RepRap: Blog

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 

The levelling arrangement on PIKA has changed significantly. The Stage has two height adjustment screws on the left: one at the top (+y) and one at the bottom. There is a third on the right at the mid point. The right one is only adjusted of either of the left ones bottom out.

Levelling with aluminium foil is having problems. When I start out with a new foil, the error detected when doing several probes at the same point (the "span" here) is relatively small:

 --- Probing corners (1000.0x1000.0)---

BL:  -93.646 µm  (span 1.000)
BR:  -73.375 µm  (span 3.218)
TR:  -76.854 µm  (span 2.000)
TL: -103.823 µm  (span 4.469)

After 17 levelling attempts, it gets pretty bad:

 --- Probing corners (1000.0x1000.0)---

BL:  -74.188 µm  (span 10.875)
BR:  -70.032 µm  (span 22.531)
TR:  -81.271 µm  (span 15.063)
TL:  -72.313 µm  (span 6.437)

The latest foil I've used is far more lumpy than the previous lot, and that may have something to do with it. However, I'm not convinced it'll be useful for anything other than rough alignment.

So I've tried lowering the probe so that it just makes a mark, and drawing 10x10 100μm grids. Looking at this one:

You can see that the left edge is fainter than the right, so I need to raise the two screws on the left slightly. But that's not too bad over 1mm. If I move the probe and re-zero the height, I'll get the same pattern when I try to make a grid somewhere else, so it's a consistent slope.

The glass surface does not seem to be micron-flat though. There are definitely places where the tip will scratch the ink before others, and I moved both the probe and the glass slide around while probing one of these patches to verify that it's not a probe positioning artefact. I'll have to look out for that, but unsure how much difference it'll make in the end.

On the whole though, the PIKA V0.02 is easier to level, and the built-in microscope pole is very much more stable - which makes it easier to track what's going on (or going wrong!) with the microscope. It is now in the "pika" directory on the RepRapMicron github.

I've got the bits together. All the little tweaks make this a much nicer build. Haven't connected up the CNC controller but don't anticipate issues with that. I took many photos for the docs, and here's what she looks like now:

What I do have problems with though is my aluminium-covered touchplate. Will have to make a new one of those before I can test levelling.