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Chapter 2: High

Volume production

systems

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Automated Production Lines� Automated production lines are used for high production of parts

that require multiple processing operations.

� Each processing operation is performed at a workstation, and the

stations are physically integrated by means of a mechanized work

transport system to form an automated production line.

� Machining (milling, drilling, and similar rotating cutter operations)

is a common process performed on these production lines, in

which case the term transfer line or transfer machine is used.

� Other applications of automated production lines include robotic

spot welding in automobile final assembly plants, sheet metal

press working, and electroplating of metals.

� Automated production lines require a significant capital

investment. They are examples of fixed automation, and it is

generally difficult to alter the sequence and content of the

processing operations once the line is built.

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• High production of parts requiring multiple processing operations

• Fixed automation

• Applications:

– Machining transfer lines

– Robotic spot welding lines

– Sheet metal stamping

– Electroplating of metals

– Electronics assembly

Features and Applications of

Automated transfer lines

Where to Use

Automated Production Lines?

• High product demand

– Requires large production quantities

• Stable product design

– Difficult to change the sequence and content of processing operations once the line is built

• Long product life

– At least several years

• Multiple operations required on product

– The different operations are assigned to different workstations in the line

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Benefits of

Automated Production Lines

• Low amount of direct labor

• Low product cost

-because cost of fixed equipment is spread over many units.

• High production rates.

• Manufacturing lead time (the time between beginning of

production and completion of a finished unit) and work-in-process

are minimized.

• Factory floor space is minimized.

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Fundamentals of

Automated Production Line

� An automated production line consists of

multiple workstations that are linked together by

a work handling system that transfers parts from

one station to the next, as depicted in Figure .

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Fundamentals of Automated Production Line

�A raw workpart enters one end of the line, and

the processing steps are performed sequentially

as the part progresses forward.

� The line may include inspection stations to

perform intermediate quality checks.

� Manual stations may also be located along the

line to perform certain operations that are

difficult or uneconomical to automate.

�Each station performs a different operation, so

that the sum total of all the operations is required

to complete one unit of work.

Fundamentals of Automated Production Line

�Multiple parts are processed simultaneously on

the line, one part at each workstation.

� In the simplest form of production line, the

number of parts on the line at any moment is

equal to the number of workstations, as indicated

in the figure.

�In more complicated lines, provision is made for

temporary parts storage between stations, in

which case there is on average more than one

part per station.

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System Configurations

Depending upon the workflow, the automated

transfer lines are classified as below.

1) In-line (straight line) arrangement of

workstations

2) Segmented in-line – two or more straight line

segments, usually perpendicular to each other

3) Rotary indexing machine (e.g., dial indexing

machine)

In-line (straight line) arrangement of

workstations

�This configuration is common for machining big work pieces, such

as automotive engine blocks, engine heads and transmission cases.

�Because these parts require a large number of operations, a

production line with many stations is needed.

�The in-line configuration can accommodate a large number of

stations.

� In-line systems can also be designed with integrated storage

buffers along the flow path.

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Segmented In-Line Configurations

L-shaped layout

U-shaped layout

Rectangular configuration

Segmented in-line arrangement of

workstations

�The segmented in-line configuration consists of two or

more straight-line transfer sections, where the segments

are usually perpendicular to each other.

�There are a number of reasons for designing a production

line in these configurations rather than in a pure straight

line, including:

1) Available floor space may limit the length of the line

2) It allows reorientation of the work piece to present

different surfaces for machining

3) The rectangular layout provides for return of work

holding fixtures to the front of the line for reuse.

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Two Machining Transfer Lines

Figure: Line drawing of two machining transfer lines: At bottom right, the first is a 12-

station segmented in-line configuration that uses pallet fixtures to locate the work

parts. The return loop brings the pallets back to the front of the line. The second

transfer line (upper left) is a seven-station in-line configuration. The manual station

between the lines is used to reorient the parts.

Rotary configuration

�The work parts are attached to fixtures around

the periphery of a circular worktable, and the

table is indexed (rotated in fixed angular

amounts) to present the parts to workstations

for processing.

�A typical arrangement is illustrated in Figure .

�The worktable is often referred to as a dial,

and the equipment is called a dial indexing

machine, or simply, indexing machine.

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Rotary configuration�Although the rotary configuration does not seem to

belong to the class of production systems called

"lines," their operation is nevertheless very similar.

�Compared with the in-line and segmented in-line

configurations, rotary indexing systems are

commonly limited to smaller work parts and fewer

workstations

�This configuration cannot accommodate buffer

storage capacity.

� The rotary system usually involves a less expensive

piece of equipment and typically requires less floor

space.

Rotary Indexing Machine

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Work Transport Systems

There are two basic ways to accomplish the

movement of work units along a manual

assembly line:

(1) manually or

(2) by a mechanized system.

Manual Methods of Work Transport• In manual work transport, the units of product are passed

from station-to-station by hand.

• Two problems result from this mode of operation are

starving and blocking.

• Starving is the situation in which the assembly operator

has completed the assigned task on the current work unit,

but the next unit has not yet arrived at the station. The

worker is thus starved for work.

• When a station is blocked, it means that, operator has

completed the assigned task on the current work unit but

cannot pass the unit to the downstream station because

that worker is not yet ready to receive it. The operator is

therefore blocked from working.

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• To mitigate the effects of these problems, storage buffers

are sometimes used between stations.

• The work units made at each station are collected in

batches and then moved to the next station. In other

cases, work units are moved individually along a flat table

or unpowered conveyor. When the task is finished at each

station, the worker simply pushes the unit toward the

downstream station.

• Space is often allowed for one or more work units in front

of each workstation. Hence, starving and blocking are

minimized.

• It can result in significant work-in-process

• Workers are un-paced in lines that rely on manual

transport methods, and production rates tend to be lower.

Mechanized Work Transport

Three major categories of work transport systems

in production lines are:

(a) continuous transport,

(b) synchronous transport, and

(c) asynchronous transport.

These are illustrated schematically in Figure.

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continuous transport,synchronous transport,

asynchronous transport

continuous transport system• A continuous transport system uses a

continuously moving conveyor that operates at

constant velocity, as in Figure (a). This method is

common on manual assembly lines.

• The conveyor usually runs the entire length of the

line. However, if the line is very long, such as the

case of an automobile final assembly plant, it is

divided into segments with a separate conveyor

for each segment.

• Examples of this kind are overhead trolley

conveyor, Belt conveyor, Roller conveyor, Drag

chain conveyor.

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• Continuous transport can be implemented in two ways:

(1) Work units are fixed to the conveyor, and (2) work

units are removable from the conveyor.

• In the first case, the product is large and heavy (e.g.,

automobile, washing machine) and cannot be removed

from the conveyor. The worker must therefore walk

along with the product at the speed of the conveyor to

accomplish the assigned task.

• In the case where work units are small and lightweight,

they can be removed from the conveyor for the physical

convenience of the operator at each station.

• Another convenience for the worker is that the assigned

task at the station does not need to be completed within

a fixed cycle time.

Overhead Trolley Conveyor

• A trolley is a wheeled

carriage running on an

overhead track from which

loads can be suspended

• Trolleys are connected and

moved by a chain or cable

that forms a complete loop

• Often used to move parts

and assemblies between

major production areas

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Belt Conveyor

• Continuous loop with forward path to move loads

• Belt is made of reinforced elastomer

• Support slider or rollers used to support forward loop

• Two common forms:

– Flat belt (shown)

– V-shaped for bulk materials

(Support frame not shown)

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Roller Conveyor

• Pathway consists of a

series of rollers that are

perpendicular to

direction of travel

• Loads must possess a flat

bottom to span several

rollers

• Powered rollers rotate to

drive the loads forward

• Un-powered roller

conveyors also available

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Skate-Wheel Conveyor

• Similar in operation to

roller conveyor but use

skate wheels instead of

rollers

• Lighter weight and

unpowered

• Sometimes built as

portable units that can

be used for loading and

unloading truck trailers

in shipping and

receiving

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synchronous transport systems

• In synchronous transport systems, all work units are moved

simultaneously between stations with a quick, discontinuous

motion, and then positioned at their respective stations.

Depicted in Figure (b), this type of system is also known as

intermittent transport, which describes the motion

experienced by the work units.

• Synchronous transport is not common for manual lines, due

to the requirement that the task must be completed within a

certain time limit. This can result in incomplete units and

excessive stress on the assembly workers.

• Despite its disadvantages for manual assembly lines,

synchronous transport is often ideal for automated

production lines. • Examples of this kind are Walking beam transport equipment and Rotary indexing mechanisms.

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asynchronous transport system

• In an asynchronous transport system, a work

unit leaves a given station when the assigned

task has been completed and the worker

releases the unit.

• Work units move independently rather than

synchronously as in Figure (c).

• Examples of this kind are Power-and-free

overhead conveyor, Cart-on-track conveyor,

Powered roller conveyors, automated guided

vehicle system, Monorail systems, and Chain-

driven carousel systems.

Workpart Transfer Mechanisms

• Linear transfer systems:

– Continuous motion – not common for automated systems

– Synchronous motion – intermittent motion, all parts move simultaneously

– Asynchronous motion – intermittent motion, parts move independently

• Rotary indexing mechanisms:

– Geneva mechanism

– Others

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Belt-Driven Linear Transfer System

Side view of chain or steel belt-driven conveyor (over and under type) for linear transfer using work carriers

• Figure illustrates the possible application of a

chain or belt driven conveyor to provide

continuous or intermittent movement of parts

between stations.

• Either a chain or flexible steel belt is used to

transport parts using work carriers attached to

the conveyor.

• The chain is driven by pulleys in either an "over-

and-under" configuration, in which the pulleys

turn about a horizontal axis, or an "around-the

corner“ configuration, in which the pulleys rotate

about a vertical axis.

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Walking Beam Transfer System

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• Many transfer lines utilize various walking beam transfer

systems, in which the parts are synchronously lifted up

from their respective stations by a transfer beam and

moved one position ahead to the next station. The

transfer beam then lowers the parts into nests that

position them for processing at their stations. The beam

then retracts to make ready for the next transfer cycle.

The action sequence is depicted in Figure.

(1) work parts at station positions on fixed station beam

(2) transfer beam is raised to lift work-parts from nests

(3) Elevated transfer beam moves parts to next station positions.

(4) Transfer beam lowers to drop work parts into nests at new

station positions. Transfer beam then retracts to original position

shown in (1).

Geneva Mechanism with Six Slots

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See Animation

D:\CIM\Geneva mechanism video\3.flv

D:\CIM\Geneva mechanism video\4.flv

• The Geneva mechanism uses a continuously rotating driver to index the

table through a partial rotation, as illustrated in Figure.

• If the driven member has six slots for a six-station dial indexing table,

each turn of the driver results in 1/6 rotation of the worktable, or 60o.

• The driver only causes motion of the table through a portion of its own

rotation. For a six-slotted Geneva, 120° of driver rotation is used to index

the table. The remaining 240° of driver rotation is dwell time for the

table, during which the processing operation must be completed on the

work unit.

In general,

Where θ= angle of rotation of worktable during indexing (degrees of

rotation), and ns = number of slots in the Geneva.

• The angle of driver rotation during indexing = 2θ , and the angle of driver

rotation during which the work table experiences dwell time is (360-2θ).

• Geneva mechanisms usually have four, five, six, or eight slots, which

establishes the maximum number of workstation positions that can be

placed around the periphery of the table.

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Given the rotational speed of the driver, we can determine total cycle

time as:

Where Tc = cycle time (min), and N = rotational speed of driver (rev/min).

Of the total cycle time, the dwell time, or available operation time

per cycle, is given by:

Where Ts = available service or processing time or dwell time (min),

and the other terms are defined above.

Similarly, the indexing time is given by:

Where Tr - indexing time (min).

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Cam Mechanism to Drive Dial Indexing Table

•Various forms of cam drive mechanisms, are used to provide an accurate and

reliable method of indexing a rotary dial table.

•Although a relatively expensive drive mechanism, its advantage is that the

cam can be designed to provide a variety of velocity and dwell characteristics.

D:\CIM\cam animation.gif

See animation of CAM

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Ratchet and pawl mechanism• A ratchet is a device that allows linear or rotary motion

in only one direction, while preventing motion in the

opposite direction.

• Ratchets are used in many other mechanisms, including

clocks, jacks, and hoists.

• Ratchets consist of a gearwheel (marked with a "b" in the

diagram to the left) or linear rack with teeth, and a

pivoting spring loaded finger called a pawl (marked with

an "a" in that same diagram) that engages the teeth.

D:\CIM\Ratchet_example.gif

See Animation

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• Either the teeth, or the pawl, are slanted at an angle,

so that when the teeth are moving in one direction,

the pawl slides up and over each tooth in turn, with

the spring forcing it back with a 'click' into the

depression before the next tooth.

• When the teeth are moving in the other direction,

the angle of the pawl causes it to catch against a

tooth and stop further motion in that direction.

• Because the ratchet's teeth can only stop 'backward'

motion at discrete points, a ratchet does allow a

limited amount of 'backward' motion, or backlash, to

a maximum of the spacing between its teeth.

Rack and pinion mechanism • A rack and pinion is a pair of gears which convert

rotational motion into linear motion.

• The circular pinion engages teeth on a flat bar -

the rack. Rotational motion applied to the pinion will

cause the rack to move to the side, up to the limit of its

travel.

• The rack and pinion arrangement is commonly found in

the steering mechanism of cars or other wheeled,

steered vehicles.

• This arrangement provides a lesser mechanical

advantage than other mechanisms such as recirculating

ball.

Click this to see Animation D:\CIM\rack1.gifD:\CIM\

Rack_pinion.gif

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Storage Buffers in Production

Lines

A location in the sequence of workstations where parts can be collected and temporarily stored before proceeding to subsequent downstream stations

• Reasons for using storage buffers:– To reduce effect of station breakdowns– To provide a bank of parts to supply the line– To provide a place to put the output of the line– To allow curing time or other required delay– To smooth cycle time variations– To store parts between stages with different

production rates

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Storage Buffer

Storage buffer between two stages of a production line

Storage Buffer

)( 1k )( 2k

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Control Functions in an

Automated Production Line

• Sequence control

– To coordinate the sequence of actions of the

transfer system and workstations

• Safety monitoring

– To avoid hazardous operation for workers and

equipment

• Quality control

– To detect and possibly reject defective work units

produced on the line

Applications of

Automated Production Lines

• Transfer lines for machining

– Synchronous or asynchronous workpart transport

– Transport with or without pallet fixtures, depending

on part geometry

– Various monitoring and control features available

• Rotary transfer machines for machining

– Variations include center column machine and

trunnion machine