Pulling Orders Through The Shop

I often talk about minimizing waste in a manufacturing operation by only producing what you really need. Of course, this, in itself, is a concept that is difficult for many to understand.

Columns From: 7/1/2003 Modern Machine Shop,

I often talk about minimizing waste in a manufacturing operation by only producing what you really need. Of course, this, in itself, is a concept that is difficult for many to understand.

The idea of a machine shop producing one product or a small quantity of products is counter-intuitive to what we may have learned in our careers. We see machines, and we think that they should be running all the time for us to make money. We know about the long changeovers involved when machining different parts, so we think, “Let’s just keep running the same part.” We think of the “efficiencies” we gain when we produce in large quantities and about how our employees get into a rhythm and can produce high quality pieces.

So why try to produce quantities “as needed”? Well, quite simply, it’s what today’s customers expect, and it’s a way of helping you reduce operating costs by controlling inventory levels and reducing the total amount of time parts are in your shop.

If we can accept the concept of producing only what we need and, in effect, pulling work through the shop, how do we execute it? The answer is that we have to change our thinking about many things.

First, making parts we don’t need just for the sake of keeping machines running is not cost effective. Setup times need not be fixed, and they can be drastically reduced. A controlled process will allow us to achieve the same level of quality on one part as we can on many. Finally, it is actually easier to schedule a pull-type of system. There are many proven techniques for doing this, but I have found that the best ones are the simplest ones. Some of these techniques are described below.

  1. Fill the container (and nothing more). Select a container of some type (tote pans work well, as do baskets, pallets, tubs, corrugated containers, rolling carts and other containers). Size the container for the quantity to be produced and write this quantity somewhere on the container. Send the container to the process that makes the parts and give instructions to produce only that many. When the container is filled, it is returned to the process that uses the parts. This technique effectively eliminates overproducing because there is nowhere to put the extra parts.
  2. Fill the empty space. Set aside an area where needed parts are kept. This can be a rack, shelving or a designated floor space. All areas should be labeled with the part name and replenishment quantity. Work centers are instructed to check the area(s) frequently and, whenever there is an empty space, they should begin to produce the part that belongs there in the specified quantity. When the parts are finished, they are immediately transported to the designated area, thereby filling the empty space and fulfilling the production requirement. If there are no empty spaces visible, then nothing should be produced at that time.
  3. Reach the line. Draw two lines on a vertical surface and mark the lower one “min” and the higher one “max.” Instruct the appropriate employees to produce parts when the “min” marking is exposed and stop producing them when the quantity made reaches the “max” marking. This works best for stackable parts, such as those made from sheet metal or bar stock. It is a useful way of initiating production based on consumption of products and, at the same time, regulating the quantities produced. Ideally, the “min” and “max” markings should be fairly close in proximity so that inventory levels can be minimized to days, or in some cases, hours, of demand.
  4. React to the visual signal (light or other marking). People tend to respond well to visual signals. Think of traffic lights, as well as lights on toll booths, checkout counters and even machine tools. Just as these signals initiate a specific action in all of us, they can initiate and regulate production.

If a downstream process needs parts, operators can activate a light that is visible to the supplying process. As long as the light remains on, parts should continue to be produced. As soon as the required quantity is reached, the light can be turned off, signaling that the requirements are met and production can cease.

Of course, lights are not the only useful signal for initiating and regulating production activity. Color-coded tags, forms, folders or stickers can have the same impact, as long as their purposes are clearly communicated to employees.

Consider trying one of these production techniques in your shop. Although they are quite simple, they are powerful and are effective ways to get your company into a “pull mode."

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