Tài liệu Century Manufacturing P2 ppt

~~u;g(
mm
Hot rulling :
,Dit:cll~ting(Al)
Shell
casting
(sleel)
Cold rolling
Forglng Isteel}
Sand casurtg tsteel)
34
Manufacturing Analysis; Some Basic Ouesttona for a Start-Up Company Chap, 2
Forging(AI.Mg);~~ (AI,Cllstiron)
Thermoplastic polymers
0.4
i
0.3
~
~
1
0.2
.1
~ 0.1
Minimum dimension of web w (in.)
FifW"lil2.7 Process capabilities related so part geometry. Very thin sections tevor
rolling and thermotorrmng: "cDunky"s<:ctiQusfavor machining and injection
molding (from fmroductivlIlIJ Manufacturing Processes by
J,
A Schey,
if)
1987.

Reprinted with permission of the McGraw-Hill Companies),
The thermoforming of plastic sheets is slightly above cold rolling in the graph.
This also creates sections that are relatively thin, and thus it competes with cold
rolled metal products for many common items that require less structural rigidity,
The middle part of the graph relates to processes that create more "chunky" looking
parts of greater thickness (the
y
axis in the figure). Finally, note that the mold making
procedures in sand casting prevent it from being selected if one of the dimensions is
less than 5 millimeters (0.2 inch),
2.3.7
Accuracy, Tolerances, and FideUty between CAD and CAM
In all fabrication processes-semiconductors, plastics, metals, textiles, or other-
wise-the physical limitations of each process have a major impact on the
echiev-
able accuracy. Each processing operation comes with a bounding envelope of
performance that is constrained
by
the physical and/or chemical processes that,
during fabrication, are imposed on the original work material. This begs the fol-
lowing question: How much fidelity will there be between (a) the specified CAD
geometry, tolerances, and desired strength and (b) the final physical object that is
manufactured? In the best case scenario, the CAD geometry will be perfectly trans-
lated into the fabricated geometry. Also, the properties of the original piece of work
material stock will be either unchanged or possibly work-hardened into an even
more preferred state.
2.3 Question 2: How Much Will the Product Cost to Manufacture
(e)?
35
Accuracy microns

TABlE
2.3 Routine Accuracies for Mechanical Processes (One "Thou" Approximately =
25 Micronsl
Accuracy inches
Hot, open die forging
Hot, closed die forging
Investment casting
Cold, closed die forging
Machining
Eleetrodischarge machining
Lapping and polishing
+f-1250microns
+
f -
500 microns
+f-75-250microns
+1- 50--125microns
+/-25-125microns
+/-12.5microns
+1-0.25 microns
+/-0.05 inch
+/-0.02mch
+/-0.003-0.01 inch
+/- 0,002-0.005 inch
+/- 0.001-0.005 inch
+/- 0.0005 inch
+/- 0.ססOO1inch
In the worst case situation, a poorly controlled process will damage a perfectly
good work material. Examples of tbis were widespread in the early days of welding,
where beat-affected zones reduced the fracture toughness of materials. Controlling

this envelope for each process is quite complex and relies on a number of factors,
which include:
•The properties of the work materials that are being formed/machined/
deposited
•The properties of the tooling/masking/forming media
•The characteristics of the basic processing machinery and its control structure
• The number of parameters in the physics or chemistry of the process
• Sensitivity of tbe process to external disturbances such as dirt, friction, and
humidity
Table 2.3 and Figure 2.8 convey the typical tolerances that can be obtained.
Note that even witbin one particular process there can be subtle differences in
performance, resulting in a range of tolerance. The darkest bars in the center of each
process are the normally anticipated values. This range is given the name natural tol-
erance (NT) of the process and is crucially important in both design and manufac-
turing work.
It cannot be emphasized enough that the cost of manufacturing, and the sub-
sequent cost of any consumer product, is related to the designer's selection of part
accuracy and dimensional tolerance.
Once the design and its related tolerances reach a factory floor, the manufac-
turers will be obliged to choose processes that deliver the accuracy and NT implicit
in the decisions made by the designer. Quite clearly, costs will rise rapidly if the
designer has been overdemanding or just thoughtless. Poor design decisions could
result in the obligatory choice of an inherently expensive manufacturing process.
The next concept to emphasize is that of process chains within a particular
family of manufacturing processes. Examples of these are also shown on the Website
<cybercut.berkeley.edu>. In general, several processes are used sequentially to
gradually achieve a highly accurate, smooth surface. A common chain in mechanical
manufacturing is to start with a flame-cut plate. a casting, or a forging to obtain the
Process
36

Manufacturing Analysis: Some Basic Ouestions for a Start-Up Company Chap, 2
in.X 10-3
100 50
Process
Traditional
Flame cutting
Hand grinding
Disk grinding or filmg
Turning. shaping, or milling
Drilling
Boring
Reaming or broaching
Grinding
Honing, lapping, buffing, or polishing
Nontraditional
Plasma beam machining
Electrical discharge machining
Chemical machining
Electrochemical machining
Laser beam or electron beam machiru
Electrochemical grinding
Electropolishing
c:::=J
Less frequent application
_Averageappllcation
2.0
0.5 0.2 0.05 0,02 0.005 0.002
z Tolerance frnrn]
F1guu ZJI Natural tolerances (NT) ~ Ihe darker bands, for a variety of common
mechanical manufacturing processes. Variations

=
the lighter bands (from
MClI1ufacrurmg Processes for Engineering Materials
by Kalpakjian,
©
1997.
Reprinted by permission of Prentice-Hall, Inc., Upper Saddle River, NJ).
bulk shape. Flame cutting could then be followed by a series of machining operations
to obtain further accuracy. These can then be followed by grinding and polishing if
high accuracy and finish are desired by the designer.
In Figure 2.8, the NTs of flame cutting, machining, and grinding are shown,
moving across from left to right with finer accuracy. Several points should be made:
•The designer should realize that these process chains exist, as summarized in
the simple diagram of Figure 2.9.
• Each additional process is needed after a certain transitional tolerance. If the
designer is unaware of these transitions, unnecessary finishing costs may be
created, as shown in Figure 2.10.The other side of this coin is that manufac-
turing costs can he saved if the designer is willing to loosen desired tolerances.
• The manufacturing quality assurance at one step in the process chain must be
carefully executed before moving on to the next process. If a "parent" process
is "ended too early," the next "child" process may have too much or an impos-
sible amount of work to do. (Imagine cleaning a rusty garden tool; heavy
2.3 Question 2: How Much Will the Product Cost to Manufacture
(e)7
37
015
015
Secondary process flat capability
FiJUre2.' Process
chains with

levelsoftojerance
grinding or heavy abrasive papers are needed before moving on to the final
polishing steps.)
2.3.8
Product Life Expectancy
Recall that part strength is listed as the third criterion in Table 2.2. It is related to the
design geometry, tolerances, material selected, and chosen manufacturing method.
These factors also have a coupled influence on the long-term in-service life. Aero-
space and structural engineers are probably the designers who are most concerned
with these long-term properties. Hertzberg (1996) and Dowling (1993) describe the
fatigue properties of metals and polymers. The influences of material composition
and local-geometry effects are also described. A fatigue failure always begins at a
stress concentration. A sharp corner, a small hole, a rapid transition in diameter are
examples of danger zones for crack initiation. Designers in such fields will specify
high integrity grades of steel and aluminum, will choose processes like forging and
forming (rather than casting) to maintain a homogeneous grain structure, and will
specify additional final finishing operations such as grinding and lapping. These
I
Drill
IEDM
I
Broach
Ream
IBm",l
Honing
I
Hole hierarchy
Flat hierarchy
c;ingl
IFinegrind

Broach
IEDMI
I
Mill
!Roughgrjnd
Secondary process hole capability
Surt rougjrin Itr-e tnche
Dtm acc in Hr-s mcnes
Dim ace in 10
_1
inches
Surf rough in Iu-e Inches
38 Manufacturing Analysis: Some Basic Questions for a Start-Up Company Chap. 2
400
Figure2.10 Finishingcostsincreaseasa
part moves from a rough casting.to a
finish-machined part, to fine-honed final
product (from Manufacturing Processes
for Engineering Materials
by
Kalpakjian,
© 1997. Reprinted by permission
of Prentice-Hail, Inc" Upper Saddle
River,NJ).
#"
300
i
~ 200
i
~ 100

additional operations lead to very smooth surfaces that give dramatically improved
long-term fatigue life.
Figure 2.10 illustrates the costs of these additional fine finishing operations.
The additional grind and hone operations add 400% more cost over the as-forged,
or as-cast, surfaces. Even in comparison with turning on a lathe, they add 200 to 300%
more cost. It is not surprising that carefully manufactured aircraft components, or the
surface of a production quality plastic injection mold, are so very expensive.
2.3.9
Lead lime
Lead time is defined for this book as "the number of weeks between the release of
detailed CAD files to the fabrication facility and the actual production of the part."
It is a small subcomponent of the total time-to-market. This broader topic will be
reviewed in greater depth in Section 2.5.For this overview, the important point is that
lead time is very dependent on the designer's decisions, which then have direct impli-
cations on the choice of manufacturing process. The desired batch size, part geom-
etry, and accuracy are the main factors. As a benchmark, a small batch of medium
complexity metal parts with +/~50 microns (+1- 0.002 inch) accuracy can be
obtained from a production machine shop with a two- to three-week turnaround
time, obviously depending on normal business conditions.
However, several weeks of lead time will be experienced as soon as a serious
mold or die is needed. For the processes like forging, sheet metal forming, and high-
volume plastic injection molding, the die making involves many extra steps. During
die design, factors such as springback for metals and shrinkage for plastics need to
be incorporated. Since the deformation stresses that build up during manufacturing
are high, the die designer also has to create supporting blocks and pressure plates.
The designer will also need to consider parting planes and the draft angles that give
slight tapers to any vertical walls: these are needed to ensure that the part can be
ejected after forming. Unfortunately, perfect analytical models do not exist yet for