UNIT 1 & 2 Machine Drafting PDF

UNIT 1 INTRODUCTION TO MACHINE DRAWING Machine drafting is a central area of specialization in technical drawing. It has its particular methods of preparing working drawings about machine parts or components. It is presented through several o...

UNIT 1 INTRODUCTION TO MACHINE DRAWING Machine drafting is a central area of specialization in technical drawing. It has its particular methods of preparing working drawings about machine parts or components. It is presented through several orthographic views so that the size and shape of the features are fully understood. Familiarization with the primary terminologies involved in machine drawings will contribute much to the technical drawing presentations. ABBREVIATIONS and SYMBOLS in MACHINE DRAFTING A/F - across flats Alum - aluminum ASTM - American Society for Testing Materials BC - bolt circle CBORE - counterbore CI - cast iron cm - centimeter/s CPL - cutting plane line CRS - cold-rolled steel CSK - countersink DP- deep DWG - drawing FH - flat head H - height Hex - hexagon HRS - hot rolled steel ID - inside diameter ISO – International Organization for Standardization L - length LD – large diameter LG – long LH – left hand m – meter/s M – Metric MAJ – major MATL – Material MAX – maximum Mm – millimeter NO. - number OC – on center OD – outside diameter PT – point QUAN – quantity RAD – radius SAE - Society of Automotive Engine SD – small diameter SFACE - spotface SI----International System of Units STD--standard  UNIT 2 ELEMENTS OF MECHANICAL MANUFACTURING Before preparing a drawing for a part's production, the drafter should consider what manufacturing processes will determine the representation of its detailed features machining accuracy. Knowledge of these processes and a thorough understanding of the part's intended use will give the relevant result. PRODUCTION MATERIALS Material specifications form a significant part of the information shown on a detailed working-drawing. The most suitable material for a particular machine part depends mainly on the use to which it will be exposed. Some of the characteristics of materials for consideration before selection are the following: Availability Cost Workability Stability Weight Machinability Wear resistance Corrosion resistance Strength Ability to further hardened or toughened COMMON METALS USED in MECHANICAL MANUFACTURING Metals are substances characterized by high electrical and thermal conductivity and malleability, ductility, and high reflectivity of light. Types of Metals 1. Ferrous Metals  Ferrous metal is one whose chief element is iron and has magnetic properties. Pig Iron is the elemental coarse iron produced during the iron ore's refining process in a blast furnace. All types of iron and steel are from pig iron. It contains several elements such as manganese, silicon, phosphorous, sulfur, and mainly, carbon. Cast Iron is a mixture of high-grade scrap iron and pig iron. It is used for parts that need casting in sand molds, which explains its name. Examples are automobile engine blocks, heads, and manifolds. Malleable Iron is the same as cast iron, except that it is specifically treated to refine its grain structure and reduce its carbon content. Refining the grain structure reduces brittleness; thus, malleable iron is used extensively for parts that must withstand shock loads. Steel is iron that contains less than 1% carbon. It is a highly refined form of iron with added particular elements to give hardness, wear-resistance, toughness, and corrosion resistance. Elements such as nickel, manganese, tungsten, and chromium, which are added to steel to provide it with desirable properties, are called alloying elements. Carbon is the second most crucial element in steel. The amount of carbon determines its hardness and its ability to be hardened further. Low-carbon steel is relatively soft steel and is often referred to as mild steel. It contains up to 0.50% carbon. High-carbon steel is hardened steel and requires special cutting tools to machine it. It contains over 0.50% carbon. 2. Non-Ferrous Metals Non-Ferrous are metals that do not contain iron, or any small amount of it. These metals are non-magnetic and corrosion-resistant. Common Non-Ferrous Metals Brass is an alloy of copper and zinc. Parts made of brass can be cast in a mold or machined to shape. Brass machines easily.  Bronze is an alloy of copper and tin. It machines easily and be cast readily. Bronze is a desirable bearing metal. Aluminum is extracted from alumina, which is found in an earthly compound called bauxite. It weighs one-third as much as steel and is used extensively for machine parts that must be light in weight. Aluminum is alloyed with copper, zinc, and magnesium to give it the hardness and flexibility qualities. Aluminum machines easily. PRIMARY PRODUCTION PROCESSES A primary production process is one that provides a machine part with its basic or characteristic shape. The main considerations in the selection of a primary production process are: the shape of the part its end use the quantity of the parts to be produced The most common primary production processes are: 1. Machining Machining is a process in which a material is cut into a desired final shape and size by a controlled material-removal process. Cylindrical parts are usually machined to shape using an automatic or semi- automatic lathe machine to form their exterior or interior contours. 2. Casting is a manufacturing process in which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process. Casting is simply giving shape to a machine part using patterns in the molds. Sand Casting Die Casting  3. Forging Forging is the process of shaping metal parts by squeezing or hammering metals into pre-shaped dies. Cold-forming refers to the process of squeezing unheated metals into pre-determined shapes. Hot-forming refers to the process of squeezing heated metals into pre- determined shapes. Usually, forging requires more than a single operation to shape a part using different dies. Machine parts subject to abnormal loads and stresses are usually forged to provide them with increased strength characteristics. 4. Stamping Flat sheet-metal stock is placed into or passes through one or more pre- shaped dies mounted in presses to stamp out the shape to the raw material. Often, parts made from sheet metal require more than one stamping operation to complete. Examples of items shaped by stamping dies are refrigerator panels, stove panels, automobile bodies and bumpers, toasters, pots, pans, knives, forks, and spoons. 5. Welding Welding is the method of joining metals by allowing localized areas to melt, intermingle with a joining metal, and then cool. This process is used to fabricate parts when the volume is too small to justify sand-casting or forging. Welding is also frequently used as a primary production process because it enables the production of components with high strength properties. SECONDARY SHOP PROCESSES The secondary production process is any shaping operation done after any primary shaping process to finish a part to the shape outlined on a detailed working- drawing. Most secondary production processes involve cutting tools to remove material on details whose primary forms are obtained by sand-casting, forging, or welding. 1. Drill Drilling is forming a cylindrical hole in solid material using a twist drill or drill bit to provide clearance for bolts, screws, pins, and rivets. It is not an accurate machining operation. Blind drilled hole Through drilled hole 2. Ream A reamed hole is a cylindrical hole finished accurately to shape and size with a reamer. Reaming requires that an undersized drilled hole be produced first. Then, the reamer is passed through the drilled hole to remove only a few hundredths of a millimeter of material to attain its exact shape and size. 3. Bore Boring is consists of enlarging an existing cylindrical hole by the use of a boring tool. 4. Counterbore Counterboring consists of enlarging an existing cylindrical hole uniformly to a required depth. It permits parts to sit below the surface to improve appearance or promote safety or space limitations. 5. Spotface A spotface is similar to a counterbore, except that the enlargement of the existing cylindrical hole is kept very shallow. Spotfacing provides a localized machined area, usually over a cylindrical hole, against which parts can sit squarely. It is a machining operation reserved primarily for details that have rough surfaces, such as sand castings and forgings. 6. Countersink Countersinking is a machining operation that bevels the opening of a cylindrical hole to accommodate a part's tapered shoulders. Like counterboring, countersinking is usually done to improve appearance or promote safety or because of space limitations. 7. Knurling Knurling is a machining operation that produces a ridged surface on a part to provide a better grip for the hand. Knurling can be coarse, medium, and fine, of either diamond-pattern knurls, or straight-pattern knurls. 8. Chamfer A chamfer is a beveled edge. Chamfers eliminate the sharp edges on parts handled manually. They also allow components to assemble quickly. They can slope at any degree, with 30,45 and 60 degrees as most expected. MACHINE SHOP TERMS AND INFORMATION Keyways and Keyseats A keyway is a slot or recess cut into the side of a cylindrical hole, parallel to the hole's axis. A keyseat is a slot or recess cut into a shaft parallel to the shaft's axis. Keys are used to securing parts, such as a shaft and a pulley, enabling them to rotate together. Bosses and Pads Bosses and pads are raised surfaces found particularly on sand castings and forgings. The term" boss" refers to a circular, raised surface of a part. Raised surfaces not circular in shape are called pads. Pads and bosses are usually machined to provide bearing surfaces against which parts can sit squarely. Pads and bosses' use eliminates the need to machine large areas of parts when only localized areas around holes are required. Finish Marks A finish mark is a symbol that indicates that a surface must be machined. Taper A taper is a cone-shaped feature consisting of two diameters of different sizes located on the same axis.