Lean manufacturing (LM), also known as the Toyota Production system (TPS), is one of the most popular techniques for quality and productivity improvement in the automotive industry [1
], although it has been also adopted by other industrial sectors, including aerospace, electronics, and services, among others [2
]. TPS is the result of many efforts from Toyota to keep updated and compete with Western automobile companies after World War II. LM is a strategy to reduce costs, especially those related to production processes [3
SMED is a tool developed by Shingo [7
] as a proposal to reduce bottlenecks caused by stamping presses in Toyota. By the time SMED was developed, these machines were not working at full capacity and, thus, were not bringing the expected benefits. Nowadays, as Ulutas [8
] points out, SMED is one of the many LM tools for waste reduction in production processes, since it offers a fast and efficient way to decrease changeover times. Changeover is the process of converting one line/machine from running one product to another and it is depicted in Figure 1
Changeover time is defined as the time needed to set up a given production system to run a different product with all the requirements [9
], and they are a typical example of waste, since changeover is a non-added value activity that incurs hidden costs [10
]. Therefore, because machines remain inactive during changeover times, this process must be reduced as much as possible [9
]. SMED is a practical LM tool that helps maximize the product value by reducing setup times [4
1.1. Stages of SMED
As previously mentioned, SMED is a tool developed by Shingo [7
] to reduce bottlenecks caused by stamping press machines that were not working at full capacity. To solve this problem, Shingo divided SMED implementation into four stages, which are depicted in Figure 2
. Each stage includes internal and external activities. Internal activities must be completed when the machine is stopped, whereas external activities have to be completed while the equipment is running.
Stage 0, called Identification Phase
(preliminary stage), does not yet separate internal and external activities. It is rather a stage to identify and study the problem. At this stage, it is necessary to make some questions regarding the production system. Some of these questions are presented below:
Is a statistical analysis performed to know time variability of the process [11
Is a statistical analysis performed to know the average process time [11
Is there a detailed analysis of the possible causes of time variability in the process [11
Have operators been interviewed about processes and the machines that they operate [12
Are operators’ activities being measured with a chronometer [12
Has the company identified activities related to changeovers [14
Stage 1, called Separation Phase, focuses on changeover and machine setup activities, since at this stage one must separate internal and external activities. Here, it is important to carry out as many external activities as possible, since they can be performed while the equipment is running. External activities at Stage 1 must be mostly planning operations, which could save between 30% and 50% in setup times.
The main activities performed at this stage are:
List the main sequential setup operations in order to identify internal activities [15
List the main sequential setup operations in order to identify external activities [15
Detect basic problems that are part of the work routine [17
At Stage 2, called Transformation Phase
, one must respond to a series of questions that would improve the work plan. Some of these questions are:
Is the previous work completed before starting changeover [17
Are visual marks used instead of making trial and error adjustments to calibrations [18
Have steps related to search of tools, raw materials, and products been eliminated [18
Have activities been reexamined to make sure none of them has been wrongly assumed as being internal [19
In addition, Stage 2 or Transformation Phase
of SMED implementation, consists in transforming internal activities into external. This implies the following actions:
Stage 3 or Improvement Phase
involves streamlining all aspects of the setup, and it includes systematic improvement of all operations. Activities carried out at this stage are:
Record key setup activities to help improve process time [16
Train operators to maintain improvement in process time [21
1.3. Research Problem and Objective
In Ciudad Juárez, Chihuahua, the manufacturing industry has been the major source of employment for more than forty years. According to the National Institute of Statistics and Geography (INEGI), in August 2015, the state of Chihuahua catered for 476 manufacturing industries called maquiladoras. Maquiladoras are foreign-owned companies established in Mexico, and they usually import raw material and export finished products to the origin country and others—such as the United States and Canada—using tax benefits from the North American Free Trade Agreement. The maquiladora industrial sector currently employs 341,374 workers in the state of Chihuahua. More specifically, Ciudad Juárez has 315 active maquiladoras representing 66% of the state’s total and employing 247,730 workers (72% of the state’s total).
Maquiladoras are characterized by high technological capacity and specific employee skills and abilities. These companies are always ready to attend production orders from parent companies located overseas. However, since such production orders usually vary in quantity and product design, changeovers occur frequently. For this reason, SMED has become a traditional technique used in the maquiladora sector. Fortunately, it seems that Mexican manufacturing companies are familiar with SMED implementation activities and benefits. However, because changeovers in machineries occur so often, the effect of SMED is usually unknown and thus rarely analyzed when reporting company benefits.
Currently, structural equation modeling (SEM) has become a popular technique among the engineering sciences to explain dependence among variables in contexts such as Just In Time [26
] and Supply Chain [27
]. From this perspective, the objective of this research is to measure the effect of all SMED stages and activities on the benefits that maquiladoras obtain by using a structural equation model. Results from the model would help managers identify critical activities from trivial ones, thereby focusing their attention on those that are relevant and important.
To achieve the proposed objective, we tried to find a dependency measure for each relationship that we proposed between SMED implementation stages and company benefits. To achieve this, we proposed and discussed six working hypotheses. As regards the first hypothesis, it is known that, as part of preventive maintenance programs, manufacturing companies keep record of the activities performed by every machine, its failures, and its effect on product delivery times [29
]. Similarly, operators are often interviewed to detect and handle equipment failures [33
]. When such events are being recorded, SMED implementation becomes much easier when companies must list the sequential activities to identify which are internal and which are external [14
]. If every activity is identified, categorization is easier [35
]. Let us remind ourselves that internal activities must be performed when the machines are stopped, whereas external activities ought to be carried out while machines are still running. In this sense, the first working hypothesis can be proposed:
Activities carried out at the Identification Phase of SMED implementation have a direct and positive effect on activities performed at the Separation Phase.
If internal and external activities are successfully identified at the Separation Phase
, it is possible to identify the whole pre-work that must be completed before the machine stops to perform the new setup [36
]. Similarly, it is possible to identify all tools required for the operation and the visual marks to be used to calibrate machines [38
]. Therefore, since activities performed at Stage 1 of SMED implementation have an effect on activities at the Transformation Phase
, the second working hypothesis can be constructed as follows:
Activities at the Separation Phase of SMED implementation have a direct and positive effect on activities performed at the Transformation Phase.
A successful list of external and internal activities also helps recognize critical operations. Activities need to be video recorded to be analyzed and identify unnecessary actions and movements that operators perform when machines are stopped. These actions are a waste of time, which is why they must be eliminated [39
]. Similarly, they represent an area of opportunity, since SMED helps evaluate setup methods that operators use to see whether they are appropriate [40
]. Once unnecessary actions are identified, such methods must be improved and programs must be implemented to train operators in these new activities [42
]. All of these tasks imply that activities properly performed at the Separation Phase
of SMED implementation have an effect on activities performed at the Improvement Phase
, which enables to propose the third hypothesis:
Activities performed at the Separation Phase of SMED implementation have a direct and positive effect on activities performed at the Improvement Phase.
A successful Improvement Phase
does not merely depend on activities carried out at the Separation Phase.
The Transformation Phase
also has an important effect. Let us remind ourselves that, at Transformation Phase
, operators make sure all tools, equipment, and raw materials are ready and at hand [40
]. Similarly, they identify instruments to calibrate machines, employ the correct visual marks, and have operation manuals at their disposal in case they need to be consulted [42
]. Therefore, since these activities are crucial for successful SMED implementation, it is concluded that they have an influence on the Improvement Phase.
The fourth working hypothesis thus states as follows:
Activities carried out at the Transformation Phase of SMED implementation have a direct and positive effect on activities carried out at the Improvement Phase.
SMED must bring a number of benefits that support its implementation. Maintenance and production managers must work hard to reduce changeover times with SMED [39
]. Thus, companies will obtain notable economic benefits if all setup tools and instruments are always ready and at hand [46
], all changeover activities are planned [45
], and the correct visual marks are used [47
]. Similarly, they would benefit from reduced setup times and errors in production and high-quality products. Moreover, availability and performance of machines will considerably increase, since more time will be dedicated to production [48
]. From this perspective, the fifth working hypothesis is constructed:
Activities carried out at Transformation Phase of SMED implementation have a direct and positive effect on the Benefits gained by manufacturing companies.
SMED benefits are not only obtained from a successful planning stage, since they also depend on properly executed activities [50
]. In these activities setup methods must be video recorded to analyze and identify unnecessary movements and actions that operators perform in the setup process [44
]. These setup methods must be improved or modified, and the top management department must propose and implement plans and programs to train machine operators [14
Benefits from execution activities mainly include reduced setup costs and small batch production as a result of increased machine productivity and availability [14
] and increased employee satisfaction [35
]. Finally, it seems that activities performed when machines are stopped have an important effect on benefits obtained from SMED implementation. Consequently, the sixth working hypothesis is constructed as follows:
Activities carried out at the Improvement Phase of SMED implementation have a direct and positive effect on Benefits obtained by manufacturing companies.
As can be seen, six working hypotheses are proposed to assess the effect of SMED implementation in the Mexican maquiladora industry of Ciudad Juárez. These hypotheses are graphically represented in Figure 3