ARCHIVE 04The Lineage of Gear Cutting Machines

Cutting Gears on Imported Machines and Making Hobbing Machines at Home (1897–1926)

The first machine cutting of gear teeth in Japan is said to have been in 1897, when cast gears were cut one tooth space at a time with a No. 2 universal milling machine from Cincinnati (US) and a form milling cutter (a cutter shaped like the tooth space)^[1][2]. Automatic gear cutting machines were bought from G&E (US) in 1901 and from Cincinnati (US) in 1903, raising the efficiency and accuracy of gear cutting^[1].

The First Hobbing Machines

Hobbing machines, which cut teeth continuously with a hob, are said to have first come to Japan around 1906, when machines by Pfauter of Germany were imported by Ishikawajima and the Kure Naval Arsenal^[1][2]. In the Taisho era they spread to private industry: in 1914 a gear-cutting specialist shop in Kujo, Nishi-ku, Osaka (Marusan Seisakusho) installed a British-made hobbing machine, said to be the first foreign-made hobbing machine installed by a gear-cutting contractor. In the same year a Pfauter hobbing machine went to the Osaka Prefectural Nishinoda Vocational School, and the following year, 1915, an American-made 600 mm hobbing machine went to Ueda Iron Works in Osaka^[5][1]. For bevel gears, generating-type bevel gear cutting machines from Gleason (US) went to Kisha Seizo (a locomotive maker) and Ikegai Iron Works in 1915–16^[1].

40-Inch Hobbing Machines: Made in Japan

When World War I cut off imports, domestic production of hobbing machines took off. According to Aida Toshio's Haguruma no gijutsu-shi (A technical history of gears), in 1917 Sonoike Seisakusho built a 40-inch hobbing machine (able to cut gears up to about 1 m). It had a differential, which made helical gears easier to cut, and was well received as better than the widely used imported machines without one. In 1918 Ikegai Iron Works built a 24-inch hobbing machine, and in 1920 Kashifuji Iron Works of Kyoto (today's Kashifuji) completed a 40-inch hobbing machine modeled on a Pfauter (Germany) machine^[1]. Kashifuji's own company history says that its founder, Kashifuji Tomesaburo, built a prototype of the first 40-inch hobbing machine in 1918 and began making repair gears for the Kyoto City trams (the company calls it "Japan's first hobbing machine"), a difference of two years^[6]. It may be the difference between a prototype and a finished machine, but we have not been able to confirm it.

For large hobbing machines, Karatsu Iron Works in Saga Prefecture built a hobbing machine for cutting steam turbine pinions in 1921, a 6-foot worm wheel hobbing machine in 1923 and a 7-foot hobbing machine in 1924. Aida concludes that "by the end of the Taisho era, domestic production of hobbing machines of various kinds was more or less complete." However, the large gear cutting machines that cut the turbine gears for the main engines of warships and merchant ships were foreign-made, by William Muir (UK), Westinghouse (US) and others^[1].

At an exhibition of Japanese-made machinery held in Osaka in 1921, a look at how the gears of the exhibited lathes were made found 15 cut on dedicated gear cutting machines, 6 cut on milling machines and 1 with ground tooth profiles^[1]. This shows that gear cutting machines were spreading even to lathe makers.

What "40-inch" and "7-foot" mean. The size of a hobbing machine is given by the largest gear diameter it can cut: 40 inches is about 1,000 mm and 7 feet about 2,100 mm. The differential is a mechanism that turns the gear a little extra in step with the hob feed when cutting helical gears; with it, the helix angle of a helical gear is easier to set with change gears.

Early Showa and Wartime: Gear Shapers and Gear Grinders (1926–1945)

In the Showa era, the range of general-purpose hobbing machines widened. Kashifuji Iron Works released the 14-inch K0 in 1928, the 24-inch K1 in 1930 and the 40-inch K2 in 1931^[6]. Gear cutting machines other than hobbing machines, and gear grinders for grinding hardened gears, also began to be built: in 1930 Okamoto Machine Tool Works completed the Type ASG-2, the first spur gear grinder made in Japan, on an order from the Kure Naval Arsenal (Chapter 3)^[7][1]. Even so, the precision gears of automobiles and aircraft were still cut on imported machines^[1].

Wartime Prototype Orders

Under the 1938 Machine Tool Manufacturing Industry Act, incentive grants were paid for prototypes of machine tools designated by the state. The eligible items included gear grinding machines and gear cutting machines (including worm gear cutting machines, but excluding hobbing machines)^[8]. When the US banned machine tool exports to Japan in 1940, the Ministry of Commerce and Industry drew up a list of machine tools to be made domestically and ordered companies to build prototypes, and gear cutting machines were built as straight copies of machines by leading foreign makers. In line with the "aircraft first" policy, the focus was on machines for cutting aircraft engine gears^[1]. In 1940 Karatsu Iron Works developed a gear shaper on special government order, and the company regards it as the first one made in Japan^[3].

A recollection by an Osaka gear maker survives on the accuracy of Japanese-made hobbing machines during the war. Cutting a module 2.5, 20-tooth gear on a Kashifuji 14-inch hobbing machine gave a span measurement error of about 0.03–0.04 mm, which is said to have been among the best at the time^[5].

Mass-Produced Automotive Gears and Technical Tie-Ups (1945–1978)

After the war, gear cutting machines became one of the machine tools most dependent on imports. From the era of controlled prices to market prices in 1949, the price of hobbing machines rose 39-fold^[8]. For automotive transmission gears, hobbing followed by gear shaving (a finishing process that pares thin layers off the tooth flanks) became the standard for mass production, but shaving cutters had to be imported and few companies could regrind them. Asked by automakers to produce them domestically, Fujikoshi Steel Industry (today's Nachi-Fujikoshi) signed a technical tie-up with Michigan Tool (US) for shaving cutter manufacturing in November 1953^[2].

The Era of Production Hobbing Machines

Japanese-made "production hobbing machines" for mass production also appeared. Kashifuji Iron Works released the KS20 in 1954 and the KS14 in 1956^[6], and the 1963 Kokusan kosaku kikai sogo katarogu (Comprehensive catalog of Japanese machine tools) lists its Types KS-14 and KS-20 precision production hobbing machines and Type KR universal hobbing machine. The hobbing machine section of the same catalog lists Karatsu Iron Works, Okamoto Machine Tool Works, Shin Mitsubishi Heavy Industries (Mitsubishi Lorenz), Toshiba Machine (large machines), Hamai Sangyo, Howa Sangyo (small automatic machines), Iwasa Tekko, Seiwa Tekko and others, and there are also sections for gear shapers, bevel gear cutting machines, gear grinders and gear finishing machines^[10]. The tool maker Nachi-Fujikoshi developed a production hobbing machine in 1961 to make the most of its own gear cutting tools, began selling it in 1962, and also released a high-speed gear shaper in 1966^[2].

Mitsubishi Lorenz and Technical Tie-Ups

In 1961 Mitsubishi Heavy Industries signed a technical tie-up with Lorenz of West Germany for gear cutting machines and began producing the "Mitsubishi Lorenz" vertical hobbing machine at its Kyoto Works (Nidec Machine Tool's "Product history" gives 1962). In 1964 it developed a spline hobbing machine, and in 1966 it began producing the Type NZA gear grinder with technology from Reishauer of Switzerland, succeeding in making it domestically in March 1968^[9][11]. In the late 1960s, as automobile production grew, Mitsubishi was building nearly twice as many hobbing machines as around FY1963, yet in FY1967 it still had a backlog of orders with delivery times of more than a year^[9].

Even so, a 1968 survey by the Japan Society for the Promotion of Machine Industry concluded that "of domestic machine tool demand, the area most dependent on foreign machines is gear cutting machines and gear finishing machines," noting that few spiral bevel gear cutting machines were sold domestically and that Gleason (US) was world-famous^[12]. Even searching the NC machine tool catalog compiled by the Japan Machine Tool Builders' Association the same year for "hobbing machine" or "gear cutting" turns up no gear cutting machines^[23].

Toward In-House Technology

In the early 1970s, makers moved from their partners' technology to their own. In 1971 Mitsubishi Heavy Industries ended its tie-up with Lorenz and switched to production with its own technology; in 1972 it released the GH series of high-performance hobbing machines (GH200, GH300), followed by the SH series of high-speed gear shapers and the Type SV gear shaving machine^[9]. In 1971 Nachi-Fujikoshi signed a technical tie-up with a West German hobbing machine specialist for internal gear skiving machines and production hobbing machines (Type SH-180 and others)^[14][2].

"Carbide hobbing", cutting fast with carbide hobs, also became practical from the machine side in this period. In 1974 Nachi-Fujikoshi showed that carbide hobbing was practical with a hobbing machine whose direct-driven hob spindle greatly reduced vibration^[2]. In 1976 Kashifuji Iron Works completed the KS300HD, a practical machine for carbide hobbing^[6]. For large gears, in 1975 Karatsu Iron Works built an extra-large hobbing machine able to cut gears 13,000 mm in diameter (the company calls it one of the largest in the world)^[3].

NC Hobbing Machines (1979–1993)

A hobbing machine has to keep the hob and the gear turning in exact step, and for a long time this was done mechanically with a master worm wheel and change gears. NC came about 20 years later than for lathes and milling machines, where it began in the 1950s. Kashifuji Iron Works converted a hobbing machine to CNC in 1979 (the company says it was "the first in the industry") and released the KB200 CNC hobbing machine in 1982^[6]. In 1980 Mitsubishi Heavy Industries developed the GH400NC NC hobbing machine. The National Museum of Nature and Science database calls it the first NC hobbing machine made in Japan, capable of heavy cutting up to module 12, with stepless hob spindle speeds of 60–350 rpm and five axes under NC control^[15][11].

The 1985 Mitsubishi Juko giho (Mitsubishi Heavy Industries Technical Review) introduces the GA series of NC hobbing machines, which used the FANUC 6MB NC unit^[16]. In November 1986 Mitsubishi's gear cutting machines reached a cumulative 3,000 units since 1961^[9]. For gear shapers, Karatsu Iron Works built a 250 mm high-speed gear shaper for automobiles and motorcycles in 1980, and began selling form gear grinders in 1989^[3].

★Dry Cutting, Skiving and Gear Grinding (1994–)

From the 1990s, gear making moved toward making the "hob → harden → grind to finish" sequence faster and cleaner. In 1994 Mitsubishi Heavy Industries released the ZG400 CNC gear grinder using electroplated CBN wheels (Chapter 3)^[17], and in 2009 the ZI20A internal gear grinder for mass production (the company calls it a world first)^[11].

Dry Cutting

In 1997 Mitsubishi Heavy Industries completed a dry cutting system for gears that uses no cutting oil at all (the company calls it a world first). In 2002 it turned this into a product, the GE series of dry-cut hobbing machines^[11].

Skiving

Skiving, a fast way to cut internal gears (a gear-shaped cutter is tilted and turned in step with the gear, paring the material away), is an old idea: in 1969 a trade magazine featured a gear skiving machine by Pfauter of West Germany^[13], and in 1971 Nachi-Fujikoshi brought in internal gear skiving machine technology^[14]. Machines and tools from Japanese companies all arrived in 2014: Kashifuji's KPS20 gear skiving machine, Karatsu Iron Works' GSV-25N gear skiving machine and Mitsubishi Heavy Industries' Super Skiving Cutter^[6][3][11], and Mitsubishi Heavy Industries Machine Tool's MSS300 Super Skiving Machine of 2016 received the JSME Medal for New Technology in 2017^[11]. The technology behind skiving and dry cutting is described in detail in the article "New technologies that solve two weaknesses of gear skiving" (dry air skiving and finishing hardened internal gears).

Today's Players

Mitsubishi Heavy Industries' machine tool business was split off as Mitsubishi Heavy Industries Machine Tool in 2015 and came under Nidec through a business transfer in 2021 (today's Nidec Machine Tool)^[18]. Karatsu Iron Works changed its name to Karatsu Precision in 2016, and in 2024 completed the Type GS-4N, one of the largest gear shapers in Japan^[3]. Among gear finishing machines, Kanzaki Kokyukoki of the Yanmar Group builds gear shapers, gear grinders and gear honing machines around its core shaving machines, and says that 90% of its users are automotive-related^[22]. Today's makers of gear cutting machines and gear grinders are listed in Makers of gear cutting tools and gear machines.

What it means on the shop floor. When reading the specs of a used gear cutting machine or an old drawing, ask: "Does it have a differential (can it cut helical gears)?", "Is it synchronized by a master worm gear or by an NC electronic gearbox?", "Can it reach the speeds carbide hobs need?" and "Does it need cutting oil?" The answers tell you the machine's generation and what it does best.