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Summary of Mixed Model Lines and Workcells

Making The Impossible Possible 

The principle of Mixed Model Production is simple enough. Designing the process and system is often more difficult. In this page we explore common roadblocks and describe some techniques to eliminate them.

In a NutshellIn A Nutshell

There are many roadblocks to mixed model production lines and assembly cells. Here are some of the roadblocks and ways to get around them.

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Reference:

LEE, QUARTERMAN, Mixed Model Production--A forgotten Aspect of Lean, IISE Annual Conference Proceedings, Philadelphia, PA, USA,2017


Mixed Model Assembly Lines

Problem Description Solutions
Work Balance

When different models have different work content, operations must be flexible enough to deal with the product mix.

  • A constantly moving line only lends itself to mixed model production when work content is nearly identical for each station. See Balancing Workcells.

  • Small Queues between stations to accommodate short-term variation.

  • Adjacent workers share tasks and help each other.

  • Fewer stations and longer task times.

Part Availability

For a mixed model assembly line to function, every part for every product must be immediately available.

  • Small quantity of every part replenished by Kanban.

Tool Availability

Tools and special fixtures must be readily available. (This is seldom a problem when the product is small. It may be a problem with large products such as refrigerators or vehicles.)

  • Multi-purpose fixtures & Tools

  • Setup Reduction

  • Dedicated cells with a permanent setup of fixtures & tools.

Fabrication Setup Reduction

Dedicated fabrication equipment is not always available. Long, expensive and/or inconsistent changeovers force large batches and high inventory between fabrication and the mixed model line.

  • Setup Reduction In Fabrication with small lots.
  • Redesign fabrication processes with more dedicated equipment, even (if necessary) at the cost of decreased labor efficiency.
  • Investigate alternate processes.

 

Dedicated Assembly Cells

Problem Description Solutions
Demand Variability

Variation in demand for each dedicated cell may be larger than the variation for a single line. (Peaks and valleys of demand often cancel when combined.)

  • Design assembly cells for multiple levels of staffing.
  • Move people between cells to balance output & demand.
  • Provide excess equipment capacity to handle peak demands.
Cell Design

Workcells appear simple and this is especially true for assembly cells. In reality they are complex and sometimes sensitive socio-bio-technical systems.

 

Integrated Cells

Problem Description Solutions
Process Incompatibility

This usually involves processes that must be isolated for environmental reasons (Dust, Dirt, Chemicals or Noise).

  • Address the environmental problem with dust control, noise control, etc.
  • Investigate alternate processes.
Process Scale

Large-scale fabrication processes have too much capacity for a single assembly cell.

  • Investigate alternate equipment and methods. (e.g. small, manual paint booth replacing a large paint line)
Special Skills

Certain tasks require extensive skills, experience and/or training but do not require a full-time operator for each cell.

  • Tools, fixtures or gages that require less skill
  • Train or cross train
  • Train cell workers in basics with consultant employee for difficult problems. The daily tasks often do not require high skills or knowledge.

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