It is the manufacturing processes that create or add value to a product. The manufac-turing processes can be classified as:
●Casting, foundry, or molding processes
●Forming or metalworking processes
●Machining (material removal) processes
●Joining and assembly
●Surface treatments (finishing)
●Rapid prototyping
●Heat treating
●Other
These classifications are not mutually exclusive. For example, some finishing processes involve a small amount of metal removal or metal forming. A laser can be used either for joining or for metal removal or heat treating. Occasionally, we have a process such as shearing, which is really metal cutting but is viewed as a (sheet) metalforming process.
Assembly may involve processes other than joining. The categories of process types are far from perfect.
Casting and molding processes are widely used to produce parts that often require other follow-on processes, such as machining. Casting uses molten metal to fill a cavity.
The metal retains the desired shape of the mold cavity after solidification. An important advantage of casting and molding is that, in a single step, materials can be converted from a crude form into a desired shape. In most cases, a secondary advantage is that ex-cess or scrap material can easily be recycled. Figure 1-10 illustrates schematically some of the basic steps in the lost-wax casting process, one of many processes used in the foundry industry.
Casting processes are commonly classified into two types: permanent mold (a mold can be used repeatedly) or nonpermanent mold (a new mold must be prepared for each casting made). Molding processes for plastics and composites are included in the chap-ters on forming processes.
Forming and shearing operations typically utilize material (metal or plastics) that has been previously cast or molded. In many cases the materials pass through a series of forming or shearing operations, so the form of the material for a specific operation may be the result of all the prior operations. The basic purpose of forming and shearing is to modify the shape and size and/or physical properties of the material.
Metalforming and shearing operations are done both “hot” and “cold,” a reference to the temperature of the material at the time it is being processed with respect to the
The most common casting method through the ages was flit cire-perdue or lost-wax process. Although expensive and time consuming, the lost-way method allows the artist to accurately reproduce the delicate nuances of the original model.
Models depicting the casting of Rodin’s Sorrow (1889).
The Lost-Wax Casting Process
1
The artist creates a sculpted model, generally made of plaster, clay, marble, stone, or wood.
2
The surface of the model is coated with a protective substance.
Then the model is put into a bed of very fine elastic material held in place by a rigid outer mold. When the model is removed, its impression remains.
The surface of this second clay model is slightly scraped away.
When this second model is returned to the mold, there is a gap between the model and the mold.
This gap is where the wax will be poured. The final bronze will be of the same thickness as the gap that is created by the scraping.
5
After closing the mold around the clay model, hot wax is poured into the gap between the model and the mold. This stage is crucial in producing a perfect reproduction of the initial sculpture. The result is a clay model covered with wax, which is then hand-finished to fidelity, incorporating the artist’s signature, cast number and a foundry seal.
6
A network of wax pipes, called sprues and gates.
is attached to the wax model. These
A finely granulated ceramic is applied to the surface of the model and its pipes until it becomes thick and coarse.
The result, now called an
"investment mold," is then dried and heated causing the wax to melt and flow out of the mold. leaving a space between the fire resistant clay model and the investment mold.
Accordingly, this method is called the "lost-wax process." liquid bronze at the top, the mold is covered with a layer of cladding (a protective broken and the metal appears. The bronze sculpture and its sprues and gates are an exact reproduction of the wax in step six.
10
The network of sprues and gates is then removed and the surface of the bronze is often chiseled and filed so that no trace of them can be seen. This process of hand-finishing the bronze to perfection is called
"chasing." Any remains of the fireproof clay model left inside the bronze are also removed now.
11
When the chasing is finished, hot or cold oxides are applied
FIGURE 1-10 A manufacturing process represents a sequence of steps. Here are the steps in the lost-wax casting process.
Metalforming Process for Automobile Fender
Sheet metal bending / forming
Hot rolling
Cold-rolling plate Rolls Slab Billet
Sheet metal Slab
Single draw punch and die
Cast billets of metal are passed through successive rollers to produce sheets of steel rolled stock.
The flat sheet metal is “formed” into a fender, using sets of dies mounted on stands of large presses.
Slide
Sheet metal Cushion pin
Upper die
Punch
Punch Blank
holder
Trim punch Die
Scrap Fender
Scrap
The fender is cut out of the sheet metal in the last stage using shearing processes.
Sheet metal shearing processes are like scissors cutting paper.
Next, the sheet metal parts are welded into the body of the car.
Punch
Punch Sheet metal
Metal shearing
Die
Die 1 2
FIGURE 1-11 The forming process used to make a fender for a car.
temperature at which this material can recrystallize (i.e., grow new grain structure).
Figure 1-11 shows the process by which the fender of a car is made using a series of met-alforming processes.
Machining or metal removal processes refer to the removal of certain selected areas from a part in order to obtain a desired shape or finish. Chips are formed by interaction of a cutting tool with the material being machined. Figure 1-12 shows a chip being formed by a single-point cutting tool in a machine tool called a lathe. The manufacturing engi-neer may be called upon to specify the cutting parameters such as cutting speed, feed, or depth of cut (DOC). The engineer may also have to select the cutting tools for the job.
Cutting tools used to perform the basic turning on the lathe are shown in Fig-ure 1-12. The cutting tools are mounted in machine tools, which provide the required movements of the tool with respect to the work (or vice versa) to accomplish the process desired. In recent years many new machining processes have been developed.
The seven basic machining processes are shaping, drilling, turning, milling, saw-ing, broachsaw-ing, and abrasive machining. With the exception of shapsaw-ing, each of the basic processes has a chapter dedicated to it. Historically, eight basic types of machine tools
have been developed to accomplish the basic processes. These machine tools are called shapers (and planers), drill presses, lathes, boring machines, milling machines, saws, broaches, and grinders. Most of these machine tools are capable of performing more than one of the basic machining processes. Shortly after numerical control was invent-ed, machining centers were developed that could combine many of the basic processes, plus other related processes, into a single machine tool with a single workpiece setup.
Included with the machining processes are processes wherein metal is removed by chem-ical, electrchem-ical, electrochemchem-ical, or thermal sources. Generally speaking, these nontradi-tional processes have evolved to fill a specific need when convennontradi-tional processes were too expensive or too slow when machining very hard materials. One of the first uses of a laser was to machine holes in ultra-high-strength metals. Lasers are being used today to drill tiny holes in turbine blades for jet engines. Because of its ability to produce com-ponents with great precision and accuracy, metalcutting, using machine tools, is recog-nized as having great value-adding capability.
In recent years a new family of processes has emerged called rapid prototyping.
These additive-type processes produce first, or prototype, components directly from the software using specialized machines driven by computer-aided design packages. The prototypes can be field tested and modifications to the design quickly implemented.
Early versions of these machines produced only nonmetalic components, but modern
The cutting tool interacts with the workpiece to form a chip by a shearing process. The tool shown here is an indexable carbide insert tool with a chip-breaking groove.
The workpiece is mounted in a machine tool (lathe) with a cutting tool.
Workpiece Cutting
tool
The Machining Process
(turning on a lathe)
The workpiece is rotated while the tool is fed at some feed rate (inches per revolution). The desired cutting speed V determines the rpm of the workpiece.
This process is called turning.
Cutting speed Original
diameter
Final diameter
Feed (inch/rev)
Depth of cut Workpiece
V
Chip Tool
FIGURE 1-12 Single-point metalcutting process (turning) produces a chip while creating a new surface on the workpiece.
Level 1
Level 1
Level 4 Level 5
Computer
Level 2
Level 3 Die
or Integrated circuit (IC) on chip Microelectronic
manufacturing
Packaging
Cover chip
PCB assembly PCB fab
Motherboard
IC Package for
connection Silicon wafer
C H I P S O N
S I L I C O N
W A F E R
FIGURE 1-13 How an electronic product is made.
machines can make metal parts. In contrast, the machining processes are recognized as having great value-adding capability, that is, the ability to produce components with great precision and accuracy.
Perhaps the largest collection of processes, in terms of both diversity and quanti-ty, are the joining processes, which include the following:
1. Mechanical fastening 2. Soldering and brazing 3. Welding
4. Press, shrink, or snap fittings 5. Adhesive bonding
6. Assembly processes
Many of these processes are often found in the assembly area of the plant. Figure 1-13 provides one example where all but welding are used in the sequence of operations to produce a computer.
At the lowest level, microelectronic fabrication methods produce entire integrat-ed circuits (ICs) of solid-state (no moving parts) components, with wiring and connec-tions, on a single piece of semiconductor material, usually single-crystalline silicon.
Arrays of ICs are produced on thin, round disks of semiconductor material called wafers.
Once the semiconductor on the wafer has been fabricated, the finished wafer is cut up into individual ICs, or chips. Next, at level 2, these chips are individually housed with con-nectors or leads making up “dies” that are placed into “packages” using adhesives. The packages provide protection from the elements and a connection between the die and another subassembly called the printed circuit board (PCB). At level 3, IC packages, along with other discrete components (e.g., resistors, capacitors, etc.), are soldered onto PCBs and then assembled with even larger circuits on PCBs. This is sometimes referred to as electronic assembly. Electronic packages at this level are called cards or printed wiring assemblies (PWAs). Next, series of cards are combined on a backpanel PCB, also known as a motherboard or simply a board. This level of packaging is sometimes re-ferred to as card-on-board packaging. Ultimately, card-on-board assemblies are put into
housings using mechanical fasteners and snap fitting and finally integrated with power supplies and other electronic peripherals through the use of cables to produce final com-mercial products.
Finishing processes are yet another class of processes typically employed for clean-ing, removing burrs left by machinclean-ing, or providing protective and/or decorative sur-faces on workpieces. Surface treatments include chemical and mechanical cleaning, deburring, painting, plating, buffing, galvanizing, and anodizing.
Heat treatment is the heating and cooling of a metal for the specific purpose of al-tering its metallurgical and mechanical properties. Because changing and controlling these properties is so important in the processing and performance of metals, heat treat-ment is a very important manufacturing process. Each type of metal reacts differently to heat treatment. Consequently, a designer should know not only how a selected metal can be altered by heat treatment but, equally important, how a selected metal will react, favorably or unfavorably, to any heating or cooling that may be incidental to the manu-facturing processes.