OPERATION OVERLAPPING

OPERATION OVERLAPPING

BRIAN WILLCOX

ACTION MRPII

Operation overlapping is a technique used to reduce the total lead time of a manufacturing order by dividing the batch into two portions and starting the second operation before the first is finished. It is either used to get an urgent batch through quicker or as a routine when big batches and/or long operation times per unit are experienced. A flow line overlaps all the operations in the line and the batch size at any one point in time at an operation is effectively one. Work cells also use this concept and again the batch size is effectively one.

Many schedulers or planners may not use this in practice but it does have its uses, especially in the job shop when the two operations are performed at work centres which are far apart. Just as a matter of interest, it often appears as a question in the Production Activity Control CPIM examination.

A time reduction is made by removing the queue time between the operations, and scheduling part of the run time of operation 1 in parallel with operation 2. In this example I have shown the total order quantity as "Qty A + Qty B" which then allows "Qty A" to be moved to the next operation whilst "Qty B" is still being worked on.

FIGURE 1 shows the typical series manufacturing process and the elapse time that goes with it. FIGURE 2 demonstrates the principle of overlapping and how the saving is made.

Figure 1:
Normal Series Manufacture for Two Operations (with no queue time)

:SU:
             Run Time       :
-----------------------
Qty A            :     Qty B

              Move : :

:SU:
             Run Time       :
-----------------------
Qty A        :        Qty B

Figure 2:
Two Operations with overlapping

:SU:
             Run Time       :
-----------------------
Qty A            :     Qty B

                   : :Move : :

                     :SU:              Run Time       :Time saved:
-----------------------
Qty A        :        Qty B

Figure 1:
Normal Series Manufacture for Two Operations (with no queue time)
:SU:	Run Time : ----------------------- Qty A : Qty B
	Move : :
	:SU:	Run Time : ----------------------- Qty A : Qty B

Figure 2:
Two Operations with overlapping
:SU:	Run Time : ----------------------- Qty A : Qty B
	: :Move : :
	:SU:	Run Time :Time saved: ----------------------- Qty A : Qty B

As the two operations may be on work centres at different ends of the factory, and only one quantity is to be passed to the second work centre before the total batch is completed at the first work centre, it is important that sufficient are passed to keep the second machine busy until the second quantity is received. That raises the question of how do you know how many to make at the first work centre before sending the parts on to the second work centre. Obviously the quantity to be passed must allow sufficient time for the remainder of the batch on work centre one to be completed and delivered to work centre two whilst work centre two is being set-up and completing the work required on operation two (run time) for the first quantity delivered. That may sound involved or confusing but it is really quite simple. To assist us there is a formula which makes it easy to calculate the minimum quantity to be made on work centre one before any are sent to work centre two.

Overlapping formula.

Qm = QRa - SUb
Ra + Rb

WHEN	Qm = minimum quantity of first batch.
	Q = Total batch size.
	Ra = unit run time, operation A.
	Rb = unit run time, operation B.
	SUb = set-up time of operation B.

For this to make real sense we will work an example;

If Q = 200 units,

Ra = 10 mins,

Rb = 5 mins,

SUb = 35 mins,

Then Qm = QRa - SUb Ra + Rb
= (200 x 10) - 35 10 + 5
= 1965 15
= 131 units.

This tells us that we need to make 131 units on the first operation before passing any to work centre two. Lets check this out so we can see that it really does work.

On the second work centre it will take the following time to process 131 units.

131 x 5 (quantity x unit run time) + 35 (op B set-up).

This equals 690 mins.

Whilst this work is being performed on work centre two, work centre one has the remainder of the batch to do.

Remainder equals 200 - 131 = 69.

The time for 69 units on work centre two is;

69 x 10 = 690 mins.

This time equals the time for work centre two, so work centre two will not run out of work.This formula may appear just a useless bit of information but in practise, if you have jobs with long process times or urgent jobs to get through, then it can be invaluable

For those of you sitting the APICS examination make sure you know how to this calculation as it appears quite regularly.

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