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
Trang 1
04
a tò T
Minimure dimension of web w (in.}
Figure 2.7 Process capabilities related to part geometry Very thin sections favor rofting and thermoforming; “chunky” sections favor machining and injection molding (from fniroduction tw Manufacturing Processes by 3, A Schey, © 1987 Reprinted with permission of the McGraw-Hill Companies}
‘The thermoforming of plastic sheets is slightly above cold rolling in the praph
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 procedures in sand casting prevent it from being selected if one of the dimensions is Jess than 5 millimeters (0.2 inch)
2.3.7 Accuracy, Tolerances, and Fidelity between CAD and CAM
In all fabrication processes—semiconductuts, plastics, metals, textiles, or other- wise-——the physical limitations of each process have a major impact on the achiev- able accuracy Each processing operation comes with a bounding envelope of 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 lated into the fabricated geonictry Also, the properties of the original piece of work more preferred state
Trang 2TABLE 2.3 Routine Accuracies for Mechanical Processes (One “Thou” Approximately =
25 Microns)
Hot, open die forging +/+ 1250: microns +/— 0.08 inch Hot, closed die forging +/— 500 microns +/~— 0.02 inch Investment casting +/~ 75-250 microns +/— 9,003-0.01 inch Col, closed die forging +f~ 50-125 microns +/— 0,002-0.005 inch
Electrodischarge machining +/— 12.5 microns +/- 0.0005 inch Lapping and polishing +/~ 0.25 microns +/- 0.00001 inch
In the werst case situation, a poorly controlled process will damage a pezfectly good work material Examples of this were widespread in the early days of welding, 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 the 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 within one particular process there can be subtle differences in performance, resulting in a range of tolerance The darkest bars in the center of each 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 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
Trang 336 Manufacturing Analysis: Some Basic Questions for a Start-Up Company Chan, 2
in x 1073 Process
Traditional
Flame cutting
Hand grinding
Disk grinding or filing
Tumming, shaping, or milling
Drilling
Boring
Reaming or broaching
Grinding
Honing, *apping, buffing, ot polishing
Nontraditional
Plasma beam machinin;
Electrical discharge machining
Chemical machining,
Electrochemical machina
Laser beam or electron beam machining
Electrochemical grinding
+Tolerance (mm)
Figure 2.8 Natural tolevances (NT) = the darker bands, for a vatiety of common mechanical manufacturing processes Variations = the lighter bands (from Manufacturing 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 foliowed by a series of machining operations 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 matrufac- turing costs can be 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
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Dim ace in 10-? inches Surf rough in 10-® inches
II
Thân
Hole hierarchy
Secondary process hale capability
Dim ace in 10 4 inches Surf rough in 10-* inches
1000
Secondary process flat capability
Figure 2,5 Process chains with levels of tolerance
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 These factors also have a coupled influence on the long-term in-service life, Aeto- with these long-term properties Hertzborg (1996) and Dowling (1993) describe the and local-geometry effects are also described A fatigue failure always begins at a 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 specify additional final finishing operations such as grinding and lapping These
Trang 5Figure 2.10 Finishing costs increase as a part moves from a rough casting, to a finish-machined part, to fine-honed final
£
Š
Surface
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, 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 Time
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.” reviewed in greater depth m Section 2.5 For this overview, the important point is that 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 (+/~ 0.002 inch) accuracy can be 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 pracesses like forging, sheet metal forming, and high- die design, factors such as springback for metals and shrinkage for plastics need to 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 ejected after forming Unfortunately, perfect analytical models do not exist yet for
Trang 6predicting the precise amounts of springback or the best draft angle This usually and-error adjustments and iterations to the die surfaces are always needed The above paragraph still pertains only to one machine and one process Several months of lead time are needed to set up the large-scale FMS systems and high-volume manufacturing processes, linked together and scheduled to make complex subassem- proportionally, At the extreme, for a completely new model of aircraft or automobile, the lead time from design to first product will run into years rather than months 2.3.10 Cost Factors Especially Related to Adjoining Parts
The following example shows how design and manufacturing keep changing to suit a rather complex interaction between (a) the availability of innovative manufacturing Miller’s (1983) quotation in Chapter 1,“CIM és the confluence of the supply elements and the demand elements.”
Twenty or even ten years ago, it would not have seemed reasonable to machine very large structures from a solid monolithic slab However, innovative machining pro- structural members that resemble giant coat hangers are spaced across the plane at This arrangement ts shown in the upper photograph of Figure 2.11 However, newer photograph This eliminates costly and unpredictable joining operations in the factory Thomas (1994) has observed that such manufacturing innovations flowing back into the design phase must be the new way of organizing the relationship between that the new innovations, or the new supply elements, include:
© Improved cutting tool technology and an understanding of how to control the accuracy of very high speed machining processes
® The availability of stiffer machine tools and very high speed spindles
* More homogeneous microstructures that give uniformity in large forging slabs
* The ability to carry out comprehensive testing and show that these one-piece structures are at least if not more reliable than multiple-piece structures Meanwhile the new demand elements include:
» Bscalating costs of joining and riveting operations, which can only be partially automated Specifically these operations often require manual fixturing of the workpieces
A preference for eliminating multiple fabrication steps, which always demand more setup, fixturing, documentation, and quality assurance
General! pressures on the whole airline industry, since deregulation, to cut costs
Trang 7
Figure 2.11 Integrated product and process design allows this aerospace
cumponent to be completely machined from the solid as shown in the lower photograph (courtesy of Dr Donald Sandstrom, The Boeing Company)
These trends introduce a great deal of complexity into the design and manu- facturing process, but on the other hand, creative companies can exploit them to their advantage The conclusion to be drawn is that no single component should be ana- improved, simplified, or made cheaper if design and manufacturing are viewed from
a slightly wider system perspective
2.3.11 Analyzing Costs in Terms of the Profit Potential
Hewlett-Packard’s return map (RM) is another method for analyzing design and questions:
* How much profit, AP will be made at any given time?
* How long, T,, will it take to make any profit?
Fignre 2.12 plots the costs or revenues against logarithmic time expelled The key curves on the chart {modeled on House and Price, 1991) are:
Trang 8
1,000
Total sales
§ RF = return factor at AT after T,,
= total investment + AP ———mm at Af after T,
: ‘Total operating
3 profit
S
2
=
& 10 fo Break-even time:
$ AP
g Tota) investment
o Break-even after
3 release
õ
Product
a definition] Product development Manufacturing and sales |
Time -
Figure 2.12 Hewlett-Packard's return map (diagram based on House and Price, 1991; Magrab, 1997)
* The total investment of dollars starting from the first instant (7) that engi- neers start dreaming up the project (see the top of Figure 2.1 at the beginning
of the chapter)
* The total sales that begin as soon as possible after the first product is manu- factured and sold, T,,, Note that setting up and debugging the manufacturing line generate no sales
© The total profit that starts to be gained at 7,,
The key points on the time axis are:
¢ T—the project initiation point, This is followed by product definition, product development, process planning for manufacturing, setting up machines, debug- point T,,
* T,,—the point where real manufacturing begins and products get sold
© T,—the break-even time from the very beginning uf the “conceptual product definition” to the point where a positive profit occurs (7, — T,), Note that the chart also shows the break-even-after-release time, which measures (T; — T,,,) and focuses more on manufacturing productivity Obviously, fast production and high volumes of product are desirable The goal is to quickly amortize all the development costs
* AT and AP—any arbitrary point (AT, AP) beyond the break-even-after-release
Trang 9calculated by dividing total profits by total investments The goal is to maxi- measure an RF" from the break-even time, 7 In terms of profitability for the time New companies with limited cash flow should focus more on the former measure
What is the income stream from the product? The following definitions are often used:
* Sales price = estimated sales price of one unit from company to distributor (not retail)
« Net sales = individual sales price X number of products sold
* Cumulative net sales = integrated net sales over several consecutive years What are the costs of being in business and producing that particular product? The following definitions are often used:
* Unit cost = prime manufacturing and related manufacturing overhead costs of
a single unit of product (see the cost of goods manufactured on the right of Figure 2.5)
® Cast of the product = unit cost * number of products sold
* Development costs = conceptual and detailed design + launch + support
* Marketing costs = a percentage of net sales (Magrab, 1997, uses 13%)
* Other promotional and running costs = a percentage of net sales (Magrab,
1997, uses 8%)
What is the potential profit or loss? The following definitions are often used:
* Gross margin = net sales — cost of product
® Percentage gross margin = gross margin / net sales x 100%
© Pretax profit = gross margin — development costs — marketing costs — other
* Cumulative profit = integrated profits (or losses) on a year-by-year basis Table 2.4 has been reproduced from Magrab (1997) to show some specific fig- ures In that example, the first two years have no sales However, the design and development costs are running up all the time showing a bottom line, rerporary loss
of $1.6 million
This particular illustration shows that by the year 2005, the product makes an impressive profit But the risks of the first two to three years cannot be emphasized market? What if the development time is too long and another company launches a are far too evident here
Also it is useful to ask, Where will the 1.6 million come from? Obviously from
a loan of some kind (new company) or a strategic investment (larger, existing com-
Trang 10TABLE 24 An Exampls of Magrab's Baseline Hypothetical Profit Model
E B Magrab Copyright CRC Press, Boca Raton, Florida.)
(Reprinted with permission from integrated Product and Process Design by
Year
1997 1998 199% 200 201 202 2003 2004 2005 j=l j=2 j=3 j=4 J=5 =6 J=7 i=8 )=9
A Sales price 365.90 $65.90 $67.90 $67.90 $67.90 $67.90 $67.90
B Number of units sold 100,000 250,000 300,000 350,000 250,000 200,000 150,000
C Net sales [= AB] $6,590,000 $16,475,000 $20,370,000 + $23,765,000 $16,975,000 $13,580,000 $10,185,000
D Cumulative net sales
[=SUM C{j)} $6,590,000 $23,065,000 + $43,435,000 $67,200,000 $84,175,000 $97,755,00 $107,940,000
E Unit cost (target) $34.00 $33.50 $33.00 $33.00 $33.50 $34.00 $34.50
F Cost of product sald
H % gross margin [=100G/C} 48.41% 49.17% 51.40% 51.40% 50.66% 49.93% 49.19%
I Development cost $800,000 $800,000 $400,000 $50,000 $50,000 $50,000 $50,000 $50,000 $50,000
J Marketing (13% net sales)
[=0.13C] $856,700 $2141,750 $2,648,100 $3,089,450 $2,206,750 $1765400 $L324/050
K Other (8% of net sales)
{=0.08C] $527,200 $1318,000 $1,629,600 $1,901,200 $1,358,000 $1,086,400 $814,800
L Total operating expense
{=I+J+K] $800,000 $800,000 $1,783,900 $3,509,750 $4,327,700 $5,040,650 $3,614,750 $2,901,800 $2188.850
M Pretax profit [=G-L] ($800,000) ($800,000) $1,406,100 $4,590,250 $6,142,300 $7,174,350 $4,985,250 $3,878,200 $2,821,150
N % profit [=100M/C} 21.34% 27.86% 30.15% 30.19% 29.37% 28.56% 27.70%
© Cumulative profit
[=SUM Mụ)] ($800,000) ($1,600,000) ($193000) $4396350 $10,538,650 $17,713,000 $22,698,250 $26,576,450 $29397/600
*Product enters market midyear.