RepRap: Blog

RepRapMicron PIKA has now left the current documented version behind and moved on to V0.03 - so if you want to get the files for building V0.02 you'll have to get them from before the V0.03 tag.

This happened because the PIKA Axis Driver is now sufficiently different to the MAUS Axis Driver that they no longer fit. The good news is that the PIKA driver is a lot cleaner, has fewer confusing holes, has easily replaced/installed tension bands, a simpler system for attaching Z flexures, and gives you a way to access all the stepper motor screws.

Changes a dropping piecemeal at present, but I've made a temporary downloadable of Printables https://www.printables.com/model/1894754-pika-v002a-sideband-axis-driver. Latest files look like this:

 

Thanks for debugging, Jon!
 

I've dropped another video. This one describing how the screw-driven Axis Driver does its thing: https://youtu.be/LRabtyVBjJM

 

Sneak peek. I'm planning on removing a bunch of extraneous screw holes in the PIKA Axis Driver. They just confuse people, make the frame weaker, and increase the print time. While I'm at it, I'm thinking of changing the way the backlash bands attach to the framework, making it easier to assemble and possible to change the bands without dismantling the whole thing. Also might make it easier to assemble in the first place. If anyone objects, let me know. Photo of the planned changes:

 

For those wanting to know more about RepRapMicron in video format, I've started putting them up on YouTube. The first two are more setting the stage. This first one explains the basics of what it can do:
youtu.be/JwNy73c8Wak

The second one gives a broad overview of the modular parts by trearing a RepRapMicron apart:
youtu.be/Chl6i1mCL2k

I've  been asked how the heck the reduction mechanism works, so I'll cover some of that next.

If you're after the UV Head details, the wiki section for that is now ready:

 

The UV beam sets resin about 6μm wide. Prusaslicer doesn't allow a layer height of more than 6μm for that, but the UV goes on for a lot further. So I have adapted fibre_process.py to use a Z_SCALE constant. This allows the model to be compressed vertically by, say, 50% in Prusaslicer and then be expanded to full height again in the filter to produce the full-height GCODE. Here's a Benchy printed that way:

If you zoom in you'll see that we have lost a bit of vertical resolution. However we have also gained resolution under overhangs, because there is less cumulative UV overshooting the edges. So Benchy's hull has more of a curve to it.

While the use of this is debatable in creating artistic models, for the grunt work of creating more or less planar mechanisms with little if any overhang it means I can print them twice as fast. Possibly more than twice.

This one I have more faith in. It turns out that the "Infill before perimeter" box had somehow become unchecked in Prusaslicer. As you can see from the images below, this 950μm long (nearly 1mm) pantograph is significantly better:

I made it 48μm thick (probably slightly over 50 in the real world) which I hope improves durability somewhat. Note that even with the assumption in the slicer that the "extrusion" width is 6μm it does not look like 65% overlap is excessive and the infill doesn't look too dense. I think there are interesting optical effects going on. I am driving the fibre somewhat out of spec after all.

As before, now to attach one end with nail gel, free the rest of it from the Krapton Tape, and see if I can flex it without breaking it. Place your bets please, ladies and gentlemen... 

I'm not holding up high hopes on this. Although I've increased the part thickness to 60μm, and set the perimeter overlap to 25%, the perimeter is still not attaching and it looks a wee bit fragile. If you look carefully at a zoom-in on the top of the micrograph below:

you will see that the stray bit of perimeter flapping around in the breeze is ribbon-like. This means that it was printed vertically, and then detached. Because we're dealing with a light beam and not extruded filament, this means the light beam did not intersect with the infill. Because it is deeper than it is thick, this strongly suggests that it is the first layer. No surprise then that subsequent layers go awry.

I'm beginning to wonder if the print width is less than 9μm. Thinking about it, an optical fibre has  a core and a sheath, so it's quite possible that the UV beam is much narrower than 9μm. I would print single lines and measure them but the bloody things don't stick!

So I have reduced the print width to 6μm and increased the overlap to 65%, and we'll take it from there. Test subject is another Benchy because I'm making them for fund raising. I didn't have to do that for the original RepRap Project, as it was funded - albeit the lowest project funding Dr. Bowyer had ever used. Less, in fact, than the cost of a 3D printer at the time.

Anyhoo the pantograph above has its lower test pad anchored to the Krapton Tape with nail gel. It now has to be prised loose and moved. Place your bets on: Broken during separation, broken during feeble articulation, actually moves, or pings off into the aether?

Still working on the print quality because larger prints seem to have more issues, however this is a print of a parallelogram flexure, otherwise known as a pantograph:

 It is 1180μm long (a little over a millimetre). This particular one is 30μm thick because I'm still playing around trying to get "large" structures printing properly and am just testing the base layers. It is scaling movement to 50% just to keep things visible.

Some of the more astute of you will have noticed a faint resemblance of this print to the flexure system in the RepRapMicron Axis Driver module:

 

The flexure part is ~100mm long. 100x smaller. I think you can see where we're going here.

Now nobody makes M0.03 screws, and the kinematics of assembly are fiddly, so I'm not going to build a direct copy. There are very basic details to be sorted out, like getting it off the build plate and anchoring it down.

I took an even worse print and experimented. To anchor it down, I place a tiny speck of "Gel Polish" UV nail art gel ("Jelly") nearby the print before I detach it, and trail a very, very fine trail up to the part. This stuff is much thicker than 'Top Coat" or 3DP resin, so it is less likely to creep and engulf the part. Before it creeps very far I turn on a UV lamp to solidify it. Then you get this:

Getting the part off the Krapton Tape is best done in a droplet of isopropyl alcohol. This seems to ease it off the Krapton and stops the part moving too quickly and flying off. Then you have a mobile mechanism firmly attached to a substrate and the experimentation can begin. Well, once I have a slightly thicker one anyway.