Page 47 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Section 1.6 Selection of Manufacturing Processes

whether the product is for industrial or consumer use, the parameters to which the
product will be subjected (such as temperatures and chemicals), and any environmen-
tal concerns that have to be addressed.
Net-shape and Near-net-shape Manufacturing. Net-shape and near-net-shape
manufacturing together constitute an important methodology by which a part is
made in only one operation at or close to the final desired dimensions, tolerances,
and surface finish. The difference between net shape and near net shape is a matter
of degree of how close the product is to its final dimensional characteristics.
The necessity for, and benefits of, net-shape manufacturing can be appreciated
from the fact that, in the majority of cases, more than one additional operation is
often necessary to produce the part. For example, a cast or forged gear or crankshaft
generally will not have the necessary dimensional characteristics, thus requiring addi-
tional processing, such as machining or grinding. These additional operations can
contribute significantly to the cost of a product.
Typical examples of net-shape manufacturing include precision casting
(Chapter 11), forging (Chapter 14), forming sheet metal (Chapter 16), powder metal-
lurgy and injection molding of metal powders (Chapter 17), and injection molding of
plastics (Chapter 19).

Ultraprecision Manufacturing. Dimensional accuracies for some modern equip-
ment and instrumentation are now reaching the magnitude of the atomic lattice.
Various techniques, including the use of highly sophisticated technologies (see
micromechanical and microelectromechanical device fabrication in Chapter 29),
are rapidly being developed to attain such extreme accuracy. Also, mirror-like
surfaces on metals can now be produced by machining with a very sharp diamond
with a nose radius of 250 micrometers as the cutting tool. The machine is highly
specialized, with very high stiffness (to minimize deflections, as well as vibration
and chatter, during machining) and is operated in a room where the ambient tem-
perature is controlled to within 1°C in order to avoid thermal distortions of the
machine.

Types of Production. The number of parts to be produced (e.g., the annual quan-
tity) and the rate (number of pieces made per unit time) are important economic
considerations in determining the appropriate processes and the types of machinery
required. Note, for example, that light bulbs, beverage cans, fuel-injection nozzles,
and hubcaps are produced in numbers and at rates that are much higher than those
for jet engines and tractors.
Following is a brief outline of the general types of production, in increasing order
of annual quantities produced:

a. job shops: Small lot sizes, typically less than 100, using general-purpose ma-
chines such as lathes, milling machines, drill presses, and grinders, many now
equipped with computer controls.
b. Small-batch production: Quantities from about 10 to 100, using machines
similar to those in job shops.
c. Batch production: Lot sizes typically between 100 and 5000, using more ad-
vanced machinery with computer control.
d. Mass production: Lot sizes generally over 100,000, using special-purpose
machinery, known as dedicated mac/vines, and various automated equipment
for transferring materials and parts in progress.