Experimental Prototypes

Electrical Discharge Machining (EDM) Tool Head

Electrical Discharge Machining, also know as Spark Erosion, would be a new tool head for the Cartesian bot that would extend its capabilities by shaping any metal without heat damage or mechanical stress to the workpiece or the tool head. EDM is a manufacturing process where a metal workpiece is shaped without the tool head actually coming into contact with the workpiece. It would extend the Darwin's capabilities by shaping any metal without stress to the workpiece or tool head. This would allow the Darwin to create some if its metal components. The process is simple to describe, but requires a precise control of voltage and current flowing through the workpiece as well as the positioning of the tool head. The metal work piece (positive polarity) is submerged into dielectric (distilled water). Then the tool head with an electrode (negative polarity) is brought close to the surface of the workpiece until a spark occurs. The spark vaporizes the metal and creates a small bubble. A small crater is left behind depending on the current which can be between 5 to 10 Amps. Industrial machines can use current up to 50A, but for a smoother finish smaller currents are preferred. The voltage is in a range of about 20 to 30 V. The metal removal rate can be 0.1 to 10 (mm³) / (min*A) depending on voltage, current and the type of material used in the tool head. A graphite electrode can be used for its thermal resistance and electrical conductivity, however other similar material can be used as well. A water pump and filter to remove impurities from the dielectric liquid.

Tool head controller boards would have to be designed to be compatible with the existing motherboard on the CNC machine. The timing of the sparks is variable and there are few modes that they can be in depending on the desired surface. Therefore, a new circuit board with a micro controller to control all these modes would have to be developed. The circuit for the spark switching is available in the literature. Further development needs careful consultation because this require software, electrical, and mechanical skills.

Literature review

Elecrical discharge machining was mainly develop as a new brach of nontraditional machining for need of parts that have very complex shape and are hard or imposible to manufacture by traditional methods. The EDM process has been very carfuly described in literature among other types of nontraditional machining. Therefore it best to rewiew this literature and extrct the relevant information for purpose of understanding the process and its effects so that the development of design for EDM tool head will be without hickups.

Existing implementation

The EDM process itself is being used mainly for dyes, molds, and parts as it can produce difficult shapes out of any metals. The wire cutting EDM is specifically used for this because it can cut away small lines through thick metal piece to produce a dye that is later used for extrusion. EDM has been implemented for making molds for casting. In addition EDM is being used in making finished parts for aerospace and automobile industry.

Development process of the EDM tool head

The EDM concept is well known and understood, but constructing a custom made tool head is a daunting task that would be of great benefit to the RepRap community. First of all, a lot of planning will be needed to find the reasonable values for the EDM process that would make it work at reasonable feed rate. Then the next step is building of the tool head from the list of parts and deciding which what materials to use. After that, testing and optimization will ensure that the EDM process is functional properly and can perform as expected. Then with the fully functional design, try to decompose the design into modules that can be simple to manufacture and if needed replace or modify. Finally, the publication of the EDM tool head to the community with cost, instructions, bill of materials, and software.

Considerations and Planing

Estimating parameter values
The most important step is to estimate and compute the values that will be required for the EDM process. These values can be derived from the literature and then applied to fit the custom version. It will be essential to find the range of values such as the voltage, current, signal function, energy, electrode material, electrode diameter, and work gap. These values will then be used it selecting the hardware which will control and regulate them.

Risk Management
Since the are high currents and charged capacitors involved, safety features will have to be implemented. One of the features could be a simple LED based display to show the status of all EDM functions and their operation status.

Rate of material removal

The right hardware need to be chosen for the required rate of material removal. The rate can vary depending on the energy in the spark and frequency, therefore the rate of removal will be limited on how much power can be delivered to the electrode.

Regulation of current, frequency, and voltage
The EDM process can produce variety of surface finishes by regulating the current it each spark. Higher current produces larger craters, so by regulating the current, different roughness surface finish can be achieved. However with smaller craters the rate of removal will be slow down so to keep it up the frequency will have to increase. Regulating the voltage is also important as it controls the gap between the electrode and the work piece. The voltage necessary to create spark depends on type of dielectric. Stronger dielectric will require higher voltage.

Power supply
Since the EDM is basically evaporating the metal, a lot of energy will be required. The right power supply must be chosen to deliver uninterrupted flow of energy to the hardware. Also, the power supply will be draining electricity from a wall outlet so wattage should be limited.

Software interference design
New software will be have to developed to control the impulse and the motion of the Cartesian bot. Unlike the extrusion, the EMD is removing the material. In addition the tool head itself need avoid obstacles as the depth will be increasing so proper coding will need to include the dimension of the tool head. The frequency of the signal for EMD process will vary with the stage, therefore the program will need to distinguish between the surface and the volume that need to be removed. Discussion should be made on what can be coded into the program and what kind of input will be required from the user. The new program should have user friendly interference with easy to understand settings since it will later given to the community.

Building EDM tool head

Once all the data are gathered, it will be easier to plan for parts and materials needed. After that, the assembly of the EDM tool head with all it components can start. During this process documentation of the assembly should be recorded for instructions given to the community.

Planning the build process
After the decision hade been made on the type of current, voltage, and frequency the EMD will operate, the selection of electronic parts and hardware should be compiled into bill of materials. Alternatives should be pointed out so that the design would have different versions.

Manufacturing and buying parts
Since the electronic components will have to be bought, the final cost of the EMD tool head will vary depending on the non-electronic parts used. Therefore all the parts that can be manufactured should be manufactured to reduce the cost. Alternatively, if the parts are too difficult to manufacture then the option of buying the parts should be considered.

Once all the components have been manufactured, the EDM tool head assembly must be documented for future reference. Assembly steps should be simple in order to reduce the difficulty in understanding the process. Any problems experienced during the assembly should be noted and solutions carefully described.

Testing and Adjusting

The expectation of the EDM process will be tested for successful design. The tests that will be conducted would show the capabilities of the design and any problems and weaknesses should be solved and adjusted for.

Rate of removal

Testing will determine the possible rates of material removal which would predict the build time. The speed at which EDM can operate depends on the spark energy and frequency, but this design is limited by non industrial environment. However despite its limitations, the EDM design can still be reasonable alternative for manufacturing parts.

Surface finish
Another set of test would determine the possible surfaces that the EDM tool head can produce. These tests would reduce the current in the spark so that smaller craters would be produced, resulting in smother surface finish. This would determine if the parts produced this way would still need any further processing

Depth of removal
An important factor in determining the possible build volume is the depth that the EDM tool head can go to. For example, the EDM process is known to not work properly when drilling deep holes because the dielectric fluid is not properly filtered resulting in uneven spark erosion. Also the walls of the work piece are interfering with electrical discharge. Therefore it necessary to determine the shapes that are difficult for this EDM design to manufacture.

Different materials
In principal the EDM can process any type of metal and does not depend on the strength of the metal. However to give some specifications for the design on how long will the process take, common metals will have to be tested. The data gained from these tests will them be used for the software coding since the rate of removal may vary.

Publication of the final EDM tool head design

After a successful completion of the whole design, all the documentation should be compiled into a paper for the community. The paper should be composed of the cost estimate, bill of materials, software used, and instruction for building EDM tool head. The cost estimate should include the possible alternatives for the components used along with a detailed bill of materials showing all of the necessary parts and the cost of each unit. If a new code or a software is developed it should be also documented in the paper. Finally, the instructions for assembling the EMD tool head should be described as a step by step process.