Slugs take flight for true EDM automation

Brass welds created by the automatic slug welding system hold slugs only at the surface and can be removed easily during finishing operations.

Shops specializing in wire electrical discharge machining (EDM) processes face a single major hurdle when it comes to configuring machines for completely unmanned, continuous operations–slug removal. Even as machine tool manufacturers transform metal cutting applications with state-of-the-art automation, operators of wire EDM machines must deal with slugs the old-fashioned way. Despite decades of effort, stopping the machine and manually removing slugs has remained the overriding norm. That is until the recent development of a slug management/removal system that incorporates the same aeronautic principles used to get airplanes off the ground.

The system, dubbed automatic slug management (ASM) by GF Machining Solutions, ensures shops can run EDMs in true unmanned, lights-out operations. This new ASM shortens cycle times and allows machines to complete both roughing and finishing operations with zero operator intervention.

Left to their own devices, slugs will fall onto the lower head of a wire EDM machine and cause a fatal crash or trigger work-stopping crash protection routines. To prevent this, CAM programs typically include tabs–usually only 0.100 in. thick–that keep the slug connected to the workpiece during cutting. Once that’s completed, operators then cut these tabs off and remove the slug by hand. On average, this task adds an extra five to six minutes to every EDM cycle and prevents unmanned operation. At best, shops will program EDMs to rough-cut parts unmanned and overnight. The next day operators remove the slugs and run the finishing passes.

Lights-out Processing

With ASM, shops can machine parts complete–both roughing and finishing passes--overnight and unattended. The system is especially well-suited for die-stamping applications and other small-cavity EDM operations. It uses Bernoulli’s principle, the same property of fluid dynamics that allows planes to fly, and GF Machining Solutions is the first company to apply it to the EDM process, an innovation presented for the first time in Hanover at EMO 2017.

The newly automated process begins with a complete roughing pass, leaving no welds or tabs behind. Instead, the machine’s Quadrex® design enables the lower head to catch the slug before it falls into the tank. A device retrofitted to the upper head then lowers and blows high-pressure air over the slug, which creates suction that pulls the slug from the part. The machine’s upper head then moves along the U and V axes to deposit the slug in a receptacle before returning to perform further operations on the part. While ASM is a somewhat slower process than its manual counterpart, that fact remains that it requires no human intervention whatsoever; manufacturers can load raw material into the machine in the evening and take fully finished parts out in the morning.

For the die-stamping die industry and other small-cavity EDM applications, shops will see benefits from the ability to run continuous machining operations, which will reduce total processing time by an average of 20 per cent.

Some Limitations

On one hand, because it essentially uses suction, the ASM system does require enough surface away from wire EDM start holes that can impede its operation. On the other hand, the system has no trouble with Z-height irregularities that often affect other suction-based slug-removal methods, but it’s naturally limited by slug weight and size. In those instances, shops can remove small slugs with a pocketing strategy that uses the EDM wire like a milling cutter to machine away the slug.

Programs involving heavy slugs and large cavities, however, such as those produced in mouldmaking operations, will continue to use such tactics as automatic slug welding (ASW). While effective, it does fall short of true uninterrupted automation despite dramatically shortening cycle times.

ASW grew from an EDM condition that most shops try to avoid. During normal operation, brass EDM wire slowly melts away and leaves behind miniscule amounts of brass buildup; excess energy, slow wire movement, or insufficient coolant can significantly increase that buildup. This buildup, in turn, reattaches the slug to the workpiece with brass.

Using Brass Accumulations

The slug welding method in GF Machining Solutions’ ASW system uses G-code commands to precisely manipulate this brass accumulation. This level of accuracy prevents top-to-bottom welds and generates 0.100-in.-thick welds that join only the surface of the workpiece and slug. The end result of the process is a slug that operators can easily knock out of the workpiece with a brass hammer, leaving behind brass that won’t interfere with a skinning pass or other finishing operation.

Unlike brass welding, the automatic slug management system removes the slug entirely without the use of tabs or welds.

Regardless of the advantages of ASW, however, it still falls short of the true automation made possible by ASM, but the ASW method does eliminate the time needed to cut slug tabs and shortens the process time to about two minutes. Unfortunately, if the wire breaks midcut, a machine without the ability to rethread inside the gap will have to restart the entire program from the beginning because of the new welds in the way. AWS also fails when presented with particularly thin sheets of material or smaller slugs, and operators must carefully select weld points.

Despite leaving the slug in the tank, as well as the other limitations, ASW still has many advantages compared to previous systems aimed at solving the slug problem. Two such early systems were a pin-based type and one that ejected slugs through a machine’s tank.

Other Removal Methods

The pin of the pin-based system inserts itself through the wire start hole and expands outward to grip the slug and pull it through the gap following the cutting program. Unfortunately, given the size of the device, it was often unable to fit through most narrow wire start holes and could securely grip only relatively heavy slugs, thus eliminating its use for small or light slugs.

The ejection-type slug management system retains tabs in the form of tiny triangular connections that hold a slug until a piston knocks it through the cavity and into a waiting receptacle. This allowed for full automation in theory, but in practice, operators had to painstakingly fine-tune the device so the slug would fall cleanly through the cavity rather than jam the machine.

ASM and ASW solve these problems and create the opportunity for fully automating many EDM processes. Future innovations will generalize these advances and further automate the process, though difficulties remain. Despite the focus on the cutting area, machine tool manufacturers know that the next step in automation involves consumable usage.

Automatic threading and rethreading are possible with current technology, for instance, but the future will deliver machines that can automatically alert operators when a machine has an insufficient supply of wire on its spool to complete a part it’s cutting to avoid production interruptions and ensure truly unmanned automated EDM part processing.

Eric Ostini is product manager at GF Machining Solutions, www.gfms.com.

GF Machining Solutions’ automatic slug management device moves under the spindle head to pull a slug from a part.