Chip Formation and Tool Life 2x1

8/10/2019 Chip Formation and Tool Life 2x1

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Manufacturing Processes:MODULE

Machine ToolsII

Lecture NotesJo eet Ghose

ChipFormation

Email: [email protected]

Senior Lectur er,Department of Production Engineering,Birla Institute of Technology, Ranchi

Chip Formation

Regardless of the tool being used or the metal being cut, the chip formingrocess occurs b a mechanism called lastic deformation. This

deformation can be visualized as shearing. That is when a metal issubjected to a load exceeding its elastic limit.

This action, shown in Figure is similar to the action that takes place whena deck of cards is given a push and sliding or shearing occurs between theindividual cards.

Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005


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Types of chip Discontinuous or segmented Continuous Continuous with a built-up edge.


Source: George Schneider,Jr. CMfgE, Cutting Tool Applications

Types of chip

FIGURE : Various chips produced in turning: (a) tightly curled chip; (b) chip hits workpiece andbreaks; (c) continuous chip moving away from workpiece; and (d) chip hits tool shank and breaks

off. Source : G. Boothroyd, Fundamentals of Metal Machining and Machine Tools .



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Types of chip (video)


Video showing different types of chip formation

Continuous Chip During cutting of ductile materials a continuous ribbon type chip is

produced. The pressure of the tool makes the material ahead of cutting edge

deformed lasticall . It generally suffers compression and shear. The material then slides over

the tool rake face for some distance and then leaves the tool. Factors favorable to its formation are ductile metals, such as mild steel,

copper etc., fine feed, high cutting speed, large rake angle, keen cuttingedge, smooth tool face and efficient lubrication system .



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As discussed in continuous chips, the temperature is higher at interface between chip and the tool during cutting. Also, the work material; slidesunder heavy pressure on the rake face before being transformed into a freechip. Therefore in these conditions some portion of the chip may stick to the

Continuous Chip with a Built-up Edge (BUE)

rake face of the tool. Because of such closed contact, it discharges its heat tothe tool and thus becomes stringer than the rest of the material following over it. Naturally it attracts more of the deforming work material and thus the sizeof the BUE goes on increasing. When it reaches a certain critical size, it

becomes unstable and portion of it may disengage and break up. These broken portions and BUE get embedded in the machine surface or get attached to theunderside of the following chip.Favorable condition are low cutting speed, excessive feed, small rake angle,and lack of lubricant.


Discontinuous Chip Discontinuous or segmented chips are produced when brittle metal such as cast

iron and hard bronze are cut or when some ductile metals are cut under poor cutting conditions.

of ductile materials. Even a slight plastic deformation produced by a smalladvance of the cutting edge into the job leads to a crack formation in thedeforming zone. With further advance of the cutting tool, the crack travels and asmall lump of material starts moving up the rake face. The force and constraintsin the motion acting on the lump make the crack propagate towards the surface,an us a sma ragmen o e c p ge s e ac e . s e oo moves ur er,this sequence is repeated.

Conditions tending to promote discontinuous chip formation include: brittle

metal, greater depth of cut, low cutting speed and small rake angle.



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Chip BreakersA continuous chip flows away from the work at high speed. If this chip isallowed to continue, it may wrap around the tool post, the workpiece, thechuck, and perhaps around the operators arm. Not only is the operator indanger of receiving a nasty laceration, but if the chip winds around the

workpiece and the machine, he must spend considerable time in removing it.A loss of production will be encountered. Therefore it is imperative that thischip be controlled and broken in some manner. Hence chip breakers are used to break up the long continuous chip in small pieces.Chip breakers may be of the following types:

. .2.Groove type: A small grove is ground behind the cutting edge.3.Clamp type: A thin carbide plate or clamp is brazed or screwed on the faceof the tool


Chip Breakers

FIGURE : (a) Schematic illustration of the action of a chip breaker. Note thatthe chip breaker decreases the radiusof curvature of the chip. (b) Chipbreaker clamped on the rake face of acutting tool. (c) Grooves in cutting tools,

.

Source Manufacturing Processes forEngineering Materials, 4 th edition,

Kalpakjian, Schmid, Prentice Hall 2003



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Tool wear Cutting tools are subjected to extremely severe cutting conditions such as:

1. metal to metal contact with chip and work 2. very high stress

.4. very high temperature gradients5. very high stress gradients

Because of all the above-mentioned factors, the tool-chip interface exhibitthe type of wears found. As tool wear progresses, cutting forces increase

. blunt edge which will result in further progressing of plastic deformationfrom the tool tip to the interior.

Cutting tool life is one of the most important economic considerations inmetal cutting.

Conditions giving a very short tool life will not be economical because


tool-grinding, indexing, and tool replacement costs will be high. On the other hand, the use of very low speeds and feeds to give long tool

life will not be economical because of the low production rate. Efforts are thus made to understand the behavior of the tool, how it

physically wears, the wear mechanisms, and forms of tool failure.

Crater Wear Typically, crater wear occurs on the rake face of the tool. It is essentially the erosion

of an area parallel to the cutting edge. This erosion process takes place as the chip being cut, rubs the top face of the tool. Under very high-speed cutting conditions and when machining tough materials, crater wear can be the factor which determines thelife of the tool. However, when tools are used under economical conditions, the edgewear and not the crater wear is more commonly the controlling factor in the life of the tool . Crater wear is caused mainly by diffusion and adhesion.



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Tool Failure Criterion

FIGURE: (a) Types of wear observed incutting tools. The thermal cracks shownare usually observed in interrupted cutting operations, such as in milling.(b) Catastrophic failure of tools. (c)Features of tool wear in a turningoperation. The VB indicates averageflank wear. Source : (a) and (b) After V.C. Venkatesh. (c) InternationalOrganization for Standardization (ISO).


Tool lifeTool life represents the useful life of the tool, expressed generally in timeunits from the start of a cut to some end point defined by failure criterion.A common method of forecasting tool wear is to use Taylors equation; his

.Taylor thought that there is an optimum cutting speed for

best productivity. This is reasoned from the fact that at lowcutting speeds, tools have higher life but productivity is low,and at higher speeds the reverse is true. This inspired him tocheck u the relationshi of tool life and cuttin s eed.

Frederick W. Taylor 1856-1915Based on the experimental work he proposed the formula for tool life.

Taylors Empirical Equation: VTVTnn

==CCWhere, T = tool lifetime; usually in minutesV = cutting velocity, m/minC = constant; the cuttin velocit for 1 minute of ela sed time before reachin the wear limit of


the tooln = constant which is considered a characteristic of the tool material, called tool life index.Note: at T = 1 minute, C becomes equal to the cutting speedEach combination of workpiece, tool material and cutting condition has its own n and Cvalues, both of which are determined experimentally


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SlNo

.

Workmaterials

Values of C for differentTool Materials

Tool life (C and n values)

SlNo

Tool Materials Values ofn

1 HSS 0.08 - 0.2

HSS Carbide Ceramic

1 Carbon steel 40 100 200 160 2500

2 Cast Iron 30 60 100 150 9000 Carbide

. - .

3 Ceramic 0.5 - 0.7

3 StainlessSteel

20 35 120 200

4 Titanium 10 20 100 150

5 Tun sten 120 - 400 - 600


160

Effect of Workpiece Microstructure on Tool Life in Turning


FIGURE : Effect of workpiece microstructure on tool life in turning. Tool life is given in terms ofthe time (in minutes) required to reach a flank wear land of a specified dimension. (a) ductilecast iron. (b) Steels, with identical hardness. Note the rapid decrease in tool life as the cuttingspeed increases.

Source Manufacturing Processes for Engineering Materials, 4 th edition, Kalpakjian, Schmid,Prentice Hall 2003


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Effect of Workpiece Microstructure on Tool Life in Turning

The tool life curves above are obtained in cuttin various ductile cast irons. Notethe rapid decrease in tool life as the cutting speed increases and the stronginfluence of the condition of the work piece material microstructure on the toollife.

Effect of cutting speed upon material removal: a material is being machined inthe a condition 265 HB; see the figure above

if the cuttin s eed is 70 m/min tool life is about 40 min and the tool has


traveled 70 m/min X 40 min = 2800 m before being replaced. if the cutting speed is increased to 120 m/min, tool life is about 5 min and the tool

travels 120 m/min X 5 min = 600m. the lower cutting speed allows more material removal per tool but the part cannot

be produced as quickly this is an economic trade-off.

Tool life


Tool life curves are usually plotted on log-log paper or are plotted taking log of theequation, from which we can easily determine the exponent n.


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Tool-Life Curves

FIGURE : a Tool-life curves for a variet of cuttin -tool materials. The ne ative inverse of the


.slope of these curves is the exponent n in tool-life equations. (b) Relationship between measured temperature during cutting and tool life (flank wear). Note that high cutting temperaturesseverely reduce tool life. Source : After H. Takeyama and Y. Murata

To account for the effect of feed and depth of cut a generalizedTaylors equation is formed:

C

Tool life

vv x ym t sT 1

=Where, S= feed in mm/min

t= depth of cut mm

vC = Modified Taylor constant depending upon the tool work pair.(Approximately varies from 270 to 325)

m= Taylors exponent n

v x v y = exponents to take care the effect of feed and depth of cut.

v x approximately varies from 015 to 0.40, and

v y approximately varies from 0.2 to 0.45



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Economics of machiningThe two most important parameter in in economics of machining arethe minimum cost per part and the maximum production rate. The totalcost per piece consists of four items:

Where C p is cost per piece, is the machining cost, C s is the cost of setting up for the machine for particular operation, C l is the cost of loading, unloading, and machine handling, and C t is the tooling cost,which includes tool changing, regrinding, and depreciation of the cutter.

t lsm p =

The machining cost is given by

Where T m is the machining time per piece, L m is the labor cost of theoperator per hour, B m and is the burden rate, or overhead charge of the

)( mmmm B LT C +=


machine including depreciation, indirect labor etc.

Economics of machiningThe setup cost, C s is a fixed amount per piece. The loading, unloading,and machine handling cost, C l is given by:

)( mmll B LT C +=Where is the time required in loading and unloading the part, changingspeed and feed etc. The tooling cost, C t is expressed as:

[ ]cgggmmc p

t D B LT B LT N C ++++= )()(1

ere p s t e num er o parts mac ne per too gr n ; c s t e t merequired to change the tool; T g is the time required to grind the tool; L gis the labor cost of the tool grinder operator per hour, B

gis the burden

rate of tool grinder per hour; and D c is the depreciation of the tool inrupees per grind.



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Economics of machiningSolving the above equation we get

speed cuttingoptimumThe

( )[ ][ ]{ }ncgggmmcn

mm

D B LT B LT n

B LC V

++++

+=)()(11

)(0

( )[ ]mm

cgggmmc

B L

D B LT B LT nT +

++++= )()(11

se,tooopt mumt ean

0

o


Economics of machiningTo find out the optimum tool life and optimum cutting speed for maximum production:

T

Which gives

0=V

s eed cuttino timumThe

( )[ ]{ }n

cT n

C V

11 0

=


( )[ ] cT nT 11,

0

o

=


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Machinability is used to refer to the ease with which a given workmaterial can be machined under a given set of cutting conditions. It isof considerable economic importance for the production engineer toknow in advance the machinabilit of a work material so that its

Machinability of metals

processing can be efficiently planned. The machinability of a materialis usually defined in terms of four factors:

Surface finish and integrity of the machined part, Tool life obtained, Force and power requirement, and Chip control.

Because of complex nature of cutting operation, it is difficult to establishrelationships that quantitatively define the machinability of a material.

In manufacturing plants, tool life and surface finish are generallyconsidered to be the most important factors in machinability.


Although not used much any more, approximate, machinability ratingsare used to get an idea about the machinability of any material.

Thus machinability is not a basic standard but is related.

Specific cutting speed is defined as the cutting speed correspondingto the predetermined tool life.

Machinability ratings can be very easily understood with the help of a

Machinability Ratings

. For example if the tool life during a turning operation under standard

condition (of feed, depth of cut, tool material and tool geometry) isfound to be 60min at a cutting speed of 100 m/min, the specificcutting speed for 60 min tool life, V 60 = 100 m/min.

Further if V t60 is the specific cutting speed for 60 min tool life for atest material and V s 60 is the corresponding specific speed for astandard material,

then the machinability ratings MR of the test material is given by

%100V

M s t60

R


60

SAE 1212 is taken as standard material for testing machinability.


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Cutting fluids, sometimes referred to as lubricants or coolants are liquids andgases applied to the tool and work piece to assist the cutting operation.Functions of cutting fluids:To cool the tool.

Cutting fluids

To cool the work piece.To lubricate and reduce frictionTo improve surface finish.To protect the finished surface from corrosion.To wash the chips away from the tool.Pro erties of cuttin fluid s: High heat absorption capability. Good lubricating quality. High flash point so as to eliminate the hazard of fire. Stability so as not to get oxidized in presence of air. Neutral so as not to react chemically. Odourless so as not to produce bad smell even when heated.


Harmless to the skin of the operators. Non corrosive to the work or the machine. Transparency so that the cutting action of tool may be observed by the operators. Low viscosity to permit free flow of the liquid. Low priced to minimize production cost.

Water: Water provided good cooling effect but is not a good lubricant. Water

is hardly used as cutting fluid because of its corrosiveness.Soluble oil or water miscible cutting f luids: These are also called water

based cutting fluids. These comprises of mineral oil or fat mixtures and

Types of cutting flu idTypes of cutting flu id

emulsifiers added to water. The emulsifier breaks the oil into minuteparticles and disperses them throughout water. These cutting fluids haveexcellent lubricating properties. It has milky appearance.

Straight cutting oils : These oils have good lubricating but poor heatabsorption properties and therefore are suitable only for low cuttings eed.These are of three types:Mineral oil: Kerosene, low viscosity petroleum fraction.Fatty oil: Lard oilCombination of mineral and fatty oil

Oils with adhesives: The benefits of mineral oils can be improved with the


, . Addition of sulphur or chlorine compounds reduces chances of chipwelding on the tool rake face. Besides these there are other adhesiveswhich are also added to improve corrosion protection, prevent any organicgrowth.


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References1. Kalpakjian, Schmid, Manufacturing Processes for Engineering Materials, 4th

edition,, Prentice Hall 20032. DeGarmo, E. P., J. T. Black, and R. A. Kohser, Materials and processes in

Manufacturing, PHI.

3. P.N. Rao, Manufacturing Technology Metal Cutting and Machine Tools, TMH.4. George Schneider,Jr. CMfgE, Cutting Tool Applications5. Amstead, B. H., P. F. Ostwald, and M. L. Begeman, Manufacturing Processes,

8th ed., Wiley, New York, 1988.6. Amitabha Battacharya , Metal Cutting Theory and Practice7. Shaw M. C. Metal Cuttin Princi les Oxford Universit Press Oxford 1984.8. Schey, J. A., Introduction to Manufacturing Processes, McGraw-Hill, New York,

1977.9. Lindberg, R. A., Processes and Materials of Manufacture,10. William J Patton, Machine tool Operations, Reston publishing company11. O W Boston, Metal Processing, 2nd edition 1951, John Wiley and Sons

-


. . . , .13. Hajra Choudhury, Elements of Workshop TechnologyVol.-II, Media Promoters

and Publishers.14. O P Khanna, Production Technology-(Vol. II)15. HMT, Production Technology, HMT

Chip Formation and Tool Life 2x1

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