RepRap: Blog

The plan was to make Probe 9 out of 0.13mm dia Nicrome 80 but that cut through very, very quickly at the meniscus when I tried etching it in nitric acid. Probably because I didn't dip it in far enough (my discoveries on etching dynamics are for a later post). Change of plan.

A length of 0.3mm dia 316 Stainless wire was immersed in the 0.5% nitric acid probe etching cell by approximately 10mm, and electrolysed for approximately one minute. The wire was checked twice, and etching was stopped when a noticeable tip had formed. By this time the sides of the wires were roughened.

The wire was then taken to a second cell containing a 5% sodium chloride solution with 0.5% hydrochloric acid. The wire was connected to the anode as usual. The very tip of the wire was then touched to the surface of the electrolyte in the cell for approximately 5 seconds, then examined under a microscope. This was repeated three times. When the probe tip had acquired the desired ogival point, the probe was washed with water, then isopropyl alcohol.

The tip was mounted in the RepRapMicron V0.05 Probe Tip Arm with cyanoacrylate adhesive. A micrograph was taken, the Probe Tip Arm was fitted to the μRepRap, the tip dipped in approximately 0.5mm of "Top Coat" resin, and another micrograph taken:

At this stage it can be seen that the roughened shaft of the probe holds a significant quantity of resin, and that fluid effects (capillary action, surface tension, tip shape etc.) cause the resin to creep towards the probe tip. The resin is likely bulging towards the bottom of frame because this was the last part of the probe to leave the resin's surface when dipped.

Probe 9 was refitted to the μRepRap, touched off on a glass slide coated with Vivid marker, and then repeatedly touched off at 40μm intervals. The intervening height was 20μm to avoid probe or fluid drag. The slide was then micrographed at the same magnification as Probe 9:

(Note: There is some spherical distortion in the images due to impromptu photography setup.)

The first few dots (left) are a little larger than the others. The rest seem a relatively consistent size. The experiment was terminated after approx 60 dots (some are out of view) due to time constraints, and resin was still being deposited.

The dot size is clearly less than 40μm, though due to the optics and imaging used the exact dimensions are not apparent. It would appear to be in the order of approximately 20μm. This is adequate, and Probe 9 will be used for layer experiments.

Unfortunately, I'm going to have to leave you with a bit of a cliff-hanger because I have to do more hospital things for a few days. I fully expect to return. Might write a few more things up while I'm gone. 

 

The resin-carrying and deposition characteristics of the probe need to be a little more quantitatively defined. To define this we need something with a simple geometry. Enter Probe 8, made from 0.13mm dia. Nichrome 80, etched in 1% nitric acid. Like previous nitric-etched probes, the point is pretty blunt, at approximately 90 degrees. If 5mm of wire is in the electrolyte, it cuts through at the meniscus in 30 seconds or so.

The probe was removed and examined periodically during etching without any particular goal other than having some kind of simple point and not thinning excessively. Approximately 5mm of wire was in the electrolyte, and total etching time was in the order of 30-40 seconds.

The slide was coated with a small amount of Vivid permanent marker to make things easier to see, and a smear of Top Coat applied nearby.

The probe was put in in the RepRapMicron, dipped in Top Coat resin, and micrographed. It was then stuck it back in the RepRapMicron which was manually controlled to drop a sequence of resin dots at 40μm intervals, skipping to 20μm high in between dots to make sure the probe was not simply dragging resin. The result:

Probe 8 (inset) seems to have a drop of resin hanging under it. This is an illusion. "Down" in that image is pointing towards the observer, viewing from above.

The probe has deposited 11 dots of fairly consistent size, then the width tapers off. The approximate width of the joined line of dots is 60μm, and the length ~160μm.

In conclusion, this is still depositing lines a bit thicker than we want. I'd hypothesise that a slightly sharper angle on the point would create a smaller dot size.

I'm unsure what to expect if the probe diameter was reduced instead. The size of the line probably relates to the angle between the sloping probe tip and the glass slide, in which case the line width would be largely unchanged, and fewer dots could be deposited using the resin coating the probe. This seems to be supported by the results from Probe 1 which produced similar line widths while being 0.3mm diameter.

It may be possible to achieve smaller dots by etching some 0.13mm Nichrome 80 in nitric acid to develop a relatively blunt point, and then switch the electrolyte to NaCl/HCl to sharpen it slightly. Even if the thinning at the tip is drastic, this will increase the distance between the face of the tip and the slide, which would also reduce the diameter of the deposited drop. It will require constant checking during the etch. I might try that next. 

The amount of resin that clings to the surface of the probe is quite small. In this particular case, too small. Most people envisage a hanging droplet, but that's not what happens. First off, the effects of gravity are minuscule on this scale compared to molecular attraction, so things don't hang like you'd expect. Secondly, this is a capillary effect and only the last 100μm or so of the probe has anything on it. Here is a micrograph of Probe 7 before and after dipping in a film of resin:

You'd be forgiven for thinking there is no resin on the probe at all. But if you look very carefully you will see a slight bulge where the distal taper starts. That's the resin loaded on the probe tip. Compared to the area of the point of contact, that's enough resin to deposit several dots per dip in the resin reservoir. This particular probe managed a whole 4 dots but I have had up to 30. This is not enough for our 30-40μm target, so we'll make another probe. I really want something between this and the rounded probe we got with nitric acid. Preferably with that really irregular surface on the sides.

Welcome to the weird world of tiny. 

By the way, this is how I position things at odd angles under the trinocular microscope. I use 10 gram 9mm lead bullets with sticky wax on their bases and a piece of perforated acrylic sheet  that can be moved around underneath the microscope without disturbing the pile of bits. Works remarkably well.

 

Probes etched in nitric acid tended to have a short, stumpy tip with rough sided. I'd found that adding hydrochloric acid to the salt etch removed cloudy precipitates, but the concentration I used was way lower than with nitric acid. So I etched Probe 7 using 50ml of 0.5% hydrochloric acid and 2 grams of salt. I also reduced the submerged length of the probe wire to 6mm.

The etch released noticeably more gas on the cathode, and the etched wire broke off just below the meniscus. For scale, that's a 0.3mm diameter 316 stainless wire:

I'm constantly surprised at the variations produced by merely changing the etching electrolyte. Good job I'm using a consistent electrolysis rig or I'd wonder what they heck was going on.

Again, the actual tip on this one is sharper than I intended. I was expecting it to have a more rapid taper like the nitric acid etched ones. However, it looks like it might be a very robust geometry, so I'm going to test it anyway. Time to mount yet another probe tip, which is much easier now I have the printed jig and less likely to result in probe destruction.

 

There was a minor oopsie and I bent Probe 6. There was swearing. There was also blood. Well, I had microscopes on hand, and I know how to make a blood smear. So here's the bent probe tip. The wee circular things are red blood cells (which don't look so red in ones and twos).

Bear in mind that Probe 6 was not made to be a particularly fine RepRapMicron probe and that a red blood cell of a more or less human is 6-8μm in diameter

 

 

With the relatively thin Probe 6 fitted, a new slide was inserted and all axes zeroed, together with Z Touch.The extrusion width in PrusaSlicer was adjusted to "1.5" (15μm) and the test "Lollipop" printed. The first layer printed perfectly, however the layer height was set to 10μm which proved to be too high and subsequent layers did not print. Images here taken at a 70 degree angle to give some impression of depth.

 

A 200μm hollow cylinder with 2 solid bottom layers was designed as the next test object, the "dipify" script modified to expose the resin to UV briefly during dips to stop the resin running around, and the second attempt was made to print. This appeared to create the first 2-3 layers, and the bottom infill can be seen:

The extrusion width was lowered to 14μm, layer height reduced to 5μm,and another attempt made. For reasons unknown, only one layer printed. It is incredibly difficult with current optics to tell if layers above the first two are being successfully deposited.

It appears that the software modifications are useful, but Probe 6 is too thin to hang on to a decent amount of resin.

Next steps:

• Put dipify code on github. 
• Make another slide.
• Make a slightly thicker probe.
• Retest. 

I have now posted the results of FPath Experiment 010. In this experiment a small stepper motor driven XY stage was created out of the medium linear actuators (documented in Experiment 008) and some LEGO bricks. 

The controlling software for the FPath project was improved and it can now position a small tool (represented in this experiment by a segment of 100 micron wire) with about 10 micron accuracy. This can be done manually (using the WASD keys) or via automatic target seeking behaviour. Furthermore, this target seeking algorythm has been extended to demonstrate path following.

I just thought that some of you might be interested in how it was done. The video explains all: https://youtu.be/rHwGZN5nuRI 

The image below shows the little COTS XY stage. (click on the image to enlarge, watch the video for context) 

 

Hydrochloric acid (Spirit of Salts) is easier to get than nitric acid because people don't tend to make explosives from it, and it is widely used in the building industry for removing concrete splat. So I tried using that to prevent the cloudy precipitate when etching a probe and it worked just fine.

I used 40ml of 5% salt solution with 5ml of 0.5% hydrochloric acid to etch a 0.3mm dia. 316 Stainless wire probe as per earlier. The result came out a bit finer than I intended, but I waited for the end of the wire to fall off and it didn't. So, um, less etch time on the next go perhaps?

Anyway, here's the point compared to a 0.5mm hypodermic needle:

[EDIT] This is now mounted as Probe 6.

I might manage slightly finer detail with that. But as I need the nail gel parts to be at least 30μm thick, this point may be overkill. Still, I've made it so I'll mount it and we'll smash it up once I've got my ducks in a row for the next attempt at layers.

Oh, I definitely need a better USB microscope over the RepRapMicron. It's nigh on impossible to see the resin dots. I might try shaving the end of the housing off this one so I can get it closer to the probe tip.