By Steven A. Melnyk and R.T. "Chris" Christensen

is frequently asked of us, namely, "What type of company are we?" To understand this question, we must first understand the origins of this question. As most of you are aware, this magazine runs software surveys/directories on a regular basis. These directories contain listings of various packages found in such categories as MRP/MRP II, finite capacity scheduling and planning, warehousing, forecasting and simulation. These directories not only identify the various packages and their vendors, they also detail various features of their operation. One of the last set of categories pertains to the specific manufacturing settings for which the various packages are potentially suitable. Found within these categories are settings such as job shop, batch, repetitive, process/continuous, mixed mode, medical/drugs, food and automotive. For many readers, these categories are a source of major confusion. They are not clear as to the differences between the various categories. In this column, we will explore these various categories of manufacturing settings and their implications. We will carry out this exploration in two waves. In this column, we will focus on the categories of job shop, batch, repetitive and process/continuous.


Major dimensions
Before we begin to explore the various manufacturing settings, it is important that we identify several of the major dimensions underlying these various settings. These include dimensions such as:

• Volume. This dimension refers to the level of output that the setting is capable of economically supporting. This dimension consists of two levels. The first is that of frequency, or the number of times during a certain period that we can expect to produce a lot of a specific part (i.e., a specific part number or stockkeeping unit). The second is that of repetition. This refers to the size of the typical production order. That is, whenever we release an order for a specific number, repetition refers to the size of the order. Using these levels, we can see that low volume can be generated by a combination of low repetition and low frequency, or very low repetition and moderate frequency, or moderate repetition and very low frequency. Volume determines the size of runs that the manufacturing setting can be expected to deal with.
• Variety. This refers to the extent to which the products built are standard or subject to modification. These variations show up in differences in routings that the manufacturing setting is expected to accommodate.
• Nature of flows. This refers to the extent to which there is a dominant flow through the manufacturing system. In some conditions, we must accommodate a setting where every order released to the floor has its own unique routing. In other conditions, nearly every order follows the same routing.
• Equipment type. This dimension refers to the type of equipment typically found in the specific manufacturing setting. Generally, this dimension runs from general purpose (GP) equipment (e.g., a drill, grinder, deburrer) to special purpose equipment.

In addition to these dimensions, it is also important to note that not every manufacturing setting is "pure." That is, we seldom see a pure job shop or a pure assembly line. Rather, we see a job shop that has elements of an assembly line in it. Under these conditions, what we tend to do is to describe ourselves in terms of that manufacturing portion that is either most critical or the largest.


Manufacturing settings
Now that we have established the major dimensions underlying various manufacturing settings, let's examine the four types of manufacturing settings to be covered this month: job shop, batch, assembly line and process/continuous.

• Job shop. This setting anchors one end of the manufacturing spectrum (process/continuous being the other end). This setting is characterized by very low volumes, with production runs consisting typically of between two to 50 units. In addition, variety is very high in this setting (a result of the small runs), with each order being very different from the others in this system. As a result, software packages targeted towards this setting are designed to cope with this high level of product variety. The combination of low volume and great product variety results in a great deal of diversity in product routings. This diversity places a great demand on the ability of management (and the supporting software package) to manage the flow of work through the shop. This means that dispatching at the work center level is a critical task.

In general, the equipment in a job shop tends to be general purpose in nature. This means that while setups are relatively short, the processing times per unit tend to be longer than what one would find in an assembly line. This short setup time is consistent with the demands for flexibility made by the orders in this setting. Just as the equipment is general, the skill level of the employees tends to be very high. In most job shops, the key bottleneck tends to be labor, not equipment.

Layout in a job shop falls into one of two formats. The first format is functional. In this format, we lay out the flow so that all of the same or similar equipment is placed together in the same area. In contrast, the second format is process. With this layout, we organize and locate together all of the equipment needed to build orders belonging to the same part family (more about this trait later on in our discussion of batch). In general, most of the lead time in a job shop is consumed by queue time (i.e., orders waiting at various work centers waiting to be processed).

• Batch. In this setting, the volume is increased, with the typical run quantity ranging from 50 to 2000 units. Variety, while still present, is less extensive as that found in the job shop. With the larger runs and less variety, we find less general purpose equipment and more specialized equipment and tooling.

An important feature of a batch environment is that of the part family and machine cells. A part family consists of a set of parts which share commonalties or similarities in terms of processing (the most important from our perspective) or design. With part families, we must now recognize the presence of sequence-dependent scheduling. That is, the setup time depends on the order in which we group the runs. The ability of a software package to recognize and incorporate this feature into the resulting schedule is an important consideration.

Typically, MRP systems are most appropriate for this setting.

• Repetitive. Within this setting, we move into a high-volume manufacturing environment where the units of output are discrete (e.g., tires as compared to gallons of paint). Here the items are either standard or assembled from standard components and options. The presence of this standardization changes the requirements placed on the planning and control system. With such large volumes, we now find ourselves with dominant routings. That is, most of the orders follow a set sequence of operations (the machines and work centers are physically laid out in a manner that corresponds to this flow). Because of the volumes, we also focus our scheduling activities at two points — the master schedule and the first operation (frequently called the gateway).

In addition, large volumes combined with very short processing lead times require a very different fashion of inventory management. In the previous systems, we could track the inventory by focusing on the points in time when it was issued to the orders. In many repetitive systems, we use a backflushing logic. That is, we determine the number of end items we produce during a period of time. We then break this level of output into its components. Ideally, we should arrive at the ending inventory by taking the beginning inventory, adding to it the receipts received during the time period, and then subtracting from it the number of components needed to cover production (as based on the number of end items produced) and scrap allowances.

Equipment and tooling capacity within the repetitive environment tends to be specialized (again, due to the high levels of volume of standard items) with labor being either unskilled or semi-skilled. Within this environment, it is equipment and tooling capacity that tends to be the bottleneck, not labor.

Capacity planning is critical within the repetitive environment. Therefore, careful attention must be paid to the strength of the capacity planning modules.

• Process. Process is very similar to the repetitive except that the units of output are not discrete but continuous. Oil and paint are examples of items produced with a process environment. Here, volume is high with the products being fairly standardized. In addition, there exists a dominant flow that every product moves through. Because setups (or the change required to go from one product to another) are fairly time consuming and expensive, care must be made to ensure that those features are incorporated into the software package and into the planning and control system. Typically, we tend to see cyclical production scheduling.

Because of the high volumes, process settings are characterized by high levels of capacity utilization. In this type of environment, planning is the critical activity. Execution tends to be a reflection of planning.


Lessons learned
In this column, we have focused on trying to help the reader understand the type of manufacturing setting in which they operate. We have seen the following:

• There should be a strong fit between the manufacturing setting and how the system is managed and controlled.
• The different settings are differentiated by factors such as volume, complexity, nature of flows and equipment type.
• The environments fall along a spectrum anchored at one end by job shop and process at the other.
• Each environment has its own requirements that must be recognized within the software decision.
• A mixed environment exhibits traits of different settings. Yet, there is a critical or dominant setting which can be used to identify the targeted setting if there is no package supporting mixed-mode production.

In next month's column, we will complete our examination of the differences in manufacturing environments.

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