Linear Motors On Multis

A primary objective for multi-spindle screw machines, or any metalcutting machine tool, is to accurately and rapidly deliver a cutting tool to a workpiece while precisely controlling its cutting action. There are many ways to achieve this. Traditionally in screw machines, that motion was prescribed by the design and construction of a cam and created by mechanical linkages between the cam and the slide.

Many machines use hydraulics to move tool slides into position. Using variable pressure valves and sometimes hard stops, these hydraulic slides precisely position the stroke and dwell of a tool slide without the need for cams.

Most CNC applications use a precision ballscrew connected to a servomotor on one end and the tool slide on the other to translate rotary motion of the screw into precise linear motion of the slide. Each rotation of the screw advances the tool a prescribed distance. This motion is programmable to very fine increments and executed by the machine’s computer numerical control.

In recent years, a new and promising actuation method has found its way into machine tool applications. These are linear motors, and they are being used as replacements for the ballscrew/servomotor actuation systems in some machine tools.

Linear motors are being given a look by builders because they offer performance that can exceed a traditional ballscrew and servomotor system. They eliminate several components used in ballscrew actuators, making them much simpler and more vibration-free. Primarily though, they can be significantly faster in terms of acceleration/deceleration than virtually any other actuation method. And they are programmable.

Basically a linear motor is not different from a conventional rotary electric motor or servomotor. Each has two main components: a rotor and a stator. As the names imply, the rotor rotates and the stator, which contains the motor’s windings, is stationary.

To understand a linear motor, visualize cutting open a conventional motor along its rotary axis and laying it flat. Now visualize a set of parallel guideways running on either side of the stator.

These ways, of course, carry a slide, which carries the cutting tool. On a linear motor, the slide does double duty as both tool carrier and stator for the motor. The guideways maintain a prescribed distance between the movable slide and non-movable flat stator.

When energized, electricity from the coils in the stator creates motion in the slide/rotor combination. This motion is linear because of the guides, and its stroke is as long as the stator coils.

At the EMO show in Hanover, Germany, Gildemeister Italiana (DMG-Chicago in Shaumburg, Illinois) is introducing a new line of CNC multi-spindle screw machines that employs both ballscrew and linear motor slide actuation systems. On the radial slides these are compound axes, so any position, from 1 through 5, can perform single point turning operations as well as plunge cuts. This is one of the first uses of linear motors on a screw machine.

It’s only been recently that the electro-mechanical technology that makes application of CNC and programmable servomotor actuation possible has become competitive with cam actuated systems. It’s a direct result of the increase in processing speed of the computer and the feedback speed of the servomotor loop.

However, what these systems bring to the party is a significant reduction in the time required to set up and tear down a machine from one job to the next. Programming is done in the shop, usually off-line. Setup consists basically of sending a program to the control, mounting pre-set tools on the radial and axial slide positions, taking a test cut, electronically comping the tools, if needed, and then running the job.

On Gildemeister’s new machines, a simulation of a programmed job can run in software, allowing the programmer to optimize a layout for cycle time before the first chip is made. These new CNC multi-spindles are fast and flexible, which have traditionally been mutually exclusive when describing multi-spindle screw machines.

Three new CNC models are in the company’s multi-spindle line. All are six spindle and accept maximum diameters of 20 mm, 35 mm, and 55 mm respectively.

On tool positions 1 through 3, ballscrew actuated compound slides are used in the radial positions. Since these are generally for heavier cuts, the torque supplied by the ballscrew system is more useful here.

In positions 4 and 5, the linear motors are applied to compound slides. According to the company, for semi-finish and finish operation, the lack of vibration from a spinning ballscrew allows for better surface finish and a more accurate final size. Because linear motors lack the mechanical connection of ballscrew or cam actuation systems, their torque curve is less. It is more than sufficient, in these machines, for the lighter depth of cuts on stations 4 and 5.

The axial tools are also under CNC and use ballscrews for their motion. Each is mounted on an independent slide so the tool drum is available for additional endworking tools or a workpiece support for long parts.

Each of the six spindles uses an integral motor that’s programmable. Maximum speeds are 8,000 rpm for the 20 mm, 6,000 rpm for the 35 mm and 4,000 rpm for the 55 mm. A contact ring, built into the drum carrier, maintains electric power connection to the spindle motors so the drum can index uni-directionally without the need to periodically “rewind” to clear motor connections.

As the precision parts manufacturing industry continues to feel the pressure of JIT inventory systems, shorter runs, tighter tolerances, fewer skilled setup people and the need for faster throughput CNC multi-spindles will increasingly become a viable alternative. Cam machines and other multi-spindle designs will always have a place in screw machine manufacturing, but as flexibility becomes more important, CNC multis will also be finding a place alongside them.