Mechanical machining methods

Mechanical Machining Methods. The inherent pliability and shear properties of plastics, particularly of thermoplastics, makes them highly amenable to shaping with traditional tools such as saws, drills, planes, shapers, lathes, sanders, and millers. The materials are easily cut by steel tools, which are harder than any plastic. For this reason, mock-up designs are often initially constructed as plastic models rather than as metal constructs, allowing design engineers to test various physical properties, such as aerodynamic stability and wind resistance, that are directly related only to the shape of the structure and not to the materials used. 

When large, flat surfaces and webs of materials are to be coated and when production reaches rates of 200 to 300 pieces per day, machine methods of application should be considered over the manual methods described above. Mechanical-roller methods are commonly used to apply a uniform layer of adhesive via a continuous roll. Such automated systems are used with adhesives that have a long working life and low viscosity. Various machine methods of adhesive application are illustrated in Fig. 17.8, and several are described below. 

In all fairness, in a survey of machine methods which are in operation, those abandoned because of dissatisfaction should be included. Ashthorpe (2) describes difficulties encountered at the Atomic Energy Research Establishment in Harwell. After two years in operation and the accumulation of 6500 Hollerith-type cards, the installation was abandoned and a multiple file of visually located index cards was established. The punched-card installation was completely dependent upon a single file of punched cards arranged in random order. It was impossible to ascertain how a document previously coded had been indexed without resorting to the punched card file. In almost all operations now used, supplementary hand files permit the rapid location of certain specific information. Ashthorpe gives an excellent description of the difficulties and frustration which may come from too complete dependence on a single punched-card file. It is a very instructive article for those embarking into the mechanical field. 

During the densification process the preforms are usually placed in the reaction chamber at a reduced pressure. As there is no pressing or other mechanical forces exerted on the fibre preform during the densification process the final shape of the preform is very close to the desired shape and there is no large amount of material to be further removed by the machining method. The starting preforms therefore can be very complex in terms of shapes as this should represent the final finished target component. 

It should be noted that the in vitro mechanical test methods most often used to date suffer from end artifacts [35-37], errors due to platen-specimen friction and machining-related damage of the specimen ends, which compromise the accuracy of most of the data. Modulus values are underestimated by at least 20% [36-38], with the error possibly depending... 

Mechanical excavation methods are labor saving and safety compared to drill and blast operations. Vertical Shaft Sinking Machine (VSM) and Shaft Boring System (SBS) can be used for top-down excavation. 

The plants are hand-cut, mowed, or pulled in developing coimtries, while mechanized harvesting methods are imder investigation in the United States. Ribboning machines are sometimes used to separate the fiber-containing bark before retting for recovery of the kenaf strands. For pulping, the kenaf is shredded or hammermilled to 57-cm pieces, washed, and screened. 

Physical pre-treatment processes are employed without chemical agent and microorganisms (Zheng et al, 2014). The common methods are mechanical machines, irradiation and extrusion (Karimi et al., 2013) with the objectives to open up lignocelluloses, reducing crystallinity, and increasing surface area and porosity, for more attacking availability of post-treatment (Behera etal., 2014). 

Shellac is another very useful adhesive of ancient development. It is a dried secretion found upon certain subtropical bushes that have been host to the Lac insect. The brown, shiny, water-resistant solid melts to a thick sticky liquid when moderately heated, and upon cooling it again becomes solid and firmly stuck to anything it touched in the liquid state. Long used by watch and instrument makers where small delicate components were to be machined, it had advantages over complex mechanical holding methods in that the part was not marked or distorted out of truth. 

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