Wire-cut eroder gives PDL productivity gains

Press, 15 June 1983, Page 29

Wire-cut eroder gives PDL productivity gains

By

NEILL BIRSS

PDL Industries, Ltd, of Christchurch, is achieving productivity gains of about 40 per cent from a technology rapidly gaining ground in New Zealand: wire-cut spark erosion. It arises from sink erosion, developed in the 19505. Both are electro-erosion technology — cutting by an electrical discharge, or spark. Sparks are generated between the tool (the electrode) and the workpiece. A special power supply generates a rapid succession of 'current pulses, which can be adjusted to suit the type of work and the metal being eroded. Each spark melts, vaporises, and removes. a tiny

part of the workpiece. An insulating and cooling fluid (the dielectric) surrounding the workpiece and the tool carries away the eroded particles from the spark gap. The fluid also acts as acoolant, preventing oxidation of the metal. The technology is used mainly in machine-tool manufacture. It allows the cutting of intricate dies for - the extrusion of metals and plastics. In old technology, for some patterns, the die would have had to be milled in soft metal, hand-finished with files, the steel hardened and then polished. Electro-erosion, or elec-tro-discharge machining, cuts steel already hardened, and with much less labour required. Sink eroders can be controlled by computers in a similar way to other CNC machines. In this manner they are in some cases able to do intricate cutting, such as putting a thread inside a hole.

PDL has a total of six sink eroders in its three toolrooms. In the Hazeldeah Road (Christchurch) toolroom it has two, one of them numerically controlled (CNC).

The CNC sink eroder can cut in three axes, and servo controls on two-dimensional table movement allow it to position itself automatically. This allows lateral surface erosion, in which the workpiece shape is enlarged by the table movement. In nonCNC erosion, a new electrode has to be cut for each machining step. This is because the erosion exactly reflects the shape of the tool’s electrode. The advent of wire eroders, first commercially available in the 19705, has given a lift to the technology, though still fewer than a dozen are in use in NewZealand.

These machines use a spool of brass wire as the- electrode; and the wire is guided along a pre-programmed track. Between the wire and the workpiece sparks are generated in an insulating fluid, so eroding the workpiece in much the same manner as in sink erosion. A table moves the workpiece in relation to the wire, following the program that has been fed into the machine. In vertical cutting, a parallel surface is produced, using X and Y coordinates. By using two further co-ordinates for the movements of the upper guides of the wire it is possible to cut tapers. PDL uses the machine to full advantage in making metal press tools and injectionmoulding tools.

Wire-cut erosion is relatively fast, and allows the making of extremely complex shapes. Hardened steel is no more of a problem to it than butter is to a hot table knife.

The PDL engineers program the machine on a Hewlett-Packard microcomputer by keying in the co-ordinates in a point, line, and circle configuration, or by drawing the plan on a plotter unit and digitising

the points for the computer to interpret and calculate the co-ordinates itself. This information is stored on tape or diskette for reuse if needed. The wire eroder’s precision is impressive: it works to accuracies of a micron when using wire of only 0.05 mm. It has flexibility, too, once the program has been entered in the machine; it can be orientated through 350 deg., or even mirror imaged. The wire eroder at the Hazeldean Road factory stands in an air-conditioned room with the two sink eroders. Together, the

machines represent an investment of $BOO,OOO. Maintenance is negligible, the machines requiring only to be set up for each job: they are then left to run by themselves.

Seiffert Kendall Engineering, Ltd, of Pakuranga, is the New Zealand agent for wire spark-erosion machines, and also uses them itself.

Mr J. B. Kendall, a director of the firm, said yesterday that applications included the making of machine tools for a Warkworth saw-blade exporter and the making of oddshaped parts for long-out-of-production cars. Recently Seiffert Kendall wire spark-eroded gears for a Kuwait airline so that it could convert its DCIO flight simulator for Boeing 747 simulation.

There was virtually no shape the process could not produce in two dimensions, ’’and now we’re working on three dimensions,” Mr Kendall said.

Wire cut spark erosion has also been used for Air New Zealand to simulate metal defects so that test equipment can be calibrated.

A spokesman for Air New Zealand’s engineering base at Auckland said that the finest such simulations had been obtained from an American firm, though electroerosion simulations were also made in New Zealand.

In the American case, the “defects” eroded were 0.8 to

0.13 mm wide, 0.76 mm long, and 0.25 mm deep. Examples of defects needed to be obtained in their very early stages so that test equipment could be calibrated. It was when the very early examples could

not be obtained that spark erosion was used to simulate them.

The spokesman said he believed that spark erosion was also used in the manufacture of some aircraft engine parts.