MIT 2.810 Fall 2016 Homework 5 Solutions 1 MIT 2.810 Manufacturing Processes and Systems Homework 5 Solutions Sheet Metal Forming Revised October 7, 2015 Problem 1. Springback Rank the metals in Table 2.2 (Kalpakjian, all editions) in terms of springback, listing those with the largest springback first. Answer: Springback is larger for materials with larger values of Y/E. Materials ranked in terms of their springback behavior E [GPa] Y [MPa] Y/E Ti 105 862 8.2 x 10 -3 Mg 43 218 5.1 x 10 -3 Steel 195 965 5.0 x 10 -3 Cu 128 588 4.6 x 10 -3 Al 74 293 4.0 x 10 -3 Ni 197 653 3.3 x 10 -3 Mo 345 1075 3.1 x 10 -3 W 375 620 1.7 x 10 -3 Pb 14 14 1.0 x 10 -3 Note: The values in this Table are averaged and therefore can deviate considerably. Example: Mild steel can have a yield strength of approximately 350 MPa, which leads to a Y/E ratio of 1.53 x 10 -3 , a value even smaller than the one for Tungsten (W).
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Problem2.ConsiderthemanufactureofthebracketshowninFigure1fromsheetsteel(availablein4in.widestrips) using a manual brake press or presses. Assume UTS = 70,710 psi. We are interested inestimatingtheunitcosttomakethesebrackets.PleaseuseinformationfromBoothroyd’s“DesignforSheetMetalworking”andthefollowingassumptions:
1. Thesheetmaterialcosts$0.25/lb.2. Thecostofapressis:Presscost[$]=2[$/lb]×forcecapacity[lb].3. All operations take the same length of time, with no breakdowns or unusual delays
(a) Describe the sequence operations needed to make this bracket, including materials, tools,
machinesandoperators.(b) Estimatethesizeandcostoftheequipmentneededtomakethispart.(c) Estimatethetimerequiredtomakeoneofthese(ignorewaitingtime).(d) Estimate the cost per bracket (materials, labor, and equipment) assuming 106 parts are
produced.Stateanyassumptions,beyondthosegiven,explicitly.(e) Estimatetheminimumbendradiusforthepart.(f) If thepart is formed to90±,makea roughestimateof the resultingangledue to the spring