ISE 311

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Organization of Workstations

• Applications in traditional and lean systems• Tradeoffs among guidelines depend on type of

production, number and types of items being produced, etc.

• Nine guidelines:1. Use Specialization Even Though It Sacrifices Versatility2. Consider Group Technology3. Consider Both Non-progressive and Progressive Assembly4. Balance Flow Lines5. Minimize Material Handling Cost6. Decouple Tasks7. Make Several Identical Items at the Same Time8. Combine Operations and Functions9. Vary Environmental Stimulation Inversely with Task

Stimulation

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G1: Use Specialization Even Though It Sacrifices Versatility

• Special-purpose equipment• Specialized materials• Specialized labor• Group technology

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G2: Consider Group Technology

• Form families of items that are almost identical. May be based on part geometry or process similarities. Classification process depends on solving coding problem. May be the basis of factory specialization.

• Imitate a flow line and shift fixed costs of the flow line to the process.

• Find common solutions to common problems.

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Cell Layout

Cell: machines and skills for producing related items placed close together.

• Advantages: Specialization Minimum material handling Simpler production control Shorter throughput times Lower work-in-process (WIP) inventory

• Disadvantages: Lower equipment utilization Loss of flexibility Cost of setting up and maintaining concept

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G3: Consider Both Non-progressive and Progressive Assembly

• Consider an assembly of N elements that requires m people to work.

Non-progressive assembly: Each worker does all N elements.

Progressive assembly: The job is split so each worker does N/m elements.

• Which should we choose?

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Non-Progressive Assembly

AdvantagesNeutral

CharacteristicsDisadvantages

• Balance delay time is eliminated.

• Scheduling flexibility is increased.

• Shocks do not have multiple effects.

• Musculoskeletal disorders are less of a problem.

• Satisfaction may increase.

• Quality may be higher or lower with each type of assembly.

• Material handling has advantages and disadvantages in both.

• Space requirements could favor either.

• Walking may be required in either.

• Direct labor cost/hour is higher.

• Skill requirements and training costs are higher.

• Equipment capital cost is higher.

• In-process inventory is higher.

• Supervision is more difficult.

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Reasons for Flow Lines

• Some of their advantages are emphasized by decision makers.

• Cost advantages that favor flow lines are visible, whereas the costs that do not favor them are lost in overhead figures.

• Many of their disadvantages can be overcome by good system design.

• Work cells in lean production can be combined into flow lines and balanced.

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G4: Balance Flow Lines

• Balance the tradeoffs between number of stations or cells and the amount produced by each station to meet production times as smoothly and efficiently as possible.

• Before balancing flow lines: Ask whether it is appropriate to balance the line. Gather the givens:

• Table of work elements and times• Precedence diagram• Required units/minute from line (Takt time)

• Determine: Number of stations/cells Number of workers at each station/cell Elements to be done at each station/cell

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Standard Balancing Technique

Example: Final assembly of an electronic organ (from Weiss and Gershon, Production and Operations Management (2nd ed.), 1993, Allyn & Bacon.)

Precedence Table:

Task Time (sec) Predecessors A 40 None B 55 None C 75 None D 40 A E 30 A, B F 35 B G 45 D, E H 70 F I 15 G, H J 65 I K 40 C, J

TOTAL: 510

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Line balancing

• Step 1: draw the precedence diagram

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Line balancing

• Step 2: Determine quantity to be made and takt time. Example:

Production goal = 200 organs/day (set by customer demand)

Cycle time = (1 day/200 organs) * (8hrs/day) * (3600 sec/hr)

= 144 sec/organ

• Estimate an approximate number of stations. Example:

Number of stations = total time required / takt time

= 510 sec / 144 sec = 3.54 = _______

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Line balancing

• Step 3: Make a trial solution. Longest task time rule: from among the available tasks, choose

the one that takes the longest. In case of a tie, choose the one with the most followers.

Station TaskTask time

Total time

Station Idle Time

Total Idle time

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Station TaskTask time

Total time

Station Idle Time

Total Idle time

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Line balancing

• Step 4: Determining the efficiency idle time = (# of stations * cycle time) - (time to produce 1 unit)

efficiency = (time to produce 1 unit) / (# of stations * longest station time)

or

efficiency = (theoretical minimum cycle time) / (achieved cycle time)

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Line Balancing

• Problems: Mean times for stations are not equal, so there is balance-delay

time. The speed at a station in a balanced line must be set at the speed

of a slow operator on a slow station. Cycle times vary.

• Options Consider element sharing. Remember that cycle times are not fixed. Remember that elements often can be redefined. Interchange elements from the assembly task and the

subassembly tasks. kanban - inventory control pull system

• parts placed in containers for next cell

• when parts are pulled, they must be replaced

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G5: Minimize Material Handling Cost

• Elements of material handling cost: Material handling cost/year = Capital cost + Operating cost Operating cost = Number of trips/year × Cost/trip Cost/trip = Fixed cost/trip + (Variable cost/distance × distance/trip)

• Reduce material handling cost by … Consider how utilization affects capital costs vs. operating costs. Eliminate peak loads through scheduling. Replace transportation with communication. Reduce number of trips through scheduling and combining. Reduce fixed costs by using line production. Move more product/labor hour. Reduce distances by efficient layout and arrangement. Use a “bus” instead of a “taxi” system. Emphasize reducing total MH costs rather than individual elements.

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Strategies to Reduce Material Handling Cost• Consider how utilization affects capital costs vs. operating

costs.

• Eliminate peak loads through scheduling.

• Replace transportation with communication.

• Reduce number of trips through scheduling and combining.

• Reduce fixed costs by using line production.

• Move more product/labor hour.

• Reduce distances by efficient layout and arrangement.

• Use a “bus” instead of a “taxi” system.

• Emphasize reducing total MH costs rather than individual elements.

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G6: Decouple Tasks

• Types of flow lines: Operation-only line Order-picking line Assembly line

• Decisions to consider: Single product or multiple products? Assemble in sub-assemblies or on the line? How to divide tasks among stations? Balanced or unbalanced?

• Characteristics of well-designed lines: Minimum idle time at the stations High quality (enough time at each station) Minimum capital cost

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Lean Manufacturing (Just-in-Time)

Reduction of work-in-process inventories to minimal amounts, to force management to solve problems by revealing them.

Little product variety Producing only what the customer wants, when it is wanted High quality High equipment availability Short setup times Cross-trained skilled workers

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Shocks and Disturbances

• Cause cycle times to vary.• May result from operator variability, station starvation,

or station blockage.• Buffers provide flexibility/tolerance.• In lean systems, the presence of inventory buffers

indicate waste opportunities to improve process goal is to eliminate the need for buffers

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Buffering Techniques

• Decouple by changing product flow Buffers at or between stations Buffers due to carrier design Buffers off-line

• Decouple by moving operators Utility operator Help your neighbor n operators, n workstations n operators, >n workstations

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G7: Make Several Identical Items at the Same Time

• Task stages: Get ready Do Put away

• Reduce cost/unit by prorating get-ready and put-away over more units.

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G8: Combine Operations and Functions

• Use multi-function materials.

• Use multi-function tools.

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G9: Vary Environmental Stimulation Inversely with Task Stimulation

• Low-stimulation tasks: Add physical movement to the task Add stimulation to the environment

• High-stimulation tasks: Make privacy possible Discourage excess conversation