Machining Shafts at Mercedes
The Mercedes 9G-TRONIC transmission—a nine-speed that features a torque converter with an efficiency of converting engine power to tractive power of 92%, which is an improvement over the circa 2003’s 7G-TRONIC’s of 85%—required the development of some sophisticated metalcutting in order to be built.
The main transmission shaft is long, measuring 550.9 mm. There are a series of holes drilled into the shaft, the most notable of which are three parallel holes, each measuring 6.1 mm. They are deep drilled into the transmission shaft with a core diameter of 16 mm to a depth of 361.5 mm. According to Mercedes, no other OEM has drilled holes like these.
Over the entire length of 361.5 mm the holes had to have a precise distance and parallelism between each other as well as to the outer surface of the shaft. The holes are internally networked with transverse holes that carry fluids that are used to lubricate and cool the planetary gear sets and shaft elements, as well as transfer the set gearshift pressure to the multi-disc clutches and brakes in the transmission.
If the deep-hole drilling task wasn’t demanding enough as it was, Mercedes manufacturing engineers decided that they would perform the machining with a minimal amount of cooling lubricants as the 370-mm long, single-fluted drill bit cut into the metal. In traditional operations they’d use approximately 18,000 liters of cutting fluid per hour. In the case of the 9G-TRONIC operation, they’re using only 0.3 liters. And realize that not only does the fine oil/air mist cool the drill, but it also has to help remove the swarf produced in machining.
In addition to which, the cemented carbide drills are running at a cutting speed of >250 mm/min, which allows the shaft to be drilled in less than 3 minutes, which is a 63% improvement compared with the cutting time required to the 7G-TRONIC PLUS.
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