A major Chinese compressor manufacturer recently ordered a machine from Holroyd Precision of Milnrow, Lancashire to be used for the roughing and finishing of outsize screw compressor rotors in the gas processing industry. Weighing in at eight tons, the stainless steel rotors need a special machine tool capable of handling diameters up to 850 mm with a length up to 4.5 m. The project posed a number of major design challenges which Holroyd tackled with the incorporation of some innovative features, according to the company's press release.

In this instance, the greatest challenge involved achieving the requisite precision of movement given the enormity of the masses to be moved and the cutting forces involved requiring maximum stiffness and damping with minimum weight and friction of all axes. To minimize the Abbe offset errors caused by the increased scale, all encoders needed to be in line with the moving axes and as close as possible to the workpiece.

The large cutter loads dictated that the machine structure should be very stiff to resist the enormous reaction loads between cutter and component. The beds were constructed from cast iron, heavily ribbed internally to reinforce the structure and minimize the weight as well as fastened to a massive reinforced concrete foundation block.

While the Holroyd team's technical expertise came into its own in devising machine guide ways that provided sufficient freedom of movement without weakening the structure itself. Amidst all this technological innovation, a spot of good ole fashioned elbow grease was required to hand-scrape the plain cast iron guide way of the transverse axis to achieve total precision of movement.

In addition, Holroyd's customer demanded flexibility. In addition to mounting 600 mm diameter cutters to machine massive components, the cutter head needed to be designed in a way to allow for the mounting of smaller cutter of only 400 mm diameter, when required, to manufacture smaller components. To avoid wind up of the long rotor shaft and the component bending its own weight, the component was gripped close to the body with its outer end supported on its bearing diameter.

A unique internal vision system with two cameras set up inside the machine itself, connected to a ‘picture in picture' LCD screen and joystick on the outside, allowing for close scrutiny of the machining area and individual cutter inserts. This was a necessary feature since operator access needed to be prevented during normal operation to comply with the latest machinery safety standards.

It's fair to say that everything was taken into account to achieve the maximum possible for efficiencies, and economies for the customer. For a start, the machine was built to very fine tolerances-it can be disassembled, transported and re-assembled without the need for extensive alignment tests.

Given the machine's size, special emphasis was also placed upon minimizing pattern costs. For example, to avoid excessively large patterns, the eight-meter bed and other patterns were divided into a number of identical and special cores which were repeated along the length.

Due to the machines sheer scale and floor plan, two operator stations were installed with special attention to safety aspects and, uniquely, only one handheld control displaying live axes data to operate the machine. It was also necessary to ensure that all functions were mutually exclusive to an active operating station and the handheld control. While the wiring of the machine meant that, if the slave station computer failed, functionality would be unaffected.