Breaking the G barrier

Are we witnessing the end of the ballscrew or are linear drives a fad? Martin Oakham, technical editor of our sister publication Metalworking Production looks at some of the arguments for and against each technology.

Machine tool designers are faced with a choice when it comes to implementing motion control systems – either they can stick with the tried and tested ballscrew/motor combination, or they can consider linear drives.

Three years ago, linear drives cost more than ballscrew systems, lacked power, had poor positional accuracy, generated too much heat, and needed elaborate emergency brake features in case of power failure. Not to mention fine particle contamination and the power they consumed.Ballscrews, on the other hand, have been a standard for years. It does not matter if they expand and contract slightly in use, because most applications requiring high positional accuracy use some form of thermal compensation algorithm or a linear encoder to measure the exact position of the slideway being driven. And, there are many ways of controlling the heat of ballscrews, such as running a coolant down the centre of them.

Of course, the principle argument for linear drives then, and still is today, the speed and acceleration they are capable of, their low inertia, friction and maintenance. Today, they still cost more than ballscrews, but many of their earlier problems have now been solved. Speed is clearly the driving factor behind new developments.

BALLSCREW ADVANTAGE

The ballscrew provides mechanical advantage as it transforms rotary motion into linear motion. The lower the lead of the screw, the higher the mechanical advantage. By the same token, the lower the lead, the lower the axis velocity for a given servo speed. For example, a screw with a 5mm lead can travel at 10m/min if the servomotor is capable of 2000rpm. A screw with a 10mm lead will travel 20m/min (about 792in/min) given the servo motor revs/min, but will require twice the motor torque to provide the same axis thrust.

For precision applications a ground low lead angle is recommended, which inherently limits the speed that a driven table can achieve. Linear motors, on the other hand, have no such restriction. It is not uncommon for linear drive manufacturers to quote traverse rates of up to 12,000in/min with accelerations up to 5g.

Two problem areas associated with ballscrews are cumulative/lead variation errors and backlash. Cumulative error over the length of a ballscrew can be compensated by a CNC control system; however, lead variation error requires a vast amount of computational power to completely correct, hence most algorithms in existence today merely ease the problem, and not fix it.

Having said that, the solution is both simple and straightforward. By making use of linear encoders, the position of the moving structure can be determined precisely with reference to the fixed structure and fed back into the controller to become part of the control loop.

Linear encoders generally use the graduated glass scale and interferential measuring principal to determine distances. By measuring the position of the axis directly, and not relying on a ballscrew/drive system as a measuring standard, backlash and cumulative/lead errors can be ignored.

LINEAR MOTORS

Linear motors are essentially rotary motors unrolled into a flat form. They are manufactured in two basic forms: ironless and ironcore.

Both systems have the advantage of not requiring support bearings, bearing blocks, motor coupling and a screw, making application designs simpler. The ironcore systems are capable of producing greater thrust than the ironless systems, making them more suitable for heavy duty applications.

Neither systems, however, are capable of producing the mechanical advantage of ballscrews. As a result, they do not deliver the same levels of continuous force. If they are pushed too hard, there is a risk of demagnetisation and failure.

Machine tool builder DMG claims to have overcome this on its new DMC85V machining centre by twinning up drives on the X,Y,Z axis. The ETEL brushless high-force motors used are cleverly arranged to prevent torsional twist.

The machine, which uses eight drives in total, has better positional accuracy than machines using ballscrews and only costs 20% more than a traditional design, according to DMG.

Each motor outputs 60Nm and is controlled by the Heidenhain TNC 426. Sinusoidal commutation or harmonic current control associated with the encoder feedback minimises force ripple giving very smooth feed. However, the most interesting aspect of the design is the way everything grows and shrinks in balance (thermo-symmetric), stabilising the machine. Thermal temperature sensors and feedback systems compensate for local growth. The motors do not add much heat to the structure either – they are separated from the gantry and surrounded by a cooling aggregate. The total feedforce is 8kN and the feed rate is up to 120m/min.

There are many examples of linear drives used in HSM applications, which generally take light cuts very quickly, but this is an example of a machine capable of moderately heavy cutting operations.

DRAWBACKS

The two greatest drawbacks to using linear motors in machine tool design are the high attractive forces created between the coils and magnets and the heat generated. The attractive forces generated are about five times greater than the axial thrust provided. These forces put a strain on the machine structure causing deformation and tend to `suck’ the working table down into the drives. Producing structures that can resist these high loads without being so heavy that they reduce acceleration is an ongoing area of development.

Another problem associated with the high attractive forces is that small particles of ferrous swarf will be attracted into the drives themselves, so it is important that the drives are well protected. It is problems such as these that prompt many machine tool builders to have the opinion that the increase in benefits to the user does not warrant the costs.

The issues of positional accuracy and cycle times can best be answered by an example from BMW who conducted a series of tests on the Urane 20 machining centre which is fitted with Krauss Maffei drives. “We reduced cycle times on various engine parts by 50-80%, compared to existing machines,” comments Lutz Schaufuss at BMW’s Methods Department.

Comparisons with three other makes of machine tool fitted with ballscrews highlighted the fact that the Urane machines produce better results in terms of cycle times. He added “The Urane also outperformed the other machines in terms of positioning accuracy and repeatability. That puts pay to the poor positioning argument and offsets the cost of the drives.”

For the future, we may well see some sort of divide, with linear drives dominating lighter applications in EDM, and ballscrews dominating the serious metal removing applications.

{{Siemens Tel: +49 9131 7 26673Krauss Maffei Tel: + 89 88 99 35 51ETEL Tel: +41 32 862 01 23}}