RepRap: Blog

I've etched another bit if 316 Stainless wire (note to self, needs to be a 70mm length) with an immersion depth of 15mm. I meant to etch in NaCl, but used the hypochlorite pot by mistake. The tip looks like this immediately after etching:

That was only etched for 45s. That's gone in a Probe Arm, with the tip bent to enter the etching well on the Stage at more or less 90 degrees. Etching in the well with NaCl produced a result that was not at all what I had expected. The tip was irregularly corroded:

I don't know if this is a side-effect of etching with hypochlorite, the annular nature of the electrode in the etching well or what. Two -pronged attack: I'll try etching that same wire manually in the big cell, and I'll put a piece of plain, unadulterated wire in a probe holder and etch that.

Oh,  but first I have to make repairs on the Z axis. I'll replace the old flexure coupling with the newer strengthened design. I tried to close the Z Axis on the microscope clamp, it made cracking noises, and now wobbles (which it didn't do in the video in the previous post). Oops. [UPDATE: That was most likely one of the Z Axis Complementary Flexure screws working loose. Fixed.]

This is just a thought. If you've used a Technical Pen you'll get the idea. If I can fit a probe wire inside a blunted hypodermic needle, I might be able to get a capillary gap between the roughened wall of the probe and the needle. This would allow a reservoir of resin in or above the needle to continuously resupply the probe tip with resin, eliminating the need to continuously dip it - but it would still need to seek safety from UV during curing:

In theory surface tension stops the resin from leaking out. Obviously there are issues with potential blockage, though there isn't a single channel to be blocked as the resin flow wraps around the probe. The probe shouldn't be any more unstable than it is at the moment. Dimensions in the drawing are a bit notional and will very probably vary in practice. I might even put a bend in the probe so I can bring it in at an angle, but that's for future experiment.

For now though I'll stick with a plain probe and muck with the Technical Pen idea once I have a repeatable probe tip scheme worked out. I've printed a bunch of spare probe arms, and the glue on the next tip assembly is currently curing.

I put the Probe 11 into a probe holder and etched it in the μRepRap Stage cell. Immediately apparent was that the hypochlorite etch at corroded absolutely huge fissures in the thing:

Way too much. Also, genius here should have bent the probe tip so that it contacted the meniscus at 90 degrees but got carried away. So having a wreck, I experimented on it anyway as you do.

If you look at the very top of the probe in the image you will see a tiny peak. This is where the probe actually contacts the meniscus. I lifted the probe so that the meniscus was pulled up from the surface, and let it etch until the meniscus dropped off. That little peak is the result.

I'm going to etch another probe in NaCl, but immersing to 15mm. This should give a more even cross-section. I'll then try a few etchings on it to attempt to smooth off about 0.5mm, then try to put a point on the very tip of that. Remembering to bend the probe for 90 degree contact this time.

I will also be buying a better microscope for the side view on the μRepRap. Using the good one to take probe photos has made me realise just how held back I have been by the cheap microscope. 

While experimenting with probes, I was thinking "I need some way of precisely dipping the probe in the electrolyte in a very precise and controlled way."

Well, duh, that's what RepRapMicron does, innit?

So I built an electrolysis cell by wiring up an M8 zinc-plated washer to a glass slide with UV Nail Gel thus:

I switched the good microscope into the horizontal position (must buy a better secondary microscope). Having made sure the positive end of the Z Touch was connected to the probe tip (a disposable grotty hypodermic one), I put salt water in the washer, set the Z Touch Retract to 500μm, and told it to probe:

 

Yes, a video on the blog! (if you can't see it, try this: https://youtu.be/GB33HMUqppU) If you look very carefully as the probe touches the electrolyte, you will see a stream of turbulent fluid heading away from the probe tip. This is electrolysis happening. I measured the probe current at ~10mA, at 5V. Interesting how it takes some time after contact with the electrolyte for the Z Touch to detect contact. Current sensing presumably.

So yes, I believe we may have a way to very controllably produce probe tips. 

Probe 9 was given a pitted surface by etching in dilute nitric acid. This allowed more resin to cling to the body of the probe, but nitric acid is restricted in many localities due to its use in illicit explosives manufacture. It gives a particularly pitted surface because it is a powerful oxidizing agent. So, I pondered on what commonly available oxidizers are in use that haven't been restricted yet. Ordinary household bleach sounded like a good starting point, it also being a starting material for a number of energetic compounds. The stuff under my kitchen sink is 42g/litre which works out at about 4%. I'd have preferred something stronger but hey. Etching as before with a 10mm immersed 316 stainless wire gives this at the tip of Probe 11:

Comparing with the wire etched in acidified 5% sodium chloride to make Probe 10, we can see that there is a promising amount of pitting (sorry, couldn't remember the magnification I used and this is about twice the magnification of the previous image):

Worth exploring further. Caution: Do not add hydrochloric acid to the sodium hypochlorite. This will immediately generate chlorine gas. Probably not in lethal amounts in the quantities I'm using, but I recommend against empirical experimentation in that direction.

Next I'll experiment with getting a regular, smooth, pointy tip. 

I need a new probe, Probe 9 having been put through the wringer a bit. So this time I'll try to quantify and document this a bit better. I also need to find an alternative to nitric acid, so I'll wrap that all into one project. The following sequence was taken of a wire (now Probe 10)  as I etched it in 5% salt water for 20 seconds at a time. Between etches the wire was washed with water, sprayed with isopropyl alcohol, then force-dried with cool air:

 I kept the magnification constant, so any apparent thinning or tapering of the wire is real.

The lower nut slot on the PIKA Motor Mounts was a bit tricky to access, so I moved them around the side where you can get at them more easily:

 Should make use of the Nut Tool more convenient too. Already uploaded to github and Printables.

Having watched Jon struggle with inserting M3 nuts into the slots on a PIKA I have decided to take pity on the poor users and create a Nut Tool. Should save makers from the occasional screwdriver stab wound:

You can find it in the library directory on github, next to the M3 parts and Metriccano libraries. I have also uploaded the STL to https://www.printables.com/model/1692745-reprapmicron-pika-micron-resolution-3d-printer

If it breaks, you get to keep both parts. But I've tested it out and it is remarkably sturdy.

I have uploaded the assembly instructions for the PIKA XY Table. Your feedback is appreciated.

https://github.com/VikOlliver/RepRapMicron/wiki/PIKA-XY-Flexure-Table-Construction

On Printables too https://www.printables.com/model/1692745-reprapmicron-pika-micron-resolution-3d-printer