Micro Drilling using EDM and Advantages over Other Micro Drilling Technologies

The process of drilling holes with diameters less than 0.020" is called micro drilling. Electrical discharge machining (EDM) is a contactless machining process that can be adopted as a highly efficient alternative to standard micro drilling and milling methods. During electrical discharge machining, sparks form in a dielectric field as a result of the electrical discharge between a tool electrode and conductive work piece. The sparks erode or remove materials from the work piece surface. Also called as wire erosion, die sinking, burning, spark eroding, or spark machining, EDM results in heating, melting, and vaporization of materials.

An EDM drilling machine consists of a round, hollow tool electrode, which rotates about the axis of a spindle and generates extremely hot sparks. A servo-controlled generator is usually used as the source for charging the tool electrode. Continuous discharge of sparks results in the formation of a number of microscopic pockets on the conductive work piece, which eventually creates a small hole. The hole size varies according to the diameter of the discharge electrode. Water is commonly used as the electrically conductive dielectric fluid, which helps in preventing premature sparking, maintaining temperature, and cleaning debris. In CNC EDM drilling machines, the position and depth of holes can be automatically controlled.

Applications

One of the key benefits of EDM drilling is extremely deep holes can be created on electrically conductive surfaces. In standard drilling methods, the ratio between the hole diameter and depth is significantly low (1:5 or 6). The performance of EDM machines are not constrained by the hardness of the work pieces. EDM machines are widely used in industries ranging from automotive and aerospace to manufacturing. Common applications of EDM drilling machines include:

  • Placement of very small/difficult holes that are not possible with conventional drilling
  • Creation of holes in fuel injectors and turbine blades
  • Removal of damaged drill bits
  • Addition of start holes in work pieces after heat treating
  • They are also used to create vent holes, cutting equipment coolant holes, as well as punch ejector holes
  • Creation of coolant holes in hard machine tool bits such as drills, end mills, and taps

Advantages of EDM over other Micro Drilling Technologies

Micro features can be imparted on work pieces through four types of EDM processes. These include wire EDM, sinker EDM, small hole micro EDM drilling, and ram-type EDM. Electrical discharge machining processes are best suited for handling extremely strong materials such as carbide, polycrystalline diamond, and magnet materials.

EDM micro drilling machines are generally available with electrodes as small as 50 µm in length and 5 µm in diameter. Pure tungsten and tungsten carbide are some examples of commonly used electrodes used in EDM micro drilling machines. Stereo microscope is another critical component in an EDM micro drilling machine. It is used to position the tool electrode on a work piece and to monitor the drilling process.

EDM micro drilling is limited to conductive and semi conductive materials. The process is ideal for handling both difficult-to-machine and easy-to-machine materials. Micro EDM drilling has the following advantages over other micro drilling technologies:

  • Compared with conventional milling processes, the walls of micro EDM drilled holes have less or no burrs. Burr-free EDM drilling process is especially suited for machining difficult holes (an example would be drilling holes in turbine blades). When machining speed and discharge energy of EDM machines are controlled, small holes can be drilled in work pieces with high levels of accuracy.
  • EDM machines have long needles as the tool electrode which are used for drilling deep holes in work pieces. For instance, the aspect ratio of EDM drilled structures can be as high as 10:1 (hole depth versus hole diameter). Conventional machining processes are not ideal for drilling deep micro holes.
  • EDM drilling machines can be used to drill materials such as soft copper and aluminum that produce gummy chips when machined.
  • Micro EDM drilling is a non-contact micromachining process. Therefore, work piece surfaces and discharge electrodes are free from any mechanical pressure during machining. This property is also advantageous in machining curved and jangled structures or ultra thin surfaces.
  • EDM machining process is best suited for drilling holes in hard semi conductive or conductive materials (ferroelectric materials and silicon are examples of ultra hard materials). Materials such as hardened tool steel and carbide are difficult to drill through conventional methods.
  • While drilling holes in angled or curved surfaces, drill bits of conventional drilling machines tend to break if torque conditions are not carefully controlled. In EDM drilling, there is no need for torque control since discharge electrodes never contact with work pieces.
  • The non-contact EDM drilling process is suited for drilling deep straight holes in work pieces, as opposed to conventional methods in which the drill bits tend to drift during deep hole drilling.
  • EDM micro drilling operations can be precisely controlled to drill accurate holes with diameters ranging from 5 µm to 300 µm.

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