5.11 - Nagare Cell Design
Introduction
As manufacturing is re-structured into cellular organisations careful choice of design principle is important. Nagare principles provide an alternative for medium to high volume and low to medium range production environments. The implementation of Nagare principles can follow a managed development path from an early rough-cut version to full perfection via continuous improvement.
The basic Nagare principles are as follows:-
Subject |
Principles |
Flow |
Material flows smoothly through the multi-process cell. Obstacles that prevent flow of materials and information are removed. Careful flowchart analysis is done first. Tools and support materials all have a pre-determined place and are maintained tidily. |
Operation |
Operator walks around the cell. Machines are loaded manually and unloaded automatically; the machines continue to run as the operator progresses around the cell. The last operation is adjacent to the first. Operations are arranged in the sequence that they will be used. |
Manpower |
Optimise the utilisation of the operators. Maximise the use of multi-skilled individuals by training. |
Output |
Produce only what is required and no more. Match the manufacturing cycle to the customer demand cycle. Add operators to increase output. |
Batch Size |
Single unit or two handed batch size - no more. Quick changeover design needed. |
Plant |
Simple proven processes with narrow machine widths to reduce walking. Reliable and easily maintained plant which is ergonomically designed for low fatigue and quick changeover. |
Quality |
High quality standard incorporating mistake proofing principles (Poka Yoke & Jidoka), inline and source inspection. |
Waste |
Expose and eliminate. Continuous improvement of standard routines and flows. |
Cycle Time |
The cell cycle time should be greater than the longest machine time. |
Example 1
For a 12 machine cell:
If each machine takes 40 seconds and the single operator takes 10 seconds to walk and load a machine. Then:-
Walking time
= 10 secs * 12 operations = 120secs.
Manufacturing time per set
= 120 seconds
Number of operators |
Manufacturing time per set |
1 |
120 seconds |
2 |
60 seconds |
Demand Cycle (units / minute)
= daily quantity required / no of walking minutes per day
Example 2
28,000 units in 312 hours / month requires a 40 second demand cycle time (pulse rate). Typically 20% extra capacity should be allowed in the preliminary design to allow for variability. Pulse rates should be 30 seconds or greater.
Number of operators
= number of operations * walk time / demand cycle time
= time for one operator to walk / demand cycle time
Example 3
Process |
|
Date |
|
No of Ops |
7 |
|
Part No |
Daily Qty Required |
Demand Cycle Time |
Operator working time |
Longest operation |
Standard Stock |
G1234 |
300 |
72 secs |
72 secs |
39sec |
Op 2 |
8 |
Seq No |
Operation Description |
Operating Time |
|
Man |
M/C |
1 |
Drill 2 Holes |
8 |
24 |
2 |
Ream 2 Holes |
7 |
32 |
3 |
Mill Pad |
7 |
29 |
4 |
Rough Bore |
9 |
21 |
5 |
Finish Bore |
7 |
30 |
6 |
Drill 4 Holes |
8 |
27 |
7 |
Tap 2 Holes |
12 |
22 |
|
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Manual operation
Machine operation
|
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Walking time is included in manual time on chart
General Benefits
The general benefits of a Nagare cell design, when compared with traditional manufacturing methods are as follows:-
Improved Labour productivity
The cell is designed to maximise the use of labour resulting in high labour productivity - typically a 20% increase over traditional functional layouts. Operator works while walking.
Lower capital expenditure
The use of simple processes will require less expenditure than complex multi-function processes - typically up to 50% less.
Machine utilisation
Wherever possible simple machines are used. The use of less costly plant reduces the importance of machine utilisation - but total process utilisation is high.
Flexibility and Responsiveness
As sales demand increases and additional operator may be added to achieve the output required. Also with simpler plant quicker changeovers are possible.
Stock Cost Reduction
Work in progress is reduced as is finished part safety stock. Kanban or material flow interfaces at cell boundary. No inter-machine stock.
Continuous Improvement
Techniques for continuous improvement, Process Changeover Reduction (see guide 5.25) and Poka Yoke (see guide 5.31), are easily applied to the critical areas which can be clearly and quickly identified.
Skill Requirement
Simpler plant requires lower levels of setting and maintenance skills.
However, all staff must be trained in detail and should be trained in continuous improvement methods (see guide 2.03).