to the work

piece

Stiffness : Deflection under load - inaccurate cuts

Dynamic response:Vibrations - chatter - rough surfaces

Horse power available: Determines the maximum material

removal rate, Determines the speeds and feeds available


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Fixture Design

Fixtures hold the work piece fixed while the cutting tool actson it

Movement under the cutting force not desirable

Deflection of the work piece under cutting force not desirable

Vibration of the work piece undesirable


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Force and Energy Consumption

Important to know Force to

Avoid excessive distortion in workpiece, tools

Distortion gives rise to inaccuracies - tolerances

Allow adequate fixturing to be designed

Determine the work done by force which ends up as heat

Important to know Power to

Choose a machine with adequate power capabilities

Estimate how long it will take to machine a part

Estimate the rate at which heat is generated


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Tolerances

Tolerances on a machine part depend on

Forces generated

Distortion of the part and its fixturing

Distortion the tool and its holder

Depends on machine and tool design

Distortion of the machine itself

Depends on the machine design

Temperature generated

Thermal induced expansion of all components in the system results inmachining errors


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Surface Finish and Integrity

Surface finish describes the geometry

Surface Integrity pertains to the mechanical properties

Fatigue life, corrosion resistance

Factors affecting surface integrity include

Workpiece temperature during processing

Residual stresses induced by the shearing

Metallurgical effects (phase transformations)

Plastic deformationTearing

Built up edge on chip


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Tool LifeVery important economic factor

Cost of tools

Cost of damaged work piece

Cost of rework due to inaccurate machining

Machinability of part has direct influence

Abrasion and high temperature cause wear on

The facemostly craters

The flank

High forces and shocks (interrupted cutting)cause chipping

Fracture of the tool

Produces holes and gouges in part

Poorly machinable materials can give a built up edge

Material adheres to edge of tool and causes inaccuracies and extra friction


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Dependent Variables

Forces and energy dissipated

Temperature rise

Tolerances of workpiece after machining

Surface finish of workpiece after machining

Wear and failure of toolType of chip produced


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Type of Chip Produced

Discontinuous chips, continuous strands, continuous

serrated strands, built up edge (on tool)

Depends on the:

machinability of the work piece

the design of the cutting toolthe design of the tool holder


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Brittle work materials Low cutting speeds

Large feed and depth of cut

High tool-chip friction

Discontinuous Chip


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Ductile materials Low-to-medium cutting speeds

Tool-chip friction causes portions of

chip to adhere to rake face

BUE forms, then breaks off,cyclically

Continuous with BUE


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Semicontinuous - saw-toothappearance

Cyclical chip forms with

alternating high shear strain then

low shear strain Associated with difficult-to-

machine metals at high cutting

speeds

Serrated Chip

Documents

1_Basic Metal Cutting