Focus 5 Systems Ltd

The Theory of Constraints & Drum Buffer Rope

1. Introduction.
Companies around the world have achieved significant improvements to bottom-line results, due date performance, lead time and quality through the use of TOC and it's five-step process of continuous improvement.

The TOC logistics concepts were first described in Dr Goldratt's best selling novel The Goal. From those ideas evolved the powerful manual scheduling technique Drum-Buffer-Rope (DBR) which was further described in The Race by Dr Goldratt and Robert Fox.

A basic principle of TOC Logistics is that we can view any organisation as a chain ....

Q. What determines the strength of a chain? .......
A. The weakest link!

There is only one weakest link in any chain, but what if your organisation is very complex? The more complex an organisation, the fewer independent chains it contains. Fewer chains means fewer weakest links. Therefore complex organisations can benefit even more from TOC.

2. Why Manage by Constraints.
Within manufacturing industry many companies are seeking to gain significant competitive advantage by ensuring Due Date Performance (DDP) greater than 98%, continual reduction in their quoted lead times (QLT) and the ability to increase volume and vary mix without jeopardising DDP or QLT. It is now widely acknowledged that this can only be sustained by implementing a true Process Of On-Going Improvement (POOGI).

Any production unit is limited either by the market demand or by very few factors - it's 'constraints'. Therefore to achieve the objectives there is a need to identify the constraints and to base the actual plans and schedules on their capabilities. Recognition of the limits set by the few constraints also leads the way to facilitate a process of on-going improvement.

Let's look at a very simple example and the questions that arise:

Product X is manufactured by starting with Raw Material X and then processing it sequentially through 5 operations using machines A to E respectively (see diagram). This is the only use that the five machines are put to. The hourly rates for each machine are given in the table.

Operation:

1

2

3

4

5

Machine:

M/C A

M/C B

M/C C

M/C D

M/C E

Hourly Unit Output Rate:

100

80

40

60

90

This begs a number of questions to help answer...... "Why manage by constraints?":

Question

Answer

What is limiting the System?

What is the maximum output per hour of Product X?

40

M/C C

By how much would the output be improved if B was increased to 90?

No Improvement

M/C C

By how much would the output be improved if C was increased to 50?

By 10

M/C C

By how much would the output be improved if C was increased to 70?

By 20

M/C D

What effect on the system if M/C A can only manage an output of 90 in one hour?

None

M/C C

What effect on the system if M/C C can only manage an output of 30 in one hour?

We lose 10 Product X

M/C C

What effect on the system if M/C B is allowed to drop to an output of 30 in one hour?

We lose 10 Product X

M/C B for that hour. Note also that the loss cannot be recovered.

3. Overview of Drum-Buffer-Rope (DBR).
The DBR logistical system is a finite scheduling mechanism that balances the flow of the system. DBR controls the flow of materials through the plant in order to produce products in accordance with market demand with a minimum of manufacturing lead time (MLT), inventory and operating expenses.

DBR is based on the TOC logistics approach and the TOC five steps of focusing. These are also the basis of POOGI and, as described in The Haystack Syndrome, say:

    1. Identify the System's Constraint(s).
    2. Decide How to Exploit the System's Constraint(s).
    3. Subordinate Everything Else to the above Decisions.
    4. Elevate the System's Constraint(s).
    5. If, in the Previous Steps, a Constraint Has Been Broken, Go Back to Step One, but Do Not Allow Inertia to Cause a System's Constraint.

and the definitions of DRUM, BUFFER and ROPE are:

  • DRUM - A schedule for the constraint.
  • BUFFER - A protection against Murphy. This is the time provided for parts to reach the protected area. The protected areas are the Drum, the due-dates and the assemblies of constraint parts with non-constraint parts.
  • ROPE - A schedule for releasing raw materials to the floor. The Rope is derived according to the Drum and Buffers; its mission is to ensure the proper subordination of the non-constraints.

The identification of all the processing, resource, and marketing constraints within the entire system is needed. These constraints become the primary focus of attention. They are then used to derive the planning, scheduling, and control of all the plant's resources. The result provides a smooth and continuous flow of materials through the plant with minimum disruptions.

Any schedule or production plan must be productive, reliable, robust and realistic. Productive in that it must relate to the market demand while contributing to and be measurable against the organisation's goal; reliable and robust in that it must reflect the capability of the resources available and stand up to the inevitable disturbances or disruptions that will hit it; realistic in that it is capable of being done with the resources available including material supply.

In any plant, there are only a few capacity constraint resources (CCRs). All CCRs are identified, and the various orders that are to be processed through them are scheduled according their capacity potential and to the market demand. The schedule established for the CCRs determines the drum beat for the system.

The essential operational steps of DBR scheduling are as follows:

  • Establish the due date requirements for the orders or demand. This provides the first and "ideal" drum to work to.
  • Identify the CCRs in the system.
  • Develop a Drum or a schedule for the CCRs which makes best use of them and is in-line with the needs of the market. The drum is effectively the master production schedule which establishes the "drum beat" and control for the entire system.
  • Protect the throughput of the factory from statistical fluctuations through the use of time buffers at critical locations. Time buffers are strategically located to protect the throughput of the entire system and to protect the due dates promised to customers.
  • Use logistical ropes tied to the CCR drum schedules for each resource. The ropes synchronise all non CCRs to generate the timely release of the right materials into the system at the right time. Ropes ensure that operations upstream of CCRs are time phased to CCR requirements and operations downstream do not subsequently impede product flow.

Buffers provide the timely protection of CCRs from any likely or expected disruption. The rope is the timely release of raw materials into the system, it is tied to the size of the buffer. Buffer Management (BM) provides the means by which the schedule is managed on the shop floor. The strategic points at which buffers are established are called the buffer origins.

4. Buffer Management & POOGI.
As said earlier a schedule clearly needs to be productive, realistic and reliable - it should also be emphasised that a schedule is only as good as the ability to manage it or "make it happen". This is where the Buffer Management (BM) control comes in and is a very important aspect of the DBR application. BM will help identify resources likely to restrict Throughput and/or increase Inventory and is also a key link in the support of the Process Of On-Going Improvement  (POOGI).

BM will firstly help to ensure the schedules for the CCRs are maintained and thereby the total schedule. Secondly, by identifying trouble-making non CCR's, it can also focus attention on these and point towards areas for improvement. If, as a result of the improvements there is less disruption from problems then buffers (the Murphy element of them) can be made smaller resulting in shorter lead times, etc, etc.

As well as the immediate purpose of managing the day to day running of the schedule, the data obtained from the buffer monitoring can be used to identify trouble making resources as a focus to direct improvements as part of the POOGI.

Note:
The above contains extracts from a paper written by A Cohen of Focus 5 Systems Ltd and T Hutchin of Constraint Management Group entitled "Implementing Constraint Based Scheduling Through Linking Computer Simulation and the Theory Of Constraints". The paper was presented at the International Symposium on Advanced Manufacturing Processes Systems and Technologies AMPST 96, University of Bradford, 26-27 March 1996 and published in conference proceedings pp411-420, ISBN 0 85298 989 X

© 1997 Focus 5 Systems Ltd.

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