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On another forum, the fellow member posted the very interesting stuff about the Nakajima problems, that were also affecting the Homare. Link to the original doc, that can be easily translated in this day and age. Also some translations provided by him (apologies for the wall of text, but this is a far better account of the major production problems than anything that can be read from the Western sources or 'sources'):
From Shibuya Ryutaro, who was inspecting private factories from September 1943 to October 1944 (pg. 442) :
As an example, looking at the differences between Mitsubishi Heavy Industries and Nakajima, Mitsubishi, with its experience building carrier-based fighters, built its aircraft in a scientific and functional way. Both the design and production teams fully understood the requirements of the navigation system, and once they had finalized the production design and production method using the gauge system, they were proceeding with production so that they could produce 100-200 units. Meanwhile, Nakajima, in what they considered their specialty, worked closely with the navigation system's engineers, and, with a mindset of making the impossible possible, they were able to achieve even the most unattainable. From our perspective, with their old-fashioned spirit, they were able to produce products that were extremely difficult to produce. It seemed as though they were creating products that were all-purpose and would overcome production difficulties through hard work. Therefore, I believe Nakajima's products were difficult to manufacture and had low yields.
More comments from Shibuya: (ibid., pg. 446)
...In a country with material shortages, the production of any weapon, machine, or aircraft required more materials than the United States, Britain, or other developed countries. While the United States and Britain used 5 tons of aluminum to build the same aircraft, Japan used 5.0-6.0 tons (domestically produced aircraft were much lighter after completion). While other countries used five machine tools to build one engine, Japan used 6-6.5. While other countries had a 92% production yield for engine castings and other weapons parts, Japan often had a yield of less than 10%.
...
For Homare Engine's aluminum carburetors, 400 molds were made at the Ichinomiya factory (molding machines), 100 of which broke when they dried, so 300 were poured into the mold and air-tested, with 100 passing. Those 100 were then sent to the Maruko factory in Tokyo for finishing and another air test, where only 7 passed. The rest were discarded, and 90 or so of the discarded parts were sent back and forth along the Tokaido to be turned into scrap at the Ichinomiya factory.
From Shibuya regarding the Homare crankcase (ibid, pg. 448):
We once had trouble with the machinery because hundreds of castings from the reduction gear case were being discarded.
We wanted to manufacture around 600-700 steel crankcases for the Homare, but each month, 200-300 steel case partition plates became scrap. The engineer in charge explained that there was a shortage of vertical lathes for cutting the partition plates, so we acquired 15 vertical lathes from Ebara Corporation, but the amount of scrap piled up, but the number of products did not. These are nothing but the culmination of poorly produced designs by designers unfamiliar with production technology, poor machining techniques, and heavy-handed measures by supervisors who are ignorant of the fundamentals of technology.
...It's true that the factory is equipped with vertical lathes to the brim, but the machining methods, particularly the way the parts are attached, are irrational, resulting in a large amount of scrap. Therefore, the cutting speed is significantly reduced.
And yet, more than 60% of the parts are scrapped, with scrap piles piling up in the corners of the factory. With this kind of usage, even if more machine tools are added, production volume will not increase.
...
So, we commissioned an efficiency engineer from the Efficiency Association, an authority on machine tool manufacturing and skilled in cutting, to conduct a detailed process analysis and improve our jig and tooling manufacturing methods based on that analysis. As a result, cutting speed doubled, errors dramatically decreased, and each process flow became smoother without backlogs. As a result, monthly production increased from 250 to 600. This actually led to delays in the supply of materials, causing machine backlogs. 394
From Kawamura Hiroyoshi, head of the steelmaking department at the IJN Air Technical Arsenal, there are two anecdotes about massive quality issues at Nakajima and the difficulties of manufacturing the Homare (ibid, pg. 443-445):
The first is a truly tragic example from Nakajima Aircraft's Tokyo factory, where the company's technology had strayed from the very basics: N Company T Factory (Nakajima Aircraft Metals Co., Ltd. Tanashi Factory), a representative large-scale foundry, once experienced a drop in yield of approximately 30% for rear covers for crankcases. The company was struggling to identify the cause. The defect manifested itself as dimensional imperfections. The factory manager unequivocally stated that nothing had gone wrong. So the leadership team first inspected the wooden molds they were using and found that they were quite worn and had already begun to deteriorate. It was a completely untrue story, but it sounded like a lie: that replacing them with new ones immediately improved yields. There were many other similar cases. It's important to remember that inspecting wooden molds is a major issue that should never be ignored. At the same time, we must also have the ability to prepare replacement wooden molds.
For about a year and a half before the end of the war, the top priority for the engine was, without a doubt, the engine. The key to production was the casting of the chassis, and N Company's T Plant produced the largest quantities. However, around the summer of [1944(?)], the yield rate for castings began to decline. The yield for poor-quality parts was around 20%, and even the best parts were around 40%. Mountains of scrap piled up at the plant, and engine factories eagerly awaiting shipments sometimes only managed to obtain one or two units a day. The engine factories were even worse off, with 20% wasted material and 20% wasted manufacturing. This meant that satisfactory engine production was no longer possible. Machine factories were forced to make catch-up production, which led to even more errors.
...
Meanwhile, I continued to encourage my cooperators, staying overnight and cooperating several times. The results were soon apparent in our performance: after one month, our yield had risen from a minimum of 40% to a maximum of 70%, and after another month, it had risen to an average of 70%. Let's examine why cooperation was so difficult at this factory. I think we can say the following:
(a) The [Homare] engine boasts world-class performance, but because of this, the structure of the cast parts was designed without taking into account the difficulty of casting, and mass production was carried out as a prototype without sufficient consultation between the designer and the foundry engineers.
(b) Because this engine went into production as a prototype, it subsequently required numerous modifications.
(c) Technical communication between the engine factory and the foundry was not smooth.
(d) Communication during inspections in particular was insufficient, and numerous dimensional defects were often discovered after the engine was brought to the engine factory.
(E) The foreman was too confident in his own casting technology and lacked tolerance for other people's advice. He was also very pressured toward his subordinates, and his subordinates' opinions were not taken into consideration.
(F) Discipline training for the entire factory was insufficient, resulting in turbulent behavior among the workers,
which clearly had a negative impact on the technology.
(G) There was no communication between superiors regarding technical command and orders. Everything was left to the discretion of the foreman.
(H) Because a three-shift system was in place, there were many shifts, which disrupted the quiet of the factory and hindered precision work. In particular, there was a tendency for workers to do "knock-down" work between shifts,
and go home without caring about the quality of the products.
(Ri) There was no organization in place to investigate the causes of defective products.
(Ju) Casting results were not properly communicated to the workers.
These problems were gradually resolved through the efforts of the leadership team and the cooperation of the company.
There was a subsidiary Nakajima plant that did much better than the main plant, but they still suffered issues when the wooden molds deteriorated and that factory was destroyed in the 1944 Tōnankai earthquake.
From Hiroshi Kawamura (川村宏矣), head of the Materials Department at the Naval Air Technical Arsenal (ibid, 446-447):
Around June and July 1944, aircraft production, albeit at a low point, was at its peak. However, there was no surplus of any of the raw materials, supplies, or parts required. Consequently, monthly production was extremely unstable, with aircraft being determined by the minimum number of maintenance required. Therefore, rather than a matter of months, production figures were a matter of daily ups and downs. Furthermore, the types of products in short supply changed constantly, making it virtually impossible to keep up with demand. At one point, a big uproar broke out over a sudden shortage of carburetors for the Honor Engine. An investigation into carburetor companies revealed extremely high levels of material and manufacturing waste. One company was generating 60% material waste and 30% manufacturing waste. The factory was practically full, with piles of materials prepared in anticipation of this and scrap in the process. We also learned that while some foundry companies were achieving extremely good results, with yields of over 90%, others were experiencing surprisingly poor results, with yields of just 10%. Among these was a factory in the Nagoya area called T (Ichinomiya Factory of Tokushu Light Alloy Co., Ltd.?). 】 was the largest carburetor casting factory, but in the worst cases, it sometimes produced the lowest yields mentioned above.
When you combine the lowest yields from casting to final finishing, the total yield ended up being just 1%. This is why the catchphrase "three hundred, three" became a kind of catchphrase at the time.
The main points of these investigations are as follows:
(a) Wooden molds are not properly cared for after use. As a result, they gradually become difficult to remove, leading to forced removal.
As a result, the sand mold becomes deformed or collapses after removal. This is then corrected with a trowel. This results in a casting that is very different from the original, leading to insufficient dimensions and uneven thickness.
(b) The sand used for the molding is generally not dried properly and is not uniform.
(c) Work is often overwhelmed by the number of tasks and lacks attention to detail.
(d) Due to the structure of the melting furnace, there is a high risk of gas seeping into the molten metal.
(e) The location of defects and their causes are not adequately identified.
The factory manager at the time readily accepted these suggestions and made great efforts to improve them. As a result, yields reached 60-70% in the first month and over 80% in the second.
...
However, we must not forget the following factors as reasons for this carburetor's shockingly poor performance, even if only temporarily.
(a) Compared to previous models, this carburetor's structure is more complex and extremely difficult to cast.
(b) As with the engine itself, production began as a prototype, resulting in subsequent modifications. Furthermore, poor communication among related parties led to numerous manufacturing discrepancies.
(c) The initial suspension of development, followed by a rapid increase in production, caused confusion in the casting technology. 392
Here, too, "as with the engine itself," a mistake reminiscent of racing engine development was made, with the prototype being moved directly into mass production. This, it has been pointed out, put undue pressure on the raw material supply department.
The president of Company N, renowned throughout Japan's special steel industry, was a man of great knowledge and exceptional skills, and a man of great self-confidence. When I once accompanied Mr. S on a factory tour, he showed me the small forged parts at the company's K branch factory. He told me that this was the company's greatest strength, and that if any defective products were to be produced, he would be fired. Shortly thereafter, a major problem arose at the factory that manufactured the engines we were most concerned about at the time: magnetic particle inspection revealed a rejection rate of over 70%. As a result, engine production was far from meeting demand. An investigation revealed that the products were a mix of several companies, and even among the products from Company [Nakajima]'s K branch factory, there were quite a few defective products.
...
Next, I would like to discuss the causes of this accident and possible countermeasures for your reference.
(a) In the die forging process, uneven tension occurred in the step just before the final die. If the part were to be forged into the final die as is, wrinkles would form, sometimes resulting in deep scratches. This requires rigorous investigation of material movement at each step and thorough die adjustments before proceeding with production. If only a small number of parts are inspected after the final die is forged, problems like this could occur when mass production begins.
(b) Since some parts already have scratches in the rough cut, it is important to thoroughly remove any imperfections in the rough cut and to design the die to facilitate removal (by increasing the number of flat surfaces whenever possible).
(c) Many parts are overlooked due to improper final inspections. This requires rigorous inspection by someone with ample experience in adequate lighting, with minor defects removed before shipping. It would be even more ideal if magnetic particle inspection were used.
The cause of this incident did not lie in any significant technical issues; rather, it involved not placing excessive trust in the factory's own technology, following the proper process order, and not neglecting necessary inspections. While N Company bears significant responsibility for causing this accident, the fact that it quickly and decisively implemented countermeasures is also due to the outstanding skills of its president.
