Lean manufacturing books have entire chapters on the pull system. These “pull systems” exist in several areas such as material logistics, manufacturing models and in marketing. Today, my pull vs. push focuses on the manufacturing model and on shop floor controls.
When doing our Lean manufacturing training, we ask one participant to push a rope through a box and then ask a second one to pull it through. We get a good share of laughs from the audience. Of course, the pushing does not really work and the pulling is easy. How do we translate this metaphor to how we run the plant?
Nowadays, furniture and cabinet manufacturers run their production schedule similarly. The days of big production runs are done. Most run on a make-to-order system. They are not producing into a big finished goods warehouse. If that is not pull, then what is?
Scheduling is issuing daily batches of customer orders to be produced one after the other. As the order is completed on a workstation, the work-in-process (WIP) moves to the next workstation. The process is repeated a few times until all the manufacturing and assembly is completed. This model is very typical in our industry. The assumption is that, let’s say in a system of daily batches, every day a day’s worth of orders is issued, and a day’s worth of orders is completed. Now what is the difference between push and pull in this type of set up?
In the push scenario orders are completed at each workstation, as much as they can, and each workstation works as efficiently as they can. As the workflow is never absolutely balanced, there are delays along the manufacturing chain. This imbalance creates the flow to backup. So what harm is there in a little backup?
As in good manufacturing tradition, we want all the workstations to be productive. All workers and machines need to work continuously at optimum speed. When they are stopped, no value is created! When going Lean, we will challenge this statement. It is one of Lean manufacturing’s paradigms, which takes a while to be accepted. We all understand traffic jams. If there is a blockage, do not bring more cars upstream on the road. Traffic control tries to reduce or meter the intake upstream.
When you have imbalance, and practically everyone has some level of imbalance, the flow gets backed up. The backing up in LEAN manufacturing terms causes one of the “7 deadly wastes” in manufacturing. It is over production!
What happens when you over-produce is the build up of WIP. This needs more space. Roller conveyors and carts start getting full. The production slows down as the operators take longer to look for empty carts and to move/park the cart/material. This starts small and is initially not much of a problem.
As the plant starts backing up, the down-stream workplaces are not always sticking to the required sequence and “sub-optimizing” to help them with their workload. This local optimization creates shortages downstream. The plant is getting fuller, but subsequent departments are waiting for the parts needed. The traditional solutions are:
- More WIP-storage required (carts, roller conveyors, floor space…)
- Bigger plant
- Longer manufacturing lead-time (factory through-put time)
- Return to batching for
- Hire more people to compensate
for the loss of productivity
Even if my scenario is a bit exaggerated, we can see and accept the direction the push model is leading us.How do we pull?
The key element of a pull system is a system of consciously restricting WIP. You start at the end of the process chain. You concentrate on optimizing the last operation.
Focus on having all parts available (see my articles on “Fit & Complete”)
Prioritize all upstream processes
on what is required downstream
Curb upstream production when they start getting ahead
When focussing on the last operation, the question might be raised what is the last operation. Is it assembly in our cabinet plant? The entire value stream does not usually end there, but it is a good start to optimize and start pulling. This, however, requires that there is a natural break. If you are shipping to dealers as you complete, or you have a sufficiently large warehouse, then pulling at the end of the assembly line will work. If. however, your installation operation is backing up and you have no space to put your product, you must solve that bottleneck first, or create a big enough warehouse.
As you pull from the end and do not overproduce, bottlenecks and areas of delay (snags) become very visible. Production management can (and must) react immediately to these delays. As you hold back overproduction, theoretically, you can use the held back resources to keep the flow going at the pinch-points.Buffers help, but are not the alternatives to Pull!
The more your production is balanced, the less you need buffers. There are two main reasons for buffers:
Inherent imbalance of processes (fast changes in product configuration and product model) make perfect and consistent balance impossible. This requires some buffers to cover for these imbalances. Also, when you tether two operations too closely together with no buffer at all, the combined capacity of these two machines will be reduced. So, some buffers are required.
Buffers are an insurance policy for imperfection. As with real insurances, what is the risk, how much are you willing to pay, and can you work on reducing the risk? This is a critical question which each company needs to answer.
The critical point with buffers and a pull system is that when the buffer is full, you stop producing more! When a well-defined buffer is full, it is usually better to stop the upstream work cell and bring the upstream resources downstream to help the flow. There is (usually) no benefit to fill the factory beyond that point.How do you start?
It is difficult to change a plant entrenched in a push system to slow down as a solution for their production problems. A simple solution, to get started, is to speed up the last operation (i.e. assembly).
Picture a sequence of belt conveyors transporting boxes. If they are all running at exactly the same speed, it will run smoothly. If a belt downstream runs slower, you will have a pile up. A pile up is solved/prevented if a downstream conveyor runs a bit faster than the one upstream. So, by putting extra resources into the last operation you start to pull.
“If we do that, we will run out of work at the assembly line,” is a common comment.
Another effect of push is that, when buffers get full, workers tend to slow down. This is beside the increased time for material transport. We are ahead, so why rush! The reverse is also true; if the workers see that their downstream co-workers (customers) are just about running out of material, it is a natural motivator to speed up. So, let them run the assembly buffer dry. First, it will surprise you how much the pull effect will refill the buffers just-in-time, and second, this brings increased visibility to what is happening on the shop floor:
- Visibility of the real bottlenecks
- Understanding how little space is actually required
- Seeing where time in factory through-put-time is lost
- Understanding the priorities for continuous improvement initiatives
Only by starting to pull will the improvement opportunities becoming visible and implementable. It is one of the initial starting points of becoming a leaner company. That the productivity and capacity increases is just a welcome side-effect.