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Unformatted text preview: ME250 Design and Manufacturing I Winter 2011 Lecture HW #5 Name: ___________________________ UMID: _________________ Lab Section #: _______ Note: You must indicate name, UMID, and Lab Section # otherwise the homework will not be returned. 1. Consider a piece of bar stock with diameter D0 [mm] to be turned down to diameter Df [mm] along length L [mm] from one end. The lathe has cutting speed v [mm/min], feed f [mm/rev], approach A [mm], overtravel O [mm], the maximum allowable cutting depth dmax [mm], and time between each pass Tr [min]. Answer the following questions. [25pts] (a) What is the range (upper and lower limits) of dmax such that the turning can be finished in exactly two passes? Hint: the limits can be expressed in terms of D0 and Df. [10pts] (b) Suppose dmax is within the range obtained in part (a). Let x be the depth of cut during the first pass. What is the range of x (upper and lower limits) such that the turning can be finished in exactly two passes? Hint: the limits can be expressed in terms of D0, Df, and dmax . [5pts] (c) Suppose dmax is within the range obtained in part (a) and x is within the range obtained in part (b). Let T be the time to complete the two passes of turning. Represent T as a function of x. [5pts] (d) Suppose dmax is within the range obtained in part (a) and x is within the range obtained in part (b). At which value of x does T become minimum, and what is the minimum value? [5pts] 2. The foreman in the injection‐molding department says that a polyethylene part produced in one of the operation had greater shrinkage than the calculation indicates it should have. The important dimension of the part is specified as 87.5 0.25 mm. However, the actual molded part measures 86.9 mm. [15pts] (a) As a first step, the corresponding cavity dimension should be checked. Compute the correct value of the nominal mold dimension and tolerances in the form d td . [10pts] (b) What adjustments in process parameters could be made to reduce the amount of shrinkage? [5 pts] 3. In sand casting, chaplets are used to support a sand core inside a sand mold cavity. The design of the chaplets and the manner in which they are placed in the mold cavity allow each chaplet to support the force of 15 N. The volume of the core = 1700 cm3, and the metal poured is steel. Assume the core is supported only by the chaplets from beneath and above as shown below. Determine the following. [20pts] core mold chaplet cavity (a) Minimum number of chaplets that should be placed beneath the core. [10 pts] (b) Minimum number of chaplets that should be placed above the core. [10 pts] 4. An L‐shaped structural section is direct extruded from a cylindrical billet with L0 = 210 [mm] and D0 = 70 [mm]. Dimensions of the cross sections are given in the above figure. Die angle = 90o. The flow curve of the work metal has K = 390 [MPa] and n = 0.20. In Johnson’s equation, a = 0.8 and b = 1.3. Determine the following. [20pts] 14 mm 60 mm
15 mm 55 mm (a) Extrusion ratio [5pts] (b) Shape factor [10pts] (c) Ram pressure at the start of the extrusion process[5pts] 5. A compound die is often used to perform blanking and punching simultaneously with one press stroke on a piece of sheet metal. A large washer is to be made with a compound die, out of 2024ST aluminum alloy sheet stock of 2.70 mm thick. The tensile strength of the aluminum alloy is 250 MPa. The outside and inside diameters of the washer are 85.0 mm and 55.0 mm, respectively. Answer the following questions. [20pts] (a) What are the punch and die diameters for the blanking operation? [5pts] (b) What are the punch and die diameters for the punching operation? [5pts] (c) Assuming three washers are made with one press stroke, what is the maximum force required for one press stroke? [10pts] ...
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This note was uploaded on 09/12/2011 for the course MECHENG 250 taught by Professor Stevis during the Winter '09 term at University of Michigan.
 Winter '09
 Stevis

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