MANUFACTURING OPERATIONS

There are certain basic activities that must be carried out in a factory to convert raw
materials into finished products. Limiting our scope to a plant engaged in making
discrete products, the factory activities are: (1) processing and assembly operations, (2)
material handling, (3) inspection and test, and (4) coordination and control.

The first three activities are the physical activities that “touch” the product as it is
being made. Processing and assembly operations alter the geometry, properties, and/or
appearance of the work unit. They add value to the product. The product must be moved
from one operation to the next in the manufacturing sequence, and it must be inspected
and/or tested to insure high quality. It is sometimes argued that these material handling
and inspection activities not add value to the product. However, our viewpoint is that
value is added through the totality of manufacturing operations performed on the
product. Unnecessary operations, whether they are processing, assembly, material
handling, or inspection, must be eliminated from the sequence of steps performed to
complete a given product.

1 Processing and Assembly Operations

Manufacturing processes can be divided into two basic types: (1) processing operations
and (2) assembly operations. A processing operation transforms a work material from
one state of completion to a more advanced state that is closer to the final desired part or
product. It adds value by changing the geometry, properties, or appearance of the
starting material. In general, processing operations are performed on discrete workparts,
but some processing operations are also applicable to assembled items, for example,
painting a welded sheet metal car body. An assembly operation joins two or more
components to create a new entity, which is called an assembly, subassembly, or some
other term that refers to the specific joining process.

Processing Operations. A processing operation uses energy to alter a

workpart’s shape, physical properties, or appearance to add value to the material. The
forms of energy include mechanical, thermal, electrical, and chemical. The energy is
applied in a controlled way by means of machinery and tooling. Human energy may also
be required, but human workers are generally employed to control the machines, to
oversee the operations, and to load and unload parts before and after each cycle of
operation. A general model of a processing operation is illustrated in Figure 2.1(a).
Material is fed into the process, energy is applied by the machinery and tooling to
transform the material, and the completed workpart exits the process. As shown in our
model, most production operations produce waste or scrap, either as a natural byproduct
of the process (e.g., removing material as in machining) or in the form of occasional
defective pieces. An important objective in manufacturing is to reduce waste in either of
these forms.

More than one processing operation are usually required to transform the starting
material into final form. The operations are performed in the particular sequence to
achieve the geometry and/or condition defined by the design specification.

Three categories of processing operations are distinguished: (1) shaping operations,

(2) property–enhancing operations, and (3) surface processing operations. Shaping
operations apply mechanical force or heat or other forms and combinations of energy to
effect a change in geometry of the work material.There are various ways to classify
these processes. The classification used here is based on the state of the starting
material, by which we have four categories:

1. Solidification processes. The important processes in this category

are casting (for metals) and molding(for plastics and glasses), in which
the starting material is a heated liquid or semifluid, in which state it
can be poured or otherwise forced to flow into a mold cavity where it
cools and solidifies, taking a solid shape that is the same as the cavity.

2. Particulate processing. The starting material is a powder. The common

technique involves pressing the powders in a die cavity under high
pressure to cause the powders to take the shape of the cavity. However,
the compacted workpart lacks sufficient strength for any useful
application. To increase strength, the part is then sintered— heated to a
temperature below the melting point, which causes the individual
particles to bond together. Both metals (powder metallurgy) and
ceramics can be formed by particulate processing.

Deformation processes. In most cases, the starting material is a ductile metal that
is shaped by applying stresses that exceed the metal’s yield strength. To increase
ductility, the metal is often heated prior to forming. Deformation processes
include forg ing, extrusion, and rolling.Also included in this category are sheet
metal processes such as drawing, forming, and bending.

4. Material removal processes. The starting material is solid (commonly

a metal, ductile or brittle), from which excess material is removed from
the starting workpiece so that the resulting part has the desired
geometry. Most important in this category are machining operations
such as turning, drilling, andmilling, accomplished using cutting tools
that are harder and stronger than the work metal. Grinding is another
common process in this category, in which an abrasive grinding wheel
is used to remove material. Other material removal processes are
known as nontraditional processes because they do not use traditional
cutting and grinding tools. Instead, they are based on lasers, electron
beams, chemical erosion, electric discharge, or electrochemical energy.

Property–enhancing operations are designed to improve mechanical or physical

properties of the work material. The most important property–enhancing operations
involve heat treatments, which include various temperatureinduced strengthening
and/or toughening processes for metals and glasses. Sintering of powdered metals and
ceramics, mentioned previously, is also a heat treatment, which strengthens a pressed
powder workpart. Propertyenhancing operations do not alter part shape, except
unintentionally in some cases, for example, warping of a metal part during heat
treatment or shrinkage of a ceramic part during sintering.

Surface processing operations include: (1) cleaning, (2) surface treatments, and
(3) coating and thin film deposition processes. Cleaning includes both chemical and
mechanical processes to remove dirt, oil, and other contaminants from the
surface. Surface treatments include mechanical working, such as shot peening and
sand blasting, and physical processes, like diffusion and ion
implantation. Coating and thin film deposition processes apply a coating of material
to the exterior surface of the workpart. Common coating processes
includeelectroplating, anodizing of aluminum, and organic coating (call it painting).
Thin film deposition processes includephysical vapor deposition and chemical vapor
deposition to form extremely thin coatings of various substances. Several surface
processing operations have been adapted to fabricate semiconductor materials (most
commonly silicon) into integrated circuits for microelectronics. These processes
include chemical vapor deposition, physical vapor deposition, and oxidation. They are
applied to very localized areas on the surface of a thin wafer of silicon (or other
semiconductor material) to create the microscopic circuit.

3. Assembly Operations. The second basic type of manufacturing operation is

assembly, in which two or more separate parts are joined to form a new entity.
Components of the new entity are connected together either permanently or
semipermanently. Permanent joining processes
include welding, brazing, soldering, andadhesive bonding. They combine parts
by forming a joint that cannot be easily disconnected. Mechanical
assembly methods are available to fasten two (or more) parts together in a joint
that can be conveniently disassembled. The use of threaded fasteners (e.g.,
screws, bolts, nuts) are important traditional methods in this category. Other
mechanical assembly techniques that form a permanent connection
include rivets, press fitting, and expansion fits. Special assembly methods are
used in electronics. Some of the methods are identical to or adaptations of the
above techniques. For example, soldering is widely used in electronics assembly.
Electronics assembly is concerned primarily with the assembly of components
(e.g., integrated circuit packages) to printed circuit boards to produce the
complex circuits used in so many of today’s products.

2 Other Factory Operations

Other activities that must be performed in the factory include material handling and
storage, inspection and testing, and coordination and control.

Material Handling and Storage. A means of moving and storing materials

between processing and/or assembly operations is usually required. In most
manufacturing plants, materials spend more time being moved and stored than being
processed. In some cases, the majority of the labor cost in the factory is consumed in
handling, moving, and storing materials. It is important that this function be carried
out as efficiently as possible. In Part II of our book, we consider the material handling
and storage technologies that are used in factory operations.

Eugene Merchant, an advocate and spokesman for the machine tool industry for
many years, observed that materials in a typical metal machining batch factory or job
shop spend more time waiting or being moved than in processing [3]. His observation
is illustrated in Figure 2.3. About 95% of a part’s time is spent either moving or
waiting (temporary storage). Only 5% of its time is spent on the machine tool. Of this
5%, less than 30% of the time on the machine (1.5% of the total time of the part) is
time during which actual cutting is taking place. The remaining 70% (3.5% of the
total) is required for loading and unloading, part handling and positioning, tool
positioning, gaging, and other elements of nonprocessing time. These time
proportions provide evidence of the significance of material handling and storage in a
typical factory.

Inspection and Test. Inspection and test are quality control activities. The
purpose of inspection is to determine whether the manufactured product meets the
established design standards and specifications. For example, inspection examines
whether the actual dimensions of a mechanical part are within the tolerances indicated
on the engineering drawing for the part. Testing is generally concerned with the
functional specifications of the final product rather than with the individual parts that
go into the product. For example, final testing of the product ensures that it functions
and operates in the manner specified by the product designer. In Part IV of this text,
we examine the inspection and testing function.

Coordination and Control. Coordination and control in manufacturing

includes both the regulation of individual processing and assembly operations as well
as the management of plant level activities. Control at the process level involves the
achievement of certain performance objectives by properly manipulating the inputs
and other parameters of the process.

Control at the plant level includes effective use of labor, maintenance of the
equipment, moving materials in the factory, controlling inventory, shipping products
of good quality on schedule, and keeping plant operating costs at a minimum possible
level. The manufacturing control function at the plant level represents the major point
of intersection between the physical operations in the factory and the information
processing activities that occur in production.