When considering the application of design and materials to turning, perhaps the oldest and most extensively used form of machining, the first question to be asked is why turn at all? The only answer is that it is justifiable only when other methods of production are less economical for the quantities required, or are inferior from a design standpoint. Therefore, alternatives that can produce to the necessary tolerances should be evaluated first, including various forms of precision casting, such as diecasting and the lost wax process, sintering, precision forging and extruding, much of which has been described in preceding chapters, or even by changing the material, eg to plastics mouldings. These processes, however, have a high initial tool cost, even for simple forms and their use is therefore restricted to high volume production. Moreover, the use of dies and moulds makes it difficult, or even impossible, to include re-entrant details such as undercuts, recesses and screw threads, the machining of which can cost almost as much as the more extensive machining required by cheaper, conventional, raw material.
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