Friday, October 10, 2025

 

Tips On Etching Tips

I got some actual 316L stainless steel wire and some Nichrome 80, after the "Art of Bulshito" non-stainless non-steel wire incident. I decided to try etching them in either 1% nitric acid or 5% salt solution using the rig and instructions here on Printables. Using 3xAAA batteries for power, immersing the wires 8mm into the liquid, and with a stainless steel spoon/cathode 40mm from the liquid's surface. Blimey, wasn't expecting this:

 

Using nitric acid seems to produce a very robust probe tip, with an almost foamed shaft. There was indeed no cloudy precipitate during etching, but the tips of the wires did not fall off. What you see in the top photos is just the eroded end of the wire.

The salt was as cloudy as ever. The 0.12mm Nichrome dropped off at the meniscus. The 0.3mm Nichrome parted about half way down the immersed section.

The tips were gently rinsed with water and sprayed with isopropyl alcohol to remove any remaining liquid.

It appears that nitric acid is best for creating a robust tip. Nichrome creates extremely fine tips, but using wire as fine as 0.12mm diameter produces a very short tip that appears uneven and particularly delicate.

I think the most impressive tip of this batch was the very fine, spire-like tip produced by the 0.3mm Nichrome in salt. The tip is exceptionally small (too fine for the camera), the length has a gentle taper, and the sides look like they would hold a good dose of resin by capillary action. Here's an image zoomed in on that fine tip, with a piece of 0.3mm wire (almost in focus - sorry, it's hard) for scale.

That tip though is really fragile, and finer than I need - I had to make a jig to protect it during assembly. I wanted something half way between nitric and salt in dimensions. Well, lets put about 5ml of 1% nitric and 35ml of 5% salt solution together and... interesting. That stops the precipitate forming, and I can see the point sharpening nicely - hydrochloric acid might do the same and is easier to get, which I'll try later.

Anyway, that combination allows me to get a good tapered point, and tune its thickness to my requirement of the day. Now to join it to the μRepRap and see if I can deposit an even smaller resin Jolly Wrencher with it. 

Doesn't look as fragile as the long thin one (foreshadowing).


Comments:
During my degree we made tips for an STM. We used tungsten wire and electrochemically etched it in a sodium hydroxide solution. The key point is that we did not try to visually inspect the etching progress.
By using a power supply we could monitor the voltage and current. By placing the wire further into the electrolyte, we could observe a voltage/current change when the end fell off and the tip had grown.
 
Yep, that works too. Just trying to keep to minimal hardware in the hope that more people will try. Some labs even go so far as to make a monitoring circuit that automatically cuts the current. But there's such a thing as "too fine' for this version of the hardware, and the manual technique works well enogh for now.
 
I commented this on the HaD article, but I may have been a little late to the party.

What about glass micropipettes? They can be made/purchased with tip diameters as small as 0.1μm. I suspect the pressure to deliver the resin through the pipette would be too high, but it could still be used in the same way as the needles. Delicate? Yes, absolutely. But I doubt the needles are being crashed into the substrate and previous layers, so it may be ok.
 
I'm only using needles because I have them. The hollow aspect isn't required. Acupuncture needless would be a fine substitute if legal in your jurisdiction.
 
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