Introducing Flow, Part II – Balancing a Repetitive Line

When you’re in the business of manufacturing parts that you sell through a catalog, chances are you’re a repetitive manufacturer. And if you produce the same product over and over on a mixed model line, SAP has great functionality for you to schedule and level your production plan – readily available in ECC 6.0. No need for extensive customizing or running a project with 5 consultants over the next 6 months; just yesterday I configured a demo into a client’s ‘sandbox system’ within 3 hours.

As we discussed in part I, to introduce flow into the production line means reducing WiP and with it cycle times. So flow is very desirable, especially if you’re in the repetitive business and to achieve it, you need to balance the operations of the line. Balancing the line means that you allow the same amount of time a product can spend on a work center for every operation. In that case there will be no buildup of WiP (no product has to wait until it can processed on the next work center) in front of any work center.

Let’s see how SAP-ERP can help you do that: First you need to set up your products for repetitive manufacturing and create at least one production version through which you assign the products to the Line Hierarchy. This represents your mixed model line. The production version relates to a routing that contains all operations and work stations the product undergoes while being produced (it ‘flows’ through the line).

The routing may be a Rate (Line) Routing that can be represented graphically…

…and has all the details necessary for lean production.

Once all products are assigned to the Line Hierarchy, you can create a Line Balance for a short term planning horizon of, let’s say, 4 weeks. In that Line Balance, you copy the demand for the next 4 weeks and calculate a takt time by which you need to run the model mix to fulfill exactly that demand. This works in the following way: The system looks at the total demand for all products in the model mix for the next 4 weeks and calculates a daily rate for the model mix, which is necessary to meet the total demand over 4 weeks. Imagine you have 3 products - A, B and C – and a demand of 200, 400 and 100 pieces over the next week for these products. If there are 20 working days in your next 4 weeks, you need to produce 10 As, 20 Bs and 5 Cs every day to fulfill the demand over the next 4 weeks (according to EPEI heijunka leveling). Therefore you need to produce 35 pieces on the line every day. If you have 7 hours available every day, you need to produce 5 pieces per hour or introduce a piece into the line every 12 minutes… this is called the takt time. That, in turn, means that every work center has 12 minutes of work content before it needs to move the product to the next work center.

In above example we have a maximum rate of 35 pieces per 7 hours and a model mix that adheres to the maximum rate. Now we have to make sure that all operations in any work center do not exceed the maximum allowable time in any work center – the takt time. That, in essence, is Line Balancing and may be carried out graphically in standard SAP.

In the Line Balance we can see that product 188 causes a takt violation with operation 0080 on work center 65030TRY (note that work station 65030TRY has more work content available. This is because we have more capacities available – either more labor or more machine capacity). You could now do either one of two things: either you increase the amount of machines used on work center 65030TRY or you could move the operation to another work center, in the flow, that has more work content available. Either way the takt violation is cured.

In any case, you have now calculated a takt time – the speed by which the line must be run to meet demand – and you can now use takt-based scheduling to determine the sequence plan as follows.

From here we can now perform a collective availability check, print the daily schedule or fill a heijunka board on the shop floor. The schedule may also be connected with a Kanban control cycle but that is stuff for another blog post…