RepRap: Blog

Probe #19 was manually etched. I put it in the etchant to a depth of 20mm for 60 seconds. Then, oh so tediously, tapped the tip on the meniscus 80 times. There's dedication. I'll put the picture later because people are bored with pictures of the tips in thumbnails.

So here's a picture of actual test squares.  The one on the right is 640μm square, the thing on the left is kind of freehanded. Dot spacing is 40μm.

The interesting illumination in that micrograph comes from me shining a light down the other eyepiece of the binocular microscope. I just happened to notice a lighting change, experimented, and this photo came out pretty good.

Drawing the dotty square shows that Probe #19is having wee issues with flow control. I suspect it was contaminated with superglue during fixing, and the slight unevenness probably isn't helping: 

 

It' slightly asymmetric at the tip. This probe is not etched to a perfect tip and then blunted, it's etched to a vaguely round end and then gradually sharpened. It may be that the sharpening process does not result in a perfect point.

Anyhoo, when I do get a dot, it's about 20μm diameter, which is pretty much what I'm aiming for. The first dot is always larger than all the others which is why, for instance, the top left corner is joined up.

The left square is just me noodling with the probe in direct contact with the slide while being moved around. As you can see, this does not result in a terribly even line width when flow control is poor. 

One other "failed" experiment: I tried drawing on a glass slide with thin conductive silver ink CircuitScribe Electon Inks electroninks.com), the idea being to etch a circuit into it in the same way as one mills PCBs. Sadly, the ink did not cover well, and I think that particular ink needs to permeate paper in order to achieve proper drying and a good thickness of conductive medium. Doesn't work well on glass or shiny cardboard anyway. Dries too fast to be deposited by probe.

Getting closer. The previous tips were far too finely pointed and were depositing micron or sub-micron dots of resin that I had difficulty positively identifying. This ogive point etch is way too efficient! So I manually took Probe #14 and blunted it a lot by dipping it 30-40 times in the meniscus of the electrolyte etch.

This produced the following tip, which I very enthusiastically drove into the glass slide. I was trying to make absolutely sure I had contact because I could not see the droplets in the USB microscope. I could not get good images on the binocular microscope and had to take the following micrographs of dots with the bench microscope:

The dots are in wavey lines because the tip displaced during my enthusiastic touchdown attempts. The dots appear to be approximately 4μm in diameter. As I want something closer to 10μm or more this tip is still to pointy. Great for etching and very fine detail, but not for building robust microscale tools.

For reference, the resin reservoir only made it up half the smooth part of the point, so about 150μm deep:

 

I'm going to have to try an even blunter tip. Last time I did this by putting a freshly-cut wire 20mm deep in the etch, which notes say produced a very rough point that had to be smoothed off by a quick dunk on the surface. So that's the next experiment.

I now have a couple of pretty identical tips:

Probe #17 received the second pass treatment (top) with probe #18 for comparison. Might need to tweak the second pass GCODE to take a little more off, but we'll see which one works best first.

I've fitted Probe #18 to a flexure tip holder, and Probe #17 to a holder that has been extended so that I can get the microscope angled more vertically over the probe:

 I'll try that one when the glue has set and I've put the Z Axis back on the PIKA.

I wanted to get the tip right, and the tips I was making just weren't ogival enough. For those going "Wut?" an ogive is the shape of a pointed gothic arch. I made another probe tip because I wasn't sure how this would play out. After the automated two-pass etching process (without roughening) it looked like this:

 

 It's too aggressive. It needs to be less pointy.

It might be fine for depositing teeny little dots, but what we're trying to do is lay down 20μm-30μm lines and build something relatively solid. The catch is that we need to do that without flooding the nascent part with a torrent of resin. The ogival point I created for Probe 10 seemed to do that well. 

I ran the second half of the automated etching GCODE that basically tried to etch off the last 2μm at a time. I noticed that it etched off roughly 8μm at each dip, so a lot of dipping time was wasted. I wrote a second piece of GCODE that dipped 5μm at a time and ran that. I took off approximately 110μm  of the point and it looked like:

Now this is much more like the desired ogival outline, and the area above the tip is roughened so it should hang on to a layer of resin by capillary action. I'll mount this one in a contact flexure and we'll try it out. [Edit: Turns out I put my monitor on top of the last printed flexure so will have to print another]

The blog has stopped accepting images for unknown reasons (bug-a-minute here), so this one is pointing to my Mastodon account on the NZOSS server. I hope it works. If it vanishes, sorry, I have the original.

 

What you should see here is four etched probe tips, numbered, etched from 0.3mm 316L SS wire. #14 was made a couple of days ago before my Z Axis died. That was given a 20 second roughening etch, then the GCODE-controlled etch as per the github repo /utils directory. It has a smooth ogival point.

#15 and #16 were the ones that ended up malformed due to the misbehaving Z Axis. I cut the tips off, and re-etched the wires using the same GCODE as #14. These display a significant stepped profile to the point. Not what I was looking for.

So I wondered. What effect is the roughening etch having on the point profile? So I etched #17 with the same GCODE as #14 but did not do a roughening etch on it. The ogival profile is very good.

All of the probes display a slightly roughened area about 0.1mm behind the tip. This is unplanned but fortuitous. I will experiment with #17 to see if the roughening process is worthwhile when using the sodium chloride etch. The nitric acid roughening etch certainly soaks up the resin, but I have not seen any of this behaviour on the sodium chloride etched tips unless they are over-dipped. Over-dipping is not good for accurate resin deposition as it results in a roughly spherical blob held on the tip by surface tension rather than a film held by capillary action.

Anyhoo, now have tips. Next step is to mount them to the Flexured Probe Holder I printed earlier. 

I took the Z axis off and followed my own instructions https://github.com/VikOlliver/RepRapMicron/wiki/Probe-Tip-Fabrication-And-Assembly to build three probes. When I started doing the second one, it was pretty obvious something was wrong because I had to raise it much higher above the etchant to break it clear of the meniscus. They came out like this:

 

 

Hmm, maybe one is usable but it looks a bit too sharp. After poking around at the Z axis I determined that the locknut at the top of the ZAxis Drive Screw was no longer in the right place. It seemed tight enough. It's possible it released and then re-tightened itself. It's also possible that the Flexure Coupling on the stepper motor has failed or compressed, or that I never had it right to begin with and just got lucky.

Anyway, it is re-tightened. I'll just chop the ends off and try again tomorrow.

I am still not capable of putting more than a couple of hours at a stretch in at the bench I'm afraid. The Doc is pleased with my progress, has upped the painkillers slightly, and - within sensible limits - is allowing me to drive again. So there's improvement somewhere anyway!

Oh, if anyone can donate me any single mode fibre cable, OS1 or OS2 or anything with a 9/125 fibre core or finer, would be appreciated. Why? Well, check out this video of someone else's micro-fabricator: https://youtu.be/UvV2CH5il3Q which is using a fibre to spot-cure resin. Not the approach I wanted to take, but they get more views than me so I have to get with the program and be relevant :) The same arrangement will work just fine on the RepRapMicron.

I printed the Probe Flexure with a 0.1mm layer height on the Prusa XL and they came out really nicely. Worth doing in the future.

Still haven't etched the new probe. Thought I'd make a proper set of croc leads with a plug on, got croc clips, Du Pont connectors etc. and then cleverly put a socket on the leads instead of a plug. Arrgh!