Sunday, November 16, 2025
Measuring Probe 8 Resin Carrying Capacity
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.
