Suppose the aforementioned transfer line could produce in two weeks all of the cylinder blocks needed for an entire year. That means the machines would be idle for 50 weeks, which would be prohibitively expensive. That's where N/C comes in. With N/C it might take three or four weeks to produce the cylinder blocks. But then the N/C machine could be quickly reprogrammed to produce cylinder heads or transmission cases or to machine body stamping dies.
N/C can machine three-dimensional contours that would be prohibitively expensive, if not impossible, to machine by conventional means. It permits engineers to design products with geometries that previously were uneconomical (and hence impossible) to produce. (There is no point in producing something if the cost of production is so high nobody can afford to buy it.)
N/C will make 10, 100, 1000, or more parts exactly the same , time after time, without deviation (except for machine and cutting tool wear). Try the same thing with a journeyman machinist using manual machine tools. No two parts will be exactly alike. And probably 10% of the parts will not meet specifications and so will require reworking or to be scrapped. Although machine tools arranged in an automated transfer line can achieve a high degree of accuracy, they are still human controlled; their repeatability cannot compete with that of N/C.
Products wear out and replacement parts must be available. In the past, the economics of mass production required extra parts to be produced and stored as spare parts in a warehouse, so that six months or five years later, customers could obtain replacements for worn-out parts. Warehoused spare parts represent capital tied up and doing nothing! Warehoused parts are often considered property for property tax purposes. When design changes occur, warehoused spare parts become obsolete. That is expensive. But N/C machines are quick and easy to set up. When an order for spare parts comes in, simply load the program into the N/C machine, make a not-too complicated setup, run the production, ship the parts, and send the customer an invoice. Rather than storing a warehouse full of spare parts, all that needs to be stored is a filing cabinet full of N/C programs on tape or disk. The investment is reduced, capital is not tied up sitting in a warehouse, spare parts are not encountering a property tax, the company cash flow is improved, and engineers can change the design as often as their hearts desire without scrapping a warehouse full of spare parts.
Tooling such as the jigs and fixtures used for conventional manual (and automated) machining processes is expensive, takes a long time to make, and is difficult to modify. That means that it takes a lot of money and time to get into production. The production process (and hence the product design) tends to become "cast in concrete." But N/C requires little if any tooling. Usually a vise or simple clamping fixture is all that is required. Also because N/C can drive a cutter to a specific location, even along a contour path, special tooling is not needed to position or guide the cutter. Again, a simple vise is often all that is needed to hold the workpiece. A change in design does not require modifying a lot of complex special tooling. All that is required is a quick change in the N/C's program. Again, engineers can change designs to their hearts content! Or a company can quickly respond to changes in the marketplace--and be a "leg up" on their competition.
N/C operators do not direct the operation of the machine tool. They simply load and unload the workpiece, perhaps load and unload cutting tools (although this is usually done automatically), push the button to start the operation, and push the panic button if anything goes wrong (like a tool goes dull or breaks). This does not require anywhere near the level of skill required of the journeyman machinist who directs the operation of manual machine tools. Operators are easier to find and train and command lower salaries, thereby improving the company's position in this very competitive industry.
| What are the Disadvantages of N/C? |
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Updated Jan. 9, 2002
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Bill Hemphill
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