Small-hole EDM

The electrical discharge machine (EDM) process lends itself to small-hole creation for several reasons. By precisely controlling the spark gap, the area between the electrode and the workpiece, and by reducing the amount of energy sent to the electrode, an EDM can create fine, controllable sparks.

With such sparks the amount of material being eroded also can be reduced, creating smaller holes.

EDMs currently being sold for fine-hole drilling produce a round hole that is accurate in placement, size, cylindricity, and in reference to the entry and exit points of the hole.

“The accuracy of the machine is paramount,” explained Jeff Kiszonas, EDM product manager for Makino. “If you don’t have an accurate machine tool, tooling, and workholding, you won’t get the desired results.”

This time-consuming process is best-suited for parts that cannot be created by another means. Drilling and laser cutting can be considered, but each also has its drawbacks.

“Anytime you have a mechanical drilling process it leaves mechanical distortion in the hole, especially at the entrance and exit,” said Kiszonas.

Bulging of the material at the exit of the hole can occur during a traditional, mechanical drilling process because of the physical forces involved in the drilling process. During this work, the cutting drill is literally moving the material.

“Some materials, such as certain stainless steels and brass, will move significantly when you try to machine them,” said Kiszonas. “This is one drawback of the physical cutting process.”

In smaller workpieces these deformations become more apparent.

While a laser cutting system can produce holes very quickly, it doesn’t do a good job in a couple of areas, according to Kiszonas.

“It doesn’t leave a truly round hole in many cases unless you have a very high-end laser system,” he said. “They are getting better in producing a true shape, but the average laser just isn’t there yet.”

Lasers use high-intensity heat to blast molten metal through the hole. This melting process can, however, also leave slag that must be removed in a secondary operation.

Like the laser, EDM uses heat but in a more controlled manner.

With EDM no physical stress is put on the part during the creation of the hole, and there is no measurable HAZ in certain materials.

“We are able to produce holes that are round within millionths, accurate from position to position within millionths, and have surface finishes that far exceed what you will see in other processes,” said Kiszonas.

Makino’s Edge2 sinker EDM has a fine-hole option and uses dielectric oil during the machining process, which allows users to have very finite control of the spark gap. This can create holes with diameters ranging from as small as 11 microns to 0.120 in.

“If you are creating diameters larger than that, you don’t really need the fine-hole option;you can use our standard sinker,” said Kiszonas.

Depth of Cut

The depth to which a hole can be created depends on the diameter of the hole and the type of electrode being used.

For example, a tube-style electrode that is 0.003 in. diameter will have a 0.001 in. channel running through it. Pumping flushing fluid through this type of tubing can create holes to depths of 25:1 to 50:1, depending on diameter.

“If it has to be an extremely accurate hole, we take a precision tube or rod electrode and spin it as fast as we can on the machine,” said Kiszonas. “As we spin the rod, it gives us a more concentric hole and also helps with chip evacuation, which helps with cylindricity of the hole.”

The Electrodes

Brass or copper electrodes with the proper finish hole diameter are used to create holes on an EDM.

“Electrodes that are hollow allow a high-pressure coolant to be pumped through them,” said Terry Meyer, applications manager for Sodick. “The electrode is charged by the EDM power supply and is fed forward under a servo control to regulate the machining feed rate. The electrode is held in location by a guide, which sits directly above the workpiece.”

Drill bits can have a tendency to “wander” as they strike the workpiece, especially if they are drilling into a sloped or hardened part. With the electrode of an EDM machine, however, this is not the case.

The rotating spindle holds the electrode in place, and a guide that sits above the workpiece ensures the correct location is held. As the electrode plunges into the material, the high-pressure flush surrounds the electrode as it drills so the hole that is being created essentially becomes an extension of the guide.

“This high-pressure flush gives you a lot more pressure than a standard sinker EDM,” said Meyer. “The advantage of the higher pressure is that the eroded material is flushed from the hole enabling deeper holes to be made.”

You can drill deeper and maintain the flush because of this higher pressure.

If flushing is poor, however, material can build up and will begin acting like part of the electrode. This will attract sparks that can arc across the dielectric fluid and reproduce their shape in the workpiece. This short-circuit, or DC arc, causes pits to be created in the workpiece and is a common side effect of poor flushing conditions.

For more information, visit www.makino.com and www.sodick.com.

Q: When should I maintain my dielectric fluid?

A: Daily. The conductivity level of the fluid must be constant to deliver the best results.

Q: When should I change my dielectric fluid?

A: A good indication is when one or more of the following occur:

  1. The machining time of a previous known part increases substantially.
  2. Pitting starts to occur.
  3. Short circuits start to interrupt machine movement.

Q: What if my dielectric fluid does not come out of the tube electrode?

A: Check the following:

  1. Make sure the tube electrode is in the collet and rubber grommet to allow flow.
  2. Change tubes to ensure that debris has not clogged inside the tube.
  3. Verify that the clamping pressure is not collapsing the tube.
  4. Check to see if the pump has lost its prime.