Cork Institute of Technology. Autumn 2007 Manufacturing Engineering. Time: 3 Hours

Transcription

1 Cork Institute of Technology Bachelor of Engineering (Honours) in Mechanical Engineering - Stage 3 (NFQ - Level 8) Autumn 2007 Manufacturing Engineering Time: 3 Hours Instructions: Answer FIVE questions Answer TWO from each of Sections A and B and ONE from section C Use a separate answer book for each section Examiners: Mr. P. Clarke Prof. M. Gilchrist Mr D. Fallon Mr. M. P.O Mahony Dr. M.E. Woods Section A Q1 (a) List FIVE advantages and/or disadvantages of thread rolling. (5 Marks) (b) Briefly describe ONE method of Thread grinding. (5 Marks) (c) A special fine thread has a nominal outside diameter of 4 inches. The pitch is 0.25in and the threads are of Whitworth Form. The thread is acceptable if the effective diameter is between nominal size and minus in. When checked with wires of diameter 0.15in the dimensions over wires was in. If the flank angle and pitch are correct, calculate the effective diameter and hence state whether the thread is acceptable. (Note Height of fundamental triangle for Whitworth H= P) (8 Marks) (d) When measuring the effective diameter of a thread, explain what is meant by the term Best Wire? (2 Marks) Q2. (a) State and define the terms in the Hollomon Equation. (5 Marks) (b) Explain Strain Hardening. (4 Marks) (c) Illustrate the THREE rules that should be followed when designing parts that are to be upset-forged. (6 Marks) (d) Determine the maximum reduction on a piece of steel 250mm thick, during cold rolling when µ=0.1 and during hot rolling when µ=0.6 for a roll radius of 500mm. Comment on how to achieve maximum draft when rolling. (5 Marks)

2 Q3. (a) List three ways of countering the effects of Springback in a bending operation. (3 Marks) (b) Fig Q3 shows the dimensions of a bracket required to be bent from a flat strip of mild steel of 450N/mm 2 yield stress. If the work is drawn to a depth of 18mm below the top surface of the die, find the punch load required for: (i) Bending (1 Marks) (ii) Overcoming friction, assuming the sides of the component are very slightly ironed, take µ=0.15. (2 Marks) (iii) Planishing at the end of the stroke (2 Marks) (c) In the context of Deep Drawing: (i) List the material properties which principally determine how well a metal may be drawn. (2 Marks) (ii) State the rules which should be observed to obtain the maximum drawing ratios. (2 Marks) (d) List FIVE guidelines for the design of Blanking/Piercing Tools. (5Marks) (e) What is SHEAR in the context of a Press Tool and what is its purpose? (3 Marks) Fig Q3

3 Section B Q4. (a) State the equations for Fick s 1 st and 2nd Laws of Diffusion and explain each parameter. [4 marks] (b) Describe with the aid of a diagram the process and application of sintering. [4 marks] (c) Aeronautical jet turbine engine blades are an example of creep limited design made from a superalloy. State the main component element in the superalloy. Describe TWO ways in which the material development of the superalloy reduces has led to a reduction in the effects of diffusion creep and power law creep. State what the effect of inclusion of chromium in the superalloy has on its properties. [6 marks] (d) A thin metallic membrane of thickness 150 µm separates two compartments of nitrogen. The compartments are maintained at nitrogen concentrations of 3.52 x atoms m -3 and 6.2 x atoms m -3 respectively. The diffusivity of nitrogen through the membrane is 5.2 x m 2 s -1. Calculate the nitrogen diffusion rate through the membrane. [6 marks] Q5 (a) State the equation for the fast fracture condition, taking care to define each parameter. Compare the relative susceptibility of mild steel and un-reinforced concrete to fail by fast fracture. [4 marks] (b) If each of the three materials in Table 1 contains a single crack of dimension, a = 1 mm, determine whether any of these materials will fail by fast fracture before plastic yielding will occur. [7 marks] Table 1 Material Fracture toughness (K c ) MN m -3/2 Yield strength (σ y ) MN m -2 Medium carbon steel Polycarbonate 2 55 (c) (i) Describe with the aid of a diagram the cleavage mechanism of fast fracture. (ii) State the formula for the local stress in the region of the crack and state the typical value of the breaking stress at which fast fracture occurs in terms of the Young s Modulus of the material. (iii) What main difference is there between the micro-mechanisms for the ductile tearing fast fracture process compared with the cleavage mechanism? How does this difference become apparent when fast fracture surfaces for both mechanisms are viewed under a microscope? [9 marks]

4 Q6 (a) What is fatigue failure in a material? [2 marks] (b) There are two categories of fatigue in materials which are initially un-cracked. (i) Name the two categories. (ii) State the relevant stress condition and ONE operational condition for each of the categories of fatigue in initially un-cracked materials. (iii) Give ONE example for each category listed in (i) of a mechanical situation where that category of fatigue failure may occur. (iv) For ONE of the named categories, describe with the aid of a diagram the process of fatigue failure in a sample. [13 marks] (c) Define friction and wear. Give ONE example where friction and wear both occur with one as a desirable effect and the other as an undesirable effect. [5 marks]

5 Section C: Quality Engineering Q7 Resistors for electronic circuits are being manufactured on a high-speed automated machine. The machine is being set up to produce a large run of resistors of 1,000 ohms each with a tolerance of ± 30 ohms. To set up the machine and to create a control chart to be used throughout the run, 15 samples were taken with four resistors in each sample. The complete list of samples and their measured values are as follows: Sample Number Sample Average X Sample Range R Fig. 7.1 Process samples for resistor values in ohms (i) Using the tables and control chart provided, develop an X chart and an R chart and plot the values. (10 Marks) (ii) Using appropriate calculations make a comment on both the control and capability of the process. (10 Marks)

6 Q8. (a) If (acceptance plans are) used for quality audit of final product as it goes out the door, they guarantee that some customers will get defective product. (i) Sketch an Operating Characteristic (OC) curve with an Acceptable Quality Level (AQL) of 2%, a producers risk of 5%, a Limiting Quality Level (LQL) of 6%, a consumer s risk of 8% and an Indifference Quality Level (IQL) of 4%. (4 Marks) (ii) Explain the meaning of Acceptable Quality Level (AQL) and describe how it relates both to the operating characteristic of a sampling plan and to the quality of the manufacturing process. (4 Marks) (b) Write notes on any three of the following; (i) Hartley s constant (ii) Cp, capability index. (iii) Cpk capability index. (iv) Attribute control charts. (12 Marks)

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