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So, how do 25 employees produce 30,000 precision gearwheels a year?


Process and Advances Their process for creating gearwheels is similar to how most in the industry now make gears, starting with a steel blank. This is a forged, soft, unhardened steel that has been turned and machined into the initial shape for the gear. A CNC machine hobs and mills the gears into each gearwheel with teeth roughed into a few 100 μm of their fi nal specifi ca- tion. Hobbing is a multipoint cutting process that rotates both tool and workpiece in precise relation to each other. It cuts multiple gear teeth at once, and is often the preferred method in complex involute designs. According to the company, cycle time at this stage is typically three minutes, including rough-cutting the gear teeth and lightening holes. Deburring is next in an automatic deburring cell. This cell also cuts the end relief chamfer that is critical for today’s gear teeth. After washing and degreasing, the next step in the pro- cess is heat treating the gearwheels. Typically heating them to over 900°C, edges are further enhanced by intro- ducing carbon in the heat treat envi- ronment. MTU oil quenches the gears after hot soaking to fi nal hardness. Following the general trend in the business, gears are ground in what MTU calls its hard machining line. Bu- cher described a continuous generat- ing grinding process, with a ceramic wormwheel. Required tolerances are down to a few microns. Grinding after heat treating instead of shaving before means a harder, more robust gear without distortion or changes of geometry from the heating process. “One of the biggest improvements we made in the last few years was in the grinding process,” said Bucher. “When I started eight years ago in the gear production, we needed something like 40 minutes to grind off the big idler gears. These are around 400 mm in diameter. With our newest machine and the new worm grinding gear technology, we need around about fi ve minutes to grind a whole gear.”


Quality and Inspection


Customer expectations—even among those who buy heavy machinery—are for robust but quiet gears. That means tolerances in the single digits of microns for each and every one of the 30,000 gears produced. Quality is enforced through typical inspection equipment. “We have a special gear measur- ing CMM,” used for SPC measurements, explained Bucher. Emphasizing the need for a stable process, he noted that on the turning machine that creates the original blanks MTU measures every part for diameter and length. At the other end of the process, the gear grinding machine uses in-process measurements on every part while grinding, adjusting the grinder parameters interactively to ensure accuracy. “Of course, we make initial samples for statistical control,” he explained. “Every fi rst part of a lot size we measure on the CMM in a First Article Inspection process, but the focus is on getting a stable process.” The idea is to engineer the need for 100% inspection out of the process. He also explained that the gears meet the International Tolerance grade of 7, or IT7.


The sparks really fl y when the gears are subjected to fi nish grinding. This grinding is commonly used today to ensure micron-level accuracy.


“It is diffi cult to quote a single number because of the differ- ent diameters of gears we make and the different dimensions around gear teeth, so we use the IT7 standard,” explained Bucher. These International Tolerance grades are referenced in International Standard ISO 286.


51 — Motorized Vehicle Manufacturing 2016


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