fundamentals of cutting

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To know the types of cutting processesTo know the types of cutting processes To know the types of chips formedTo know the types of chips formed To be able to calculate the cutting forces & power To be able to calculate the cutting forces & power

involvedinvolved To acknowledge the temperature distribution in To acknowledge the temperature distribution in

the cutting toolthe cutting tool To distinguish between different types of wear To distinguish between different types of wear

mechanismmechanism

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1.1. A family of shaping operations, the common A family of shaping operations, the common feature of which is removal of material from a feature of which is removal of material from a starting work material so the remaining part has starting work material so the remaining part has the desired geometrythe desired geometry

Machining Machining

– – material removal from work material by a sharp material removal from work material by a sharp cutting tool.cutting tool.

- Divided into Divided into

a)a) Cutting – single point -turningCutting – single point -turning

- multipoint - milling, drilling- multipoint - milling, drilling3

Abrasive processes Abrasive processes

- material removal by hard, abrasive particles, - material removal by hard, abrasive particles, e.g., grinding e.g., grinding

Nontraditional processes Nontraditional processes

- various energy forms other than sharp cutting - various energy forms other than sharp cutting tool to remove materialtool to remove material

- Waterjet, Laser machining- Waterjet, Laser machining

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Cutting action involves shear deformation of work Cutting action involves shear deformation of work material to form a chip material to form a chip As chip is removed, new surface is exposedAs chip is removed, new surface is exposed

(a) A cross sectional view of the machining process, (b) tool with negative rake ‑angle; compare with positive rake angle in (a).

MACHININGMACHINING

1.1. Closer dimensional accuracy Closer dimensional accuracy Casting not very accurateCasting not very accurateProduce holesProduce holesImprove surface finishImprove surface finish

2.2. Produce internal and external profilesProduce internal and external profilesSharp corners & flatness can not be produced by Sharp corners & flatness can not be produced by forming & shaping processforming & shaping process

3.3. Heat treated parts have rough surface finishHeat treated parts have rough surface finishNeed grinding to improve surface finishNeed grinding to improve surface finish

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3.3. Special surface characteristics Special surface characteristics like copper mirror requires diamond cutting like copper mirror requires diamond cutting tooltool

4.4. More economicalMore economicalIf quantity of machining is largeIf quantity of machining is large

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1. More energy, capital & labor is used

Material removal process – waste of material

2. Removing material – takes more time than other processes

3. Material removing can have adverse effects on the tool and surface of work material if not removed properly

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1.1. Most important machining operations:Most important machining operations:

TurningTurning

DrillingDrilling

MillingMilling

2.2. Other machining operations:Other machining operations:

Shaping and planningShaping and planning

BroachingBroaching

SawingSawing

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Single point cutting tool removes material from a Single point cutting tool removes material from a rotating work material to form a cylindrical shape rotating work material to form a cylindrical shape

Three most common machining processes:

(a) turning

Used to create a round hole, usually by means of a Used to create a round hole, usually by means of a rotating tool (drill bit) with two cutting edgesrotating tool (drill bit) with two cutting edges

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(b) drilling,

1.1. Rotating multiple-cutting-edge tool is moved Rotating multiple-cutting-edge tool is moved across work to cut a plane or straight surfaceacross work to cut a plane or straight surface

2.2. Two forms:Two forms:

12 peripheral milling peripheral milling face millingface milling

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1.1. Single-Point ToolsSingle-Point ToolsOne dominant cutting edgeOne dominant cutting edge

Point is usually rounded to form a nose radiusPoint is usually rounded to form a nose radius

Turning uses single point toolsTurning uses single point tools

2.2. Multiple Cutting Edge ToolsMultiple Cutting Edge ToolsMore than one cutting edgeMore than one cutting edge

Motion relative to work achieved by rotating Motion relative to work achieved by rotating

Drilling and milling use rotating multiple cutting Drilling and milling use rotating multiple cutting edge toolsedge tools

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(a)(a) A single point tool showing rake face, flank, and ‑A single point tool showing rake face, flank, and ‑tool point; andtool point; and

(b)(b) a helical milling cutter, representative of tools a helical milling cutter, representative of tools with multiple cutting edges.with multiple cutting edges.

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1. Three dimensions of a machining process:

Cutting speed v – primary motion

Feed f – secondary motion

Depth of cut d – penetration of tool below original work surface.a) Radial

b) Axial

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2. For certain operations, material removal rate can be computed as

where v = cutting speed; f = feed; d = depth of cut

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MRR = v f dMRR = v f d

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Radial

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Axial

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1.1. In production, several roughing cuts are usually In production, several roughing cuts are usually taken on the part, followed by one or two finishing taken on the part, followed by one or two finishing cuts cuts

2.2. RoughingRoughing - removes large amounts of material from starting - removes large amounts of material from starting

work materialwork materialCreates shape close to desired geometry, but Creates shape close to desired geometry, but leaves some material for finish cuttingleaves some material for finish cuttingHigh feeds and depthsHigh feeds and depths, low speeds, low speeds

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3. Finishing - completes part geometry

Final dimensions, tolerances, and finishLow feeds and depths, high cutting speeds

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Simplified 2-D model of machining that describes the mechanics of machining fairly accurately

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Orthogonal cutting:

(a) as a three dimensional process. ‑

Chip thickness after cut always greater than before, so chip ratio always less than 1.0

Where

r = chip thickness ratio;

to = thickness of the chip prior to chip formation;

tc = chip thickness after separation

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c

o

tt

r

Based on the geometric parameters of the orthogonal model, the shear plane angle can be determined as:

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where r = chip ratio, and = rake angle

sincos

tanr

r

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Shear strain during chip formation: (a) chip formation depicted as a series of Shear strain during chip formation: (a) chip formation depicted as a series of parallel plates sliding relative to each other, (b) one of the plates isolated to parallel plates sliding relative to each other, (b) one of the plates isolated to show shear strain, and (c) shear strain triangle used to derive strain show shear strain, and (c) shear strain triangle used to derive strain equation.equation.

Shear strain in machining can be computed from Shear strain in machining can be computed from the following equation, based on the preceding the following equation, based on the preceding parallel plate model:parallel plate model:

where where = = shear strain,shear strain, = = shear plane angle, andshear plane angle, and = = rake rake angle of cutting toolangle of cutting tool

= tan( - ) + cot

More realistic view of chip formation, showing shear More realistic view of chip formation, showing shear zone rather than shear plane. Also shown is the zone rather than shear plane. Also shown is the secondary shear zone resulting from tool chip ‑secondary shear zone resulting from tool chip ‑friction.friction.

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1. Discontinuous chip

2. Continuous chip

3. Continuous chip with Built-up Edge (BUE)

4. Serrated chip

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Brittle work materials

Low cutting speeds

Large feed and depth of cut

High tool chip friction ‑

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(a) discontinuous

Ductile work materials

High cutting speeds

Small feeds and depths

Sharp cutting edge

Low tool chip friction‑

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(b) continuous

Ductile materialsDuctile materials

Low to medium cutting ‑ ‑Low to medium cutting ‑ ‑speedsspeeds

Tool-chip friction causes Tool-chip friction causes portions of chip to adhere portions of chip to adhere to rake faceto rake face

BUE forms, then breaks off, BUE forms, then breaks off, cyclicallycyclically

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(c) continuous with built up edge‑

Semicontinuous - saw-tooth appearance

Cyclical chip forms with alternating high shear strain then low shear strain

Associated with difficult-to-machine metals at high cutting speeds

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(d) serrated.

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Groove

Friction force, Friction force, FF and Normal force to friction, and Normal force to friction, NN

Shear force Shear force FFss and Normal force to shear and Normal force to shear FFnn 35

Forces in metal cutting:

(a) forces acting on the chip in orthogonal cutting

1. Orthogonal cuttingCutting tool edge perpendicular to movement of tool

Chip moves up to the face of tool

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2. Oblique cuttingCutting tool edge is at an inclined angle, iChip moves laterallyIncreased i, chips become longer and thinner

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Coefficient of friction between tool and chipCoefficient of friction between tool and chip:

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Friction angle related to coefficient of friction as follows:

NF

tan

Shear stress acting along the shear plane:

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sinwt

A os

where where AAss = area of the shear plane = area of the shear plane

Shear stress = shear strength of work material Shear stress = shear strength of work material during cuttingduring cutting

s

s

AF

S

1. F, N, Fs, and Fn cannot be directly measured

2. Forces acting on the tool that can be measured:

Cutting force Fc and Thrust force Ft

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Forces in metal cutting: (b) forces acting on the tool that can be measured

3. Cutting force, Fc Act in the direction of the cutting speed, V and

supplies the energy required for the cutting

4. Thrust force, Ft Acts in a direction normal to the cutting velocity perpendicular to Fc

5. Ft and Fc produced the resultant force, R

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6.6. R can be resolved intoR can be resolved into friction, Ffriction, F normal force, Nnormal force, N

7. The relationship can be derived as:

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F = R sin βN = R cos β

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1.1. Equations can be derived to relate the forces that Equations can be derived to relate the forces that cannot be measured to the forces that can be cannot be measured to the forces that can be measured:measured:

F = FF = Fcc sin sin + F + Ftt cos cos

N = FN = Fcc cos cos F‑ F‑ tt sin sin

FFss = F = Fcc cos cos F‑ F‑ tt sin sin

FFnn = F = Fcc sin sin + F + Ftt cos cos

Based on these calculated force, shear stress and Based on these calculated force, shear stress and coefficient of friction can be determinedcoefficient of friction can be determined

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1.1. Of all the possible angles at which shear deformation Of all the possible angles at which shear deformation can occur, the work material will select a shear plane can occur, the work material will select a shear plane angle angle that minimizes energy, given by that minimizes energy, given by

2.2. Derived by Eugene MerchantDerived by Eugene Merchant

3.3. Based on orthogonal cutting, but validity extends to Based on orthogonal cutting, but validity extends to 3-D machining3-D machining

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1. To increase shear plane angle

Increase the rake angle

Reduce the friction angle (or coefficient of friction)

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1.1. Higher shear plane angle means smaller shear plane Higher shear plane angle means smaller shear plane which means lower shear force, cutting forces, power, which means lower shear force, cutting forces, power, and temperatureand temperature

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Effect of shear plane angle : (a) higher with a resulting lower shear plane area; (b) smaller with a corresponding larger shear plane area. Note that the rake angle is larger in (a), which tends to increase shear angle according to the Merchant equation

1.1. Temperature in cutting causes:Temperature in cutting causes:Strength, hardness & wear resistance lowStrength, hardness & wear resistance lowCauses dimensional changes & low accuracyCauses dimensional changes & low accuracyInduce thermal damage the machined surfaceInduce thermal damage the machined surfaceCause distortion of machine, poor dimensional Cause distortion of machine, poor dimensional controlcontrol

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2.2. Temperature increases during machining Temperature increases during machining because of:because of:

Strength of material increaseHigh speedLarge depth of cutLow specific heat & thermal conductivity, k

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As speed increases, a As speed increases, a large proportion of large proportion of heat generated is heat generated is carried away by the carried away by the chip & little heat goes chip & little heat goes into the work material.into the work material.

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1. To measure temperature & its distribution in its cutting zone, thermocouples are used

It is embedded into the tool and /or the work material

2. Thermal emf at the tool-chip interface as a hot junction between 2 different materials

3. Radiation pyrometer – infrared radiation

Used to measure surface temperature

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1. Group consists of 3 students 2. Submit the name of group members 2. Collect the assignment's title 3. Submission date

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WEDNESDAY 9 OCTOBER 2013

Please download Tutorial 1 – Fundamental of Cutting from LMS

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