EDM drilling picks up speed

Modern Machine Shop, June, 1991 by J.L.C. Wijers

Drilling is not just one of mankind's earliest machining activities. Today, it is also the most prevalent by a wide margin. New material developments, on-going miniaturization, a need for ever greater length-to-diameter ratios, the growing need for small-diameter holes, and the wellknown limitations of mechanical drilling have resulted in a continuous search for alternative drilling methods.

Mechanical drilling with the common twist, spade, or similar drills is running up against three problems. As workpiece materials get harder and tougher, the difference between the drill and workpiece becomes smaller, thus, making the operation more difficult. Except for gundrilling, the length-to-diameter ratios are practically limited to about 5 to I at most, with any degree of accuracy. And gun drills can't produce holes a few thousandths of an inch in diameter. Also, the common twist drill, with its spiral flute, is inherently weak, which makes it unsuitable for small diameter holes or those with larger length-to-diameter ratios.

It was the aerospace industry's need to drill small and deep holes in nickelbased and titanium alloys that led to a search for alternative drilling methods. Lasers, electron-beam methods, ultrasonic techniques, and several electro-chemical processes have all been used with some degree of success (Table 1).

Since its development, EDM has been used for hole drilling, even though penetration rates were quite slow. Its main advantage as a hole-drilling method was the ability to accurately drill holes in surfaces that were slanted or spherical. This is possible because there is no direct contact between the EDM electrode and the workpiece to generate mechanical forces. The alternative drilling methods, including EDM, account for about five percent of current hole-making production.

EDM's Growing Role

Although EDM first became recognized as an excellent alternative method of producing 3D cavities for mold and die work, many of the very early applications were devoted to hole drilling. Not only did EDM work well on non-flat surfaces, but material hardness ceased to be a factor. The EDM erosion spark doesn't care whether the part is made of soft 1012 steel or hardened carbide. The only requirement is electrical conductivity in the workpiece.

EDM has been remarkably improved over the last decade. Numerically controlled wire cutting that generates complex profile shapes is now enjoying an explosive growth for tools and dies and multiple-part production work. Workpiece removal rates have shown dramatic increases, although the rate that EDM removes workpiece material still cannot compare with conventional chip generating methods such as mechanical milling and drilling.

Much of EDM's increased performance can be traced to improved power supplies and CNCs (computer numerical controls). This has led to improved stock removal rates, finer finishes, the ability to generate contours and undercuts even with ram-type EDM units, automatic tool (electrode) changing, and the ability to work long hours (even over a weekend) untended. Adaptive control is on the horizon, whereby the user will program factors such as depth, material to be machined, and the final surface to be achieved. The control unit and power supply will then adapt themselves to maximize removal rates for roughing work and cut back for the final finishing.

With all its advantages, EDM basically is still a slow machining process. There are not many standard electrode shapes available (most are custom-machined). EDM technology is still rapidly developing; it is limited to electrically conductive materials, and electrode wear still is not always predictable.

On the other hand, EDM can do most 3D die and mold sinking operations with a single axis movement. Workpiece hardness is not a limiting factor in successful EDM. The process can do both internal and external machining. With EDM, there is no mechanical contact between the tool and workpiece. A broad range of surface conditions, from very rough to almost mirror smooth, can be achieved with EDM. The process offers good repeatability, and it is totally buff-free.

Drilling With EDM

EDM originally was used as the machining method of last resort. When nothing else worked, the EDM unit was called in. This certainly was true for drilling. Deep holes or holes in very hard materials were turned over to the EDM unit even though the machining rate was slow. With those early units, it was not possible to rotate the electrode to aid in dielectric flushing and smooth out electrode wear that occurs mainly on the front face. Dielectric flushing is essential to remove the detritus generated by the process.

Flushing in the small gap, which may be only a "thousandth" or two on the side, becomes increasingly difficult as the electrode goes deeper and deeper into the hole. However, holes do not have to be round. They can be square or any other shape for which an electrode can be fabricated.

A significant leap forward in roundhole drilling with EDM was made when the ability to rotate the electrode became available. Even if there is no dielectric fluid hole in the center of the rod, rotation greatly aids in the flushing action and in balancing out electrode wear. This has led to even more accurate holes and fewer form defaults as the rotation tends to center the process and even out all disparities.