Construction of a One-by-one Production Systemfor Resistance Welding Electrodes
抵抗溶接電極の 1 個流し生産体制構築
Tatsuro YAMAGUCHI山口達郎

Kazutomi NAGANO

長野和臣

Taro KUROKI

黒木太郎

Yousuke SAKURAI

櫻井陽介

Yasunari TANAKA田中泰成

Yuuki OHTA大田祐樹

Key words: TK Electrode, Toyota Production System (TPS), lead time, one-by-one production system
キーワード； TK 電極、トヨタ生産方式 (TPS)、リードタイム、1 個流し生産方式
Abstract
The number of electrical components in automobile parts has increased in recent years, and demand isincreasing for our products, especially "TK Electrode", our representative resistance welding electrode.
Owing to large fluctuations in demand for such electrodes, a production system that can flexibly respondto demand was urgently required. We constructed a production system adopting concepts from theToyota Production System. As a result, we could shorten the production lead time of TK Electrode byconstructing one-by-one production system, and cope with increased demand and respond flexibly todemand fluctuations.
近年、自動車部品における電装品の増加に伴い、当社の抵抗溶接電極の需要が増加している。抵抗溶接電極は需要変動も大きいため、需要量に対し柔軟に対応できる生産体制の構築が急務であった。我々は、当社の抵抗溶接電極の代表製品である「TK 電極」に対し、TPS の考え方を取り入れた生産体制構築に取り組んだ。取り組みの結果、１個流し生産方式を適用することにより TK 電極の製造リードタイムを短縮し、需要増加及び需要変動に柔軟に対応することができた。
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1. Introduction
1.1 BackgroundOur resistance welding electrodes belong to a group of products mainly used in the automobileindustry. Recently, demand for TK Electrode, our representative product, has increased rapidly.
Considering future increases in demand, we expected that, using our conventional production system, it
would be difficult to meet to customers' desired delivery dates. Our company typically employed the lotproduction system, in which products move between processes in lots (batches). For the TK Electrode,each lot contained several hundred units. Thus, we were unable to respond flexibly to large fluctuations
in demand for the TK Electrode. Therefore, it was urgently required to shorten production lead times (theduration required from the start of manufacturing until all processes are completed), to cope withincreased demand and to establish a production system that can flexibly respond to demand fluctuations.
1.2 About resistance welding electrode
Resistance welding electrodes are used for welding of insulated coated copper wires, metal terminals
and wire harnesses (copper wire bundles). Such electrodes are indispensable for welding electricalcomponents for use in automobiles.

Fig.1 Resistance welding electrode1.3 Purpose of the study
In this study, we changed the production system of TK Electrode from a lot production system to a
one-by-one production system in which products move between processes one at a time. This eliminatedthe residence time between each process. The aim of the study was to shorten production lead time andto build a production system that can flexibly respond to demand fluctuations.
1.4 Production lead time
Production lead time is the sum of working time and residence time. Working time can be divided into
net working time (processing time) that adds value and incidental working time (setup time) without
adding value. Residence time is the time spent waiting for processing.
1.5 Toyota Production System and the seven types of waste
In this study, we conducted activities based on the Toyota Production System (TPS). TPS is a
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production method proposed by Toyota Motor Corporation. Its purpose is to meet customer demand in
the shortest time and most efficiently. The basic idea of TPS is to thoroughly eliminate overburden,inconsistency and waste.
Table 1 shows the seven types of waste that form a key part of TPS. The present study focused onthese seven types of waste. We analyzed the TK Electrode production line and implementedimprovements.
In addition, TPS utilizes a one-by-one production system. In the lot production system, setup andprocessing are carried out on a lot basis. Thus, the process is intermittent, and long residence times
occur when the processing equipment has insufficient capacity. In contrast, in the one-by-one production
system, setup and processing are carried out on a one-by-one basis and all processes are continuously
carried out. Therefore, residence time between the processes is minimized or eliminated. In addition, it isa flexible production system that can respond to demand in increments a single unit. Therefore, even if
the per-unit working time (setup time + processing time) is the same for each production system, theproduction lead time will be shorter for one-by-one production owing to the shorter residence time.
Table 1 Seven types of waste in the Toyota Production System

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2. Improvement method
2.1 Identification of waste
Considering the seven types of waste mentioned above, we analyzed the production process of TK
Electrode and identified areas of waste. Fig. 2 shows the outline of the TK Electrode production process
and the lead time. To identify waste, we analyzed lead times, focusing on the residence time between
each process. We also analyzed the actual work process, using video footage. As a result of the twoanalyses, we identified waste in unnecessary processing, unnecessary transport and unnecessary
inventory in the TK Electrode production process. To eliminate these types of waste, it was necessary tochange from the lot production system to the one-by-one production system.
2.2 Setting of target
We set a target production lead time to produce the required quantity of TK Electrode and meet thecustomer's requested delivery date. Table 2 shows the current and target lead times. To achieve thetarget lead times for the material process and for the product process, we attempted to reduce the wasteidentified in each process.

Fig.2 Outline of the TK Electrode production process

Table 2 Current and target lead times– 33 –

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2.3 Lead time analysis of the material process (waste due to unnecessary processing)Fig. 3 shows the current and target material process lead times. The current material process lead timeis 26 days. Among the parts of this process, the lead time of the grinding process ( ② in Fig.2) was thelongest, at 16 days, accounting for 62% of the lead time. Therefore, by shortening the lead time ofgrinding process, we aimed to shorten the lead time of the material process by 13 days. As a result ofanalyzing work during the grinding process ( ② in Fig.2), we found that the processing efficiency was low.
To shorten the lead time of the material process, we implemented measures to improve the efficiency of
the grinding process.

Fig.3 Material process lead time
2.4 Analysis of carrying distance during the product process (waste due to unnecessarytransport)
After analysis of the product process, we found large distances between the processing equipment,
leading to the product being carried for unnecessarily long distances. Fig. 4 shows the layout of thecurrent product process. We aimed to minimize the carrying distance by aggregating equipment for theone-by-one production system.

Fig.4 Layout of the current product process

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2.5 Analysis of residence time of product process (waste due to unnecessary inventory)In the current lot production system, setup and processing are carried out intermittently, and longresidence times occur when the processing equipment has insufficient capacity. Fig. 5 shows the current
product process lead time. The residence time accounted for most of the lead time (83%). We set a targetof 5 days, to eliminate residence time. To achieve this we worked to construct a one-by-one production
system with increased capacity.

Fig.5 Product process lead time

3. Improvement results
3.1 Overview of improvement
By solving the waste problems outlined in sections 2.3, 2.4 and 2.5, and by constructing a one-by-one
production system, the production lead time was greatly shortened. Each improvement is described
below.
3.2 Improvement of the grinding processing method (reduction of waste due to unnecessaryprocessing)
The grinding process ( ② in Fig.2) part of the material process was conventionally processed by onlygrinding. To improve this, we divided the processing method into two stages: cutting and grinding.
Processing efficiency of cutting is higher than that of grinding; thus, the lead time can be greatlyshortened. However, in ordinary cutting work, the tool life is short and there is a problem of increased
tooling costs. Therefore, we selected tools to prolong the tool life and set a target life based on thenumber of machining processes and the amount of tool wear. Fig. 6 shows the test results. Table 3 showsa matrix evaluation of each processing method. From the matrix evaluation, it was confirmed that theimproved combined cutting and grinding process has better processing and lead times than the othermethods, for a small increase in tooling cost. We shortened the material process lead time by 15 days
(58%) by changing the processing method. Fig.7 shows the improvement in the material process leadtime.

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Fig.6 Cutting test result
Table 3 Matrix evaluation by processing condition

Fig.7 Improvements in material process lead time
3.3 Layout change（reduction of waste due to unnecessary transport）
In the conventional product process, products were carried for long distances between processing
equipment stations. Although it was desirable to move processing equipment into a smaller area toNippon Tungsten Review Vol.41 (2017)– 36 –

reduce unnecessary transport, there were problems implementing this:The processing equipment used in roughing process (2) ( ⑦ in Fig.2) was large and heavy, making it
impossible to change the layout. Therefore, we constructed smaller processing equipment to allow theequipment to be rearranged and moved closer together.
Processing equipment used for roughing process (1) ( ⑥ in Fig.2), the thickness process ( ⑧ in Fig.2),
the shaping process ( ⑨ in Fig.2), and the finishing process ( ⑩ in Fig.2) were easily moved, but wereunsuitable for a small working area, owing to their large size. Therefore, new smaller processing
equipment was selected and introduced.
As shown in Fig. 8, by arranging the smaller processing equipment in a U shape, the distance walked
by the worker while transporting products was shortened by 80% compared with that for theconventional layout.

Fig.8 Facility layout before and after the layout change to the new U-shaped line

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3.4 Reduction in residence time (reduction of waste due to unnecessary inventory)
To reduce the types of waste identified in sections 3.2 and 3.3, we greatly shortened the residence time byconstructing a one-by-one production system. (Fig. 9)
By reducing the residence time, we achieved a process lead time of 5 days. Moreover, we have been able tofurther reduce the processing time, to just 2 days, by improving the setup method and optimizing theprocessing conditions. As a result, we shortened the product process lead time by 28 days (93%). Including thereduction in the material process lead time, we shortened the total TK Electrode production lead time by 43days (77%). (Fig. 10 and Table 4)

Fig.9 Improvements in production process lead time

Fig.10 Production lead time after improvement
Table 4 Lead time after improvement

Nippon Tungsten Review Vol.41 (2017)– 38 –4. ConclusionsThe aim of the present study was to shorten the production process lead time of our TK Electrode. We
constructed a new production system, adopting concepts from TPS and eliminating waste.
The main results are as follows:
1） In the material process, we reduced the lead time by 58% by dividing the grinding process into
two processes: a cutting process and a grinding process, and by choosing appropriate processing
conditions.
2） In the product process, we shortened the lead time by 93% by adopting a one-by-one production
system with an improved layout and dedicated equipment.
3） In total, we shortened the production lead time of the TK Electrode by 77%.
These improvements allow us to meet to customer's short delivery dates and to cope with demand
fluctuation. This production system can be adapted for other products, and we will continue to shorten
production lead times and improve productivity.

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