When a wire EDM operation is initialised, achieving accurate results in sharp corners or tight radii depends on a multitude of factors. These include wire type, diameter, tension, and feed conditions; workpiece material and thickness; dielectric type, flow, and temperature; and spark power and duration.
Traditionally, fine-tuning an EDM process required cutting a test part and then removing it from the machine for measurement on a CMM. After remounting the part on the machine, the operator consulted the CMM data to note deviations from the desired part dimension and made changes in the machining parameters with the hope of achieving an acceptable part.
A newer, and more reliable and faster approach uses a closed-loop vision system to measure the part and make in-process corrections.
While the part remains in the machine, the system’s camera scans the part contours and compares them to the part’s DXF CAD file. If the part is out of specification, the system employs what is called a ‘contour modifier’ that directs the machine control to correct the EDM path to produce a part within 2.5 µm of the required dimensions.
A vision system can also facilitate with the quick location of a part before machining begins, saving the time and wire invested in the traditional part location method that uses the wire itself to locate the part on the machine.
Control of the wire’s path when producing inside and outside corner radii is another important element of EDM process accuracy. When the EDM wire rounds a corner, it can act somewhat like an articulated lorry making a turn -where the front wheels track perfectly but the rear of the trailer swings wide.
While EDM wire guides hold the wire’s position above and below a workpiece, the wire’s middle section between the guides may bulge or swing wide during the cutting operation.
Advanced EDM machines and software provide methods of anticipating and eliminating such a swing, thus reducing the risk of exaggerated corner paths.
Twenty years ago, controlling corner machining required manually updating the CAM program to reduce power as the wire entered a corner. Reduced power/spark intensity then permitted the application of greater wire tension to eliminate swing-out and increase accuracy without causing the wire to break.
Today’s EDM software packages have a look ahead facility and can change machining parameters in anticipation of upcoming part features such as corners and radii. On some machines, for example, the software automatically reduces the intensity of the spark to prevent over-cutting on curves. However, spark intensity not only influences metal removal rates, it also dictates required offset amounts. When power settings lower, the offsets must change accordingly.
In many cases, however, decreasing power and also the flushing volume will increase the heat of the operation and keep material removal rates steady, eliminating the need to adjust the offset. For this reason the most common form of corner control involves pairing reduced power with adaptive flushing.
On a machine with a vision-based in-process measuring system, corner control can consist of reversing the wire path after traversing a corner and backtracking to complete the desired amount of metal removal. The reverse pass will be faster than the initial pass because a minimal amount of material is being removed.
Because the lower power employed in corner control slows the overall machining process, shops often opt to turn automatic corner control off when the slower cutting speeds are impractical. For example, implementing corner control when rough machining the multiple teeth of a gear generally consumes an unacceptable amount of time. An operator might choose to rough the gear shapes at higher cutting speeds and then return for cleanup passes that can run more quickly.
Another factor affecting the accuracy of EDM operations in terms of corners, radii and angles is the thermal condition of the cutting environment and machine. Wire guides set up first thing in the morning when a shop is 68 degrees F will perform much differently in the afternoon when that same shop is 78 degrees F and the machine is thoroughly warmed up. A typical negative effect of inconsistent thermal conditions is cone-shaped radii.
As a solution, some machines are thermally stabilised. The control system minimises thermal variations with the use of temperature-stable dielectric to cool the casting of the machine in conjunction with the machine’s X, Y, U, V, and Z glass scales.