Last updated March 20, 2005

We decided to use the PIC16F877 Microcontroller because of its popularity, ease of use and it has the characteristics for design completeness with a minimum of external components. The 4 MHz chip mostly operates at one instruction per microsecond, with the exception of branch instructions which are designed to allow isochronous operation and operate at two. The unit has a built in Analog to Digital (A/D) converter which has a high conversion speed to handle the performance requirements, and maintain continuous sparks. It has enough I-O lines to handle simultaneous the LCD display (12), Stepper Motor (2), Push Buttons (4), A/D (1), and status LED (1).

Theory of Operation

EDM is a very different method of machining. It can be used to machine materials that are too hard for conventional machining, and can do things that are difficult for conventional machining such as make square inside corners. One limitation is that only electrically conductive materials can be machined. In its basic form and EDM consists of an electrically isolated head called a ram. The ram moves up and down and holds a cutting device called an Electrode. Ram electrodes are typically made out of Copper or Graphite and are manufactured to the shape of the material to be cut away. EDM cuts by spark erosion. The spark has a typical temperature of about 10,000 degrees centigrade. This basically vaporizes the work piece, and then the EDM oil solidifies the vapor and carries it away by means of flushing. Flushing can be accomplished by several means. Forcing EDM oil through the electrode or on the sides of the work piece is one method. Another method is to agitate the EDM ram or the work piece. The spark also causes the debris to moved away. Typically the Electrode and work piece are immersed in an oil that has characteristics suitable for EDM operation. These fluids are called EDM oils. Starting fires is a constant problem with EDM that use flammable oils, and oils that have high flashpoints are desirable. EDM oils that are non-toxic create a safer workplace and less employee sick leave. The electrode has to be precisely placed over the work piece, not only in the X and Y axis, but in the Z axis as well. The Z axis has to be positionable in the 10 thousandths of an inch range to control the spark, especially for fine surface finishing operations where the EDM electrode has to be very close to the work piece. The EDM works by generating a high frequency DC pulse, which is adjustable in frequency to provide optimal operation of no wear cycles, finishing, and difficult material erosion. In some cases agitation of the X and Y axes, aid in the improving the surface finish and in flushing operations. To optimize material removal, Microprocessors are used to do smart operations. The control unit has at minimum a need to detect if the EDM is sparking and to detect a voltage short where the head has come into physical contact with the work piece. It also needs to be able to move the ram up and down. A debris removal cycle sometimes needs to be done for particle removal especially in deep cavity sinking where flushing gets stuck and can not get the machined away particles out of the cavity. Another Feature of the control unit is to allow in-process adjustment of parameters.

Features of our EDM

In addition to basic features, our EDM has some routines which are required for smooth operation. Changeable Interrupts, depending on the routine were needed to mesh with the LCD display and other internal operations. Switch debounce routines are required to avoid false inputs. Constant monitoring of feedback voltage on all downward motions was an invaluable help in the Home setup, Zero Positioning, and Erosion to prevent head/work piece collisions which would cause and loss of position since the stepper motor is open loop. Another feature is the ability of the EDM controller to save operational parameters such as motor speed, short voltage, and spark voltage.