RepRap: Blog

As one might have expected, the signal-to-noise ratio caused by motor vibration makes attempts to use a piezo on the probe to detect the bed impractical. Still, had to give it a go. Getting well bored with this Covid thing by now.

I had heard potassium hydroxide was the electrolyte of choice for etching titanium wires. I tried it on my Nichrome. It is much less ... aggressive. I had to run it at 12V to get anything to etch, and then, well, I bring you The Demon Probe:

Totally unusable, but very eldritch. There's a lot of gnarly corrosion on there that won't boil off.

Conclusion: salt works better for Nichrome wire tips. But I will try to source some titanium wire and see what that does. Anybody got donations?

I just stuck a standard piezo disc on the stage (with a 1M2 resistor across it) and prodded at it with the probe. I'm getting a 0.02V response when the probe hits it with a positional accuracy of within 40μm - astounding considering the roughness of the piezo element and the fact that it's held down with a tiny piece of blue tape.

So, might be worth taking a side-trip to get some form of Z-height automation going on. So far the best algorithm seems to be to lower the probe until values happen, then lift the probe until they stop happening. There is a zero point in between, which complicates things and stops me just using it as a Z contact, and I have no idea what happens with fine probes (but I suspect it won't work so well...).

I guess the next step is to make a probe arm that holds a piezo with a tip stuck onto it and see what the heck happens. If anyone has been here already, please pipe up.

There is remarkably little noise introduced by the movement of the stepper motors.

I tried using the thickened resin again, and turns out manual flickering of the UV led is not accurate enough for a consistent result. Probably also temperature dependent. I noticed that the drawn-out lines of solidified resin warped and moved under my breath, but not a blowtorch draft, so the material seems to be very moisture sensitive in thin layers.

I got a thicker point which has heavy corrosion on it from not being cleaned properly after the electro-etching, and tried that as a dip pen. Just using a drop of resin off the end of a dental probe was too much material. The resin blobs when the point goes in, so using tissue paper I wiped most of it off, leaving a smeared layer approximately 0.3-0.4mm thick. This allowed me to dip the probe tip without causing resin to splurge everywhere.

Once dipped, I moved the tip away 1mm and lowered it to the slide surface. Not particularly accurately, just give or take 0.05mm. Raising the tip left a droplet roughly 100μm in diameter (don't know how thick yet). Moving back to the smear of resin, moving forward, new droplet, repeat, produced this line. Smear can be seen as darker area on the right.

 

An unforeseen problem is that the size of the smallest reservoir drop I can easily make is about the size of my work area, so I'm working around that by making the reservoir and manually aligning the probe with the edge of it before engaging the stepper drivers. Anyway, the Covid is starting to win again, so that's it for now.

Sorry folks, got the covids. Normal service resuming when I can figure out what normal is.

Capillary action gets a bit too carried away when you dunk the probe in a blob of 3D printer resin. It's designed to flow nicely, and for our purposes we don't want it flowing quite so far and enthusiastically. So to thicken some I gave it a half-second burst of UV, which allowed me to tease out a line less than 0.1mm wide - about 50μm in this photo. Back to micron-scale again!

I think the inclusion might be Sharpie marker off a dirty probe tip. I only noticed it after I washed some dust off with IPA, but this might be a way of making things easier to see. Unfortunately said IPA also washed off some finer detail I had done by bouncing the point on the slide, so I didn't get to measure those dots. It took a lot of other stuff off too. Still more things to learn to handle this material, obviously.

This was pretty slapdash, but I thought it was time to put some resin on a probe and see what the heck happens. I used a wooden toothpick to put on what I thought was a small blob but was about 100x what I needed. Then I dunked the probe into the very edge of the blob and trailed it around to see how far it would draw out. Then I exposed the whole thing to UV light for a couple of minutes to set it good and solid, and this is what it looked like:

The image on the right is before UV exposure, the one on the left after it. For scale, the white circle is 6mm in diameter, so we're looking at about 0.1-0.2mm line widths for the most part. I was moving the probe around in 0.3mm increments, and as it moved to the left needed to drop the probe by 0.015mm to keep contact.

Note that the resin has contracted and balled up under UV exposure. This is not ideal, because the smallest surviving blobs look to be bigger than 50μm across - sorry, I totally wrecked it trying to remove the resin. The big blob came off easily, but the fine stuff pulled apart and I totally failed to pull a strand off. Multiple layers next time, I guess.

Anyway, a change from the usual scratchings, and more data points.

I wondered if there might be useful diffraction effects if I illuminated the probe with a laser that might make it easier to see when the probe was near the surface. The result was ... scary. Fortunately it didn't fry the optics.

Note how it lights up all the little bitty dust grains on the slide.

Oh, I've added an LED under the stage, so this is the view I get on my monitor when flying the thing. Having a functional "Home" on the controller is making a noticeable difference in repeatability.