ta 202-a (lecture 8) instructor: shantanu bhattacharya

TA 202-A (Lecture 8)

Instructor: Shantanu Bhattacharya

Introduction (Mechanics of machining operation)

Idealized model; Orthogonal; 2-D cutting with a well-defined shear plane; also called Merchant model

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In idealized model, a cutting tool moves to the left along the workpiece at a constant velocity, V, and a depth of cut, to, Chip thickness, tc

Orthogonal and Oblique Cutting

• When the cutting edge is perpendicular to the direction of velocity of the tool, the cutting is orthogonal.

• When the cutting edge is at an angle not equal to 90 deg. to the direction of velocity of the tool, the cutting is oblique.

Mechanics of Cutting:Types of Chips Produced in Metal Cutting

• Types of metal chips commonly observed in practice (orthogonal metal cutting)

• There are 4 main types:

a) Continuous chip (with narrow, straight, primary shear zone)b) Continuous chip with secondary shear zone at the tool-chip interfacec) Built-up edge, BUE chipd) Serrated or segmented or non-homogenous chipe) Discontinuous chip 4

Mechanism of Chip Formation

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1. The uncut layer undergoes severe plastic deformation in the primary shear zone.

2. Just after formation, the chip flows over the rake surface of the tool and strong adhesion between the high temperature chip and the rake face results in some sticking. This is secondary shear zone.

3. At low speed, lower uncut chip thickness, large rake angle and suitable cutting fluid chips get produced as continuous and ribbon like.

4. At higher speeds, uncut chip thickness and smaller rake angle, the temperature increases and the tendency of the plastically deformed material to adhere to rake face increases and a lump is formed at the cutting edge called built up edge (BUE).

5. After the BUE builds up to a certain size it gets deadhered due to the increased force exerted on it by the surrounding flowing material. The broken surface adhete to the finished surface and makes it rough.

6. If the material is brittle ruptures during high speed cutting occur intermittently resulting in discontinuous chips.

Built-up Edge (BUE) Chips• Consists of layers of material from the workpiece

that are deposited on the tool tip• As it grows larger, the BUE becomes unstable and

eventually breaks apart– BUE: partly removed by tool, partly deposited

on workpiece• BUE can be reduced by:1. Increase the cutting speeds2. Decrease the depth of cut3. Increase the rake angle4. Use a sharp tool5. Use an effective cutting fluid6. Use cutting tool with lower

chemical affinity for workpiece material

Hardness distribution with BUE chip

Note BUE chip much harder thanchip

BUE: turning

BUE: milling

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BUE deposited

Cutting tool

work pieceBUE deposited

Built up edge formationBuilt up edge formation



Mechanics of chip formation

Numerical Problem• During an orthogonal machining operation with a cutting

tool having a rake angle of 10 deg., the chip thickness is measured to be 0.4mm, the uncut thickness being 0.15mm. Determine the shear plane angle and also the magnitude of the shear strain.

Numerical Problem• During an orthogonal machining operation with a cutting

tool having a rake angle of 10 deg., the chip thickness is measured to be 0.4mm, the uncut thickness being 0.15mm. Determine the shear plane angle and also the magnitude of the shear strain.

Sol. The cutting ratio is 0.15/0.4 = 0.38

Temperatures in CuttingTemperatures in Cutting

• Temperature rise (due to heat lost in cutting raising temp. in ⇒cutting zone)

Its major adverse effects:

1. Lowers the strength, hardness, stiffness and wear resistance of the cutting tool (i.e. alters tool shape)

2. Causes uneven dimensional changes (machined parts)

3. Induce thermal damage and metallurgical changes in the machined surface ( properties adversely affected)⇒

• Sources of heat in machining:

a. Work done in shearing (primary shear zone)

b. Energy lost due to friction (tool-chip interface)

c. Heat generated due to tool rubbing on machined surface (especially dull or worn tools)

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Temperatures in Cutting

Temperature Distribution• Sources of heat generation are concentrated in

– primary shear zone, and– At tool–chip interface– ⇒ v. large temp. gradients

in the cutting zone (right)

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Temperatures in Cutting

“Techniques for Measuring Temperature”

• Temperatures and their distribution can be determined using

– thermocouples (placed on tool or workpiece)– Measuring infrared radiation (using a radiation pyrometer)

from the cutting zone (only measures surface temperatures)

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Tool Life: Wear and FailureTool Life: Wear and Failure

Tool wear is gradual process; created due to:

1. High localized stresses at the tip of the tool

2. High temperatures (especially along rake face)

3. Sliding of the chip along the rake face

4. Sliding of the tool along the newly cut workpiece surface

• The rate of tool wear depends on

– tool and workpiece materials

– tool geometry

– process parameters

– cutting fluids

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Tool Life: Wear and Failure:Flank Wear

• Flank wear occurs on the relief (flank) face of the tool• It is due to

– rubbing of the tool along machinedsurface (⇒ adhesive/abrasive wear)

– high temperatures (adverselyaffecting tool-material properties)

• Taylor tool life equation :

CVT n V = cutting speed [m/minute]T = time [minutes] taken to develop a certain flank wear .n = an exponent that generally depends on tool material (see above)C = constant; depends on cutting conditionsnote, magnitude of C = cutting speed at T = 1 min Also note: n, c : determined experimentally

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• To appreciate the importance of the exponent, n, Taylor tool life equation, rearranged:

– Thus, for constant C : smaller n smaller tool life⇒

• For turning, equation can be modified to

where,

n

V

CT

/1

CfdVT yxn

d = depth of cut (same as t0)f : feed of the tool [mm/rev ]x, y: must be determined experimentally for each cutting condition

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Tool-life Curves• The exponent n can be

determined from tool-life curves (see right)

– Smaller n value as ⇒ V increases tool life decreases faster⇒

– n can be negative at low cutting speeds

• Temperature also influences wear:

– as temperature increases, flank wear rapidly increases

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Cutting FluidsCutting Fluids

Any liquid or gas applied directly to machining operation to improve cutting performance

• Two main problems addressed by cutting fluids: 1. Heat generation at shear zone and friction zone 2. Friction at the tool‑chip and tool‑work interfaces

• Other functions and benefits:– Wash away chips (e.g., grinding and milling)– Reduce temperature of work surface for easier handling– Improve dimensional stability of work surface.– Reducing the coefficient of friction at the chip tool

interface due to the formation of a weaker compound at the interface.

– Protection of the finished surface from corrosion 20

Characteristics of An ideal cutting fluidCharacteristics of An ideal cutting fluid

• Have a large specific heat and thermal conductivity

• Have a low viscosity and small molecular size

• Contain a suitable reactive constituent.

• Be nonpoisonous and non corrosive

• Be inexpensive and easily available

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Cutting Fluid Functions• Cutting fluids can be classified according to

function:– Coolants - designed to reduce effects of heat in

machining– Lubricants - designed to reduce tool‑chip and

tool‑work friction

Types of cutting fluid:

Water based fluids (Coolant)Mineral oil based fluids (Lubricants)

Types of cutting fluid:

Water based fluids (Coolant)Mineral oil based fluids (Lubricants)

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Coolants• Water used as base in coolant‑type cutting fluids• Most effective at high cutting speeds where heat

generation and high temperatures are problems • Most effective on tool materials that are most

susceptible to temperature failures (e.g., HSS)

• Usually oil‑based fluids• Most effective at lower cutting speeds• Also reduces temperature in the

operation

Lubricants Lubricants

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Economic Advantages Using Cutting Fluids

• Reduction of tool costs– Reduce tool wear, tools last longer

• Increased speed of production– Reduce heat and friction so higher cutting speeds

• Reduction of labor costs– Tools last longer and require less regrinding, less

downtime, reducing cost per part• Reduction of power costs

– Friction reduced so less power required by machining

ta 202-a (lecture 8) instructor: shantanu bhattacharya

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