RepRap: Blog

Layers, like an ogre (ref. Shrek et al., 2001). As with most experiments, before attempting a multi-layers print I conducted a trial run using a robust, relatively blunt old probe just to find my unknown unknowns.

The test object was an 8x8 hollow square of voxels spaced at 30μm, layer height set to 10μm. The test pattern turned out to be largely irrelevant due to the size of the probe tip, and an ill-defined square was what ended up being printed, and I ran out of resin at the end of the square. No matter, trial run isn't it? The experiment continued.

A single layer object was left as a one layer control, the probe moved over 500μm, and a second object printed in the same manner.

After the first "layer" the probe retreated to the resin reservoir and the UV LED was activated for 12 seconds. The probe was then dipped, and became over-saturated so I wiped it on the reservoir edge. Not ideal, never mind, soldier on.

The probe then returned to the object origin and was manually lowered until a change in the meniscus of the resin on the probe was seen, indicating contact. This occurred at a probe height of 10μm. The test pattern was then recompiled and printed at that height. Set as before with UV.

After recharging the probe a third pass was made. As anticipated, contact was made at 19μm height, and the pattern compiled/printed. In theory, resulting in a 30μm high object. This was then set for 3 minutes under a 4W UV LED lamp 40mm distant.

So, the result. The slide was put in a slide holder, and inclined at a slight angle under the binocular microscope to get a side-on view of the ill-defined blob. Here I have a comparison between the single-layer and three-layer objects:

Glass substrate is on the left side.  Looks like the layer heights determined manually are approximately correct. Doesn't look so nice from the front, so I'm not calling this a "successful" print, but it was a useful experiment.

Takeaways:

Repositioning of the probe after dipping is accurate to within detectable error.

The aluminium foil reservoir adequately protects the resin, and shields the probe so it doesn't set solid.

The UV LED does not take out the microscope camera.

Blunt probes are no good for accurate resin deposition.

Resin layer with a blunt probe depositing >30μm diameter dots roughly 10μm thick.

More care needed to keep the resin reservoir film at an absolute minimum thickness (swab with a cotton bud if needed).

Printing needs to happen centred more over the UV LED in the bed light well to speed curing time.

Even if accurate layering turns out to not be possible, it appears that creating thicker areas can be achieved, allowing for foldable structures to be generated.

Need to print a proper jig to hold slides at an angle for imaging.

 

What's next? The olive harvest. Nature does not wait for the schemes of men.

Had a thought while trying vainly to sleep last night. I've tried using a parallelogram flexure before to make a pantograph, which in theory works but in practice is not sufficiently stable. I realised that if I made essentially an asymmetric pantograph I could at least achieve stable and constrained motion in one axis:

There's probably a proper technical term for this arrangement but I don't know it. When moving the free end (yellow) towards the (blue) pivot, which would be done with a drive screw, the joint indicated by the green arrow also moves linearly towards the pivot over a reduced distance. This mechanism would be relatively easy to constrain and hold firmly.

The result would be a linear actuator that didn't pivot, unlike the current design which basically relies on a rotating lever which effectively gets shorter towards the limits of its range of motion.

Assuming I can get a 10 degree total movement out of the flexures, and want to have an 8mm range of motion with the same 3:1 reduction ratio, my vague maths suggests the shortest linkage needs to be about 23mm long. That would make the long beam 92mm long, which is a bit longer than the current beam but not outrageous. I suspect the flexures would survive more than this but might not be linear at the extremes, so the beam could be shorter.

Thought: Might have to extend the drive screw to 60mm.

I don't intend to pursue this immediately, so feel free to have a go at it yourselves. I still have much to learn from the existing mechanism, which while not perfect is quite good enough for this stage of the project.

I've been making a few new probe tips. Keep 'em all safe in a little box like this:

You can't tell with the naked eye what the tip looks like. I thought I'd try making a couple consecutively and see how different they are. I've experimented with just running the electrolysis on the probe wire until the tip just falls off, as in this picture:

And letting it run for a just bit longer like this:

 

Both of these are 0.29mm 306 stainless wire from the same batch, etched in 5% salt solution with 3xAAA batteries in the same jig, and the photographs have been taken at the same magnification with the same microscope and lighting.  While the first one has a much longer thinned shank, it is interesting to note that the tips end up coming to much the same size and sharpness.

Frankly I was amazed at the difference in profile from much the same process. I do not yet know how this will affect the probes' resin-carrying ability, but I suspect the thicker probe would deflect less when used to mechanically interact with items on the print bed.

 Better make some new print beds and test them out...

Fast approaching the point where μRepRap is going to need an interface for direct manual manipulation rather than micron precision. Using conventional CNC jog interfaces just won't cut it (ahah). It's going to need a more complex motion control system, and I say gamers know about that already. So enter pygame.org

The idea here is that this is a widely used platform that's relatively easy to use. μRepRap seems to have fallen into python somehow anyway so may as well make the most of it. The graphics we'll need are pretty minimalist as the user will mostly be looking at a microscope, and pygame provides interfaces to the joystick/controller of your choice.

That's the vision, commentary and criticism welcomed. Two things need to be done before I start on that: FAB25 μRepRap presentation in Prague, and demonstration of multiple layers. So if I'm quiet on the blog for a bit, it's because I'm flat out doing that.

 

That day in 2008 when we announced successful replication to the world. Made a bit of a splash.

 

The plan: Get some of this fine fake gold foil, which is basically ultra-thin brass, and burnish it onto a glass slide like I do 30μm aluminium foil.

Then carve into it with the probe tip to make conductive traces, either in lines or little dots much as I've done with Sharpie marker. I have already poked holes in foil, and others have made macro scale circuits from fake gold foil, so the principle is sound. You can even solder onto it.

If I do all this near manually scored lines, I should be able to construct contact pads. Then I can use the probe to do things like add ionic gels or active resins etc. across conductors and create circuits. With gels that would be done by diffusing the copper into the gel electrolytically. Other options include creating photosensitive components from copper sulfide and so forth.

It's on my Amazon wish list...

 

The previous Flexure Coupling was a bit too stiff, and also perched the connecting nut right on top of the coupling. This in theory allowed a bit of play for self-centring but but in practice this needed a lot of manual (and physical!) adjustment. As I noted when building a μRepRap for someone else with proper limit switches etc., it also tends to strip when you hit the limit, and have forgotten to connect the switch. Amazing how your attention focuses when you're actually going to hand something over to someone else. So, new version:

 

This one completely engulfs the nut in the tapered cavity, and has much thinner flexures. This gives more lateral play for removing wobble. Note that this flexure design doesn't tend to twist and introduce backlash. Quick test shows they centre pretty well, so I'll install a set on Maus #2 and see how they go.

The buttpain here is that the coupling now needs to be printed with a brim as it is too delicate to stay on the bed otherwise. So I have to redo all the build files in git, rebuild and upload the build plates, take new photos, and update the build and calibration documentation. Damn.