RepRap: Blog

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. 

I kinda got some layers in a recognizable form. This attempt was a shakedown of the hardware and software pending the fitting of a finer probe. There were two print attempts of a 400μm "lollipop" sliced with PrusaSlicer and the dipify_gcode.py script with Top Coat nail resin. I changed the dipify script so that the reservoir is now at (0,-1000), and as long as I set (0,0) past the edge of the foil reservoir the probe should get dunked.

The first one was intended to be 35μm tall in 10μm layers, the second 50μm tall in 8μm layers. Neither worked to plan. I tilted the slide under the trinocular microscope so you can get some idea of it in 3D:

Sorry about the fuzzy, but the microscope has a limited depth of field.  The purple blob is a marker so I can find things, made from UV resin and a bit of Sharpie (actually a Bic Vivid). It set solid, and I might try integrating marker ink again. I can't honestly tell if the closest object is actually 30μm tall. I strongly suspect that the probe only contacted for the first layer. So I tried again with a thinner layer. The one at the back is definitely thicker and I'd believe 50μm. I could see contact for the layers so it was layering. but it blobbed out. Why? Well, let's look at the probe:

 

It's covered in gelled resin (which mostly washed off). This caused it to dump a lot more resin than intended on the second object. In the future I'll have to get the probe lower and further back to stop the reflected UV from gelling the resin.

Here's what it looks like from the top. Remember, the circles are 0.2mm across in this one:

The To Do list looks like:

• Label and store this attempt.
• Make a new slide.
• Make a new probe.
• Modify dipify script to lower probe before UV exposure.
• Try again. 

I've sorted out a niggle with the RepRapMicron's ground plane. It was tiresome to get the spring-loaded clip onto the aluminium foil that's used as a reference touch plane and UV shield for the reservoir. I've soldered the ground wire to the M8 washer that magnetically clamps the slide down. You clamp the slide in place, and it's grounded!

I've stuck the wire to the other side of the washer with hot melt glue as a nod to strain relief.

As I've increased the length of the slide covered in foil from 25mm to 30mm, the edge of the foil/reservoir can go right up to the UV LED, with the probe at (0,0). This means I do not have to set the reservoir location in code every single time. I just relocate the part on the slicer's print bed. I get more working volume too.

You do have to make sure the probe goes a fair bit over the reservoir to stop the UV LED underneath setting the probe tip into a solid block!

The "dipify" code had a round of debugging, mostly to do with maintaining safe Z height.

I'll try the new slide and the latest dipify out before making a finer probe.