Programming for ‘life on the small wire’

The smallness and complexity of wire- EDMed components and component features are driving demand for thinner wire. That’s how 0.008"- and 0.010"-dia. wires became the “popular kids.” Then shops downsized to 0.002" and 0.004" wires.

The author operating a 0.0008"-capable wire EDM at Top Tool Co. Image courtesy Top Tool.

Today, with miniaturization driving the need to tame challenging 0.0008" wires, micro EDMing is often only as good as its programming. With significantly less margin for error, and the higher cost of small-wire time, normal (large) wire strategies are not sufficient.

Life on the 0.0008" wire would have been inconceivable just a few years ago. Today, it’s standard in micromanufacturing operations thanks, in part, to improved power supplies, higher-quality wire and more powerful processors. In our business of stamping microparts, when someone is skeptical that it’s even possible to stamp “something that small and complex,” a significant part of our explanation focuses on what the evolution of small-wire EDMing has added to engineering and building microstamping tools—such as the ability to maneuver through a workpiece having a 25:1 material-thickness-to-thinnest-feature ratio.

The NC programming that allows a 0.0008" wire to perform to expectations starts with the orchestration of just three G-code commands: proceed straight, clockwise or counterclockwise. Of course, there are also advanced algorithms and high-impact CAD/CAM capabilities that make routines more intelligent in terms of an EDM programmer being able to capture original design intent. But another element critical to small-wire programming is on-the-part experience, otherwise known as learning from trial, error and retrial. That includes, as someone once said, “the death of a few good parts.”

The following are suggestions for “balancing on the small wire,” culled from programming experience at Top Tool Co.

See what the wire will see. Get a step ahead by assembling, before an EDM operation begins, your best theory on what will occur during the process. What did the outcome from your last application tell you? You can’t always be certain what caused a failure, but experience gradually builds a preroutine menu of indicators, variables and scenarios that help anticipate what the wire and machine will encounter—and what the results will be.

The stress characteristics and grain direction of the workpiece material come into play, as well as indicators such as the appropriate power setting for the required detail. In addition, be sure you know what the highest pressure setting possible is without bending the part, and if a radius dimension needs to change to avoid having the wire cut off the corner.

Determine correct direction. You chase material stress when wire EDMing. The end point of the cut is where the stress will be. Cutting releases energy, which can cause the part to move. So, the best direction for the wire path is the one that best anticipates and manages that stress. You want to start and stop EDMing on the least-critical workpiece feature.

Stress energy stored in the material is the product of multiple factors, including the material type and grade, casting method, grain direction and previous machining or heat treating. Wire-path direction also depends on workpiece geometry. For example, when EDMing a hook shape, it might be best to cut the outside of the feature first to make the stress-releasing process work with you, not against you. Even the workholding arrangement can impact the side you start on, and which wire-path direction to take.

Program to be slug-free. A wire EDM is just one dropped slug (a remnant cut from the main workpiece) from a machine crash requiring an expensive repair. And, yes, even slugs from small-wire projects can be catastrophic. Slugs also affect production efficiency and cost, because an operator must stand by, stop production and manually remove the slug. Coreless programming, also called “pocketing,” doesn’t create slugs. This strategy also enables unattended machining.

Pocketing essentially turns material into dust by burning the desired shape a very small amount at a time. Predictably, a micropart with complex geometries increases the difficulty when programming to erode material without creating slugs. CAD/CAM programming systems typically offer preprogrammed, coreless macros, but it’s possible you’ll need to program a custom pocketing routine. In that case, predetermined standards—such as when to pocket and when not to—streamline the process.

Always simulate, but know the limits of the process. One of the key questions when EDMing is whether the machine will do what you want it to do if it uses the numbers you input. Offline simulation—a programmed dry run—is the first line of defense in preventing scrap. It’s a useful tool and should be an automatic step for any shop that EDMs.

CAM-program-based simulation software for wire EDMing has improved dramatically. It uses the offsets in a program and signals an alarm if the program is flawed.

However, even with improved software, a simulation only checks the operation and verifies the computer path. It can’t overcome “garbage in/garbage out.” And it can’t correct a bad file, such as a geometry that is incorrect in size and scale or the wrong power setting.

Personal discipline is key. Finally, programming wire EDMs, and small-wire machines in particular, is a lesson in patience and intestinal fortitude. Expect to program and reprogram numerous times before finding the optimal toolpath, especially on complex microparts. Optimization requires “wire-path forensics” and deductive reasoning.

Execute your theory, cut the sample part and then determine how to compensate for what you observed. An inside corner not sharp enough? You may need to go back and create a dwell that keeps the wire there as briefly as three-thousandths of a second longer. The machine couldn’t achieve what the geometry asked it to do? Change the 0.0002" radius to 0.0001", then repeat the process. Eliminate errors until none remain.

That is the key to living well on the small wire. µ