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EMEC2313 Tutorial3

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EMEC2313 Tutorial3
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    Tutorial 3EME 2313 Manufacturing Processes1. a.What is the difference between bulk deformation and sheetmetal working?Bulk deformation is characteried b! significant deformations and massi"e sha#e changes and the $bulk$ refers to work#arts with relati"el! low surface area%to%"olume ratios.sheetmetal working is the forming and related o#erations #erformed on metal sheets& stri#s& and coils. 'igh surface area%to%"olume ratio of starting metal& which distinguishes these from bulk deformation [4 marks]  b.(ist the #ro#erties of metals that suitable for bulk deformation. )esirable material #ro#erties* a.(ow !ield strength  b.'igh ductilit!These #ro#erties are affected b! tem#erature* )uctilit! increases and !ield strength decreases when work tem#erature is raised+ther factors* ,train rate and friction  [4 marks] c.)escribe the differences between cold& warm and hot working of metals. [6 marks] d.What is the effect of tem#erature on flow stress for a t!#ical metal? Effect of tem#erature on flow stress for a t!#ical metal. The constant C  & as indicated b! the intersection of each #lot with the "ertical dashed line at strain rate - 1.& decreases& and m  /slo#e of each #lot0 increases with increasing tem#erature.   1   [4 marks] e.What is the function of lubricants in metal forming? What are the considerations in choosing a lubricant for a metal forming #rocess?Metalworking lubricants are a##lied to tool%work interface in man! forming o#erations to reduce harmful effects of friction Benefits* a.educed sticking& forces& #ower& tool wear  b.Better surface finishc.emo"es heat from the toolingonsiderations in hoosing a (ubricanta.T!#e of forming #rocess /rolling& forging& sheet metal drawing& etc.0 b.'ot working or cold workingc.Work materiald.hemical reacti"it! with tool and work metals e.Ease of a##licationf.ost [6 marks] 2.a. #late of co##er /the melting #oint is 14  0 is rolled at the tem#erature of 5  . )etermine whether the #rocess is cold rolling or hot rolling. [3 marks](a) %2 mm in thickness of co##er #late /melting #oint of 14  0 will be rolled to reduce the thickness at the tem#erature of 5  . )etermine whether the #rocess is classified as cold rolling or hot rolling. [5 marks] 6,WE*T recr!talliation - 7 T m  - 7 /14 8 2930: - ;9;.5 : - <3.5   . 2 marks Because working tem#erature is higher than T recr!talliation & hence it is considered as hot rolling. 3 marks  b.(ist the #ossible conse=uences of too high of a s#eed and too slow of a s#eed in rolling #rocess.   olling at high s#eed is ad"antageous in that #roduction rate is increased& but it has disad"antages as below*  The lubricant film thickness entrained will be larger& which can reduce friction and lead to a slick mill condition where the rolls sli# against the work#iece. This can lead to a damaged surface finish on the work#iece.  The thicker lubricant film associated with higher s#eeds can result in significant oil #eel& or surface roughing&   2     Because of the higher s#eed& chatter ma! occur& com#romising the surface =ualit! or #rocess "iabilit!.  There is a limit to s#eed associated with the motor and #ower source that dri"e the rolls.olling at a too low s#eed has conse=uences such as  Production rate will be low& and thus the cost #er unit weight will be higher.  Because a thick lubricant film cannot be de"elo#ed and maintained& there is a danger of transferring material from work#iece to the roll thus com#romising surface finish.  The work#iece ma! cool e>cessi"el! before contacting the rolls. This is  because a long billet that is rolled slowl! losses some of its heat to the en"ironment and also through conduction the roll con"e!er. [4 marks] c.)escribe the factors that influence the magnitude of the roll force&  F  & in ig 2.1. The formulas which can be referred* 02/0/ 01/ −−−−−= −−−−−−−=   f  oavg  hh R L  LwY  F  [6 marks] d.)escribe the im#ortance of controlling roll s#eeds& roll ga#s& tem#erature& and other #rocess "ariables in a tandem%rolling o#eration& as shown in figure 2.2.eferring to the tandem rolling o#eration in the figure abo"e& we note that mass continuit! has to be maintained during rolling. Thus& if the roll s#eed is not s!nchronied with the work#iece thickness in a #articular stand& e>cessi"e tensions or slack ma! be de"elo# between the stand@ some ma! be sli#.Af tem#erature is not controlled #ro#erl!& work#iece thickness will change& thus affect the roll ga# and roll deflections. om#le> control s!stems ha"e  been de"elo#ed for monitoring and controlling such o#erations at high rolling s#eeds.  [6 marks] 3.a.What is the difference between cold& warm& and hot forging? Cold Forging  3    old forging in"ol"es either im#ression die forging or true closed die forging with lubricant and circular dies at or near room tem#erature. arbon and standard allo! steels are most commonl! cold%forged. Parts are generall! s!mmetrical and rarel! e>ceed 25 lb. The #rimar! ad"antage is the material sa"ings achie"ed through #recision sha#es that re=uire little finishing. om#letel! contained im#ressions and e>trusion%t!#e metal flow !ield draft less& close%tolerance com#onents. Production rates are "er! high with e>ce#tional die life. While cold forging usuall! im#ro"es mechanical  #ro#erties& the im#ro"ement is not useful in man! common a##lications and economic ad"antages remain the #rimar! interest. Tool design and manufacture are critical. Warm Forging Warm forging has a number of cost%sa"ing ad"antages which underscore its increasing use as a manufacturing method. The tem#erature range for the warm forging of steel runs from abo"e room tem#erature to below the recr!stalliation tem#erature& or from about 4 to 1&4. 'owe"er& the narrower range of from 1& to 1&33 is emerging as the range of #erha#s the greatest commercial #otential for warm forging. om#ared with cold forging& warm forging has the #otential ad"antages of* educed tooling loads& reduced #ress loads& increased steel ductilit!& elimination of need to anneal  #rior to forging& and fa"orable as%forged #ro#erties that can eliminate heat treatment. Hot Forging 'ot forging is the #lastic deformation of metal at a tem#erature and strain rate such that recr!stalliation occurs simultaneousl! with deformation& thus a"oiding strain hardening. or this to occur& high work#iece tem#erature /matching the metalCs recr!stalliation tem#erature0 must be attained throughout the #rocess.  form of hot forging is isothermal forging& where materials and dies are heated to the same tem#erature. An nearl! all cases& isothermal forging is conducted on su#er allo!s in a "acuum or highl! controlled atmos#here to #re"ent o>idation. [6 marks]  b.E>#lain the features of a t!#ical forging die and state the factors ma! cause failures to a forging die.The features a t!#ical forging die are*   #arting line is at the largest cross section of the #art   gutter is #re#ared to allow flash material flows into it and #re"ent increase the forging load  )raft angles are necessar! in order to facilitate the remo"al of the #art from the die  ,election of the #ro#er radii for corner and fillet is im#ortant in order to ensure smooth flow of the metal into ca"it! and to im#ro"e die life.   <    ailure of die in manufacturing o#erations generall! results form one or more of the following causes*  Am#ro#er die design  )efecti"e or im#ro#er selection of die material  Am#ro#er manufacturing and im#ro#er heat%treatment and finishing o#eration  +"erheating and heat checking  E>cessi"e wear   +"erloading  Am#ro#er alignment of the die com#onents with res#ect to their mo"ements  Misuse  Am#ro#er handling of the die [6 marks] c.Wh! is the control of the "olume of the blank im#ortant in closed%die forging?Af too large a billet is #laced into a die in closed%die forging& #resses can Dam and thus not be able to com#lete their stroke. An turn& this would cause high loads to the #ress structure. 6umerous catastro#hic failures in #resses ha"e  been attributed to such e>cessi"e loads. Af the blank is too small& ob"iousl! the desired sha#e will not be com#letel! im#arted onto the work#iece. [4 marks] d.'ow can !ou tell whether a certain #art is forged or cast?orged #arts e>hibit generall! greater ductilit! when subDected to a tension test& and generall! tougher than cast #arts. )e#ending on the #rocesses and heat treatments used& grain sie will usuall! be smaller in forgings& and the grains will ha"e undergone deformation in s#ecific directions.ast #arts will be more isotro#ic than forged #arts. ,urface characteristics are also likel! to be non%uniform& de#ending on the s#ecific casting #rocesses used and factors such as the condition of the mold or die surface. [4 marks] e.Adentif! two casting design rules that also can be a##lied to forging.There are se"eral rules that a##l! e=uall! to forging and casting*,har# corners and angles should be a"oided.(arge flat areas should be a"oided&   5
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