Manufacturing Methods and the Drawing
A manufacturing department starts with what might be called raw stock and modifies this until it agrees with the detail drawing. Before preparing a drawing for the production of a part, the drafter should consider what manufacturing processes are to be used. These processes will determine the representation of the detailed features of the part, the choice of dimensions, and the machining or processing accuracy. Principal types of metal forming are (1) casting, (2) machining from standard stock, (3) welding, (4) forming from sheet stock, and (5) forging. Knowledge of these processes, along with a thorough understanding of the intended use of the part, will help determine some basic manufacturing processes. In sand casting all cast surfaces remain rough textured, with all corners filleted or rounded. Sharp corners indicate at least one of the surfaces is finished and finish marks are shown on the edge view of the finished surface. In drawings of parts machined from standard stock most surfaces are represented as machined. In some cases, as on shafting, the face existing on the raw stock is often accurate enough without further finishing. Corners are usually sharp, but fillets and rounds are machined when necessary. For example, an interior corner may be machined with a radius to provide greater strength. On welding drawings the several pieces are cut to size, brought together, and then welded. Notice that on drawings lines are shown where the separate pieces are joined. On sheet-metal drawings the thickness of material is uniform and is usually given in the material specification note rather than by a dimension on the drawing. Bend radii and bend reliefs at corners are specified according to standard practice. For forged parts separate drawings are usually made for the diemaker and for the machinist. All corners are rounded and filleted and are so shown on the drawing.
Although a number of different casting processes are used, sand molds are the most common. Sand molds are made by ramming sand around a pattern, and then carefully removing it, leaving a cavity that exactly matches the pattern to receive the molten metal. The pattern must be of such a shape that it will pull away from the sand. The plane of separation of the two mold halves is the parting line on the pattern. Since shrinkage occurs when metal cools, patterns are made slightly oversize. The patternmaker accomplishes this by increasing the pattern size by predetermined amounts, dependent upon the kind of metal being used in the casting. Due to shrinkage and draft, small holes are better drilled in the casting, and large holes are better cored (cast-in) and then bored. The patternmaker receives the working drawing showing the object in its completed state, including all dimensions and finish marks. Usually the same drawing is used by the patternmaker and the machinist; hence, it should contain all dimensions and notes needed by both.
Welding is a process of joining metals by fusion. Although welding by lasers is becoming popular in sophisticated plants, arc welding, gas welding, resistance welding, and atomic hydrogen welding of stock plates, tubing, and angles are more commonly used. Forging is the process of shaping metal to a desired form by means of pressing or hammering. Generally, forging is hot forging in which metal is heated to a required temperature before forging. Some softer metals can be forged without heating, and this is cold forging. Draft, or taper must be provided on all forgings.
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Heat-treating is when heat is used to alter the properties of metal. Different procedures affect metal in different ways. Annealing and normalizing involve heating to a critical temperature range, and then slowly cooling to soften the metal, thereby releasing internal stresses developed in previous manufacturing processes. Hardening requires heating to above the critical temperature followed by rapid coolingquenching in oil water, brine, or in some instances in air. Tempering reduces internal stresses caused by hardening also improves the toughness and ductility. Surface hardening is a way of hardening the surface of a steel part while leaving the inside of the piece soft. Surface hardening is accomplished by carburizing, followed by heat-treatment; by cyaniding, followed by heat-treatment; by nitriding; by induction hardening; and by flame hardening. Lasers are widely used for transformation hardening of selected surface areas of metal parts. Laser hardening applies less heat to the part than do traditional methods, and thermal distortion is greatly reduced.
The two main families of plastics are known as thermosetting and thermoplastic. The thermosetting plastics will take a set and will not soften when reheated, whereas thermoplastics will soften whenever heat is applied. Typical plastic processing operations include extrusion, blow molding, compression molding, transfer molding, injection molding, and thermoforming. An extrusion-molding machine transfers solid plastic particles, additives, colorants, and regrinds from a feed hopper into a heating chamber. The extruder screw conveys the material forward through the barrel toward a die. The contour of the die will determine cross-sectional shape of the extruded member. Typical uses of the extrusion process are in compounding and pelletizing of bulk plastics, pipe and profile extrusion, blown film and sheet manufacturing, and blow molding.
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