RepRap: Blog

Shaping is more complicated when you try to automate it blind. The probe tip gets shorter as you etch it, and its electrical characteristics change. Eventually though, I managed something replicable:

Etched 0.3mm dia. 316 stainless steel wire points

A spot of corrosion on the lower one. Must've forgotten to wash it after I made it late last night. That's not coming off. But the profile? Pretty darn close, even if not exactly what I was aiming for. Repeatable probes means repeatable experiments, scientific method an' all. Nice to see the machine making its own parts already too.

The breakthrough was when I noticed that using a previous (badly) etched probe gave a better point second time around. By sheer bloody-mindedness and tinkering I have devised the following scheme, all using the new acidified NaCl etch:

Roughing

This roughens the body of the probe and takes the worst of the cut marks off the end. 30 sec etch at 15mm is what I'm using just because it's a known starting point. Roughly 3.8V at 160mA.

Shaping Pass

This starts off with  0.5mm dips, then 0.15mm, then another 0.15 mm but pushing deeper 5μm at a time. As the probe shortens a lot of these miss the surface, but shaping happens (I suspect there's some self-correction going on here). Current wobbles around a lot ~50mA to start ~5mA at the end. The probe end is roughly pointed but may be rounded at this stage, it varies. I have GCODE to do this to ensure repeatability of motion.

Finishing Pass

The probe is zeroed to the surface and the Shaping Pass repeated. Probe characteristics are less variable now, so a more even shape emerges.

Notes:

The wire but be cut as cleanly and squarely as possible. If the cut end is kinked, this indicates your cutting tool is blunt and not cutting cleanly enough. Good, sharp side-cutters please. 

Roughening is just done with an accuracy of "minute of Sharpie". Mark the wire at 15mm and clamp it with that mark on the meniscus. 

By "zeroing" I mean advancing the probe towards the meniscus until it contacts. If you overshoot, you need to back off about 300μm so that the meniscus drops off, and slowly sneak back on your assumed zero. For the Shaping pass I figured 5 microns was close enough, and got to 1 micron for the Finishing Pass. Don't know how critical those measurements are.

I found it important to wrap the anode lead croc clip in aluminium foil to give its grip some compliance, or the wire drops into the pot.

The anode lead also needs to be secured to the Z Axis Driver to reduce unwanted movement of the probe by the lead - I'm just hanging it over a screw on the Axis Driver Frame Trio.

They look much nicer if you wash them in very hot water immediately after etching.

There are probably optimum shaping and finishing passes but for now I'm keeping it simple. Don't know if more passes help as I've not tried it.

# posted by vik-olliver @ 5:58 AM