Electrochemical Machining

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The electrochemical machining (ECM) process electrochemically dissolves material -- "reverse electroplating" -- until only the desired shape remains. Similar processes are Electrical Discharge Machining (EDM) and Electrochemical Discharge Machining.

A RepRap has the potential to make metal parts or even its own circuit boards. The process has no forces on the tool head, requires no unusual "chemicals" and commonly available voltages, so it's well suited for the not so rigid RepRap machines.

The Process in a Nutshell

It's simple. Connect the work piece to the positive voltage of an electric power source, the tool head to the negative one. Submerge both in a bath containing an electrolyte and make sure they don't touch each other. As you switch on the voltage supply, bubbles appear on the tool electrode, the machining has already begun. It also works well to have the voltage supply switched on all the time, but to retract the tool electrode from the bath for stopping the machining.

An indirect requirement of the process is to constantly flush clear electrolyte between tool head and work piece. The process generates some slurry, which can hinder the electrolysis to happen or even create a short circuit between both electrodes.

From the practical point of view, one of the best electrolytes is table salt (NaCl), which is dissolved in tap water. Typical voltages are between 2 V and 25 V, so the 5 V or 12 V from a PC PSU work fine. Required currents are proportional to the surface area of the tool, so small tool tips (needles) get away with one or two Ampére. The tool head can be made of any conductive material, e.g. steel, copper or solder tin. The tool head experiences no tool wear.

Forum discussion

There is a thread on mentioning electrochemical board milling at:
http://blog.reprap.org/2006/11/universal-pcb-test.html

From that thread, Adrian mentioned that:

The Reprappers had an alternative idea: put a (bunch of) electrode(s) in a RepRap head and use electrochemical machining to make the PCB. Best electrolyte, according to my colleague Tony Mileham who's a world authority, would be sodium chlorate. Bit dodgy as it's

  1. Horribly Poisonous, and
  2. A fierce oxidsing agent...

Vik mentioned: I'm,er, familiar with sodium chlorate. I've been accidentally poisoned by it and to be honest it wasn't a big deal. I wasn't even really sick after accidentally consuming a teaspoonful or so of the stuff. Wouldn't recommend it though, even if the LD50 is a few hundred grams (1200mg/kg in rabbits,apparently).

I must say, it's very easy to produce though. Fire 12V into salty water through a couple of old carbon battery rods and there she is.

The proximity of clorates and finely divided long-chain ketones would be a problem, however.

-- Main.EnriquePerez - 04 Dec 2008

Is it possible to make PCBs using electrochemical machining (ECM)? With this method, you move the tool head over the isolation gaps near (but not touching) the copper clad board, immersed in a shallow pan of saltwater. A positive voltage wired to the copper clad board and a negative voltage wired to the tool head drives a current through the saltwater that dissolves the workpiece near the tool head.


Perhaps: Use the extruder board's current-limited stepper motor driver to, instead, drive a constant current. Perhaps: use the thermistor ADC to, instead, measure the analog voltage from the PCB (positive) to the stiff wire tool head (negative).

  • wire touching PCB gives zero volts: too close, drive the Z axis to pull the wire away from the PCB
  • wire far from the PCB gives high voltage: too far away, drive the Z axis to push the wire closer to the PCB.

(Once an isolation gap is cut completely around a region, it's impossible to ECM inside that region. How to avoid painting yourself into a corner?) (Is it possible to use the ECM tip as a probe to detect holes already drilled in the PCB, and semi-automatically compensate for any small misalignment to make perfectly centered pads and fan-out traces?)

Comparison ECM vs EDM

The electric discharge machining (EDM) process cuts away material using dielectric breakdown (sparks) until only the desired shape remains.

Forrest Higgs is considering "Going for an EDM toolhead"[1]

Wikipedia: electric discharge machining

The equipment for EDM milling appears similar to the equipment for ECM milling -- a high-power electric power supply, a submersed block of raw material in a water-tight tray, and a tool head focusing the electric current on the unwanted material to be eroded away. Both systems are non-contact -- the machine maintains a small but nonzero gap between the tool and the workpiece. (And therefore puts much less stress on the positioning system than mechanical milling). Both system require electrically conductive raw material.

The differences are less obvious:

  • ECM requires a conductive fluid (typically salt water), while EDM requires non-conductive fluid (typically oil).
  • ECM requires the tool head to have a negative voltage, while EDM works with positive, negative, or AC voltage.
  • ECM has no tool head wear, while EDM has heavy tool head erosion.
  • ECM pulls a more or less continuous current from the power supply, while EDM pulls a pulse of current for each spark.

Recommended Reading

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