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Single-Station Manufacturing Cells

Sections:

1. Single-Station Manned Workstations

2. Single-Station Automated Cells

3. Applications of Single-Station Cells

4. Analysis of Single-Station Cells

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Single-Station Manufacturing Cells

• Most common manufacturing system in industry

• Operation is independent of other stations

• Can be manned or automated

• They are used for either processing or assembly operations

• Can be designed for: • Single model production

• Batch production

• Mixed model production

• Two issues that must be considered in the planning of single station systems • (1) how many workstations are required to satisfy production requirements, and

• (2) how many machines can be assigned to one worker in a machine cluster*.

*A machine cluster is a collection of two or more identical or similar machines that are serviced by one worker.

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Classification of Single-Station Manufacturing Cells

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Manned cell

Automated cell

Single-station manufacturing

cells

Fully automated machine (M<1)

Machine cluster (M<1)

Semiautomated machine (M=1)

Manually operated machine (M=1)

Hand tools and portable powered

tools (M=1)

Single-Station Manned Cell

One worker tending one production machine (most common model)

• Most widely used production method, especially in job shop and batch production

• Reasons for popularity: • Shortest time to implement

• Requires least capital investment

• Easiest to install and operate

• Typically, the lowest unit cost for low production

• Most flexible for product or part changeovers

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Single-Station Manned Cell Examples

• Worker operating a standard machine tool • Worker loads & unloads parts, operates machine (engine lathe, drill press)

• Machine is manually operated

• Worker using hand tools or portable power tools at one location (arc welding gun, powered handheld drill)

• Worker operating semi-automatic machine • Machine is controlled by some form of program (CNC machine)

• Worker loads & unloads parts, starts semi-automatic work cycle

• Worker attention not required continuously during entire work cycle

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Variations of Single-Station Manned Cell

• Two (or more) workers required to operate machine • Two workers required to manipulate heavy forging at forge press

• Welder and fitter in arc welding work cell

• One principal production machine plus support equipment • Drying equipment for a manually operated injection molding machine

• Trimming shears at impression-die forge hammer to trim flash from forged part

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Single-Station Automated Cell

Consists of fully automated production machine capable of unattended operation for time period longer than one machine cycle

• Worker not required except for periodic tending (move the completed part away from the processing head and move the raw work part into position in front of the work head)

• Reasons why it is important: • Labor cost is reduced

• Easiest and least expensive automated system to implement

• Production rates usually higher than manned cell

• First step in implementing an integrated multi-station automated system

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Enablers for Unattended Cell Operation

• For single model and batch model production: • Programmed operation for all steps in work cycle

• Parts storage subsystem

• Automatic loading, unloading, and transfer between parts storage subsystem and machine

• Periodic attention of worker for removal of finished work units, resupply of starting work units, and other machine tending

• Built-in safeguards to avoid self-destructive operation or damage to work units

• For mixed model production: • In addition those enablers

• Work unit identification: • Automatic identification (e.g., bar codes) or sensors that recognize alternative features of

starting units

• Capability to download programs for each work unit style (programs prepared in advance)

• Capability for quick changeover of physical setup

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Parts Storage Subsystem and Automatic Parts Transfer

• Necessary conditions for unattended operation

• Given a capacity = np parts in the storage subsystem, the cell can theoretically operate for a time

UT = npTc where UT = unattended time of operation • In reality, unattended time will be less than UT because the worker needs time to

unload finished parts and load raw workparts into the storage subsystem

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Parts Storage Capacity

• Typical objectives in defining the desired parts storage capacity np: • Make npTc = a fixed time interval that allows one worker to tend multiple

machines

• Make npTc = time between scheduled tool changes

• Make npTc = one complete shift

• Make npTc = one overnight (“lights-out operation”)

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Storage Capacity of One Part

• Example: two-position automatic pallet changer (APC)

• With no pallet changer, work cycle elements of loading/unloading and processing would have to be performed sequentially

Tc = Tm + Ts where Tm = machine time and Ts = worker service time

• With pallet changer, work cycle elements can be performed simultaneously

Tc = Max{Tm, Ts} + Tr where Tr = repositioning time to move the completed part away from the processing head and move the raw work part into position in front of the work head

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Storage Capacities Greater Than One

• Larger-storage capacities allow unattended operation, as long as loading and unloading the parts can be accomplished in a reasonable time

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The figure depicts a single-station machining cell interfaced with an automated pallet

storage and handling system

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Storage Capacities Greater Than One

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Machining center and automatic

pallet changer with pallet holders

arranged radially; parts storage

capacity = 5

Machining center and in-line

shuttle cart system with pallet

holders along its length; parts

storage capacity = 16

Machining center with

pallets held on indexing

table; parts storage capacity

= 6

Machining center and

parts storage carousel

with parts loaded onto

pallets; parts storage

capacity = 12

Applications of Single Station Manned Cells and Automated Cells

• CNC machining center with worker to load/unload

• CNC turning center with worker to load/unload

• Cluster of two CNC turning centers with time sharing of one worker to load/unload

• Plastic injection molding on semi- automatic cycle with worker to unload molding, sprue, and runner

• One worker at electronics subassembly workstation inserting components into PCB

• Stamping press with worker loading blanks and unloading stampings each cycle

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• CNC MC with APC and parts storage subsystem

• CNC TC with robot and parts storage carousel

• Cluster of ten CNC TCs, each with robot and parts storage carousel, and time sharing of one worker to load/unload the carousels

• Plastic injection molding on automatic cycle with robot arm to unload molding, sprue, and runner

• Electronics assembly station with automated insertion machine inserting components into PCBs

• Stamping press stamps parts from long coil

Manned Cells Automated Cells

Machine Cluster vs Machine Cell

• Machine cluster: A collection of two or more machines producing parts or products with identical cycle times and service (usually loaded and unloaded) by one worker • The semiautomatic machine cycle is long relative to the service portion of

the cycle that requires the worker’s attention

• The semiautomatic cycle time is the same for all machines

• The machines that the worker would service are located in close enough proximity to allow time to walk between them

• The work rules of the plant permit a worker to service more than one machine

• Machine cell: Consists of one or more machines organized to produce a family of parts or product, like groups technology or flexible manufacturing system.

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Machine Cluster

• Assumption: worker is always available when service is needed, so the machine is never idle. Total cycle time of a machine is Tc = Tm + Ts

• If more than one machine assigned to the worker, a certain amount of time will be lost while the worker walks from one machine to the next, Tr, repositioning time. The required for the operator to service one machine is therefore Ts + Tr and the time to service n machines is n(Ts + Tr )

• For the system to be perfectly balanced in terms of worker time and machine cycle time n(Ts + Tr )= Tm + Ts

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Analysis of Single-Station Systems

• Determining number of workstations required

• Number of setup is known

• Number of setup is unknown

• Including availability, utilization, and defect rate

• How many machines for one worker?

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