Advancing EDM Accuracy
As users of electrical discharge machining (EDM) equipment know well, everything in the process is a tradeoff, the biggest being time (which is closely tied to money) and accuracy (the definition of a successful part). While process fundamentals remain largely the same for both wire and die-sinker EDM machining, technology providers continue developing motion control, software and additional process adjustments that are making the tradeoffs not so severe.
A brief reminder: EDM works by eroding conductive hard workpiece materials such as tungsten or tool steels either using an electrode (die sinker or cavity EDM) or a wire (wire EDM), both submerged in dielectric fluid to produce the eroding electric current. The advantages of using EDM are the ability to produce complex geometries with good surface finishes in hard-to-machine materials. Disadvantages are the process is typically slow and finer dimensions are difficult to maintain due to electrode wear.
For example, machining high-tolerance inside or outside radii on a wire EDM machine requires close control so as to not wash out the corners. Fukuoka, Japan-based EDM machine manufacturer Seibu, sold and serviced in North America by KGK International (kgki.com), has combined wire speed and vibration control together with flushing pressure and wire tension adjustments it calls the “Corner Control Circuit.”
Simply slowing down when entering the corners isn’t enough, the company says. Reducing flushing pressure and increasing wire tension work with the machine’s wire brake system to reduce wire vibration for higher-precision results. When selected (Corner Control Circuit is an on-off function), a slider function permits the operator to lengthen or shorten distance moving in or coming out of corners in both roughing or skimming passes.
Linear Shaft Motors
Most linear motors used on EDM machines are the flat-plate type—two facing plates with an air gap between them. The bottom plate is lined with permanent magnets and the top (forcer) plate is lined with iron-core magnetic coils. Applying electric current to the coils creates electromagnetic energy that reacts with the magnetic flux of the permanent magnets to pull the forcer plate along the bottom plate.
In contrast, cylindrical drive technology (CDT) available on Mitsubishi (mcmachinery.com) MV Series machines employs linear shaft motors (LSM). These motors operate on the same basic principles, but rather than flat plates, they consist of a cylindrical shaft and forcer unit fitting together like a telescope with an air gap between them. The OD of the solid interior shaft is embedded with permanent magnets, while the interior of the hollow forcer unit surrounding it is lined with electromagnetic coils. Mitsubishi EDM says this configuration facilitated easy substitution for previous machines’ ballscrew systems, with the inner shaft replacing the screw and the forcer shaft replacing the nut.
The LSMs’ cylindrical configuration ensures full use of the available magnetic flux for motion because the magnetic coils are wrapped 360 degrees around the permanent magnets. In contrast, flat-plate systems are limited to using only part of the available flux, the portion along the top side of the bottom plate, meaning flat-plate systems need more energy to generate the same amount of motion. More energy creates more heat, and counteracting that heat requires the use of multiple liquid cooling systems, increasing design complexity. LSMs require only one cooling device for the forcer to remove what little heat is generated.
Additionally, the lack of iron cores in an LSM’s forcer provides smoother motion because there is no risk of cogging due to magnetic attraction. LSMs are also less susceptible to dust or other shop contaminants because the air gap is larger and much less critical.
Compared to ballscrew drives, LSMs require no lubrication, create no friction or noise, and are not susceptible to thermal expansion. Available for retrofitting ballscrew-driven EDM machines, LSMs are also not susceptible to wear, nor do they require pitch-error or backlash compensation.
FANUC’s (available through methodsmachine.com) take on positioning accuracy and repeatability in the company’s new CiB series of Robocut EDM machines involve a moving work table. Unlike equipment with a moving column that can be subject to pitch and yaw variations, the CiB Series column and lower arm are fixed, meaning the work table and workpieces move together for better positioning. Rapid traverse of 78.74 inches per minute is more than twice the speed of previous models.
Also new is a 3D compensation feature that measures workpiece flatness and automatically tilts the wire to make it perpendicular to the workpiece surface, increasing throughput efficiency. A new 31i-WB CNC controller includes CUT LINKi 2.1 remote machine monitoring that can monitor processing efficiencies, remaining consumables life, and provide maintenance and other alerts for up to 32 FANUC wire EDM machines. Maximum workpiece size is 49.2 inches by 38.4 inches with Z-axis travel from 11.8 inches up to 20.1 inches available. Factory tests on thermal displacement compensation on CiB Series machines show 0.0001-inch repeatability on long EDM programs, meaning owners won’t be doing a slow burn on part quality.