Fast movers

Developments in software and control engineering are making parallel kinematic machine tools viable, while traditional designs are also benefiting. Which technology will dominate the future? Martin Oakham reports.


Growing competition from the East means the engineering sector in the UK and Europe must invest in automation and the latest machine tool technology to retain an effective manufacturing base.

The European Commission wants flexible manufacturing systems with ‘greener’ production and the ability to absorb technology quickly. It also wants to see the development of new, more productive machine tools incorporating parallel kinematic (PK) technology.

Few people have heard of parallel kinematics, but the technology has plenty of potential. It combines rotary motion with simple linear movements, allowing it to offer fast overall speeds. All the forces are in compression or are tensile, therefore it is possible to manufacture extremely stiff yet light structures.

The majority of the metal-cutting sector believes there will soon be far more developments along the line of cross-technology hybrids, which combine processes to reduce set-up times and floor space. There are now many examples of five-axis machining centres incorporating lasers, grinding wheels, and even EDM die sinking.

For the end user, advantages include reduced capital outlay in the purchase of one machine rather than several tooling arrangements and, in some instances, fixtures. It also eliminates component transfer problems between machines and reduces setting and component inspection.

For the machine tool manufacturer, the major benefit is in the production of one generic machine system; able to be configured to accommodate one or more machining processes, without the need for separately-designed and manufactured units.

In addition to being highly automated, these types of machines are likely to incorporate expert systems, helping the machinist with every aspect of programming in much the same way as today’s EDM machines are set up.

As this trend develops we will see more help with tooling choices, cutting strategies, cut depths and speeds and feeds. It would be within reason to have all the expert knowledge of a high-end office-based CAM system on the machine tool controller, making the distinction between what should be handled by the CAM developers and the controller manufacturers even fuzzier.

At present both parties are fighting for the upper hand in controlling the dynamics and ease of programming toolpaths. However, a beautifully thought out user interface is worthless without the control software that drives the machine tool’s kinematics and, conversely, new kinematic technologies are worthless if they are too difficult to implement in a part program.

There was a time when a CNC was responsible for little more than relatively slow axis positioning and basic tool management. However, as machining centres have evolved, axis-positioning requirements have drastically increased while tool management and other functions have become vastly more complex, demanding far greater processing power to handle them.

Parallel processing technology allows the use of two or more microprocessors to be employed simultaneously. This is one way machine tool developers are alleviating the pressures of multi-tasking.

Fadal, for example, uses a dedicated processor for each axis to enhance processing speed. This is a highly effective solution but the speed of the processor and its architecture also need considering as part of the machine tool design process.

Today’s CNC systems also extend functionality for general use. Tool offsets can be set with the push of a button. Servomotors can be tuned on the fly. Broken tools can be detected and parts inspected on the machining centre.

Just as the interface software depends on good control software, the control software depends on machine build, minimising the effect of expansion and contraction and loss of stability due to low mass. This applies right across the board.

According to Graham Gough, engineering manager of machining contractor mdc-Mtec, modern control systems such as the Siemens 810D help but you still need an accurate machining platform to perform the type of work produced. Here, Gough says, mdc-Mtec’s newest machine, a Dugard Eagle 660 is proving its worth; with repeatability of +0.003 mm it is well able to hold typical tolerances of 0.02mm while most programming involving up to 100,000 lines of information is performed at the machine using Siemens ShopMill user interface.

High-speed control builder Fidia supports this philosophy. Its president, Dr Giuseppe Morfino, sees development of its machine tool line as a natural progression of CNC manufacture. Making high-performance machine tools helps the company achieve better, high-performance CNCs. Fidia also manufactures a CAM package for high speed machining that ties together the control and the machine tool for high-speed milling of moulds and dies.

Other trends such as the use of linear drives, linear scales, automated palleting systems and ‘chipped’ tooling systems are all aiding the drive for faster production times and are therefore trends.

So in the face of so many general developments, is the European Commission right in its enthusiasm for parallel kinematics? Despite the obvious promise of the technology, complications with the technology arise due to the nature in which the motion is achieved. Whereas the axial movements of a traditional machine are directly controllable, parallel kinematics motion in a given direction involves the simultaneous adjustment of several struts and an ability to predict the end result.

In a working machine, these ‘kinematic transformations’ must be carried out at intervals of just two milliseconds to achieve the required smoothness of translation and acceleration. Determining the mathematical foundation for this real-time computer control and for calibration and error compensation on individual machines is a major challenge.

However, Fatronik has overcome these and has the essential methodology in place to adapt this technology to different machine tool types.

The axes in Chiron-Werke ‘Vision machining centres’ move with a rapid speed of up to 120m/min, acceleration up to 3g and vectorial axis acceleration of 5.2g. It has a chip-to-chip time of 1.2 seconds.

Stability may be a problem. Each ‘strut’ is critical for the stable motion of the machine. Take one out through a system failure and the entire machine becomes potentially unstable. However, with only 32 examples in existence this is in its infancy and may form the basis of a new generation of machine tools.

In contrast, most conventional serial kinematic architectures, with each axis of movement supporting the following axis and providing its motion, must be heavy enough to provide the stiffness to inhibit the flexing movements that limit machine accuracy. This restricts dynamic performance and reduces operating flexibility.


Sidebar


Vinten picks a winner
When Vinten Broadcast, part of the global Vitec Group, had to meet a surge in orders for one of its products, the choice to keep the work in-house centred on the acquisition of a new twin-pallet horizontal machining centre.

After an extensive shortlist was narrowed to two candidates, the camera support product manufacturer opted for a Kitamura Mycenter HX400iF from sole UK agent, Leader CNC. The set of parts in question comprises three complex aluminium components — a ‘body’ part, a right-hand ‘arm’ and a left-hand ‘arm’.

Previously it took Vinten seven hours and 50 minutes to complete one set on an 11-year-old horizontal machining centre. Today, using the Kitamura it takes just three hours and 30 minutes.



Sidebar


TGM gets lift-off
When aerospace subcontractor Techni Grind Machining (TGM) won a project to manufacture 1,200mm x 800mm cargo door linings for the Airbus A340 aircraft, no machine tool maker could compete with the bed size and machine capabilities of the Cincinnati FTV1040-3700.

TGM initially machined the door linings on an alternate machining centre and although it was a powerful machine with a large bed it was insufficient for the continual batch quantities required.

The Cincinnati FTV1040-3700 has cut the processing time from 15 hours to less than 12. When manufacturing batches of up to 20-door linings a month, this has proven a significant saving.