University of Saskatchewan College of Engineering Dept. of Mech. Engineering ME Final Exam Solution December 19, 2018

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1 University of Saskatchewan College of Engineering Dept. of Mech. Engineering ME Final Exam Solution December 19, 2018 Time: 3 Hours Open Book Exam. Instructor: Chris Zhang Name: Student Number: This exam question set consists of two parts. Part I consists of 14 questions. Part II consists of 2 questions. There is an extra work asked for the peer evaluation of your group members. Attempt all questions. ****************************************************************************** Peer Evaluation Group member (name) Score (1-5) (1: poor; 5: excellent) Part I (40 marks) Question I.1 [3 marks] Explain the physics of ECM and EDM for material removal. List one pros and one cons to each of them from the physics, respectively. 1

2 ECM is based on the electric-chemical effect that is two electrodes (tool, work) are in a bath. After an electric circuit on the electrodes (tool is with the negative electrode, and work is with the positive electrode) is formed, the ions from work will transport to tool. (0.5) EDM is based on the electric-induced arc on the small clearance; electric energy thermal energy melt the metal material. (0.5) One pros of ECM is: good surface quality (due to chemical removal). (0.5) One cons of ECM is: need of disposal of the electrolyte sludge after the process. (0.5) One pros of EDM is: hardness and strength are irrelevant to this process. (0.5) One cons of EDM is: poor surface quality (due to thermal removal). (0.5) Question I.2 [3 marks] Compare two additive manufacturing processes: SLS and SLM in terms of material and mechanical property. Explain the similarity of SLM versus SLS and Fusion welding versus solid state welding. SLS: starting material is powder, and laser strikes on the powder, and the temperature is below the melting temperature of powder. (0.5 mark) SLM: starting material is powder, and laser strikes on the powder, and the temperature is above the melting temperature of powder. (0.5 mark) SLM corresponds to fusion welding, in which the material is melted. (1 mark) SLS: corresponds to solid data welding, in which the material is not melted. (1 mark) Question 1.3 [3 marks] Washer The figure on the right shows a washer. Explain the role or function of the washer and the physics (principle) of the washer for achieving its role. The role of the washer is to enhance the strength of the non-permanent assembly. (1 mark) The principle of the washer is the elastic energy stored while the washer is assembled into between the nut and part, and after the assembly, the elastic energy in the washer is released to create pressure on the two pressing surfaces. (2 mark) 2

3 Question I.4 [3 marks] Explain the physics (principle) of resistance welding and elaborate why it does not need filler and is usually used for lap joints from the physics (principle). The principle of the resistance welding is to form an electric circuit for two parts (A, B), and then the interface between A and B serves as a resistance, on which high temperature is induced and the heat will then melt the material on the interface of A and B. (1 mark) The material melted is not a large amount, so no need to have filler to fill in the gap. (2 Mark) Question I.5 [3 marks] Take casting process as an example to define the process parameter, system parameter, and product parameter. Process parameter: pouring temperature. (1 mark) System parameter: mold material. (1 mark) Product parameter: casting. (1 mark) Question I.6 [3 marks] Give the definition of additive manufacturing and the definition of 3D printing. Additive manufacturing is to make a part by layer by layer. The starting material can be liquid, powder, and sheet. (1 mark) 3D printing is a nickname to AM, but it is in fact a kind of AM, in particular, the starting material is solid and then heated into melt, and when it is dropped onto a surface with room temperature, the melt solidifies. (2 mark) 3

4 Question 1.7 [3 marks] Explain any process in the AM (additive manufacturing) technology that may work with metal. If a 3D printing process is with powder as its starting material, it has potential to make a metal part. (3 mark) Question I.8 [3 marks] The figure on the right shows a part that is to be made by the casting process. Please explain the criteria used to make decision on parting line or surface and then determine the parting line for this part. The criteria are: (1) no or less cores and (2) no or less draft. (2 marks) The parting line is shown in the figure. (1 mark) Question I.9 [3 marks] Explain the operating principle of investment casting. Explain why the wax material is used to build mold. The operating principle of investment casting lies in how the pattern and mold are made. The pattern is made by wax, and then refractory material is coated on the wax pattern. After that, when the pouring of molten metal onto the wax pattern, the metal occupies the pattern, as the wax was melted away. (2 marks) The reason to use the wax is because it has the low melting point and easy to build a complex 3D pattern. (1 mark) Question I.10 [3 marks] In plastic injection molding, please (1) explain some potential defects with the process and (2) explain fundamental reasons responsible for these defects. 4

5 (1) 1 marks (2) 2 marks Question I.11 [3 marks] For the notation of 50 F7/h8 which may appear on a design drawing, please (based on the specification) (a) determine the type of fit, (b) determine whether it is basic shaft or hole system, (c) determine the quality of manufacturing, and (d) determine the tolerance for the hole and shaft. (1) Clearance fit based on the block diagram in module 10b. (1 mark) (2) Basic shaft system. (0.5 mark) (3) The quality of manufacturing is determined based on 7 and 8, which show a moderate accuracy. (0.5 mark) (4) (1 mark) Question I.12 [2 marks] In the plastic extrusion process, please (1) explain the meaning of maximum flow rate and the maximum head pressure, respectively, and (2) explain the concept of the operating point of the extrusion process. (1) Maximum flow rate happens when the head pressure is zero, and maximum head pressure happens when the flow rate is zero. (1 mark) (2) The operating point is represented by Q and P, and they represent the actual Q and P in the system. (1 mark) Question I.13 [2 marks] Explain the failure modes in the process of bulk metal drawing process. Two failure modes: 5

6 (1) The stress in the product at the exit of the die larger than yield stress. (1 mark) (2) The maximum power is larger than the power capacity. (1 mark) Question I.14 [3 marks] Explain the two most attractive features of the spot welding (i.e., less heat affected zone and good controllability of heat energy delivery) from the operating principle of the spot welding. The principle of spot welding is a kind of resistance welding. The principle of it is to have an electric energy be converted to heat energy (1 mark) From this principle, we can derive some features: (1) no filler and (2) sheet and sheet welding. (2 marks) Part II (60 marks) Question II.1 (20 marks) A casting product is a 2 in 4 in 6 in rectangular plate. Suppose that in the Chvorinov s rule formula as presented in the text, n=2. Suppose that two cylindrical risers (A, B) are used. A is on the top of the casting and B is adjacent to the casting. Both risers are embedded in the mold. The TSTC (TST of the casting) is 3.5 minutes. Please do the following: (a) Suppose that the height/diameter (H/D) ratio of the riser A is 1.5 and the H/D of the riser B is 2 and the diameters of the two risers are the same. Determine the geometry for the two risers by assuming TSTAB = TSTA + TSTB = 1.25 TSTC, where TSTA: solidification time of the riser A; TSTB: solidification time of the riser B; TSTAB: solidification time of the two risers together. [5 marks] (b) Determine the optimal geometry for the two risers, respectively, under the same assumption as (a). [15 marks] Question II.2 (40 marks) A hot rolling mill has rolls of diameter = 24 in. It can exert a maximum force equal to 500,000 lb. The mill has a maximum horsepower of 100 hp ( in lb/min). It is desired to reduce a 1.5 in.-thick plate by the maximum possible draft in one pass. The starting plate is 10 in. wide. In the hot condition, the work material has its strength coefficient equal to 20,000 lb/in 2 and strain hardening exponent of 0. In the cold condition, the work material has its strength coefficient 6

7 equal to 20,000 lb/in 2 and strain hardening exponent of Suppose the friction coefficient between the work and the roll is 0.15 (both for hot and cold rolling). Please do the following: (a) Determine the maximum draft for the hot condition and for the cold condition, respectively. [15 marks] (b) Determine the maximum speed of the rolls (ft/min) for the hot condition and cold condition, respectively. [10 marks] (c) Compare the hot rolling and cold rolling in terms of the maximum draft that can be achieved. [5 marks] (d) Suppose that the final thickness is required to be 1.2 with the hot rolling. Propose an optimal solution with justification to meet this new requirement of reduction. (You are not allowed to change the power supply system, nor allowed to change the roller). [10 marks] - THE END 7

A given material (shapeless or a simple geometry) Rolling, extrusion, forging, bending, drawing (plastic deformation)

A given material (shapeless or a simple geometry) Primary shaping processes Metal forming processes Metal cutting processes Metal treatment processes A complex geometry (shape, size, accuracy, tolerances,