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Miltiadis A. Boboulos

Exercises

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Miltiadis A. Boboulos

Manufacturing Processes and Materials:

Exercises

Manufacturing Processes and Materials: Exercises

Contents

Contents

Summary 6

Question 1: Non-conventional manufacturing processes 7

Question 2: The Electro-discharge Machining (EDM) process 14

Question 3: Factors causing tool wear 20

Question 4: Acceptance sampling 33

Question 5: Principles of the Resin Transfer Moulding (RTM) 45

Question 6: Fibre reinforced plastic composites 50

Question 7: A cutting test on a steel bar 55

Question 8: Electro-discharge machining (EDM) requirements & properties 58

Question 9: Hard and soft automation 62

Question 10: Surface integrity of manufactured surfaces: properties & applications 65

Question 11: Bored holes - plug and gap gauges 68

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Exercises Contents

Question 12: Integrated Manufacturing Systems: a facility of large and small machines 73

Question 13: Simulation method, determining the time to complete the batch and the machine utilisation 79

Question 14: Plain carbon steels and high strength low alloy steels (HSLA) 91

Question 15: Ferritic stainless steel & the mechanism of corrosion 97

Question 16: Impact Strength (J) to Test Temperature 102

Question 17: Metal and polymer material applicability 108

Question 18: Maximum tolerable through thickness crack size 112

Question 19: Glass fibres production - Reinforced composite design 117

References 122

Endnotes 124

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Exercises Question 1: Non-conventional manufacturing processes

Question 1: Non-conventional manufacturing

processes

You are a Manufacturing Engineer employed by a toolmaking company whose main business is in sub-contract manufacture of a wide range of tools used in the injection moulding and forging industries. There is also a specialist division machining small batches of precision components for the aerospace industry. Component workpiece materials include most toolsteels, high duty alloys and a range of sintered materials, non-ferrous materials stainless steels and ceramics. The existing manufacturing facility include all the usual conventional machine tools including a number of stand alone CNC multi-tool machining centres.

Your Managing Director, through his trade association and by glancing through technical journals is aware that competitors of the company are introducing non-conventional manufacturing processes to their facilities.

You have been requested to submit a brief report covering the following issues:

a) What is meant by the term non-conventional manufacturing processes? [2 Marks]

b) What are the areas of application of such processes? [8 Marks]

c) How do non-conventional processes compare with the companies existing process facilities in respect to:

Feature capability, surface finish, surface integrity, material removal rate, tool wear, environmental issues and skill requirements. [12 Marks]

d) What particular non-conventional process might be suitable for the companies current product portfolio? [3 Marks]

Exercises Question 1: Non-conventional manufacturing processes

Question 1a

Conventional and wide spread machining processes include: mechanical cutting operations, material removal techniques – chipping off, forging, casting, stamping, engraving. Additionally, conventional processes include turning, milling, drilling, grinding etc. mechanical operations. Back in the 1940s the needs of the defense industry, aviation and space industry, electronics and other industries necessitated machining techniques to be adopted for processing thin, fragile or special and very thin products that could not be manufactured using the conventional processes or this would have been rather impractical and costly. Therefore, a new group of “non-conventional” manufacturing processes emerged to provide improved, convenient and economically advantageous means for specific types of production. These were based on latest scientific and technical achievements and some new findings for using laws of nature relating to light – lasers, sound – ultrasonic processes, magnetism, atomic physics – plasma, electronics and new “powder” metallurgy materials.

Non-conventional processes include:

a) Chemical machining (CM) b) Electrochemical machining (ECM) c) Electrochemical grinding (ECG) d) Electrical discharge machining (EDM) e) Wire electrical discharge machining (WEDM) f) Laser-beam machining (LBM) g) Electron-beam machining (EBM) h) Water-jet machining (WJM) i) Abrasive water-jet machining (AWJM) j) Abrasive-jet machining (AJM) (using air, sand or beads)

Additionally, we could include here Ultrasonic machining (UM) and Deburring processes.

Example: A typical non-conventional process is the machining of abrasive discs using diamond (adopted in 1955) or synthetic tools – cubic boron carbide (1970). In some applications these processes replaced almost completely the aluminium oxide processes (1893) and the green silicon carbide processes (1891). Other non-conventional processes include powder metallurgy processes used to produce hard-alloy cutting tools made of tungsten carbide, titanium carbide, cobalt carbide, etc.

Exercises Question 1: Non-conventional manufacturing processes

The depth of the cutting plates is adjustable to up to 300mm. The tool (the wire) is usually made of copper, brass or tungsten and of outside diameter 0.25 mm.

Another optional EDM technique is the electrical discharge grinding where a conventional internal grinding machine is used the grinding stone of which is a conductor material (brass, graphite) playing the role of the electrode and the part being machined is any conductor material. Mostly used for grinding hard carbide alloys of titanium, tungsten, cobalt and tool steels; for machining fragile and brittle small-size components, surgical tools, optical devices, electronic devices, etc.

f) Laser-beam machining (LBM) is used for similar applications to those stated above – cutting, drilling, marking and for surface machining and welding operations involving various materials: metals, ceramics, plastics, leather, textiles, composite materials (in the aircraft industry, etc.).

g) Electron-beam (plasma) machining (EBM) is used in similar applications to those described for LBM but performed in a vacuum surrounding medium: precise cutting and welding of various materials.

h) Water-jet machining (WJM): This technique is used for dynamic cutting and machining various materials: plastic, rubber, foodstuffs, paper, leather, insulation materials, composite materials of up to 25mm thickness. Finds application in the food industry and the production of plastics.

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Exercises Question 1: Non-conventional manufacturing processes

i) Abrasive water-jet machining (AWJM): for “shooting” under pressure and applying dynamic action to the surface of the machined component part. Used for the same applications and materials as those described for WJM.

j) Abrasive jet(gas) machining (AJM): Applied for machining small holes, cleaning surfaces from removing sand or scale in foundry applications, stamped forgings and also for non-metal and fragile materials, as well as for deburring operations.

Question 1c

As described in paragraph 1b above, machining operations feature similar or various spheres and sites of application. Non-conventional manufacturing processes are applied where conventional methods are not applicable, such as cutting and machining very hard, fragile, brittle or small-size component parts.

ii) Based on the particular characteristics of the process we can select the most suitable technique for each specific application. For example, cutting the internal cavity of an average-sized temperature treated high-hardness die is usually slow and expensive when using conventional machining techniques. A suitable non-conventional process for such an application is the EDM method. We select the suitable method based on the material hardness, brittleness, part size and material type. If we have to cut thick steel plates along an external contour that could be of a complicated shape the suitable method is the WEDM process. For drilling and welding various materials we select the LBM method and for drilling holes of outside diameter smaller than 0.1 – an operation which is almost impossible to perform using conventional techniques- the EDM or ECG process.

Machining rough or corroded (oxidized) external surfaces is best performed using the AJM manufacturing process.

iii) Quality comparison: Several quality characteristics are important here and these include surface roughness (Ra), dimensional tolerances, structure of the material in the cutting area. To examine these parameters we use data from tables, graphs, formulae and process studies.

For example, these include the Roughness (Ra)/ process type relation charts as shown in Figure 2 and the tolerance/process type relation chart, as well as the average and extreme repetition probabilities for their values.

iv) Comparison based on structure: Some manufacturing processes, like for example the LBM and EBM result in distortions of internal material structure in the cutting area, so other techniques are to be preferred when this is not desirable, such as CM or ECM, EDG and EDM.

Exercises Question 1: Non-conventional manufacturing processes

Question 1d

The current average-size toolmaking range of the company can preferably employ any conventional equipment and techniques used for this type of production along with the EDM, WEDM, AJM, ECG and ECM non-conventional manufacturing processes. The production of more complicated small-size component parts for the aerospace industry should preferably employ the EBM. LBM, EDM, ECM and EDG manufacturing processes. A precise estimate of process efficiency should be made when selecting the suitable type of process taking into account relatively expensive machines and equipment involved in the EDM, LBM, EBM, etc. processes.

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Exercises Question 2: The Electro-discharge Machining (EDM) process

Question 2: The Electro-discharge Machining

(EDM) process

a) The Electro-discharge Machining (EDM) process is widely used m the toolmaking industry. In this context discuss the advantages and the disadvantages of the die sinking EDM process. [14 marks]

b) With the aid of an annotated sketch illustrate a typical EDM single voltage pulse, indicating voltage levels and timescales. Explain why a characteristic pulse has this profile. [6 marks]

c) (Explain the principles and features of the "orbital technique" as applied to the EDM process. [5 marks]

Question 2 a

When applying the EDM manufacturing process the workpiece is machined either “sunk” into a specific fluid or not, the fluid which covers the workpiece in the cutting area being a dielectric.

The method which involves die-sinking uses a work table specifically made airtight (a sinking bath where the fluid is provided). The type of fluid most widely used is kerosene (petrol), distilled water or deionized water. This arrangement of the application of the EDM manufacturing process provides for the electrical discharge between the tool [electrode (-)] and the cathode (+), which in fact is the machined workpiece, to take place in dielectric fluid medium. The method features the following advantages and disadvantages:

2a 1 Advantages:

1. The fact that the process takes place in a fluid medium improves the removal of metal chips from the cutting area and enhances cooling characteristics of the tool and workpiece.
2. Improved cooling and fast discharge resulting from switching off of the electrical impulse (frequency between 50 and 500 KHz) improves the wear resistance of the electrode (tool) and improves surface integrity (Ra) of the machined surface.
3. Due to the electrical discharge the process eliminates almost completely the emission of harmful gases into the atmosphere.
4. The process allows for “heavy” duty operation in higher frequency and current (A) values which results in increased process efficiency.

Exercises Question 2: The Electro-discharge Machining (EDM) process

Question 2b

1. Corona and spark discharge

This type of discharge process occurs when a relatively high pressure is available and electrodes are featured by very high non-uniformity of the discharge area (gap). Ionization takes place only in a thin layer around the electrode characterized by a small radius of the curve. This is called an ionizing layer. In real practice these are “corona” cylindrical wires. When DC voltage is supplied, a negative (-) or positive (+) corona is available depending on the polarity of the corona electrode. As the voltage on the corona electrode increases, the corona undergoes arch discharge or spark discharge (when the source output power is insufficient to maintain stationary arch discharge – constant electrical arch. * When a positive corona is applied spark discharge occurs at lower voltage (U) compared to negative corona. The temperature of the gas in the spark “channel” reaches 10000K. This allows for thermal ionization to occur. This phenomenon does not fall within the category of the theory of the “avalanche”- type of discharge and is explained by the theory of “streamers”.

The first condition for the formation of “streamers” is the following:

(1) d

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p

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p

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1 ppd 8 3

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Exercises Question 2: The Electro-discharge Machining (EDM) process where a – coefficient of volumetric ionization [M -1^ ]; p – gas pressure [Pa] d – distance between electrodes E 3 – voltage in the area between electrodes [V/m].

From (1) we can conclude U 3 = E 3 d (2), Where U 3 is the voltage required for a spark to occur [V], and E 3 is the voltage in the area between electrodes [V/m].

Thus calculated, the voltage U 3 is exactly identical with experimental data measured at p.d > 250 [MPa].

The second condition for streamer formation is: (3) n i ≥ 7. 10 20 [ions/m^3 ], where: n i is the concentration of ions in the avalanche head.

The condition (3) refers to relatively short spark gaps (times) and is always met when (1) is met as well.

Study case 1:

Calculate drilling voltage between flat electrodes in air medium at p = 10 5 [Pa] (760mm mercury column) and T = 293K (20C):

• for d = 1 cm (0.01m) U 3 = 31.35 [kV]
• for d = 2 cm (0.02m) U 3 = 58.10 [kV]

2. Diagram of voltage pulse

According to (2) above the diagram of a single voltage pulse is as follows:

Figure 1

Exercises Question 2: The Electro-discharge Machining (EDM) process

2 c 2. Applying the “orbital” technique to the EDM process

The “orbital” technique is applied to the EDM manufacturing process with the tool, the electrode (-), performing a similar movement to the described in Figure 1 above. An example of one such application is illustrated in Figure 2.

Figure 2

1. Tool holder 1.1 Tool (electrode)
2. Workpiece (round grinding machine)
3. Machine chuck

Let us consider an application of the EDM process for a machine featuring horizontal spindle and chuck 3 used to support the workpiece to be machined, with the holder 1 feeding the tool 1.1 along the X direction, the feeding of the tool resulting in gradual change of the angle  from 0 to . The workpiece 2 rotates around X – X with the chuck 3. This results in shaping (cutting) the cavity of an OD = D, having the shape of a pear. Different cutting shapes can be achieved through synchronizing the movement of the workpiece, tool and tool feed.

The “orbital” movement of the electrode can also be applied to conventional machines of vertical tool axis and die-sinking. This will require the rotational movement of the spindle of the machine to provide for the “orbital” movement in point O 1 (operational end) of the electrode. The workpiece can alternatively be fixed to the machine worktable. Thus, smaller size (diameter) electrode could be used to cut or grind larger-size cavities or holes of shapes which are completely different from the cylindrical shape, as well as toroid-shaped, etc. cavities – Figure 3.

Exercises Question 3: Factors causing tool wear

Question 3: Factors causing tool wear

a) In conventional metal cutting process tool wear is inevitable. Discuss the most significant factors that cause tool wear and explain why cutting tool failure is difficult to predict. [8 marks]

b) Describe four different methods that might be used for the on-line monitoring of tool wear, indicating the possible problems associated with each method and justify the method that you consider to show the most promise. [12 marks]

c) A steel ring outside diameter 600mm and an internal diameter of 200mm is being faced on a vertical CNC lathe. The machine is capable of maintaining a constant surface speed, as the face of the ring is being machined and the feedrate is set to 0.25 mm/rev. From tests when v = 50n/min Tool Life T is 60 mins n = 0.3. Given Taylor's empirical tool life relationship VT = C. Determine the number of components that can be machined per tool for a tool life of 50 mins. [5 marks]

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