Machining: The Versatile Giant
Machining processes make up the broadest range of options for parts manufacturing techniques. Machined parts and machining services are available from a large and highly competitive supplier base. This article will help buyers understand the various major machining types and will help lead to informed purchase and selection decisions.
Machining is unquestionably the most versatile of all manufacturing processes. More properly, the concept of machining encompasses a broad range of processes, including a number of radically new techniques.
Machining is most frequently considered as comprising a family of processes that involve relative motion between a tool and a workpiece resulting in material removal to form a desired shape. This group of processes can be found in most machine shops and includes operations such as:
Milling is where rotating cutters feed in to a stationary or traversing workpiece. Milling machines may have vertical or horizontal spindles. Milling-type machines can usually perform a variety of operations, including drilling, tapping (cutting internal threads), boring, or reaming.
Drilling is where a rotating tool feeds into a stationary workpiece. Special variations on drilling machines include gundrills, used for drilling deep holes such as gun bores or cooling channels in injection mold bases. Multiple-spindle drilling machines permit rapid processing of parts with multiple holes in the same plane.
Turning is where a single-point tool feeds into a rotating workpiece. Machines in this broad category are called lathes. Engine lathes have a gearbox to permit cutting external threads. Screw machines are a family of turning machines automated either mechanically with special cams or by CNC for producing batch parts. Swiss-type automatics are used for small precise parts, while other automatic screw machines can handle parts up to about 3" in diameter. Vertical turret lathes use a rotating worktable and are useful for cutting features such as bearing raceways into large parts.
Boring is where a single-point tool rotates and feeds into a workpiece to generate a precise, round hole. Boring machines are usually very heavily built in order to eliminate vibration and to provide the rigidity necessary to cut large, true holes. These machines also generally have very accurate positioning systems to allow precise location of holes. Often, such holes are required in large parts such as mold or die bases, machine frames, and so on.
Grinding is a process in which small abrasive particles imbedded in a grinding wheel, belt, cylinder, or stone accomplish material removal. Many different relative tool/workpiece motions are used in various grinding machine types. Grinding is ordinarily used only for removing small amounts of material at a time. Grinding permits very accurate tolerances of 0.001" or less, and allows for very smooth surface finishes. While grinding is often considered a secondary operation, i.e. one that is done after a part has been substantially formed by some other process, many parts like the kinds of precision shafts used in many business machines can be machined entirely by grinding machines.
Centerless grinding is a good choice for shaft work. Instead of being held between centers, (as on a lathe) the work rests on a roller and a stationary supporting blade while an abrasive roller wheel removes material around the work's true axis. Centerless and other cylindrical grinders are good machining choices when concentricity is important.
A variety of grinding machines will generate smooth surfaces in flat planes to within a few ten-thousandths of an inch, such as the horizontal-spindle or vertical-spindle surface grinder. Of these, the vertical-spindle type are usually more suitable for larger surfaces. Honing is a specialized process that uses an abrasive stone mounted on a shank similar to a boring bar to grind the inside surface of holes. Lapping is another specialized process for producing very highly finished flat surfaces.
Grinding is also a good choice for machining very hard metals such as carbides, inconel, and tool steels. These kinds of operations are most often applied to metal, but are appropriate to a wide variety of materials including plastics and various composites. The traditional machining processes usually result in forming chips or small particles (swarf) of the removed material.
All of the traditional chip-cutting techniques are capable of being automated to at least some extent with numerical control (NC) or computer numerical-control (CNC).
Some of the newer machining techniques do not employ a cutting tool at all, at least not in the usual sense. Of the newer techniques, (EDM) is the most widely available. EDM is also the most mature of the newer machining techniques.
Developed for first practical use during the 1950's, the process erodes metals and other conductive materials with a controlled electrical arc. Thus, a three-dimensional carbon electrode can erode its reverse image as a cavity in a block of material. Because of this unique capability, and because the soft carbon electrodes are easier and cheaper to sculpt than expensive tool steels, EDM was originally used almost exclusively for making die and mold cavities. A new dimension literally came to EDM with the development of the traveling wire EDM in the 1970's. The traveling wire electrode permitted EDM cutting in a manner similar to a bandsaw or jigsaw, but with vastly better precision and surface quality. Today, both solid-electrode (also known as ram-type) and traveling-wire machines are used effectively in batch production and for one-off operations such as toolmaking.
Lasers are another radically new technique for machining. Lasers can be used for drilling, cutting, engraving, and similar straight-line operations. Lasers are also finding increased applications on the shop floor in measurement and calibration systems and other applications besides material removal. Laser-drilled holes tend to be flared (bell-mounted) at the entry point. Although this condition can be controlled to a point, it may be a drawback in some cases.
Abrasive water-jet machining is another brand-new technique that already competes with laser-cutting as a choice of manufacturing technique. Abrasive particles are suspended in a jet of water under pressure to cut material to a desired two-dimensional form.
Unlike the more traditional processes, suppliers offering laser machining and abrasive water-jet cutting are fewer in number and more widely dispersed.
Machined parts can usually be produced with a minimal tooling investment. Such tooling is usually limited to inexpensive consumable cutting tools. While some jobs may require development of special jigs, fixtures, or an NC or CNC program, these kinds of up-front requirements are usually cheaper and less time-consuming than designing and building permanent tools, dies, molds, and special machines.
Because special tooling is not required, machining is often the only economical way to make a single item or a small batch. The use of NC and CNC also greatly facilitates the ability of typical small machine shops to produce small batches in repeat orders economically. The same technology also helps meet just-in-time systems and contributes to improve quality.
Parts machined from a solid piece of material often are structurally stronger than parts with identical geometry made by other processes. Castings, for example, tend to be of relatively soft materials and have little "grain" or directionality of the material structure. Machined parts, on the other hand, can be made of very hard materials, and can take advantage of the grain properties of rolled or even forged material.
Because machined parts, even in great quantity, do not require complicated tooling, changes to the part design or specification can be made quickly and economically. The use of CNC and computer-aided- design and computer-aided-manufacturing (CAD/CAM) help many machining companies accomplish customers' changes routinely.
There are some 12,000 tooling and machining companies dispersed throughout America's industrial regions, and most of these companies are quite small. A recent Air Force study revealed that about 65 percent of the content of a military aircraft is manufactured in these small shops.
Machining companies often specialize in particular market niches (such as turbine blades or fuel injection nozzles) or in single services (such as grinding specialists or EDM shops). Companies that specialize in building tools, dies, and molds use machining techniques to build their tools and frequently perform job-shop machining as well, particularly when the job requires high precision, hard materials, and other tough requirements.
Many buyers prefer to deal with local suppliers of technical products like machined parts. While current express shipment services and electronic communication capabilities are reducing the dependence upon the local base, with 12,000 companies nationwide, there is a good corps of machining and tooling companies close to your own plant.
Your machining supplier should have a plan for quality control, and that plan should be in writing. Such a quality manual needn't be fancy, but it should reflect what actually happens on the shop floor and in the front office.
A walk through the shop can tell you a lot about a company. Is it clean? Well lighted? Does the operation appear well organized? Has the company or its employees received any special awards, distinctions, certifications, training achievements, or other special recognition?
The company you select should be at least partially NC or CNC equipped. Presently, and for the foreseeable future, manually-operated equipment, in the hands of a smart operator can compete effectively with a CNC approach on a single job, and within a few other constraints. Particularly in skill-intensive, one-of-a-kind situations such as prototype work or toolmaking, traditional manual methods may even sometimes be faster and cheaper (i.e., if a CNC program doesn't have to be developed first). But the technological arena is full of moving targets, and nothing stays the same for very long anymore. The more-advanced technologies of CNC and all its permutations will give the sophisticated user a long-term advantage in price, quality, turnaround time, and overall customer satisfaction. In fact, CNC permits the manufacture of parts whose geometry is so complex that no other method can even be considered.
Currently, attempts at transferring CNC machine programs from a customer's host computer to a supplier's machine tools via a modem or other such device have met with only limited success. Problems include dissimilar standards and machine interfaces, and differing toolchanger capacities and control requirements. Most progressive machining companies prefer to receive a data base of the parts geometry instead of toolpath data. In this way, the supplier can develop the most efficient program for the part, using the full capability of the equipment at hand.
A recent study by the National Tooling & Machining Association (NTMA) showed that buyers and users of machined parts and machining services rated delivery and scheduling as among their chief problems with suppliers. Many NTMA member companies, including many very small ones, report success with shop scheduling and control software packages that will run on small computers. A growing number of such packages are available, and NTMA members are able to compare notes on their success and failures with these software systems through an information-sharing program conducted by the Association.
NTMA is made up of 3300 member companies in 62 local chapters throughout the industrial U.S. The nonprofit association is governed by a Board of Trustees representing the local chapters, and directed by a panel of committees made up of member volunteers. Daily management and administration are handled by a small staff at NTMA's Fort Washington, Maryland headquarters office.
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