The inductance of the power cable between the arc generator and the workpiece limits the rate at which the current can change.
The time it takes for the current to reach the desired level is:
time=(desired current level / Voltage across inductor) * inductance of
The maximum output voltage of the BX17 arc generator is 150V. When arcing takes place, the combined voltage across the arc and ohmic losses in the wire contacts is roughly about 50V, which means that there is 100V across the inductance of the cable. The inductance of the power cable has been measured (as twisted pair with shorted end), and this is exactly 1 uH for a 2 meter length.
Now, the maximum current setting of the arc generator is 30A. This
means that it takes (30/100)*1e-6=0.3 microseconds to reach 30A with a 2
meter long cable.
The shortest pulse on-time that the arc generator can make is 1 us.
This means that during that 1 us, 0.3 microseconds is used as current
rise time, 0.3 micro seconds is used as current fall time, and for 0.4
microseconds the current is actually at the desired level.
So you can clearly see, that the length of the cable affects the pulse rise and fall time. If the cable would be twice the maximum length, the inductance doubles, in that case the shortest BX17 pulse would be distorted too much.
If however, the BX17 is set for pulses that are longer (so machining at lower frequencies and/or higher duty cycles), than the distortion by the cable contributes far less and a longer cable can be used.
The main reason why a maximum cable length of 1.5m (1.5m twisted pair + 0.5 m untwisted for the C-arc = ~2m) is specified, is that only then it can be guaranteed that the pulse (on the workpiece side of the power cable) is within specification for the entire operating range of the BX17. Note that inside the BX17, there is also about 0.4m of cable length.
The example EDM machine from BaxEDM is a wire cutting machine, but the BX17 can also be used for other EDM applications. For instance, the popular Youtube channel Applied Science has built an EDM drilling machine with the BX17 arc generator:
The BX17 can also be used for die-sinking EDM. Note that the BX17 is intended for small shops and therefore does not require 3-phase mains. The BX17 is therefore a medium power generator. Medium power is more than sufficient for wire cutting, EDM drilling and sinking with small electrodes. For large surface area electrodes in EDM sinking, very high power industrial size generators are normally required for fast sinking, for such an application the BX17 is less suited. It will work however, but will take more time to make a cut.
It is important to note that the BX17 is an arc generator only. It cannot drive stepper and/or servo motors. For that, a (CNC) control solution is required in combination with the BX17. The BaxEDM YouTube tutorial series on DIY EDM explains this in all detail.
The BaxEDM machine uses these (marked by arrow) brushed DC motors in combination with a 1:84 ratio planetary gear head:
These motors were picked simply because they were salvaged from another build. When choosing motors for a new WEDM build, similar but low cost versions from another brand can be used as well. The used motors in the BaxEDM example machine are overdimensioned and can easily break the wire by the large torque that they can deliver. The important thing to note when selecting motors is that they should be backdriveable. This rules-out the use of worm wheel reduction gear boxes.
The industrial wire guides in the BaxEDM example machine were found on Ebay a few years ago and are originally from a Fanuc machine. Ebay does however not list many. By far the best offers can be found on AliExpress. A good query is “EDM wire guide S103 0.255 mm” This will lead you to wire guides from Sodick, which can be found for prices as low as 25 Euro/piece. The YouTube user TessenCNC also uses these. Here’s what the Sodick S103 guides look like:
The typical rate at which the wire is renewed lies in the range of 1 to 3 meters per minute, for 0.25mm diameter wire. Depending on the power at which the arc generator is set and on the flushing conditions used, a different speed should be used. It is not possible to renew the wire too fast. The only effect in such a case is that you are wasting wire, cutting will work just fine. If the wire speed is too slow however, you risk braking the wire. A good indicator for seeing if you have chosen the correct wire speed is by taking a piece of spent wire and pulling on it by hand. If it requires little force to break, increase the wire renewal rate. Typically if you have found a speed that works well for you, you will rarely change it, unless you make big changes to the generator power. For the BaxEDM example machines, the used speed is 1.5 meter/min.
As EDM does not exert any forces on the work piece, clamping the work piece to the table is quite easy. The only forces exerted on the work are due to the water flushing. In The BaxEDM example machine, work is clamped to the table by simple screw type clamps that are adjustable in height:
The clamps themselves are of course Wire EDM cut by the machine itself 🙂
An EDM machine requires a control loop for controlling the feed rate.
The CNC motion controller should adjust the feed rate continuously such that the setpoint error remains as small as possible. In order to keep the error small, the CNC controller needs to command the motors to speed up if the error is negative and slow down if the error is positive. The motion controller needs to be able to continuously change the speeds of the motors, and also the direction in which the motors are turning. Any CNC controller that is able to do this, and has the flexibility to program the behavior of the control algorithm is a very good solution. The CNC controller of Dynomotion meets these requirements. There might be other solutions that meet these too, however these are unknown to BaxEDM.
Most simple commercial CNC controllers do not allow the user to program their own control loop, nor are they able to dynamically reverse the feed rate. For such controllers, the only option is to use a control strategy in which the motors are stopped if the error goes positive. Although it is possible to machine with such a control strategy. This is not desired as the arc stability will be quite poor, which has a negative impact on the part finish. Next to that, the machine won’t automatically find the correct feed rate. In such a stop/go control scenario, the operator has to estimate the feed rate, which is difficult for each material type and thickness, which is clearly not desired.
Upon startup of the Dashboard app, the application needs to load the parameter library from the “EDMparametersLib.json” file. The Dashboard app searches for this file in on the same path as the executable. If it is not there, it cannot load the library and will raise the “Failed to execute script WED4” error:
To resolve this, make sure that you have copied the json file to the same path as the exe file.
The differential inputs of kanalog are digital, the generator control signal differential output is analog, so you need an analog differential input, which Kanalog does not have. So the answer is no, you need the converter (or make one yourself).