Interleaving Operations

I was sitting in the dentist’s chair earlier this week. It leaves one with a lot of time to think – because it’s really hard to talk with all of that stuff in your mouth. Although my appointment was for an hour and a half, I was getting attention from my dentist only a fraction of that time. It occurred to me that I was not the only person with an 8:30 appointment that morning. And yet, I felt comfortable that he was taking good care of me.

He was doing a masterful job of interleaving tasks. That is, he was serving me and perhaps several other patients at the same time. He was making excellent use of what would otherwise have been idle time; waiting for anesthetic to take effect, impressions to harden and etc.

How often are we able to plan our own operations to use idle time more productively?

Perhaps the most common application of interleaving that we encounter is combining the retrieval of a pallet with a pallet put-away task, eliminating the empty travel back to the dock. Of course, to make this work the shipping and receiving docks need to be relatively close to one another. A full function WMS will be able to direct the put-away to a location near the pick, or find a pallet to be retrieved close to the one just stored.

In a project on a machining floor, one of the bottleneck processes was the de-burring department. This operation followed every milling and drilling step. Deburring is a fast operation requiring very little capital equipment. By acquiring a few more deburring machines, it was possible to put one within the work cell of each major machining process. The operator had sufficient time while the machine was running to debur the last piece, interleaving the task into the machine load/unload cycle.

In a warehouse, one of the most common orderpicking methods is a serial zone system. Order cartons, or totes are conveyed from one zone to the next for products required there. One of the difficult tasks in this system is keeping a level workload in each zone. If there is no queue of work coming into the zone, then the picker is idle. If there is a large queue, to provide continuous work, then orders move more slowly from one zone to another, potentially starving the downstream zones. 

We have found that in distribution centers where there are many single line orders, or orders that can be completed in a single zone that these orders can be held back and used to fill lulls in the flow of multi-zone orders. Priority should be given to processing the orders that need to move on to another zone. When there are no orders entering the zone, single line/zone orders can be interleaved into the normal waiting time.

Another picking application where pickers move along the face of a flow rack or down an aisle of bin shelving can be improved considerably by reversing the sequence of locations for the next order to be picked so that the return trip is used productively. Most modern WMS packages can do this with little trouble, especially for picking directed by radio frequency terminals.

Check your own operations, and even your office for interleaving opportunities. They are practically free.

ABOUT THE AUTHOR

James M. Apple, Jr. is a Director in The Progress Group. Prior to co-founding The Progress Group in 1991, he was a Partner with Coopers & Lybrand's SysteCon division. During 1992-1995 he served as a Senior Systems Advisor with Vanderlande Industries, a major conveyor and systems provider in Europe.

Jim is an internationally recognized thought leader in the area of facility design and integrated distribution systems. His contributions to the improvement of distribution practices have been recognized by his receipt of the prestigious Reed-Apple Award, which is given for lifetime contributions to the advancement of the material handling profession. Jim has also received the Institute of Industrial Engineers' Facilities Planning and Design Award. He has written numerous articles and handbook chapters on warehousing and logistics operations and is a popular speaker on logistics seminar and conference programs.

Prior to SysteCon, Jim worked as an Industrial Engineer with IBM, was Supervisor of Facilities Planning for the Oldsmobile Division of General Motors and was Executive Vice President for an automotive aftermarket parts supplier. He holds B.S. and M.S. degrees in Industrial and Systems Engineering from the Georgia Institute of Technology.