Linear Motor Brings Accuracy to EDMing of Deep Ribs

A ram electrical discharge machine may not be the place most observers would expect to see a high-speed linear motor. Linear motors seem more appropriate for the axis drives of milling machines, where high feed rates complement high spindle speeds in high performance machining. But high performance is a goal for EDM builders as well. Hence, Sodick Inc. (Buffalo Grove, Illinois) has introduced linear motors on the Z axis of its new AM35L and AM55L ram machines. These are medium-sized machines with work tanks 29.5 by 21.6 by 12.6 inches and 37.4 by 28.5 by 16.1 inches respectively.

According to the company, linear motors enhance both the speed and accuracy of the EDM process. Where these machines have been displayed at trade shows, the rapid motion of the Z axis (vertical travel of the ram) is obvious. At WESTEC in Los Angeles, California, for example, a blade-like electrode could be seen plunging in and out of a cavity up to eight times faster than on nonlinear models. This in-and-out motion had the apparent effect of vigorously churning the dielectric fluid, effectively sweeping out the chips and ash produced by the EDM process.

"In the past, flushing was a requirement to EDM ribbed cavities of more than 1 1/2 inches," says P J. Naughton, Sodick's Marketing Manager. "The linear motor delivers faster jumps and reacts instantly to minute spark gap changes to eliminate auxiliary pressure flushing in extremely complex dies and molds. This motor provides ultra-fast response time at speeds of 1,440 ipm with four millionths resolution," he adds. "A new motion controller has replaced the driver used to control more traditional motors. The linear motor has fewer parts to wear out and won't slow down over time like most other motors. It also has less vibration and more rigidity for smooth, quiet and reliable EDM production."

Unlike traditional motors, Sodick's linear motor operates without ball screws, couplings and other mechanical parts, thus eliminating backlash and lost motion inherent in these designs. That means better speed, acceleration and torque for faster, more accurate metal removal and finishing, the company contends.

The new models use the linear motor in the Z axis. The Z-axis column is made of ceramic material, giving it light weight, high rigidity, and less thermal distortion than non-ceramic structures. The magnets are installed on both sides of the ceramic column. The magnetic force on the coil side ensures smooth motion of the Z axis while preventing distortion.

The outer covering of the Z-axis column incorporates the coil unit, where the heat characteristic of linear motors is generated. Coolant is piped into the coil to counteract excessive heat buildup. The machine uses a built-in air cylinder rather than a counterweight to balance the weight of the movable ram. The air cylinder maintains this balance even when the unit is powered down and a clamping mechanism automatically locks the ram's position to prevent the Z-axis unit from dropping.

Effective evacuation of chips and enhanced motion accuracy translate into faster metal removal. In one test, which the company says is typical, a conventional EDM cut a 40 mm deep cavity in 1 hour, 10 minutes while a linear motor EDM cut a cavity 70 mm deep in 2 hours, 15 minutes without auxiliary flushing.

On these new machines, Sodick's Mark 30 CNC handles virtually all of the decision making without operator input once the workpiece is set up. The metal removal and finishing process is based on a set of algorithms that recognizes machining conditions and overcomes poor conditions by automatically adjusting the linear motor power settings.

The control adjusts conditions including on and off times, current intensity and linear servo intensity of the EDM burn with orbital motion and computer controlled pulsing. If needed, an interactive screen allows operators to monitor and modify all machining parameters during the rough and finish cuts.

Sodick apparently has further plans for linear motors on EDM units. At the recent EMO show in Paris, the company was displaying the AQ35L, a model with linear motors on all three axes.