RepRap: Blog

By carefully folding the edge of the aluminium foil contact plate on the slide, I was able to form a reservoir for UV resin. This is the view from the USB microscope. You can just make out the array of dots on a 100μm grid on the slide below the dyed resin in the foil.

These were deposited with the Nichrome 80 tip. Here is a rather grubby micrograph of a 3x3 area with a hair next to it for scale. Unfortunately the hair oil got on the slide, and it blurs out some of the dots. More unfortunately the slide was not recoverable.

The dots of resin appear to be roughly 10μm in diameter. The purple blotch is a marker so I can find the damn things.

I cleaned the slide and ran again. The probe was a bit closer, so the dots were bigger, but I seemed to pick up a bigger resin load. This allowed me to not only make an array, but to use the dots to build up a square 300μm on a side. Horizontal rows all deposited with one dip in the reservoir. Had to tweak the colours and contrast to make it visible.

This was all done by manual control of the CNC interface, because I wasn't sure of the locations of everything and don't have code to adapt to using a reservoir yet. Also that Y axis is playing up again. However, this is quite the step forward.

# posted by vik-olliver @ 2:17 AM 1 comments

My first physical step down the Feynman Path is a pantograph.

“What”, you say, “something that stopped being hi-tech in the 1700’s is somehow relevant on a path to Nanotechnology”. Well, no and yes. No, the pantograph is not really an optimal solution – I think flexures such as are being developed for the RepRapMicron are the more promising avenue… and... Yes. I do think machinery at the micro and nano levels will resemble massively parallelized versions of older solutions - maybe not the 1700’s but certainly the 1800’s. I will have more to say on this complexity vs simplicity issue in future blog posts.

Back to the point. My primary interest at the moment is how errors might be removed as big devices build smaller devices. The pantograph is a simple thing built out of LEGO bricks and has a reduction ratio of slightly over 5:1. The goal is to create a really nice, visible circle using closed loop feedback to iron out the errors as extremely inaccurate large scale actuators move the tool head about. The tool head has a barrel and lead scavenged from a mechanical pencil mounted on it to record the path.

The image above shows the pantograph. The macro end is on the left (red arrow) and the toolhead is in the center at the micro end (green arrow). 

Closed loop control was used to drive the macro and micro ends around a circular path. The image on the left shows a typical path taken by the toolhead when the control was applied to the macro end (it is also supposed to be a circle). This forms kind of a baseline case representing the inherent accuracy of the hardware. The image on the right shows the path taken when the control was applied to the micro end of the pantograph.

It is fairly clear from the image on the right that the errors attributable to the pantograph mechanism have been greatly reduced by applying closed loop control to the toolhead. Please see the webpage (FPath_Ex006) and short (8 min) video for more a more complete discussion.

Interestingly, both Heinlein and Feynman proposed using pantographs to have large machines make smaller machines. One reason I decided to use a pantograph as my “first device” is to acknowledge their ideas – a kind of homage if you will.  

Labels: Feynman Path, FPath, Nanotechnology, Pantograph, Stigmergic Path Following

# posted by OfItselfSo @ 5:35 PM 2 comments

I fixed the Y axis backlash, and put a temporary brace on the Z axis. Good move, must build a proper one. The brace eliminated probe wobble, though the touchdown point on the slide when the probe makes contact is still larger than I want it to be. That one is going to need some study, but the next major phase is going to be building with UV resin and for that the probe doesn't actually have to touch the build surface. I'm going to enlarge the hole in the XY Table under the build surface, because the aluminium foil contact plate can be used to shield the resin from the under-slung UV light.

I've tested out the latest Nichrome 80 probe, and I think an 8mm tip is too long. I put a dollop of hot glue on the shaft to help stop it wibbling around, but the slide is still dragging the point about. Again, future plan is not to hit the slide. Bear in mind that the probe isn't just touching a slick glass surface here, but trying to plough through about 5μm of solidified Sharpie as well. It has reasons to be behaving badly.

Good to see that the probe works well with the aluminium foil touch plate, and that the tip survived being accidentally over-driven into the slide surface by 50μm. I haven't tested it yet, but the surface of the 316 stainless steel probe is much rougher. That might actually help when it comes to transferring UV resin.

Finally, I've been talking to Eddie about doing the video, checking the AV equipment available. Sorting out the order of things, workspace, editing software, the logistics of file transfer etc. We have a plan. All considered, the first step is for me to come up with at least an approximation of the assembly instructions. I've done assembly manuals before, and there's a process to doing it right. The bad news is I don't think I can get it all down before the 16th, which is when the trip to Everything Open begins. I'll release printable STLs of what I have before I go, whether the instructions are ready or not.

# posted by vik-olliver @ 7:56 AM 0 comments

In an attempt to make a more rigid probe, I started with some 22 gauge (0.6mm) "Nichrome" wire. This took nearly a quarter of an hour to corrode away in the electrolysis cell, and when the end finally fell off it looked like this:

You do not have to be an expert metallurgist to see there's something ... interesting going on in the manufacture of this brand. There is a clear spiral pattern to the erosion, and fibres of incompletely mixed metal in the alloy. While sold as Nichrome, the packaging itself only states "Resistance Wire" and it appears that resistance is futile.

I'd recommend getting wire from a manufacturer that states the percentage of nickel and chromium, and make sure the numbers add up to 100.

# posted by vik-olliver @ 3:12 AM 0 comments

I found a limitation with jscut in that it always closes a path, so if you draw a 'U' it will join the two ends at the top. So jscut is not suitable. After looking for something that works, I found that PrusaSlicer imports SVG paths (though they need re-scaling for undetermined reasons).

So I wrote a RepRapMicron printer configuration for PrusaSlicer based on Marlin and told it not to use the extruder. It put 'A' commands and 'M' commands in the GCODE though, so I used a post processor to strip them out:

/bin/sed "s/ A[0-9.-]*//";grep -v "^M"

This allowed me to import an SVG line drawing and produce this:

 Due to the probe impact problem producing a bloody great horizontal line every time it hits the slide, some things are a bit obscured, but that's a 500μm line with end marks and the letters "500um" underneath it.

So now I can draw open-ended paths and can get on with creating some calibration patterns manually.

I also really need to print a microscope slide storage box to take to Everything Open. Must remember to set the printer type back...

# posted by vik-olliver @ 12:13 AM 0 comments

Happy New Year! Is there anything that I should put at the top of my New Year's Resolution list to release, document, or improve things that you want to see? 43 people have checked the new git repo, please do tell.

I wrote the GCODE for a calibration zigzag, 500μm lengths, 50μm spacing, repeated along X & Y with a staircase gradually lifting the probe by 2μm for each segment. I ran it twice, and took an image of the calibration slide with the same camera settings:

Immediate things, X scale is off, the lines are only 415μm long, not 500, fixed that in GRBL config.

Lighter lines are generally more consistent, which figures as less stress on the probe.

Y axis seems to have consistency issues being +/-30μm. Needs an overhaul. I'll probably do a diagonal calibration slide later to test the overhaul. Even so, that's literally within a hair's breadth.

[Edit: It seems one of the bands used for backlash has broken, which is going to tilt the drive nut...]

But the X axis, other than the scaling issue, seems to be positioning +/- 10μm absolute which I am moderately happy with for now.

Start point divots are reduced, possibly by a 5x reduction in plunge speed. I noticed a lot of vibration of the probe through the microscope. Not sure how much of that is the axis mount, and how much flexibility in the probe, but both need to be addressed.

I'll take a peek to see what's up with the Y axis. Now I have had experience in handling the setup, I think I might risk a decent probe with a more symmetrical cross section and see if that improves things.

This all brings to the fore the question of end stops, so that I can guarantee the same starting point every time and thus experimental consistency.

# posted by vik-olliver @ 2:33 AM 0 comments

Now I have a binocular microscope. Ho-Ho-Ho. Santa was exceptionally kind to me this year (thank you very much, Suz). It's a Konus Crystal-45 with a 2x objective, so it has 90x zoom magnification - even more if I jam my old Barlow into the camera port and peek through that in monovision!

I've not got the camera attachment working properly yet. Working on it. I suspect 3D printing will happen. However, I have MacGyvered the phone onto it, and it's not half bad.

Anyhoo I decided to try a backlash test on the X & Y axes, and a Z height drag test. The hypothesis was that I would be able to see line displacement along the axis as it changed direction, and from that calculate backlash.

The method was to make clear contact with the slide using an expendable 24ga hypodermic tip, uplift, and move to (0,0,100μm). There the probe was lowered to Z=0, moved along the +X axis by 0.5mm, down -Y for 0.05mm, back on -X for 0.5mm, down on -Y for another 0.05mm and repeat until I get fed up with it.

The procedure was repeated at 90 degrees to exercise the Y axis. Following that the probe was raised 1μm at a time and stepped along -X and -Y, unfortunately not on a regular spacing (sorry). Results look like this:

As you see, colossal improvement in image resolution. Shame about the probe movement. Not at all what I was expecting, which is why we experiment, right?

Analysing that, we see a big divot at the probe start point in the top left. The downward velocity of the probe may have been a bit high. It also does not line up on the X axis with subsequent lines. As this was done manually, it may be operato,r error (Note: Write actual GCODE with slower Z descent, and some repeatability).

Also obvious is that motion on the Y axis is notably more consistent than the X axis. There is a suggestion that motion on the X and Y axis is not proportional.

There is a significant curve on the zigzags where there should be sharp corners. This persist as the Z lift manoeuvre progresses along the staircase.

My overall conclusion here is that the probe probably dug in too deep, and is being dragged around as it scratches the marker off the slide. X/Y distortion differences may be due to the geometry of the probe, which is aligned along the Y axis. As a side note, the removed material can be seen accumulating at the end of the longer movement segments.Nonetheless, the major lines are meant to be 50μm apart ("human hair"), and so positional accuracy under these condition still appears to be less than 25μm.

it would appear that processes that have high drag on the probe are not conducive to accurately measuring backlash or probe positioning capability, but that with more consistent test procedures the effect could be better analysed.

The GRBL is now being driven in 1mm = 1μm scaling, and as a separate experiment I drew the 400μm logo again. The regular distortion on the arc segments of the logo is no longer present indicating that those were indeed artefacts caused by attempting to use GRBL for very small movements. Other distortions exist, but may be due to the effects described in the first experiment.

# posted by vik-olliver @ 6:03 AM 2 comments

Late Xmas present for you. I've put the OpenSCAD model files for the Maus prototype on github https://github.com/VikOlliver/RepRapMicron/tree/main/maus and they should be buildable. If they're not, I'd like to know.

First, download all the files in the "maus" and "library" directories.

[something something]

 and then it looks like this:

Well, not quite, because I've made the XY Table base mount simpler, but you get the idea.

I'll be putting the STL files up on Printables and getting some kind of HOWTO together, but there's a bit of prep work for trip to Oz and a presentation on.

# posted by vik-olliver @ 4:30 AM 0 comments