Planetary gears are a key component in automatic transmissions (AT). The shift from manual to automatic transmission required improvements in the production of these essential gears, and the large-diameter helical broach was developed to meet this need. The helical broach ensures the consistent machining required throughout the process, from roughing to finishing. We interviewed staff at the Gear Cutting Tools Manufacturing Department about the history of its broach development, including innovative products developed ahead of the global competition.
Broaching is a method of processing designed to enable the creation of special forms on the interior surface of the cylindrical bore, forms such as splines and key grooves. The edge of a broach is similar to the round-hole on the material. The teeth are gradually formed as it proceeds toward the centre. And finally formed into their completed geometry by the time the broach is pulled all the way through. Thus the entire gear forming process, from roughing to finishing, is performed in a single process.
Creating the best cutting conditions for the individual processes required to cut the high-precision gears used in automatic transmissions - the roughing, semi-finishing, and finishing - all three steps in one process, significantly increases productivity.
Broaches with many teeth whose forms gradually change from roughing to finishing profiles have the following features:
• Ease of the procedure to pull the broach through a workpiece on a broaching machine shortens the time required for processing.
The sharpness of the broach and precision of the edge are directly reflected on the workpiece. The higher the broach performance is, the higher the quality of the surface and dimensional accuracy of the finished product becomes.
• It is possible to process complex axial gears such as those with helix angles.
• Since the amount of cut per edge and total amount of cutting can be set in advance when designing the broach, operators do not require special skills to pull the broach through a workpiece.
• Since the pressure created during cutting works towards clamping the workpiece, it is not necessary to have special jigs to clamp it.
After 1955, the rapid growth of the Japanese manufacturing industry spurred demand for cutting tools. To meet this demand, Mitsubishi Materials opened its Akashi Plant in 1962. The Akashi Plant had a wide range of leading-edge equipment for specific processes such as polishing, quenching. As well as the inspection facilities to aid manufacture of various cutting tools, including drills, end mills, reamers, and broaches. Broaches in particular were expected to offer significant merits for customers since gears could be processed with accuracy and efficiently with them. Therefore, Mitsubishi Materials started working on the development of broaches early on.
As the implementation of automatic transmission accelerated in the 1990s, production efficiency of planetary gears that have relatively large diameters had to be improved. Before the development of broaches, gears were cut with gear cutters. Gear cutting required three processes, roughing, semi-finishing and finishing that required approximately 2 to 3 minutes per gear. Broaching required 30 seconds or less for each gear, which meant a 4- to 6-fold improvement in productivity. In addition, broaching achieves much better precision than gear cutting and it only requires the simple operation of the broach being pulled through a workpiece on the broaching machine.
To take full advantage of these benefits, Mitsubishi Materials started development of a large-diameter helical broach, an advanced model of the existing spline broach. The first prototype was an assembly of the roughing gear (main body) comprising a cutting blade for the outer periphery and a finishing gear (shell), which was a cutting blade with thick teeth. Due to the large size, a big enough precision measurement device at that time was unavailable, meaning it was necessary to separate the prototype into the main body and the shell. The form of the broach end is directly transferred to the workpiece. It was difficult to achieve the required precision with the assembly-type broach. Several prototypes were provided to automatic transmission manufacturers; however, most of them were returned due to their poor precision. For the assembly type, the performance of the blade in the shell part affects the precision of the gear teeth. This required making micron-level adjustments to the form of the blade in the shell. Mitsubishi engaged in a process of trial and error to improve the blade until a stable level of precision was achieved in 1995.
Mitsubishi Materials started mass production of assembly type broaches in the 1990s and also began development of a new type of broach. The work started on the development of an one-piece broach that combined the main body and the shell part. Due to not having a device to measure the precision of the teeth on an one-piece broach, it was impossible to grind the teeth with high precision. The large-diameter helical broach used to manufacture planetary gears for automatic transmissions would have an outer diameter of ø100 – 180 and a total length of 1,500 – 2,000 mm. This necessitated the adoption of an assembled type of broach that separated the shell part, whose teeth had to be very precise, but due to the smaller, size precise measurement was possible with a gear measurement device. However, precise measurement of a large one-piece broach demanded development of a new measurement device. A Mitsubishi Materials engineer succeeded in creating an innovative on-board measurement device for the tooth form. This was the first attempt in the world, and Mitsubishi Materials’ achievement was recognized with the Japan Society of Mechanical Engineer Encouragement Prize. The engineer received his doctorate in engineering at Osaka University Graduate School of Engineering. A summary of his study was as follows: “Precise grinding of the form of the teeth used for spur and helical gears requires measurement of grinding errors caused by grindstone and tooth formation, analysis of the data, creation of an automatic error correction program, and immediate feedback to the grinding machine. Combining these systems together can secure the required precision in gear grinding.”
The use of this new on-board measurement device for tooth form on the CNC grinding machine, made it possible for the one-piece broach to have the tooth form to be ground with high precision. This led to the successful development of the world’s first large-diameter helical broach. The one-piece helical broach can significantly reduce manufacturing costs through one step manufacturing of both body and shell and realize high-precision gear machining. In addition, the cutting load on each blade can be optimized, which reduces abrasion on the entire broach and increases the tool life thereby extending the time before regrinding is required. Furthermore, the disassembly, reassembly and phase adjustment of the shell required for regrinding on the assembly-type broach are unnecessary with a one-piece broach, which also reduces costs. These merits pleased our customers and the reduction of work for regrinding in particular is highly regarded by overseas automatic transmission manufacturers.
While promoting the development of the one-piece broach, Mitsubishi Materials has also worked to improve the assembly-type broach. Groove types include a ring groove (right angle to the axis) and a helix type (off-normal). The ring groove type causes a significant change in cutting load compared with the helix type, causing shorter tool life. The helix type causes less change in cutting load, which improves both the precision of the tooth form and increases tool life; however, it requires special equipment for regrinding.
There are three assembly-type broaches. One is a combination of both main body and shell at a right angle to the axis. The second is a combination of the main body at a right angle to the axis while the shell is an off-normal type. The third is a combination of both main body and shell that are off-normal types. The number of grooves also varies. For example, the teeth on the main body are reduced to from 4 to 6 while those on the shell are increased to from 8 to 10, which improves sharpness.
The one-piece broach also has the same three types of teeth, and a one-piece broach whose teeth are changed on the main body and shell has been under development.
The manufacture of high-precision broaches requires the strict control of temperature during grinding. Since the grinding requires a significant amount of time, the temperature needs to be maintained within a narrow range to prevent the grinding machine from expanding and contracting that could cause variations which would influence pitch accuracy in the final product. Mitsubishi Materials continues its search for ways to manufacture even higher quality and higher precision broaches while reducing even slight changes in temperature.
Nishikawa: The Gear Cutting Tools Manufacturing Department, which manufactures broaches is in close contact with our customers. Since the cutting tools are made to be used by our customers, their feedback after use and problems they have are very important to us. Sometimes they have complaints, which we take very seriously; but in addition to addressing any issues they may have, it is important to develop tools that are even more useful for them. The repetition of this cycle has provided the foundation for our growth.
Kohno: Precision tool manufacturing does not always follow theoretical expectations. This is perhaps the most enjoyable part of manufacturing broaches. We have developed large-diameter helical broaches, and some of these exceed 2 meters in length. Only a small difference in the blades of a broach has a big influence on tooth precision in the final product. For example, the very slight honing of the cutting edges by hand, sometimes improves precision. This cannot be explained by theory, and cannot be done by just anyone. This is why it is important, that such precision tools can be adjusted in such an analogue way.