FEATURE LINEAR MOTION SETTING THE


RECORD FOR HIGH SPEED MILLING


system whereby the preload level was not affected by the cooling at any time during operation.


RA linear


guides ensure high accuracy in all main axes


Needing linear drives for a new high speed machining centre it was developing, a


Swedish manufacturer turned to NSK for the solution


B


ased in Sweden, Modig Machine Tool develops and manufactures


high performance CNC machines that are used by aerospace manufacturers across the world, including Airbus, Boeing, SAAB, Sikorsky and Volvo Aero. Chip removal speed is a key parameter


for machining centres used in the aircraft and aerospace industries because many of the central, very large components – like wing ribs and other structural parts – are milled from solid material. As an example, parts weighing over 100kg could be made from blocks of aluminium weighing four tonnes or more, which means that 97.5% of the material has to be removed. While its machinery has a reputation


for high speed and precision, the company had two goals when developing its latest machine, as David Modig, CEO, explains: “One of our goals was to develop the first 2-in-1 machine in this performance class; in other words a milling machine for high-speed roughing and finishing. And the second goal: the RigiMill had to be the fastest milling machine in the world.”


MACHINE DESIGN With the aircraft industry booming, the company wanted to supply its customers with gantry systems that have even shorter machining cycles. Machining the


18 FEBRUARY 2019 | DESIGN SOLUTIONS


During a customer demonstration, the RigiMill machine (above) removed 16.4 litres of solid material in just one minute


structural components of an aircraft also requires precision at the highest level, so the stiffness of the machine axes is just as important as speed. So, when designing the new machine,


the company needed linear drives for the axes and turned to NSK for the solution. To meet the requirements, the machine therefore uses twin drives on the Y and Z axes with a positioning accuracy of 0.5μm in each case. The two electric motors work in master/slave mode. In co-operation with Swedish


authorised distributor Acumo, specialists from NSK in Ratingen configured the drive units, taking into account factors such as torque, speed, axis acceleration forces and the temperatures generated in the spindle drives during high-speed machining. The HMD series ball screws with nut


cooling were found to be the optimal solution as they have been designed for use in high-speed machining applications, supporting travel speeds of up to 120m/min. With a newly developed recirculation system, it was possible to increase the d x n speed rating to 160,000. Cooling the linear drive through the


nut (precisely where the heat is generated), guards against temperature increases which could potentially impair the accuracy of machining. There were, however, a number of


design challenges to overcome. As an example, they had to solve a problem caused by the cooling, which made the material in the body of the nut contract and affect the preload of the ball screw. After extensive tests, NSK development engineers came up with a cooling


THE SOLUTION Ball screws with nut cooling from the HMD series are used in the RigiMill’s Y and Z axes. The Y axis has a travel distance of 1560mm, with 900mm in the Z axis. HMD ball screws are equipped with axial angular contact ball bearings from NSK’s BSBD series that were developed precisely for this type of application. The double-row configuration of these bearings, with a contact angle of 60˚, is capable of withstanding high axial loads in both directions. They simultaneously offer the stiffness and accuracy required for modern precision machinery. The NSK system solution is completed


by RA roller guides in all the main axes of the RigiMill. They, too, are characterised by extreme stiffness at maximum load capacity and create the conditions for the spindle axes to run smoothly, with low vibration in spite of the high speeds and high forces generated by machining. The ideal line contact of the roller keeps the displacement force low because a pure rolling movement adds no friction to the drive. Explaining that the components and


drive system met the stringent demands, Modig commented: “They make a significant contribution to ensuring that the RigiMill not only works with the highest precision but also at extremely high speed, which in turn lowers machining times and cost per part.” The resulting machine can produce


more structural aerospace parts in a given time. Take the example of a wing rib, where the machining time of 384 minutes has been reduced to only 74 minutes. In addition, when performing finishing, the machine can produce a surface quality of up to 0.8Ra. The RigiMill high-speed machining


centre now holds the world record for high-speed milling. During a customer demonstration, the machine removed 16.4 litres of solid material in just one minute. The spindle was running at speeds of up to 30,000rpm driven by a 180kW motor.


NSK www.nskeurope.com / DESIGNSOLUTIONS


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