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Repairing Your Own Machine Components
|Many industrial concerns have workshops of their own. For the repair of worn shafts, the lathe machine is excellent. Keyway slots can be machined by using a milling machine, while a shaping machine can do machining of large flat areas. A drilling machine does drilling of holes.
A skilled Maintenance Engineer should know how to use all these machines in order to make his own repairs in a safe manner. Very often he has to supervise machinists. The information below should be useful for that purpose.
The lathe machine uses a single-point-cutting tool for a variety of turning, facing, and drilling jobs. Excess metal is removed by rotating the work piece over the fixed cutting tool to form straight or tapered cylindrical shapes, grooves, shoulders and screw threads. It can also be used for facing flat surfaces on the ends of cylindrical parts.
The work piece is clamped onto a horizontal rotating shaft by a 3-jaw or 4-jaw chuck. The latter chuck can be used to cut off-centreed cylinders. The rotating horizontal spindle to which the chuck is attached is usually driven at speeds that can be varied.
The cutting tool is fixed onto a tool rest and manipulated by hand. It can also be power driven on straight paths parallel or perpendicular to the work axis. This is useful for screw cutting.
Internal turning known as boring results in the enlargement of an already existing hole. The holes are more accurate in roundness, concentricity, and parallelism than drilled holes. A hole is bored with a single-point-cutting tool that feeds along the inside of the work piece.
The shaping machine is used to machine flat surfaces, grooves, shoulders, T-slots, and angular surfaces with single-point tools. The cutting tool on the shaper oscillates, cutting on the forward stroke, with the work piece feeding automatically toward the tool during each return stroke.
The drilling machine is used to cut holes in metal with a twist drill. By changing the cutting tool, they can be used to do reaming, boring, counter boring, countersinking, and threading.
The milling machine uses a rotating cutting tool to cut flat surfaces, grooves, and shoulders, inclined surfaces, dovetails, and T-slots. Cutters of many shapes are changed to cut different grooves.
Metal-cutting tools are classified as single point or multiple point. The lathe and shaping machine use single point cutting tool while the milling and drilling machines use multiple-point-cutting tools.
Metal is cut either by moving the work piece like in the lathe or by moving the tool like in the shaping machine, drilling or milling machine. Clearance angles must be provided to prevent the tool surface below the cutting edge from rubbing against the work piece. Rake angles are often provided on cutting tools to cause a wedging action in the formation of chips and to reduce friction and heat.
In order to remove chips from a work piece, a cutting tool must be harder than the work piece and must maintain a cutting edge at the temperature produced by the friction of the cutting action.
Carbon steel tools even though comparatively inexpensive tend to lose cutting ability at temperatures around 400 degree F (205 degree C).
High-speed steel, containing 18 percent tungsten, 4 percent chromium, 1 percent vanadium, and only 0.5 to 0.8 percent carbon, permits the operation of tools twice or three times the speeds allowable with carbon steel
Cast-alloy cutting-tool materials containing cobalt, chromium, and tungsten are effective in cutting cast iron and retaining their cutting ability even when red hot.
Cemented Tungsten Carbide
The hardness of Tungsten Carbide approaches that of a diamond. Tungsten carbide tools can be operated at cutting speeds many times higher than those used with high-speed steel.
Ceramic, or oxide, tool tips consist primarily of fine aluminum oxide grains, which are bonded together. These are very hard.
An overheated tool can become blunt and soft very fast. Therefore very often, cooling fluids cools the cutting points of the tool. This serves to lubricate and cool.
Water is an excellent cooling medium, but it corrodes ferrous materials. Sulfurized mineral oil is one of the most popular coolants as it can both cool as well as lubricate. The sulfur prevents chips from the work from melting on to the tip of the tool.
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