Metal Fabrication & Machining: Operations
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As metal stock flows through the shop one of the first stop it makes is at processes that reduce the size of the stock to create individual workpieces. The most common cutting processes are:
- Shearing - mechanical, hydraulic or pneumatic shears capable of cutting larger sheets of metal into smaller workpieces
- Power Hack Sawing - short, straight blade that is drawn back and forth over the workpiece.
- Band Sawing - continuous band with small teeth that perform one-directional cutting.
- Circular Sawing - ideal for cutting billets, forgins, extrusions, bars, tubes, and similar stock, generally five inches in diameter or less.
- Acetylene Torch Cutting - hand held torch that uses heat to cut through metal workpieces when close tolerances are not required.
When the metal stock is cut to desired dimensions the workpiece is now ready to be shaped. In metal shaping, the workpiece is formed into its desired geometries through the application of mechanical force. This process is typically called forming and the most common types of equipment to achieve a formed part are:
- Press Brakes - hydraulic, pneumatic and electrical machines that use force to bend the workpiece into flanges, contours and other geometries.
- Rolls - machine to bend the workpiece to desired radius
- Spinners - process of forming axially symmetrical shapes
- Punching - punches various shapes into the workpiece
Metal workpieces are then machined to meet customer specifications. The machining process gives the workpiece its final geometry through the removal of metal by a number of processes including:
- Broaching - a metal removal process that is performed on flat, round, or contoured substrates. The multi-toothed cutting tools used in broaching have teeth that are generally higher than the preceding tooth, each removing more material as it passes over the substrate.
- Boring - a precision metal removal process that create internal cylindrical holes that are generated using a single-point or multiple-edge cutting tool.
- Twist Drills- drilling is the production or enlarging of holes in a workpiece by the relative motion of a cutting tool.
- Turning - in turning operations, a workpiece is rotated about its longitudinal axis on a machine tool called a lathe. Material is removed by tools mounted on the lathe to create the desired shape.
- Counterboring, Spotfacing, and Countersinking - enlarging a hole for a limited depth is called counterboring. If the cut is shallow, leaving only a finished face around the original hole, it is called spotfacing. The cutting of an angular opening into the end of a hole is known as countersinking. A counterbore tool-bit usually has straight or helical flutes for the passage of chips or fluids.
- Reaming - process is used for enlarging, smoothing, and/or accurately sizing existing holes by means of a multi-edge fluted cutting tool.
- Milling - involves the removal of metal in small, individual chips made by each milling cutter. In face milling, chip thickness varies from a minimum at the entrance and exit, to a maximum along the horizontal diameter.
- Grinding - an abrasive process, that unlike other metalworking processes, removes metal from the work piece by abrasive grains.
The category of nontraditional machining covers a broad range of technologies, including some that are used on a large scale, and others that are only used in unique or proprietary applications. These machining methods generally have higher energy requirements and slower throughputs than traditional machining, but have been developed for applications where traditional machining methods were impractical, incapable, or uneconomical.
Non traditional machining methods have inherent P2 benefits and are typically divided into the following categories:
- Mechanical - Ultrasonic Machining, Rotary Ultrasonic Machining, Ultrasonically Assisted Machining
- Electrical - Electrochemical Discharge Grinding, Electrochemical Grinding, Electrochemical Honing,Hone-Forming, Electrochemical Machining, Electrochemical Turning, Shaped Tube Electrolytic Machining, Electro-StreamTM
- Thermal - Electron Beam Machining, Electrical Discharge Machining, Electrical Discharge Wire Cutting, Electrical Discharge Grinding, Laser Beam Machining.
- Chemical - Chemical Milling, Photochemical Machining
Tool and Workpiece Cooling
Metalworking fluids perform numerous functions in metal fabrication and machining processes. Fluids provide the following:
- Metalworking fluids provide cooling action to the workpiece and the tool. This prevents thermal damage to the workpiece, minimizes thermal stressing and brittle hardening of the tool bit, and prevents built-up edge (BUE). In place of fluids for cooling, some operations may use pressurized air, a combination of pressurized air and fluid, or other means.
- Chips that are formed during chip-making processes can have a tendency to build up in the cutting area and cause undue stress on the machine tool. Fluids are used to flush chips out of the work zone.
- The amount of lubrication provided by the fluid largely depends on the type of fluid and fluid additives used. Straight oils provide fair lubricity, but when compounded with active chemical agents such as chlorine, sulphur, or phosphate, they provide a great deal more lubrication.
Most metal fabrication and machine shops have processes that join metal parts together. The most common methods of joining metal parts are:
- Nearly all metal fabrication processes employ some form of welding. Welding processes vary by heat source, pressure, and filler metals.
Brazing and Soldering
- Brazing and soldering use heat and filler metals to connect metal surfaces without melting the base metals.
- Adhesive joints are prepared by applying adhesives and then curing or setting the adhesive.
Surface/Substrate Treating (excluding organic and non-organic coatings)
During the manufacture of fabricated metal parts, the surface (or substrate) of the metal workpiece is often treated in order to: protect the part from cracking; improve machinability; relieve stresses, enhance mechanical properties; provide impact resistance; and alter the electrical or magnetic properties of the substrate.
The following methods and processes are used in treating the surface of metal workpieces:
- Heat Treating - Heat treating may take place at various stages throughout the fabrication process. Parts are heat treated using a number of different methods to supply the heat. Furnace types include: salt-bath, vacuum, fluidized-bed, induction, laser, and electron beam.
- Case Hardening - Similar in nature to heat treating, surface or case hardening involves heating parts to impart certain surface characteristics. Surface hardening is a thermochemical treatment in which the chemical composition of the steel surface is altered.
- Quenching - Metals are cooled after heat treating at varying rates and to varying degrees depending on the type of metal and the desired characteristics. Cooling media may be air, oil, polymer, water, or molten salts, and may be applied in streaming gaseous form, liquid baths, fog, or mist.
- Buffing/Polishing - Buffing and polishing are performed to smooth and shine the surface of parts to give the product its finished look. A variety of buffing and polishing machines are used, including polishing and buffing lathes, high-speed polishing machines, or off-hand buffing and polishing pads.
- Tumbling and Vibratory Finishing - Tumbling and vibratory finishing are performed for cleaning, oxide removal/descaling, polishing, brightening, and edge-breaking/burr removal. There are wet and dry tumbling and vibratory processes that use a variety of media including ceramics, stone, glass beads, metal shot, nut shells, corn husks, hardwoods, and plastic/resin beads.
Please refer to Automotive Repair topic hub for more information on maintenance processes and pollution prevention opportunities.