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INTRODUCTION
Type III-A or I-A Manufacturing System
~ Designed to perform fixed sequence of assembly steps on a specific product
~ Used for high production of products that require limited number of parts to
be assembled
~ Stations are integrated with MH system
~ These are examples of fixed automation
Requirements
High product demand
Stable product design
Limited number of components
Product is designed for automated assembly
Comparison wi th transfer lines
Assembly operation vs processing operation
Smaller work units are produced
Less mechanical force is required and also the power
For the same number of stations AAS tend to be smaller in physical size
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Applications
Typical Products made by Automated Assembly:
~ Alarm Clocks~ Ball Bearings
~ Ball Point Pens
~ Electrical Plugs and Sockets
~ Gear Boxes
~ Light Bulbs
~ Locks~ PCB Assemblies
~ Small electric motors
~ Spark plugs
~ Wrist Watches
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Processes
Typical Assembly processes/activities performed in
Automated Assembly:
~ Adhesive bonding
~ Insertion of components
~ Placement of components
~ Riveting
~ Screw Fastening
~ Snap Fitting~ Soldering
~ Spot welding
~ Stapling
~ Stitching
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INTRODUCTION
Subsystems of AAS:
~ One or more assembly workstations
~ Parts feeding devices at individual workstation
~ Work handling system (Base part or partial assembly transfer)
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System Configurations
System Configurations:
~ In-line~ Dial type Assembly Machine
~ Carousel Assembly System
~ Single station assembly machine
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System Configurations
System Configurations:
~ In-line~ Dial type Assembly Machine
~ Carousel Assembly System
~ Single station assembly machine
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System Configurations
System Configurations:
~ In-line~ Dial type Assembly Machine
~ Carousel Assembly System
~ Single station assembly machine
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System Configurations
System Configurations:
~ In-line~ Dial type Assembly Machine
~ Carousel Assembly System
~ Single station assembly machine
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Work part Transfer Systems
Workpart Transfer Systems:
~ Synchronous~ Asynchronous
Parts Delivery at Workstations:
Elements are:
~ Hopper
~ Parts Feeder ~ Selector and/or Orientor
~ Feed track
~ Escapement and placement device
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Work part Transfer Systems
Workpart Transfer Systems:
~ Synchronous~ Asynchronous
Parts Delivery at Workstations:
Elements are:
~ Hopper
~ Parts Feeder ~ Selector and/or Orientor
~ Feed track
~ Escapement and placement device
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Vibratory Bowl Feeder
Most versatile of hopper feeders for small parts
Consists of bowl and helical track
Parts are poured into bowl
Helical track moves part from bottom of bowl to outlet
Vibration applied by electromagnetic base
Oscillation of bowl is constrained so that parts climb upward along
helical track
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Selector and/or Orientor
Purpose - to establish the proper
orientation of the components for theassembly workhead
Selector
Acts as a filter
Only parts in proper orientation
are allowed to pass through to
feed track Orientor
Allows properly oriented parts to
pass
Reorients parts that are not
properly oriented
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Feed Track
Moves parts from hopper to assembly workhead
Categories:
Gravity - hopper and feeder are located at higher elevation than workhead
Powered - uses air or vibration to move parts toward workhead
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Escapement and Placement Devices
Escapement device
Removes parts from feed track at time intervals that are consistent with the
cycle time of the assembly workhead
Placement device
Physically places the parts in the correct location at the assembly workstation
Escapement and placement devices are sometimes the same device,
sometimes different devices
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Escapement and Placement Devices
(a) Horizontal and (b) vertical devices for placement of parts onto dial-indexing
table
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Escapement and Placement Devices
Escapement of rivet-shaped parts actuated by work carriers
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Escapement and Placement Devices
Two types of pick-and-place mechanisms for transferring base parts from
feeders to work carriers
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Work part Transfer Systems
Parts Delivery at Workstations:
Elements are:
~ Hopper ~ Parts Feeder
~ Selector and/or Orientor
~ Feed track
~ Escapement and placement device
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Quantitative Analysis of
Assembly Systems~ Parts Delivery System at Workstations
~ Multi-station Automated Assembly Systems
~ Single Station Automated Assembly Systems
~ Partial Automation
Parts Delivery System at Workstations:
Parts feeder feed rate
Probability of components passing through the selector
Effective rate of delivery of components from the hopper into the feed trackDelivery rate > Cycle rate
Low level sensor
High level sensor
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Quantitative Analysis of
Assembly Systems~ Parts Delivery System at Workstations
~ Multi-station Automated Assembly Systems
~ Single Station Automated Assembly Systems
~ Partial Automation
Parts Delivery System at Workstations:
Example:
The cycle time for a given assembly workhead = 6 sec. The parts feeder has a
feed rate = 50 components per min. The probability that a given component fedby the feeder will pass through the selector is 0.25. The number of parts in the
feed track corresponding to the low level sensor is 6. The capacity of the feed
track is 18 parts. Determine:
(a) How long it will take for the supply of parts in the feed track to go from high
level to low level? and
(b) How long it will take on average for the supply of parts to go from low levelto high level?
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Quantitative Analysis of
Assembly Systems~ Parts Delivery System at Workstations
~ Multi-station Automated Assembly Systems
~ Single Station Automated Assembly Systems
~ Partial Automation
Parts Delivery System at Workstations:
Example:
A feeder-selector device at one of the stations of an automated assembly
machine has a feed rate of 25 parts per minute and provides a throughput ofone part in four. The ideal cycle time of the assembly machine is 10 sec. The
low level sensor on the feed track is set at 10 parts, and the high level sensor is
set at 20 parts.
(a) How long will it take for the supply of parts to be depleted from the high
level sensor to the low level sensor once the feeder-selector device is turned
off?(b) How long will it take for the parts to be resupplied from the low level sensor
to the high level sensor, on average, after the feeder-selector device is turned
on?
(c) What proportion of the time that the assembly machine is operating will the
feeder-selector device be turned on? Turned off?
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Quantitative Analysis of
Assembly Systems~ Parts Delivery System at Workstations:
~ Multi-station Automated Assembly Systems
~ Single Station Automated Assembly Systems~ Partial Automation
Multi-station Automated Assembly Systems:
~ Synchronous transfer system
~ In-line, dial indexing, and carousel configuration
~ Constant element times, although the times are not necessarily equal at allstations
~ No internal storage
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Automated Assembly Systems
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Quantitative Analysis of
Assembly Systems Multi-station Automated Assembly Systems:
qi probability that the component to be added during current cycle is
defectivemi probability that a defect results in a jam at the station and consequent
stoppage of the line
Three events occur at a particular workstation:
The component is defective and causes a station jam, pi = qi*miThe component is defective but does not cause a station jam = (1 mi)*qiThe component is not defective = (1 qi)
The probabilities of the three possible events must sum to unity for any
workstation
mi qi + (1 mi) qi + (1 - qi) = 1
For n station assembly line
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Automated Assembly Systems
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Quantitative Analysis of
Assembly Systems Multi-station Automated Assembly Systems:
Two of the three terms of earlier equation represent events in which a goodcomponent is added at the given station.
miqi indicates that a station jam has occurred, and thus a defective component
has not been added to the existing assembly.
( 1 - qi) means that a good component has been added at the station.
The sum of these two terms represents the probability that a defective
component is not added at station i.
Multiplying these probabilities for all stations, we get the proportion of
acceptable product coming off the line Pap.
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Automated Assembly Systems
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Quantitative Analysis of
Assembly Systems Multi-station Automated Assembly Systems:
The proportion of assemblies containing at least one defective component isPqp.
Other performances are:
machines production rate
proportion of uptime and downtime
F = Frequency of downtime occurrences per cycle
If each station jam results in a machine downtime occurrence, F can be
determined by taking the expected number of station jams per cycle
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Example:
A ten-station in-line assembly machine has an ideal cycle time of 6 sec. The base part is
automatically loaded prior to the first station, and components are added at each of thestations. The fraction defect rate at each of the ten stations is q = 0.01, and the
probability that a defect will jam is m = 0.5. When a jam occurs, the average downtime is
2 min. Cost to operate the assembly machine is Rs. 2500 /hr. Other costs areignored.
Determine:
(a) Average production rate of all assemblies
(b) Yield of good assemblies
(c) Average production rate of good products(d) Uptime efficiency of the assembly machine, and
(e) Cost per unit
Quantitative Analysis of
Assembly Systems
q m Rp(pc/hr) Yield Rap(pc/hr) E Cpc($)
0 0.5 600 1 600 100% 0.07
0.01 0.5 300 0.951 285 50% 0.15
0.02 0.5 200 0.904 181 33.3% 0.23
0.01 0 600 0.904 543 100% 0.08
0.01 0.5 300 0.951 285 50% 0.15
0.01 1.0 200 1 200 33.3% 0.21
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Keypoints:
As fraction defect rate increases, meaning that component quality gets worse,all five measures of performance suffer.
The effect of m is less obvious.
At low values of m (m = 0) for the same component quality level q, production
rate and machine efficiency are high but the yield of good product i.e.proportion of acceptable products, is low.
Instead of interrupting the assembly machine operation and causing downtime,
all defective components pass through the assembly process to become part of
the final product.
At m = 1, all defective components are removed before they become part of the
product. Therefore yield i.e. proportion of acceptable products, is 100% but
removing the defects takes time, adversely affecting production rate, efficiency
and cost per unit.
Quantitative Analysis of
Assembly Systems
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Instantaneous Control vs. Memory Control:
Memory control (m = 0) requires sorting station and instantaneous control(m = 1) stops the machine when a defect occurs.
Memory control has to consider cost of additional sensors, controls and
sortation devices and will be more useful if sortation station is 100% effective.
Instantaneous control has to consider the cost of lost production time due tostopping of the machines
The selection of control may be justified using the cost per unit factor.
Quantitative Analysis of
Assembly Systems
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Quantitative Analysis of
Assembly Systems~ Parts Delivery System at Workstations:
~ Multi-station Automated Assembly Systems
~ Single Station Automated Assembly Systems~ Partial Automation
Single-station Automated Assembly Systems:
~ A single workstation with several components feeding into the station to be
assembled to a base part
~ Increasing the number of elements in the assembly machine cycle resultsin a higher cycle time, decreasing the production rate of the machine
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Single-station Automated Assembly Systems:
ne = number of distinct assembly elements that are performed on the machine
Tej = element timeTh = handling time (loading of base part and unloading of assembled product)
Ideal cycle time Tc can be expressed as:
Instead of component addition if certain activity is carried out (welding,
fastening) then pj can be used as the probability of station failure duringelement j.
Quantitative Analysis of
Assembly Systems
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Example:
A single-station assembly machine performs five work elements to assemble four
components to a base part. The elements are listed in the table below, together with thefraction defect rate (q) and probability of a station jam (m) for each of the components
added (NA means Not Applicable).
Time to load the base part is 3 sec and time to unload the completed assembly is 4 sec,
giving a total load/unload time of Th = 7 sec. When a jam occurs, it takes an average of
1.5 min. to clear the jam and restart the machine. Determine:
(a) Production rate of all products
(b) Yield of good product(c) Production rate of good products
(d) Uptime efficiency of the assembly machine
Quantitative Analysis of
Assembly Systems
Element Operation Time (sec) q m p
1 Add gear 4 0.02 1
2 Add spacer 3 0.01 0.63 Add gear 4 0.015 0.8
4 Add gear and mesh 7 0.02 1
5 Fasten 5 0 NA 0.012
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Quantitative Analysis of
Assembly Systems~ Parts Delivery System at Workstations:
~ Multi-station Automated Assembly Systems
~ Single Station Automated Assembly Systems~ Partial Automation
Partial Automation:
~ Combination of automated and manual workstations
~ Reasons:
Automation is introduced gradually on an existing manual lineCertain manual operations are too difficult or too costly to automate
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Quantitative Analysis of
Assembly Systems Partial Automation:
Assumptions:
~ workstations perform either processing or assembly operations~ processing and assembly times at automated stations are constant,
though not necessarily equal at all stations
~ synchronous transfer of parts
~ no internal buffer storage
~ upper-bound approach is applicable
~ station breakdowns occur only at automated stations~ human adaptability
~ Tc remains constant over time
The ideal cycle time is determined with the bottleneck station (manual)
Because of random variation in any repetitive human activity Tc will show
certain degree of variation
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Quantitative Analysis of
Assembly Systems Partial Automation:
na = number of automated stations
nw = number of stations operated by manual workersn = na + nw = total station count
Casi = Cost to operate automatic workstation, i (Rs/min)
Cwi = Cost to operate manual workstation, i (Rs/min)
Cat = Cost to operate the automatic transfer mechanism (Rs/min)
Co = total cost to operate the line