As we passed through a residential area and entered the industrial zone, the Chiba Factory of Sumitomo Construction Machinery, a member of the Sumitomo Heavy Industries Group, came into view. Just inside the main gate is the LS-25000ALJ hydraulic excavator, the first model the company developed and sold in 1969. Having worked tirelessly in the field, it now stands proudly on display.
“Sumitomo Construction Machinery produces between 8,000 and 9,000 hydraulic excavators and asphalt pavers each year,” explains Mr. Keisuke Sakamoto, Manager of the Chiba Manufacturing Section and the Manufacturing Planning Section of the Manufacturing Department. “The Chiba Factory is currently the only domestic production site—our mother factory—that manufactures the full range of hydraulic excavators, from small to large models. Around June 2025, we began ramping up operations at our Yokosuka Factory in Kanagawa Prefecture, with plans to transfer production of large excavators in the 35- to 80-ton class there. By 2028, our production system will consist of two sites in Japan, along with plants in China and Indonesia.”
Wearing a helmet, workwear, and a protective mask, I toured the factory, guided by Mr. Sakamoto. At the Chiba Factory, excavators ranging from 7.5-ton to the 80-ton class are produced, with models of different sizes moving along the same production line. In the case of automakers, production efficiency is typically improved through leveling and automation using robots to enable the mass production of identical models. However, this approach does not work at the Chiba Factory, which produces a wide variety of models in small quantities.
“At our factory, we have a rule that each production process is carried out within a 14-minute cycle. This cycle was established by breaking down the workflow in order to achieve a daily output of around 40 units. Depending on the model and size of the machine, however, the time required for the same process can vary, and work on larger machines does not always fit within the 14-minute window. We therefore schedule smaller models after larger ones, designing our production plans to level the overall working time across the entire line, including the preceding and subsequent processes,” says Mr. Sakamoto.
The production process is broadly divided into four stages: welding, machining, painting, and assembly. Each plays an essential role, but welding is considered particularly critical. If the quality of welding is inadequate, it could lead to accidents at work sites where heavy earth and debris are excavated and loaded, or where buildings are demolished. “Not everyone working here is a veteran. Our workforce is diverse, including new hires, mid-career hires, and foreign technical intern trainees, regardless of gender” says Mr. Sakamoto.
To produce such a large number of machines while maintaining high quality, all employees must acquire a high level of technical skill. To this end, the Chiba Factory has established a three-pronged skills development system for each stage of the production process—comprising the “Welding Dojo,” “Machining School,” and “Assembly Academy.” “New workers learn the fundamentals through training before being assigned to the shop floor. It takes at least three months to master a single process, and experienced employees provide hands-on support to ensure that new workers improve their skills while maintaining production quality,” explains Mr. Sakamoto.
Production at the Chiba Factory does not rely solely on human senses and skills. For example, even when bolt tightening in the assembly process appears to have been performed correctly, foreign matter lodged in the threads can result in insufficient clamping force, potentially leading to serious accidents later on. To address this risk, areas where detachment would be particularly dangerous— such as the joints between the upper and lower structures and mounting points for rear counterweights— are designated as “critical tightening” points. A system has been introduced to automatically evaluate these points based on three parameters: torque (the force that twists an object around an axis), the rotation angle after tightening begins, and the time required for tightening. Records are retained for each unit, enabling traceability in the event of a problem.
Outside the building, functional checks are carried out on excavators whose assembly has been completed. The sight of these brand-new machines—swinging their massive arms up and down, rotating their cabs, and moving on their brand-new crawler tracks—is truly impressive. “Excavators that have passed the functional checks are lined up in a large open area. Machines in a variety of colors—including Sumitomo Construction Machinery’s signature yellow, bright orange models for Europe, and red Link-Belt machines primarily sold in North America—quietly await shipment,” says Mr. Sakamoto. For machines destined mainly for overseas markets, the buckets at the end of the arm are typically attached at the destination, not installed at the factory, to meet customer requirements. Watching the machines take shape one after another, I appreciated anew the role they play at construction sites.
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