Spark erosion with a welding inverter

The video about spark erosion with a welding inverter

About this machine

Figure 1:
After explaining the theory behind Spark Erosion Machining in earlier videos, I am now taking a big step towards a practical machine. As usual, I still take a rather unconventional approach to machine hard metals with rather "soft" machines. Here I am machining a steel sheet using the mechanics of an old 3D printer in combination with an inexpensive welding inverter.
The machine is still far from fully developed, but at least it can be used to process steel in principle. Therefore, in this chapter there is only a rough description. A build instruction, which deserves the name, is available in a following chapter. The aim is to develop a machine that is capable of processing steel with a precision that won't be perfect, but good enough for my purposes while being easy to replicate - nothing more, but also nothing less.

The mechanics are based on a decommissioned Creality CR-10 printer, which can process an area of around 30x30cm. There is a tub made of 2mm acrylic panels on the print bed.

Welding inverter

Figure 2:
The Mecpow inverter that I purchased delivers currents of up to 120A, but for the erosion process I am dialing to the lowest setting of just 10A. For spark erosion, the device is operated in "MMA" mode, which the manufacturer intends for electrode or arc welding. When the circuit is open, a direct voltage of 64V is detected across the two output terminals. If the circuit is closed, pulse width modulation ensures that no more than the preselected current flows - the digital multimeter therefore shows a lower voltage of only 0.13V. The welding inverter can therefore be treated as a short-shot-proof DC voltage source.

Figure 3:
If the electrode is brought very close to the workpiece, a spark jumps over and the circuit is closed via the plasma. I talked about the process in detail in the previous chapters of this series, so here's just the short version:
The plasma of the spark melts the metals of electrode and workpiece. While welding means adding material, our goal is to remove material. During welding, mostly the metal of the electrode wire melts and solidifies again on the workpiece. However, the water jet on my spark erosion machine washes away the molten material and removes it primarily from the workpiece, but also from the electrode.
I use a 1.6mm tungsten rod as electrode.

Figure 3:
After about 110 seconds, a hole with a diameter of about 1.9mm has been eroded into the 2mm thick steel. All in all, the bore is nice and round, but the edges are not perfectly smooth. I had also shown in previous chapters that the lower the energy transmitted per spark, the smoother the edges are. The disadvantage of the welding inverter is that I have already selected the lowest setting of 10A. In order to get smoother edges, the electronics of the device would have to be modified, which I may do in later versions of this spark erosion machine.

Figure 5:
I need a total of 512 holes for a sieve to produce plastic powder for my granule 3D printer. The hole spacing is 5mm. The steel sheet was drilled through without any problems, although it was anything but well fixed. The force on the axes of the printer mechanics and the workpiece is negligible, which is the biggest advantage of spark erosion.

If you look closely at the finished sieve, you can see that not all the holes were made with the desired diameter - some holes are significantly smaller. The reason for this is that not only the workpiece, but also the 1.6mm tungsten electrode is eroded by the sparks, which can be clearly seen through the microscope. I have already mentioned this fact several times in previous videos. The problem is that the electrode does not erode evenly, but that a tip has formed at the originally blunt end. From time to time I stopped the drilling process and manually pushed the electrode further down to compensate for the erosion.

Figure 6:
Tip of the electrode seen through a microscope.

Electronics

Figure 7:
When it comes to electronics, in addition to the welding inverter, I use an Arduino Mega with an attached Ramps board in version 1.4 - both components are widely available and easy to source. An old computer power supply provides the 12V for these components and the stepper motors.

Figure 8:
The water pump is made with a 3D printer, the drive motor is sourced from an old inkjet printer.

Figure 9:
As soon as a spark jumps from the electrode to the workpiece, pulse width modulation begins and the falling edge of the resulting signal is recorded via a digital pin. However, since the welding machine interrupts the voltage from time to time even without sparking, this type of feedback is not as clear as it should be. But the same applies here: Although not optimal, it is sufficiently good and easy to implement.

What else the maschine is intended to do

Figure 10:
In addition to drilling holes, the tungsten electrode can be used to cut parts from the steel sheet.

Figure 11:
For round parts I implemented a small lathe.

Build instructions

If you would like to support me financially in the further development work on this and other open source machines, in addition to more information and the build instructions, there is also a donate button on my pages. Thank you to all the great people who have already used it to send me an Obol!

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