Milling the Monoliths

A complete rethink of milling machine design aims to increase production rates ten-fold. Roger Brownlie reports

Aerospace engineers have long understood the potential of making monolithic parts – huge aircraft structural components hogged out of a single billet of metal, usually aluminium. The vision is to make floor beams, wing ribs, fuselage frames, and other components as single pieces rather than as multi-part assemblies. The advantages of this method include eliminating hundreds of bolts, rivets and other fasteners, increasing production speed, eliminating tooling, reducing the number of component parts, and a reduction in costly assembly operations. The challenge has been how to machine these large parts efficiently.

Cincinnati Machine began to look for a solution in 1995 with Boeing. The result of that research and development effort is the 5-axis HyperMach, a unique design incorporating technology that has the potential to re-define high-speed machining.

‘Within the next 15-18 months we should have a marketable product. We’ve brought the technology a long way but before we are prepared to manufacture a very large profiler we need a big launch order from a customer,’ says Randal Von Moll, HyperMach product manager.

To make HyperMach Technology pay practical dividends for the aerospace industry, Cincinnati Machine engineers concentrated on developing a machine that would transcend current technology constraints on acceleration and velocity. The primary method was the use of linear motors. By adopting linear motors for all 5 axes, the HyperMach contours at feed rates of 60 m/min and rapid rates of 100 m/min. The machine changes direction at 2Gs – almost 800 inches/sec2 and cuts with spindle rpm of 40,000 plus, targetting 60, 000.

‘We are beginning to use the HyperMach term to describe a family of high performance machining products. Some of the lessons we have learned about very high speed path following capabilities are being applied to current product launches. HyperMach does not simply describe our linear motor developments but our entire high performance family.’ Historically, machine tool makers have over designed their machines, the solution being to add more mass. But this philosophy does not work if you are aiming for 2G acc/deceleration and contouring feed rates over 60/min.

Much of the research focused on the machines dynamic characteristics. HyperMach would be subjected to a whole new set of forces that have the potential for intolerable vibration, structural deformation and lack of accuracy.

With its minimal structural mass, the HyperMach technologies make for an unconventional looking machine tool – not of the hexapod form- that challenges the metalworking industry’s status quo. The moveable portion is structurally challenged and this is reflected in its design and appearance with members built for either tensile or compressive loads.

In tests the HyperMach has milled thin-walled aluminium structures from a single billet of material in less than 30 minutes. The same process would have taken at least three hours on a state-of-the-art, high-speed CNC machine, or as long as eight hours on a conventional CNC mill. Spindle bearing technology was another design consideration.

‘The whole idea of HyperMach originated as the desire to create a platform that could carry a 40/40 spindle, which is fairly well known, at speeds that were not possible before. Our idea behind the design of the machine is to be able to implement incremental advances in the enabling technologies as they become available. When the spindle manufacturers deliver the next generation of high speed spindle, we want to be able to put it in the machine and test it.’ Using hybrid ceramic (silicon nitride) ball bearings, HyperMach’s low vibration HSK-63F motorised cartridge spindle delivers high levels of accuracy, rigidity, bearing life, and temperature stability.

Despite the extra demands, the linear guides are also of standard type. Von Moll explains: ‘We are continuing our design focus on the roller type as opposed to a ball type. These are straight out of the catalogue. One of the drivers for the HyperMach programme is not to create any new technology where we don’t need it. All of the pieces that we are using are available out of anyone’s catalogue. There is enough risk in the overall interface of these different components without introducing additional risk by developing specialised versions of these. We’ve taken what we consider ‘the-best-of-the-best’ of commercially available components and have applied them just a little bit more intelligently than some have in the past.’ The Siemens 840D Digital CNC/Digital Servo drives, combined with an innovative structural design, provides very high dynamic stiffness. Extremely accurate 5-axis contouring at ultra-fast metal removal rates is possible. These factors are critical to high performance machining.

‘Part of the design initiative was to create modules that could be mixed and matched to the customers specific application.’ One application of HyperMach’s modular technologies is a horizontal profiler with X-axis travel expandable in 144″ increments. This machine could produce parts from plate material up to 150′ long and up to 80″ wide, ideal for aerospace industries.

‘Boeing is not buying at the moment and has not for the last two years. But we have these modules available when they are interested in really gearing-up for a major new programme, like Airbus 3XX or whatever Boeing’s answer is going to be to 3XX – and we know there will be one.