Batch manufacturing is the ‘traditional’ form of manufacturing where production is completed in lots of various size, and the lots are passed along en masse to the next step. Typically, layouts in batch manufacturing are done by function—a weld shop, a paint shop, a fabrication shop, etc.
Another name for batch manufacturing is ‘batch and queue’. It gets this name for obvious reasons. Products are produced, and then are shipped to the next process, where they sit in line waiting to be worked on.
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On the surface, batch manufacturing has a lot of logic behind it.
Big, efficient machines tend to have a long setup time, so you have to spread out the changeover across a lot of parts to make it worthwhile. For example, consider this situation: a 10 minute setup time and a 1 minute cycle time for a part. When you spread the 10 minutes out, a lot size of 1 (single piece flow) would take 11 minutes per part, a lot size of 10 would take 1.1 minutes per part. Seems like a no-brainer.
Batching deals with long processes (i.e curing paint) by using big equipment that can handle large batches.
When different functions are far from each other, batching helps lower the impact of transportation time. The effect is the same as setup—the more parts on a cart, the less time moving parts around takes per part.
When assembly people do many parts at one time (like hose assembly or cutting wire for electrical harnesses), they can get into a rhythm that makes them really fly.
When a machine is hard to adjust, it saves time to just run a whole bunch of pieces once the settings are ‘dialed in’.
If the night crew doesn’t know how to make the WidgetMax 3000, the day shift can run enough to supply their whole evening’s production.
OK, if I stopped here, I would convince myself of the joy of batch manufacturing. So what is the problem with each of these above arguments?
Each assumes that the underlying problem is not solvable. In the first example, the problem is that the team feels that they can’t reduce the setup time. Lean says to take the setup time down to single digit minutes (SMED), or eliminate it altogether. In the long process example, they assume that there is no way to make a smaller oven, or cure the paint faster. In each of these cases, there is some underlying assumption that precludes flowing product smoothly.
So, why is batch manufacturing bad? Well, first of all, there is lots of inventory, and inventory costs money. It also takes up space. It requires heavy equipment to move it. It takes a long time to cycle through all of the product groups to meet customer needs. The list can continue, but the point is made. And these are all best cases.
In the worst case situation, batching can have a huge impact on quality. Imagine that a punch breaks on the first fabrication machine in a value stream, and a large batch is produced with every part missing a hole. The problem might not be noticed until the products snake their way all the way through to the last station of the assembly line, right before an important client’s shipment is scheduled to go out the door. Then people end up scrambling to try to track down all the bad parts, which all have to be scrapped. In Lean, with less inventory, the problem would be noticed much sooner, minimizing the problem.
The reason that batch manufacturing is still around is that it is intuitive. When I got married, I had been in Lean manufacturing for a few years. I helped my wife assemble our wedding invitations. I thought I was being efficient. I figured out how to get the computer configured to print all of our envelopes, so I got them all done at once. Then we put all the stamps on at once. Then we got ready to put together all of the invitations at once. Things were going great until my then-fiancé told me that I had printed the addresses on the inside envelope instead of the outside envelope. I didn’t even know there was a difference! OK, that didn’t really happen, but it could have.
Well, you get the point. After all my Lean training, I immediately reverted to a batch process at home. It just makes sense when looked at with the knowledge that most of us accumulate in a lifetime. It is one of the reasons that Costco and Sam’s Clubs are so popular. We think we are getting a deal when we buy 300 rolls of toilet paper all at once. We forget that we needed to by the Super-Size SUV to transport it into our oversized house with enough closet space to hold it. We forget that the extra money we spent on it could have been earning interest in our account, instead of sitting in a hall closet in the form of paper goods.
I once had a team member tell me—“We are doing one piece flow—we just do it 5 at a time.” There is just something ingrained in people along the way that batching is good business. Getting Lean, and really striving for continuous improvement requires developing an ability to take these paradigms and throw them out the window. Look at the process with an eye on how it should be set up if no problems existed, and then figure out how to get rid of the problems.