Friday, May 4, 2012

Mehmet Özer, 030070050, Bonus Words


1. Centerburst (Deformation)

There is no previous entry about Centerburst.

New Answer
This defect is an internal crack that develops as a result of tensile stresses along the centerline of the workpart during extrusion. Although tensile stresses may seem unlikely in a compression process such as extrusion, they tend to occur under conditions that cause large deformation in the regions of the work away from the central axis. The significant material movement in these outer regions stretches the material along the center of the work. If stresses are great enough, bursting occurs. Conditions that promote centerburst are high die angles, low extrusion ratios, and impurities in the work metal that serve as starting points for crack defects. The difficult aspect of centerburst is its detection. It is an internal defect that is usually not noticeable by visual observation. Other names sometimes used for this defect include arrowhead fracture, center cracking, and chevron cracking.

(Groover M.P., Fundamentals of Modern Manufacturing: Materials, Processes, and Systems 4th Edition, pp. 429-430)



2. Water Jet Cutting (Manufacturing Method)


There is no previous entry about Water Jet Cutting.


New Answer
Water jet cutting (WJC) uses a fine, high-pressure, high-velocity stream of water directed at the work surface to cause cutting of the work, as illustrated in Figure 26.2. To obtain the fine stream of water a small nozzle opening of diameter 0.1 to 0.4 mm (0.004 to 0.016 in) is used. To provide the stream with sufficient energy for cutting, pressures up to 400 MPa (60,000 lb/in2) are used, and the jet reaches velocities up to 900 m/s (3000 ft/sec). The fluid is pressurized to the desired level by a hydraulic pump. The nozzle unit consists of a holder made of stainless steel, and a jewel nozzle made of sapphire, ruby, or diamond. Diamond lasts the longest but costs the most. Filtration systems must be used in WJC to separate the swarf produced during cutting.
Cutting fluids in WJC are polymer solutions, preferred because of their tendency to produce a coherent stream. We have discussed cutting fluids before in the context of conventional machining (Section 23.4), but never has the term been more appropriately applied than in WJC.
Important process parameters include standoff distance, nozzle opening diameter, water pressure, and cutting feed rate. As in Figure 26.2, the standoff distance is the separation between the nozzle opening and the work surface. It is generally desirable for this distance to be small to minimize dispersion of the fluid stream before it strikes the surface. A typical standoff distance is 3.2mm(0.125 in). Size of the nozzle orifice affects the precision of the cut; smaller openings are used for finer cuts on thinner materials. To cut thicker stock, thicker jet streams and higher pressures are required. The cutting feed rate refers to the velocity at which the WJC nozzle is traversed along the cutting path. Typical feed rates range from 5 mm/s (12 in/min) to more than 500 mm/s (1200 in/min), depending on work material and its thickness [5]. The WJC process is usually automated using computer numerical control or industrial robots to manipulate the nozzle unit along the desired trajectory.
Water jet cutting can be used effectively to cut narrow slits in flat stock such as plastic, textiles, composites, floor tile, carpet, leather, and cardboard. Robotic cells have been installed with WJC nozzles mounted as the robot’s tool to follow cutting patterns that are irregular in three dimensions, such as cutting and trimming of automobile dashboards before assembly [9]. In these applications, advantages of WJC include: (1) no crushing or burning of the work surface typical in other mechanical or thermal processes, (2) minimum material loss because of the narrow cut slit, (3) no environmental pollution, and (4) ease of automating the process. A limitation of WJC is that the process is not suitable for cutting brittle materials (e.g., glass) because of their tendency to crack during cutting.
(Groover M.P., Fundamentals of Modern Manufacturing: Materials, Processes, and Systems 4th Edition, pp. 630-631)



3. Screw Press (Tooling)

There is no previous entry about Screw Press.

New Answer
In screw presses [18] the upper ram and die are connected to a large vertical screw that can be rotated by a flywheel, so that the ram can move up and down relative to the fixed die in the bed of the machine (Fig. 14.18c). The ram has a limited amount of energy for each stroke; thus multiple blows are usually employed similar to hammers. Screw presses are available in ratings from 0.63 MN to 63 MN (63-6300 tons).
(loan Marinescu, Product Design for Manufacture and Assembl, pp. 615-616)

Screw presses apply force by a screw mechanism that drives the vertical ram. Both screw drive and hydraulic drive operate at relatively low ram speeds and can provide a constant force throughout the stroke. These machines are therefore suitable for forging (and other forming) operations that require a long stroke.
(Groover M.P., Fundamentals of Modern Manufacturing: Materials, Processes, and Systems 4th Edition, pp. 415)

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