RepRap: Blog

Now I can actually move around and scratch things with the probe I'm noticing an awful lot of variation in the Z axis. Whenever I move to a new bit of the slide, the height is out. I don't know if this is just the setup I have, or if it's a thing with the delta stage in general.

However, I've got a lot of data and micron-scale experience from the delta stage. Time to move on to the XYZ stage that has a smaller work area (4x4x2mm) but higher accuracy. Allegedly down to 250nm. That'll take a while to do, so I'll be playing with the delta for a bit longer while I sort it out. The good news is that the XYZ stage should work with standard 3D printer firmware and I can ditch a lot of the more painful delta work.

So there are a few things I still want to explore with the delta, mostly involving photopolymer resin tests. I have resin, and a 3W UV lamp is on order. I doubt I'll break any resolution records in the initial tests, but if I can make objects smaller than a millimetre that don't look like a jellyfish I'll be happy.

Here's how it works so far. Getting the Z height dialled in is tricky, and I can no longer really see what I'm doing - I'm looking for a spot of light coming through the Sharpie layer on the slide. I have moved to a different marker pen which has a much finer grain to the ink.

So this image shows a trail of Z height adjustment leading up to a 50μm square with a line drawn in the middle of it (actually a 20/30μm split). Note that the Z height is moving within the thickness of a layer of Sharpie ink as I guide it in. Probe tip appears to be about 5μm.

Then disaster. I switch from 0.001mm steps to 0.1mm steps to guide the probe out of the way, go the wrong direction on Z, and plough into the slide. The huge gash resulting (top right) is about the thickness of a human hair. The image was taken on a turret microscope after I removed the slide. My best USB macro camera has a fraction of that magnification, and I'm working on a new mount to get it closer. I'm guiding the Z height initially by looking at a side view from another USB macro camera. I'll post a picture of the setup when I fire it up next.

Upside: I'm now showing a feature 20μm in size, which beats the resolution of a high-end resin printer if I can keep it up.

Here is a cross made from 0.1mm squares, or 100 microns on a side. Etched into sharpie on a glass slide with a 0.3mm base conical nichrome tip. This is not my sharpest tip but at least the lines are relatively consistent in width on X and Y unlike the hypodermic needle tip. I'm now running into the limits of Sharpie as a substrate. I think I can call it micron scale now with some plausible justification.

Calibration slide shows 0.05mm or 50 micron squares.

Nothing like proving your results. On the left we have my notional 0.2mm square (somewhat dusty by now) scratched into a sharpie-covered slide. On the right we have my new 0.01mm/div calibration slide. The small squares in the centre are 0.05mm a side each. So yes, I'm at about 0.2mm and can prove it. I'll freely admit I need to get my steps/mm dialled in a bit better but my efforts are pretty much within the error of line widths. With the big microscope though it's possible to see that even with the (relatively) chonky hypodermic needle tip the interior corners of the square are nice and sharp.

We can also see that the hypodermic tip is cutting a swath about 25 microns in one direction and 50 microns in a perpendicular direction, which one might expect as the things are obliquely sharpened.

The image of the square is at an angle basically because I mounted the slide in the μRepRap at an angle when I cut it. Bits of equipment just got in the way...

I've made headway into being able to set up the Z axis in software, so I might risk a slightly sharper, conical tip. I'll make one out of 0.3mm wire and try not to make it too pointy.

EverythingOpen conference presentation done, back to project work. I'm implementing CNC-like controls to home the axes and set the zero positions for XY and Z to allow CNC-like control.

This is not a straightforward as it seems. I can't just tell the GRBL to home because the XYZ on the GRBL does not correspond to the XYZ of the stage. So to do a HOME I have to first position the stage XY to zero, then move stage Z to zero. This stops me dragging the probe over the stage, and I think that bit works.

Setting new zero positions for XY and Z before beginning "work" has a similar problem, so I have to design a relative zero for all axes and factor that in for all move commands. Haven't done that yet.

Finally, I'll want to read in a gcode file and move the stage around. Again, this requires translating stage positions into relative positions, then moving the towers with GRBL. I'll experiment with using jscut http://jscut.org to generate the gcode from SVG files. I'll probably have to implement some kind of scaling system for the gcode because I'm not sure how jscut will handle very tiny numbers (Inkscape certainly fails on this).

Minimal updates for a week or two as I'm preparing for and heading to EverythingOpen in Gladstone, Australia to present on the Quirkey accessibility keyboard. This takes me away from the hardware which is a bit of an embuggerance in the development department. I'm deliberately not fixing a broken axis at the moment so I concentrate on the presentation, trip prep., instructions for house-sitter, last minute house maintenance etc.

There is some new software I'm working on though, to create test gcode patterns. I have that and new stage designs to work on while stuck in airports and hotels, so expect to be fully occupied.

Just posted this on the wiki, and posting here for comment:

"As with the original RepRap, it is hoped that once the first μRepRap is operational, it will be able to replicate. Unlike the original, the replicant will not be identical to its parent but will be a much smaller functional equivalent. Control of the replicant will be difficult, but not impractical. Options include perpendicular magnetic fields, inducing vibrations to operate ratchet mechanisms or tuned structures, or at its most basic cranking the mechanism with the tip of a probe until something better can be worked out.

It might also be possible to fabricate a μRepRap that is scaled down further. Assume that a RepRap has a precision of approximately 0.2mm on a 200x200mm work area, and a μRepRap has approximately 1 micron resolution on a 10x10mm work area. We see roughly two to three magnitudes in reduction in scale. If flexure systems also scale, then it might be possible to use a μRepRap to create a smaller printer with a resolution on the nanometre scale. Unlike current attempts to operate with nanometre precision with macro scale hardware, errors in the device due to thermal expansion and other material-related noise would be much reduced. It would at least be interesting to see how far this limit could be pushed."

I keep snapping the 'O'-rings on the OpenFlexure stage, probably because I'm pushing the range of movement more than the average microscope user. That's going to have to change, probably using anti-backlash nuts like the old Darwin printer's Z stage. There are a few other things I want to hack about: Proper motor mounts, extra platforms and attachment points, new flexures, better gearing, more access the the mechanism, cable guides etc. but it's going to have to wait until I've finished presenting at EverythingOpen. Once I've decompressed from that trip I'll be able to focus on μRepRap properly. I have some photosensitive resin ready for dip-pen tests too, but blowed if I can find my stash of UV LEDs anywhere.

I plan on having a darkened resin reservoir on the slide, and illuminating from below. This will avoid having to shield the probe or the resin reservoir during UV exposure cycles. Hopefully cheap soda lime glass slides will let enough UV through.