"More bang for your machine tool buck?" (Ron Loeffler on the Three 'R'S").

Although linear motors first came on the scene back in the '70's, recent advances in drive amplifier technology make them more suitable for machine tool use.

Linear motors' growth as replacements for servomotors and their associated components continues as numerous builders now offer linear motor-driven machine tools as standard products.

Basics:

Simply put, a linear motor is a rotating motor with its rotor and stator peeled back and laid flat. They are then installed opposite each other on either the moving or stationary surfaces of a machine tool's axis, depending on the machine's design. Once the motor is energized, a magnetic field is generated, and the axis moves.

Comparisons:

Conventional CNC machine tool axis motion is accomplished using a servomotor integrated with machine-mounted hardware like a ballscrew or a rack and pinion. It is through this hardware that the servomotor's rotation is translated into linear machine motion and its torque converted into thrust. Backlash, friction, ballscrew wrap up, and wear are typical to these systems due to their direct mechanical linkage to the machine. These degrade through machine use and must be compensated for to maintain machine accuracy and repeatability.

While some ballscrew systems reportedly can reach accelerations up to 3 G's, the acceleration and maximum speed of a machine tool depends on factors including inertia, frictional forces, ballscrew pitch and diameter, motor torque, inertia, and maximum motor speed.

A linear motor is a non-contact device that directly produces linear thrust. There is no backlash or additional friction imparted to the machine and the system itself does not wear, promising a long operating life.

For optimum linear motor system performance, the machine tool must be both lightweight and stiff. Lightweight materials and appropriate construction methods coupled with components such as recirculating ball linear guides are essential.

A ballscrew-designed machine considered stiff may flex with linear motors installed because the magnetic forces between the linear motors' two elements can exceed the thrust by as much as 3 - 4 times.

Linear motors are touted as capable of acceleration rates into the 10 G range. Although this may be "specsmanship", by design linear motors are capable of performance levels higher than their servomotor counterparts.

Considerations:

Before choosing between a conventional and a linear motor machine tool, it's important to first project the machine's short and long-term application. Will the machine you're considering provide the continuous cutting forces required? Do its speed and acceleration satisfy your requirements, or are they overkill?

A machine tool must be designed for its intended use. While linear motor specifications are quite impressive, the motors alone will not ensure high-level machine performance. All machine elements are important. Consider, for example, linear scales, which are typically used for position feedback. The accuracy and repeatability of axis positioning is dependent on the scale's ability to count pulses at any speeds. Errors will occur if the slew rate (frequency) of the scale is exceeded. Look for the weakest link in the performance chain because that will determine your machine's performance.

If at all possible, request the machine supplier to run cutting tests on the same model of machine you intend to purchase. Impressive specifications don't make profits.

A Word About Retrofits:

Linear motor systems are generally not used in retrofits. The intended machines are typically not stiff enough, and are too heavy. This will directly affect a completed machine's accuracy and compromises the benefit of the system's speed. Substantial remachining is usually required to adapt the motors and provide proper heat transfer, without which thermal contraction and expansion will alter the machine's dimensional integrity.

The costs involved to retrofit it with a linear motor system would be prohibitive. The money would be better invested in a new machine.

Benefits:

While the price of a linear motor-fitted machine may be as much as 15% higher than a conventional CNC, there are some important benefits. Improved throughput because of higher speeds, high accuracy and repeatability, and less down time due to fewer components will definitely contribute to your profit picture.