ME445 INTEGRATED MANUFACTURING SYSTEMS

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ME-445 INTEGRATED MANUFACTURING SYSTEMS

• PROCESS PLANNING

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The 21st century engineering response to world
competition is concurrent engineering.

• Concurrent engineering requires the integration
  of all aspects of the product life cycle, that
  is
• design,
• manufacturing,
• assembly,
• distribution,
• service,
• disposal

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• Two important areas in the life cycle of a
  product are design and manufacturing. Process
  planning serves as an integration link between
  design and manufacturing.
• Process planning consists of preparing a set of
  instructions that describe how to fabricate a
  part or build an assembly which will satisfy
  engineering design specifications.

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The resulting set of instructions may include any
or all of the following

• operation sequence,
• machines,
• tools,
• materials,
• tolerances,
• cutting parameters,
• processes (such as how to heat-treat),
• jigs,
• fixtures,
• time standards,
• setup details,
• inspection criteria,
• gauges,
• graphical representations of the part in various
  stages of completion.

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• Process planning emerges as a key factor in
  CAD/CAM integration because it is the link
  between CAD and CAM.
• After engineering designs are communicated to
  manufacturing, either on paper or electronic
  media, the process planning function converts the
  designs into instructions used to make the
  specified part.

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CIM cannot occur until this process is automated
consequently, automated process planning is the
link between CAD and CAM.
CAM
CAD
Process design Process planning (CNC codes) Tool
selection Facilities management
Conceptual design Mathematical analysis Geometric
data (graphical representation)
CAPP COMPUTER AIDED PROCESS PLANNING
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Some typical benefits of automated process
planning include

• 50 increase in process planner productivity
• 40 increase in capacity of existing equipment
• 25 reduction in setup costs
• 12 reduction in tooling
• 10 reduction in scrap and rework
• 10 reduction in shop labor
• 6 reduction in work in process
• 4 reduction in material

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If the process planners productivity is
significantly improved

• More time can be spent on methods, improvements
  and cost-reduction activities.
• Routings can be consistently optimized.
• Manufacturing instructions can be provided in
  greater detail
• Preproduction lead times can be reduced.
• Responsiveness to engineering charges can be
  increased.

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The development of process plans involves a
number of activities

• Analysis of part requirement
• Selection of raw workpiece
• Determining manufacturing operations and their
  sequences
• Selection of machine tools
• Selection of tools, workholding devices, and
  inspection equipment
• Determining machining conditions and
  manufacturing time

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ANALYSIS OF PART REQUIRENTS

• The part requirements can be defined as
• part features
• process determination
• steps of processes
• dimensions
• machine tool size
• tolerance specifications
• machine tool capability
• CNC code generation

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SELECTION OF RAW WORKPIECE

• It involves such attributes as
• shape
• standard materials
• rod
• slab
• blank
• profile
• pre-shaped materials
• cast
• forged
• extruded
• size
• machine tool size
• material
• cutting conditions
• tool selection

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DETERMINING MANUFACTURING OPTIONS AND THEIR
SEQUENCES

• selection of processes
• availability
• accuracy requirement
• suitability
• cost
• sequence of operations
• work holding method
• cutting tool availability

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SELECTION OF MACHINE TOOLS

• work piece related attributes
• part features
• dimensions
• dimensional tolerances
• raw material form
• machine tool related attributes
• process capability
• size
• mode of operation
• manual
• semiautomatic
• automatic
• CNC
• tooling capabilities
• type of tool
• size of tool
• tool changing capability
• manual
• automatic

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EVALUATION OF MACHINE TOOL ALTERNATIVES

• Machine tool capability

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MACHINING CAPABILITY

• MC lt 100 capability is good
• MC 100 process is just
  acceptable
• MC gt 100 It is not acceptable (
  or parts produced would have to be sorted)

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PROCESS CAPABILITY

• PC 1/MC
• PC tolerance/6s
• PC gt 1 process is acceptable

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unit cost of product

• The distribution of the size of finished parts
  are assumed to be normal.

where Zu and Zl are the standard normal
variates for the upper and lower tolerance
limits, tu and tl are the upper and lower
tolerance limits m is the mean of the
population s is the standard deviation
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• portion of accepted parts (AP) F(Zu) - F(Zl)
• where F(Zu) is the probability of parts
  having the dimension less than the upper
  tolerance value
• F(Zl) is the probability of parts having
  the dimension less than the lower tolerance
  value

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portion of rejected parts (SC) 1- APSC 1-
F(Zu) F(Zl)
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• where ki and ks are the technological
  coefficients

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• material balance
• Yi ki Yo
• Ys ks Yo
• cost of a part
• Xi Yi Yi f(Yi) Xo Yo Xs Ys
• Xo ki Xi - ks Xs ki f(Yi)
• where
• Xi is the unit cost of a raw part
• Xo is the unit cost (value) of a machined
  part
• Xs is the unit value of a scraped part
• f(Yi) is the processing (machining) cost per
  unit

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• average manufacturing lead time
• T S t ki Yo
• where
• T is the average lead time
• S is the setup time
• t is the average machining (processing) time

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EXAMPLESuppose 500 units of a shaft are to be
manufactured within mm. Suppose there are
three alternative machine tools as follows
Unit raw material cost 10.00 Unit salvage
value 2.00 Process average
25.038 mm
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Determine the most suitable machine tool for the
job. (Take the turret lathe case first)

• Use a normal distribution table to determine the
  scrap rate.
• F(Zu) 0.5832
• F(Zl) 0.2611

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• of parts above
• upper tolerance limit (1 - 0.5832) x 100
  41.68
• of parts below
• lower tolerance limit (0.2611) x 100 26.11
• total scrap SC 0.4168 0.2511 0.6779

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technological coefficient of scrap

• technological coefficient of input
• ki 1 ks 1 2.1047 3.1047
• number of units scraped
• Ys ks Yo 2.1047 x 500 1052
• number of raw part required
• Yi ki Yo 3.1047 x 500 1552

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• manufacturing lead time
• T S t Yi 151.00 x 1552 1567 min
• unit output cost
• Xo ki Xi - ks Xs ki f(Yi)
• Xo 3.1047 x 10.00 - 2.1047 x 2.00 3.1047 x
  7.00
• Xo 48.47 /part (for turret lathe case)

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Turret lathe should not be the choice. However
there is a trade-off between the unit cost and
the number of units of scrap as well as the
manufacturing lead time for the engine lathe and
automatic screw machine.
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SELECTION OF TOOLS, WORKHOLDING DEVICES, AND
INSPECTION EQUIPMENT

• Tools
• tool material
• shape
• size
• nose radius
• tolerance

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• Workholding devices
• The primary purpose of a workholding device is
  to position the workpiece accurately and hold it
  securely.
• manually operated devices
• collets
• chucks
• mandrel
• faceplates
• designed devices
• power chucks
• specially designed fixtures and jigs
• flexible fixtures used in flexible manufacturing
  systems

• Inspection equipment
• on-line inspection equipment
• off-line inspection equipment

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DETERMINING CUTTING CONDITION AND MANUFACTURIN
TIMES

• Machining conditions
• cutting speed
• feed rate
• depth of cut
• Object is to set the cutting conditions in such
  a way that the economically best production state
  is achieved.

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• What is the economically best production state?
• It is
• 1- Minimum production cost
• or
• 2- Maximum production rate

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CHOICE OF FEED

• Finishing cut Proper feed rate to provide
  desired surface quality (relatively low)
• Roughing cut Feed rate is not effective as
  cutting speed over tool life, therefore, feed
  should be set to maximum possible value
• limitations
• maximum tool force that the machine or the tool
  can stand and the maximum power available

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CHOICE OF CUTTING SPEED

• Cutting speed is set to provide the optimum tool
  life.
• High V low tool life
• high tool cost
• high production rate
• short production time
• Low V high tool life
• low tool cost
• low production rate
• long production time

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MINIMUM COST PER PIECE

• Cost per component,
• Cu nonproductive cost
• machining cost
• tool changing cost
• tooling cost

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• where
• co labor and overhead cost (/min)
• ct tool cost per cutting edge (/edge)
• tl nonproductive time (min/piece)
• tc machining time (min/piece)
• td tool changing time (min/edge)

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For a single pass turning operation

• where
• tc machining time (min/piece)
• L length of workpiece (mm)
• D diameter of workpiece (mm)
• v cutting speed (mm/min)
• f feed rate (mm/rev)

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Taylors equation for tool life

• where
• v cutting speed (mm/min)
• T tool life (min/edge)
• n Taylor exponent
• C cutting speed for one minute of tool life
  
• (mm/min)

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Combine the above equation one can get the cost
per piece equation
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Differentiating this equation with respect to
cutting speed and equating to zero, then solving
for cutting speed will give the cutting speed for
minimum production cost.
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MAXIMUM PRODUCTION RATE

• Time per piece Tu nonproductive time
• machining time
• tool changing time

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Differentiating Tu with respect to v and equating
it to zero, then solving for v will give the
cutting speed for maximum production rate
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MANUFACTURING LEAD TIME

• Lead time S Tu Q
• where S major set up time
• Tu production time per piece
• Q lot size

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EXAMPLE

• A lot of 500 units of steel rods 30 cm long and
  6 cm in diameter is turned on a CNC lathe at a
  feed rate of 0.2 mm/rev and a depth of 1 mm. The
  tool life is given by
• vT0.2 200 (m/min)
• The other data are
• Machine labor rate 10 /hr
• Machine overhead rate 50 of labor
• Grinding labor rate 10 /hr
• Grinding overhead rate 50 of grinding labor
• Workpiece loading and
• unloading time 0.5 min/piece
• Tool Brazed insert
• Cost of tool 27.96 /tool
• Grinding time 2 min/edge
• Tool changing time 0.5 min/edge
• Tool can be ground only five times before it is
  discarded.

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• Determine
• Optimum tool life and optimum cutting speed to
  minimize the cost
• Optimum tool life and optimum cutting speed to
  maximize the production rate
• Minimum cost per component, time per component
  and corresponding lead time
• Maximum production rate, corresponding cost per
  component, and lead time

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SOLUTION1.
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2.
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3. Minimum cost
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• Cu 0.25 /min x 0.5 min/piece
• 0.25 /min x 3.43 min/piece
• 0.25 /min x 3.43 min/piece
• x (1/84.56) edge/min x 0.50 min/edge
• 5.16 /edge x 3.43 min/piece
• x (1/84.56) edge/min
• Cu 1.20 /piece

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Time per component

• Tu 0.5 min/piece 3.43 min/piece 3.43
  min/piece
• x (1/84.56) edge/min x 0.5 min/edge
• Tu 3.95 min/piece
• Lead Time 500 units x 3.95 min/piece
• Lead Time 1976.4 min

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4. Maximum production rate
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Production time per piece

• Tu 0.5 min/piece 1.62 min/piece
• 1.62 min/piece x (½) edge/min
• x 0.5 min/edge
• Tu 2.53 min/piece
• Lead Time 500 units x 2.53 min/piece
• Lead Time 1264.4 min

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Cost for maximum production rate

• Cu 0.25 /min x 0.5 min/piece 0.25 /min x
  1.62 min/piece
• 0.25 /min x 1.62 min/piece
• x (1/2) edge/min x 0.50 min/edge
• 5.16 /edge x 1.62 min/piece
• x (1/2) edge/min
• Cu 4.82 /piece

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THE PRINCIPAL PROCESS PLANNING APPROACHES

• Manual experience-based process planning method
• Computer-aided process planning method

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MANUAL EXPERIENCE-BASED PROCESS PLANNING METHOD

• most widely used method
• time consuming
• inconsistent plans
• requires highly skilled, therefore, costly
  planners

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COMPUTER-AIDED PROCESS PLANNING METHOD

• it can systematically produce accurate and
  consistent process plans
• it can reduce the cost and lead time of process
  planning
• less skilled process planners may be employed
• it increases the productivity of process planners
• manufacturing cost, manufacturing lead time and
  work standards can easily be interfaced with the
  CAPP system

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There are two basic methods used in
computer-aided process planning

• Variant CAPP method
• Generative CAPP method

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The Variant CAPP Method

• process plan is developed for a master part which
  represent the common features of a family of
  parts
• a process plan for a new part is created by
  recalling, identifying, and retrieving an
  existing plan for a similar part and making
  necessary modifications for the new part
• to use the method efficiently, parts classifying
  coding system must be used

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Advantages of variant process planning

• efficient processing and evaluation of
  complicated activities and decisions, thus
  reducing the time and labor requirements
• standardized procedures by structuring
  manufacturing knowledge of the process planers to
  companys needs
• lower development and hardware costs and shorter
  development times

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Disadvantages of variant process planning

• maintaining consistency in editing is difficult
• it is difficult to adequately accommodate various
  combinations of
• material,
• geometry,
• size,
• precision,
• quality,
• alternative processing sequences,
• machine loading
• The quality of the final process plan generated
  depends to a large extent on the knowledge and
  the experience of the process planners

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The Generative CAPP Method

• In a generative approach, process plans are
  generated by means of
• decision logic
• formulas
• technology algorithm
• geometry based data
• to perform uniquely the many processing
  decisions for converting a part from raw material
  to a finished state

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There are basically two major components of
generative process planning system

• a geometry based coding scheme
• process knowledge in the form of decision logic
  and data

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Geometry Based Coding Scheme

• The objective is to define all geometric
  features for all process-related surfaces
  together with feature dimensions, locations, and
  tolerances, and the surface finish desired on the
  features.
• The level of details is much greater in a
  generative system than a variant system.

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Process Knowledge in the Form of Decision Logic
and Data

• In this phase, part geometry requirement is
  matched with manufacturing capabilities in the
  form of decision logic and data.
• Selection of
• processes
• machine tools
• tools
• jigs and fixtures
• inspection equipment
• sequence of operations
• are achieved.
• Finally, operation instruction sheets (for
  manual operations) or NC codes (for CNC) machines
  are generated.

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DECISION TABLES

• Decision tables provide a convenient way to
  document manufacturing knowledge.

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EXAMPLE

• Consider the problem of the selection of lathes
  or grinding machines for jobs involving turning
  or grinding operations. Data on conditions such
  as lot size, diameter, surface finish and
  tolerance desired are available.

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They are compiled in form of a decision table as
shown below.
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FUTURE TRENDS IN COMPUTER-AIDED PROGESS PLANNING

• One of the major strategies for reducing cost and
  lead time is to integrate various functional
  areas such as design, process planning,
  manufacturing, and inspection.
• There are a number of difficulties in achieving
  the goal of complete integration.

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• For example, each functional area has its own
  stand-alone relational database and associated
  database management system. The software and
  hardware incompatibilities among these systems
  pose difficulties in full integration. There is a
  need to develop a single-database technology to
  address these difficulties.

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• Other challenges include automated translation
  of the design dimensions and tolerances into
  manufacturing dimensions and tolerances
  considering process capabilities and dimensional
  chains, automatic recognition of features, and
  making the CAPP systems affordable to the small
  and medium-scale manufacturing companies.

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