How to do line balancing (dedicated lines)?

Line balancing is a power tool to remove waste if you do it properly. Here are four steps for balancing a dedicated line (versus a shared or mixed product line) by using a simple example:

1. Identify the process steps, materials and process time. This step is usually done using a Process Graph (see figure 1.)
Figure 1
A Process Graph depicts each material or component in the sequence they are assembled together to create the final product. The process time at each assembly or process step is also included. 
In the figure 1, materials and components are in blue (i.e. "Tubes") starting from top to bottom and from left to right. Assembly steps (i.e. "P1") are in yellow while the process time (i.e. "16 seconds") are in green. 
For example, process P1 take 16 seconds to assembly Tubes using screws while process M2 requires the sub-assemblies completed by process M1 and P2 and it takes 6 seconds.
The final assembly is presented at the bottom (i.e. "Assembled Plug") 

2. Calculate the takt time. Takt time gives you the production time required to meet the customer demand (the pace of the demand.) For our example we will assume 29 seconds per plug

3. Group operations. Starting at the top of the process graph (figure 2), add the the process time until the total time is equal or less Takt time. In the example, 
Figure 2
  • Workstation 1: P1+M1+P2+M2 = 29 seconds
  • Workstation 2: M3+M4 = 28 seconds
  • Workstation 3: M5+M6 = 28 seconds

4. Build a Balancing Chart or Yamazumi Chart by using magnets or post-it for each process within each workstation operation as shown in figure 3.

How to use the Balance Chart? 
The benefit of building a Yamazumi or Balance Chart is that actually you can "see" waste and opportunities for improvement. For instance,

  • Any improvements to reduce the total cycle time in workstations 2 and 3 will not increase the overall line's throughput. In fact, improvements in these two stations will create more waste as idle time or waiting which is one of the 7 deadly wastes. Why? Because workstation 1 is the bottleneck (the one with the longest cycle time) "holding" the flow of production to 29 seconds even workstation 2 and 3 can run faster. 
  • On the other hand, workstations 2 and 3 have 1 second each per every cycle. if we improve workstation 1 to 28 seconds, the overall line can produce one unit every 28 seconds. In that case, the line is evenly balanced and there is not waiting time. 
  • However, if each workstation produces at 28 seconds, the line will be overproducing  given that the takt time is 29 seconds. As you might know, overproduction is the worst waste as that would create additional waste such as inventory, transportation and others.