How Testing Was Conducted
EDM Test Equipment
The EDM machine used in this testing program is a state-of-the-art CNC machine. Although the machine was equipped with optimizer controls, they were disabled for this test program. The orbiting feature was only activated for fine finish tests. The machine was equipped with MV (mean voltage — finish machining) and EP (efficiency pulse — high speed machining) servos.
Note: Far machines without these parameters, follow the on-time/off-time parameters.
Electrodes were machined into a standard shape, 1" x 1" x 2" with a 0.220" diameter flush hole lengthwise through the electrode for cuts up to 55 amps. Electrodes for cuts above 55 amps were machined 1.5" x 1.5" x 1.5" with a 0.220" diameter flush hole.
Flat-ground rectangular plates were selected as workpieces for economy of cost and storage; they were thick enough to make test cuts on both sides. Data sheets were designed to systematically document the data that was to be generated.
Figure 1. Test electrode.
In order to obtain general overall performance characteristics for each work metal/electrode material combination the following procedure was followed. During a test series the preset peak current was held constant with machining taking place typically at four on-time settings with replications at each on-time setting. In each test, the off-time was adjusted to a minimum value thereby giving the highest duty cycle possible, consistent with good machining stability.
The servo mechanism control was set to maintain a constant average machining voltage. After a series of data points had been obtained the procedure was repeated using the remaining peak current settings in both polarities.
Through the electrode flushing pressure for all tests was 2-3 psi. The only exception was tungsten carbide where the flushing pressure was increased to 14 psi.
Test duration varied depending on the rate of metal removal. It was considered essential to have a cavity of 0.005" or deeper to ensure accurate measurement.
A number of techniques were required to determine performance results.
Measurements were made using a Taylor Hobson® Surtronic® 3P instrument. Ra (roughness average measured in µinches) was chosen as the standard method for defining surface finish. EDM surfaces can be measured with a stylus instrument, but due to the nature of the surface, the end results are not directly comparable with conventionally produced surfaces.
Figure 2. Taking a surface finish reading
End wear was measured in the following manner: Prior to each cut, the overall length of an electrode was measured with an electronic height gauge and the reading recorded. After each cut, the electrode was then measured to determine its overall length. This length was then subtracted from the measurement made prior to the cut, and the remainder indicated the amount of end wear.
The end wear ratio is a more important figure than the actual end wear, since it is a work/wear ratio.
Figure 3. Taking end wear measurements
Corner wear measurements were determined by placing the electrode on an optical comparator, which projected the image of the electrode's silhouette, magnified 10 times, onto a ground-glass screen where a grid was superimposed to obtain apparent corner wear; the end wear value for the same cut was added to the apparent corner wear to get the true corner wear.
The corner wear ratio indication is better than end wear alone since it is also a work/wear ratio.
Figure 4. Taking a corner wear reading on the optical comparator
Metal Removal Rate
Metal removal rate, which is the volume of metal removed from a workpiece in a specified period of time, is often expressed as cubic inches per hour (in3/hr). To determine the volume of metal removed, the depth of each cut was measured with a depth micrometer. The starting width and length of the electrode was measured with a digital gauge. The product of these measurements (width x thickness of the electrode x depth of the cut cavity) equaled the total volume of the cut, with one correction: Since a circular flush hole was used, it was necessary to subtract its volume from the total volume to determine the amount of metal actually removed.
Figure 5. Measuring the depth of a cut
Figure 6. Redressing electrode for next series of cuts
How to Read and Use the Graphs
The performance data on the graphs that follow were generated from actual tests run in POCO's EDM Laboratory. POCO has selected the graph as the most straightforward and simple method of presentation. Since it displays information visually, the user will find it easy and convenient to use—for comparing one set of data with another, or for selecting optimum operating values for a particular application. This method arranges the data in the form most useful to EDM machinists, designers, industrial engineers, and estimators; specifically, it emphasizes Metal Removal Rate, Electrode Wear, and Surface Finish, the three most important indicators of electrode performance.
By plotting different values of peak current, on-time, and off-time, the information which best describes an electrode material's performance is summarized visually on a graph or set of graphs. In selecting this form of graphic presentation, POCO has found that the performance of any electrode material can be accurately displayed.
The electrode material used to produce the graphed data is listed at the top of each page—see figure 7. The workpiece metal, the polarity and the servo setting used to produce the data on the page is presented in the upper left corner of each page. The plotted lines on each of the three graphs represent different peak current values. Each value is plotted in a discrete color and line symbol that is identified in the lower right corner of each page.
Top Graph — This graph compares the metal removal rate at different machine settings. The horizontal axis is the on-time scale and the vertical axis is the metal removal rate scale. On most graphs three peak current values are plotted with the off-time noted above the plot points.
Middle Graph —This graph displays Corner Wear (CW) and End Wear (EW). The horizontal axis is the on-time scale and the vertical axis shows the percent wear scale. Corner wear is represented by broken lines and end wear is represented by solid lines.
Bottom Graph —This graph displays surface finish. The horizontal axis is the on-time scale and the vertical axis is the surface finish scale.
Comparing plot points on the three graphs will allow the user to select the best setting to obtain the results desired for a particular application.
The performance graphs that follow are grouped by workmetal. Each tabbed workmetal section has a table of contents to aid in the location of graphed data for a specific electrode grade, polarity and peak amperage .
EDM Performance Graphs (Negative and Positive Electrode Polarity)
Two basic kinds of graphs are necessary to show electrode material performance data: Negative polarity graphs and Positive polarity graphs. These graphs represent the polarity of the electrode material, and the polarity is marked on each page. Color coded headings at the top of the page make it easy to identify the polarity of the electrode. An orange EDM Performance heading indicates positive and a green EDM Performance heading indicates negative polarity.