Are You Using Swing Machining on CNC EDM Machines Correctly?

On a CNC die sinker, a straight Z plunge removes material only under the electrode face. The side gap stays narrow, and the dielectric has to push eroded particles up through that gap. As depth grows past roughly two to three times the gap width, the flow stalls and you get arcing, taper and burnt corners.

Swing machining adds a horizontal orbit to the servo path, usually programmed as a circular, square or vector pattern with a set amplitude. The electrode still feeds down, but the orbit widens the effective cutting envelope. That extra width lets fresh dielectric reach the bottom of the cavity and carry sludge out along the sides.

Two numbers describe the orbit. Amplitude is how far the electrode center travels from the cavity center at the widest point. Offset is the programmed shift between electrode and finished wall. Operators often confuse them, then wonder why the cavity comes out oversize.

The servo reacts faster than most people expect. When the orbit sweeps, average gap voltage rises because part of the cycle is cutting air or partly flushed material. The control sees this and advances the Z axis. Set the amplitude too high and the machine feeds down hard, then stalls at the corners.

Eroded debris is the main reason a die sinker stalls. Particles are conductive. Once they collect in the gap, they bridge the electrode and the workpiece, and the next discharge happens through that bridge instead of through the dielectric. The result is a pit, a black spot or a broken corner.

An orbit gives those particles a way out. As the electrode sweeps sideways, the gap opens on one side and closes on the other. Dielectric flows into the widening side and pushes sludge toward the opening. On a 40 mm deep cavity in a hardened tool steel insert, that alone can cut the number of retract cycles in half.

Thin ribs and slots behave differently. A rib that is 0.8 mm wide with a 0.15 mm side gap leaves very little room for orbit. Push the amplitude too far and the electrode bends or the rib washes out at the tip. Many shops keep the orbit tight on ribs and rely on a jump cycle instead.

Deep pockets with a small radius at the bottom are the hardest case. The orbit has to be small enough to fit that radius but large enough to flush. On our own die work we usually start at 0.02–0.05 mm amplitude and raise it in steps once the first few millimeters are cut.

Undersize is calculated before the electrode is cut, not after. A roughing electrode needs to be smaller than the finished cavity by twice the total gap, and that gap is the sum of the spark gap and the orbit amplitude. Get the arithmetic wrong and the finishing electrode cannot clean up the wall without cutting into tolerance.

A practical order is: fix the finish cavity size, pick the finishing orbit from the required surface finish, then subtract the spark gap. The roughing electrode is then built from the roughing orbit, which is usually two to three times larger. On a cavity with a ±0.01 mm wall tolerance, keeping these numbers separate matters more than any single setting.

Watch the corner radius. A circular orbit rounds inside corners by the amplitude value. If the drawing calls for a sharp 0.3 mm corner and you run a 0.1 mm circular orbit, the corner will never clean up. Vector or square orbits reduce this effect, but they load the servo differently and can chatter on tall electrodes.

Amplitude is not a constant. It is a starting value you trim by depth. Shallow cuts tolerate more orbit because the electrode is short and stiff. Past about 50 mm of electrode length, reduce the amplitude and add a jump cycle. Copper tungsten holds its shape better than graphite here, which is why it still gets used on deep, fine work.

Orbiting is not free. Every sweep adds a small amount of side wear to the electrode, and that wear shows up as taper on the wall. On a part with a tight straightness call over 100 mm of depth, a straight plunge with a good jump cycle can hold the wall better than a wide orbit.

Sharp internal corners are another limit. A circular orbit cannot produce a corner tighter than its own radius. If the drawing shows a true sharp corner, the electrode must be dressed to that shape and run with a very small orbit, or the corner is finished by milling before heat treatment.

Graphite wears in a different way than copper. Under a long orbit, graphite tends to lose its corners first, so the cavity gets rounder as the electrode wears. Copper and copper tungsten hold corners longer but cost more to machine. For a short run of five cavities, the difference rarely pays back.

Small features below about 0.2 mm across are usually better served by wire EDM or by a dedicated micro-hole machine. The electrode is too fragile to orbit, and the flushing benefit never arrives because the gap is already at the minimum the dielectric can pass.