In order to meet the requirements for ever smaller engines with increasing performance and reduced_{CO2 emissions}, various adjustments are being made. For example, new materials are being developed that are lighter and yet more stable. Or engine components are modified in order to reduce the rotating mass and, in turn, fuel consumption. For example, crankshafts for different engines are designed with a lightening bore. This results in a significant reduction in the weight of the component.

A new machining process

For the production of crankshafts, however, the relief bore means an additional machining process. At Daimler, the process development, planning and tool management departments work closely with the production department on an interdisciplinary basis. However, in addition to the internal participants, suppliers are also needed to provide the optimum tools for reliable machining.

When the process was first designed, tools from different suppliers were considered. Several manufacturers developed concepts for drilling the hole through the crankshaft bearings. The solution from Mapal was the most convincing. Employees from the precision tool manufacturer are constantly on site at the production facility in Untertürkheim to resolve problems and provide support during production. In addition, the company has carried out development work and preproduction tests throughout the entire time leading up to series production, as there is often not enough time for this in large-scale production.

Facilitation drilling in four steps

The process for machining the relief bore of the micro-alloyed steel crankshaft is realized with a total of four specially designed, double-edged tools. Cooling takes place with minimum quantity lubrication. The first solid carbide drill (diameter 19.2 millimetres) is used to pre-machine the cover bore. The second solid carbide drill (diameter 18 millimetres) produces the pilot hole for drilling through the first bearing. A replaceable head drill (diameter 18 millimetres) is used as the third machining step. It finishes the first bearing and drills through the second bearing. Finally, the remaining bearings are machined with another long TTD replaceable head drill (diameter 18 millimetres). This threads through the first two bearings at a slow speed and low feed rate, supports itself in them and thus achieves good results when machining the remaining bearings despite the long overhang. This specially designed replaceable head drill consists of a steel holder with CFS interface, the CFS replaceable head holder with a base body length of 119.2 millimetres and finally the TTD replaceable head made of solid carbide. When machining the crankshaft of a 6-cylinder engine, the holder with CFS interface alone is 480 millimetres long.

Burr formation when starting up the series

The four-stage machining process delivered the desired results in the tests and series production with low quantities. However, a problem arose during the ramp-up of series production. A burr formed at the entry and exit of the individual bearing bores after around 100 bores, which grew larger with each machined bore. As a result, the drill heads of the third and fourth drills had to be replaced after just 100 holes. This was because the burr would not have guaranteed the mechanical cleanliness of the component. There must not be any metallic particles on the component. These could come loose during operation of the unit. For this reason, any residual dirt and metallic particles on the components must be categorically excluded.

Four drills are used to machine the relief bore of the crankshaft at Daimler AG. Hydraulic chucks from Mapal ensure that the tools are securely clamped. © Mapal

A deburring cutter is the solution
Those responsible approached Mapal with this problem. The burr formation was to be eliminated without using an additional tool for deburring. This would have increased the cycle time and the process would have had to be restarted. "We worked intensively on solutions in our development department and equipped the replaceable head drill, which finally machines all bearings, with a deburring cutter," explains Tobias Moser, the sales representative responsible for the project. The deburring cutter is spring-mounted directly after the interface to the replaceable head on the CFS replaceable head holder. The bore entries are deburred during the forward movement and the bore exits are deburred accordingly when the drill retracts.

The long drill therefore enters the hole at a feed rate of 200 millimetres per minute for drilling. At a defined point, the feed rate is reduced to 125 millimetres per minute over a distance of four millimetres. Over this distance, the deburring cutter creates the chamfer at the bore entry point and is simultaneously pressed into the interior of the holder by the suspension. The bore can then be machined at a feed rate of 375 millimetres per minute. The same happens when retracting.

Off to mass production

At the beginning of 2017, Mapal tested the new concept in the Research and Development department using an original crankshaft. "We were able to work well with these results and further optimize the tool," says Moser. Things finally got serious in the summer. The modified tool with deburring edge was tested on the machine for series production. Here, too, the results were good and the process, including forward and backward deburring of the bores, has been in use in large-scale production since September 2017. "Thanks to the deburring cutter, the service life of the tools is also much longer than before," says Moser. The first two drills are replaced after 2,000 holes, the drill heads of the two replaceable head drills after 800 holes.

And the deburring blade? It only has to be changed after 25,000 bores. This process is therefore the new standard for the lightening bore of the crankshaft and is currently being transferred to two other lines.

Longer service life thanks to PVD coating

However, despite the reliable and satisfactory process, work is of course continuing on improving and optimizing the tools. The first step was to achieve process reliability in large-scale production. Now the task is to make machining even more economical. A first step in this direction has already been taken. "We initially used the deburring cutting edge uncoated, but now it has a special PVD coating for even longer tool life," explains Moser.

In order to be able to achieve higher work rates and thus further reduce the cycle time, the same process is also currently being tested on a test machine with the three-edged counterparts, different versions of the Tritan drill, to the drills previously used. With extremely promising results.

Intec, Hall 3, Stand G05