Ball screws and roller guides used in innovative machine tool

Growth markets such as wind power, offshore technology and marine engineering require ever-larger annular components for turret rings, flanges, seals and bearings. In the past, their fabrication was hindered by severe tool wear, heavy energy consumption and significant material wastage. A more efficient process for separating rings from the feedstock has therefore been sought.

The PTZ Weidner Giant Ring Cutter (GRC) reduces cutting times by 80 per cent, with 70 per cent less energy consumption, material savings of up to 60 per cent and 40 per cent in tooling costs. This is in addition to significantly reduced rework for cast, rolled and forged rings.

This processing concept must meet a stringent set of specifications:

• Extremely fast, high-precision cuts with low heat input and with the smallest possible kerf to give low material consumption
• Maximum parallel planarity, down to a few tenths of a millimetre, at the best possible surface quality
• Stiffness and precision while machining
• Maximum occupational safety with positive fixing and removal of the rings after separation, together with short, dry chips that present no hazard
• Flexible equipment concept, with the option to retrofit existing turntable-type lathes and milling machines
• Automation concept with gripper arm, designed for ring diameters of 4-8m and the capacity to carry rings weighing up to 20 tons

With the GRC, the gripper arm first moves the blank onto the cutting device. Three chucking jaws, driven by Rexroth ball screws, hold the ring in position during rotation. Rexroth ball screws are extremely accurate since they feature precision spindles and preloaded, zero-play single and double nuts. And the stiff Rexroth roller guides, in size 65, reduce vibration sufficiently that noise emissions are just 70 to 74dB(A), even in the non-enclosed version.

Sawing motion

A saw blade, the insertion depth of which is set by a roller guide, approaches the rotating blank and begins the separation procedure. Here only two to four teeth - depending on the material - are in contact. Thanks to the tangential motion of the two circular paths, the process achieves superb parallel planarity and optimises the use of the costly material. A further advantage is the low amount of heat developed during the separation process. Exactly defined tooth engagement in the material permits higher cutting speeds without the need for cooling lubricants. Thus, in contrast to all other processes, the length and thickness of the swarf can be carefully controlled. At the same time, this cutting concept, even in the initial step in production, achieves a very smooth surface finish.

This process is available in standalone machines - either open or enclosed - and as a retrofit kit for turntable-type lathes and milling machines. In addition, a machine to process tubular and solid material (diameters of up to 480mm and lengths of up to 4m) is in the development stage. Here, again, Rexroth components will ensure precision both in drive mechanisms and in handling.

A patent-pending prototype machine for the process, sized to handle rings of up to 1.45m in diameter, has been running at full speed since the start of 2011 - and has earned the German Material Efficiency Prize. This is thanks to the GRC that not only slashes fabrication times, but saves a large amount of material. The new process can separate rings made of stainless steel and other alloys at diameters of up to 9m and wall thicknesses as much as 300mm. Those dimensions are said to be inconceivable with band saws. And the only alternate available to date - oscillating sawing with a jigsaw or similar equipment - leaves striations on the ring. These then have to be cut away, and that means a great waste of material and time.

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