RepRap: Blog

I fired up the PIKA and ran a test grid. Been having a few camera issues after the laptop upgrade, but that's sorted now. More generally useful stuff on that in a later post, TL;DR vlc, mpv etc. aren't doing what I want so I've hand-rolled something.

The python script for creating the grid GCODE is here: https://github.com/VikOlliver/RepRapMicron/blob/main/utils/grid_gcode.py

Here's a 10x10 grid of 100μm squares etched into marker using Probe 9:

 

Spacing is pretty good on the Y axis, but the X axis needs a bit of love. The X deviations seem to be regular, so I suspect I just need to centre the Drive Screw a bit better.

Line widths are different on the X and Y axis this is because my particular PIKA prototype has the Z Tower 2mm too close to the centre and I have to tilt the probe at an unholy angle to get it in contact. The PIKA chassis takes a lot of printing, so I'll probably print a temporary probe assembly to work around that, which is a much smaller part.

The actual consistency of line width (given this is subtractive scraping) is reasonably constant over a 1mm square, though does fade out a bit towards the top right. My office got unreasonably hot yesterday, so I think I may need to re-level. 

Just to help Jon Oxer out during https://www.youtube.com/watch?v=r19WoewJ_Ys. These are the photos of the V0.01 PIKA Base assembly and Axis Driver integration that I took for the docs:

 

No code changes, just ran it. The old 30μm/pixel version, 1.2mm long . So, backwards compatible.

 Used a hypodermic needle probe, so not the most elegant or fine dots, but well spaced.

The RepRapMicron probe needs to be within the print bed surface to within a micron or so. Levelling a bed on an ordinary 3D printer manually is a non-trivial exercise, so how the heck is it done on a μRepRap? Well, the PIKA has an answer to that: springs. Under the Stage there are four springs (using 6.5 dia x10mm ones of 0.5mm wire at present) that can be screwed down to adjust the height of the corners. The exact spring dimensions don't matter, but they have to have a compressed length of 2mm or less, a normal length of 5mm or more, and fit over an M3 screw. Yes, you can cut springs up, but fold the end so it doesn't dig into the plastic. The setup looks like this:

Functional PIKA files are now on github. 

The long screws are used as a guide to set the Stage height to roughly 3mm and are removed afterwards. I did try spring washers, but they don't have enough reliable range of motion.

To make life easier, I have written a very basic python script  https://github.com/VikOlliver/RepRapMicron/blob/main/utils/levelling_probe.py that will touch the probe on 4 corners of a 1mm square and report the high and low ones. A turn on a levelling screw equates to about 8 microns at that scale, so setup is humanly possible.

The conductive target is a "Touch Plate" slide onto which aluminium kitchen foil has been burnished and stuck with resin. The process is:

• Fit Touch Plate to Stage and connect grounding washer to the foil-covered end.
• Using your CNC GUI home all axes and position the probe to within 0.2mm or so of the surface.
• Close CNC program, run levelling_probe.py for a rough result (you'll need to edit the USB port in the code to match your μRepRap's). Run it 2 or 3 times to let things settle in.
• Screw down high corner, raise low corner until they are all within 20 microns.
• Undo the Y Axis Driver and re-secure to remove any tension built up during adjustment.
• Repeat  levelling to within 10 microns, remove tension in Y Axis Driver again.
• Level to within 4 microns. 

The script will return the probe to the starting position so it can be run in the same pattern repeatedly. You want to run it multiple times as the probe tip will push the foil down onto the Touch Plate slightly when it contacts a new spot. The Touch Plate will also spring back to some extent over time, so you always need to repeat.

The aluminium foil Touch Plate isn't reliable beyond 2 microns, so don't go hunting perfection at this point. This should be good enough to get going.

Caution: Don't rest the end of the slide on the Y Axis flexures. Easy to do by mistake.

Here's hard data. The first pass was done last night after I'd done levelling. The second pass was done this morning: 

vik@haast:~/uRepRap/utils$ ./levelling_probe.py 

--- Probing corners ---
BL: -602.094 µm
BR: -602.844 µm
TR: -605.688 µm
TL: -605.625 µm

--- Relative height to average ---
BL:   +1.969 µm
BR:   +1.219 µm
TR:   -1.625 µm
TL:   -1.562 µm

Highest corner: BL
Lowest corner : TR
Total variation: 3.594 µm

Returned to (0,0,Z_SAFE) for next run

[Rerun done first to re-level the touchplate surface - omitted] 

vik@haast:~/uRepRap/utils$ ./levelling_probe.py 

--- Probing corners ---
BL: -598.250 µm
BR: -600.844 µm
TR: -598.969 µm
TL: -597.750 µm

--- Relative height to average ---
BL:   +0.703 µm
BR:   -1.891 µm
TR:   -0.016 µm
TL:   +1.203 µm

Highest corner: TL
Lowest corner : BR
Total variation: 3.094 µm

Returned to (0,0,Z_SAFE) for next run

The initial hack for PIKA had a few shortcomings. Notably if you moved it you'd lose calibration, and it was extremely difficult to put the Stage mounting nuts in. I've addressed that and put the V0.01 on github (V0.00 wasn't really a version so it's not tagged).

I've recessed the screws in the flange on the bottom of the XY Table so they don't hit the Axis Drivers, and simplified the Base design so that separate feet are not needed.

 

This has made the whole thing much simpler to assemble, and I hope to tackle that bit tonight.
 

RepRapMicron is two years old. Just saying...

Updated maus_pika.scad and metriccano.scad on the github  with a reinforced XY  Table, microscope pole clips, and Stage nut holes that you can poke nuts into without superpowers.

Sorry for the gap in posts, lots going on in my personal life, and the printer is running commission prints. Anyway, PIKA had a few problems. The PIKA frame was not sturdy enough, the Z Tower was too close, it was insanely difficult to insert the Stage mounting nuts, and the Microscope Brackets weren't as stable as I'd like. So, changes:

 

The base now has a much thicker edge, and there is a stiffening band 15mm down from the top edge. That should stop things going out of alignment if the unit is moved. The Z Tower has been moved back a bit, and extra bracing thrown on to keep that corner stable. There are now only 4 holes to mount the stage, and the mounting nuts are inserted from the more easily accessible side (at 45 degrees, sorry, only way I could do it given the print-in-place slope). The Microscope Brackets have been moved closer to the centre, and the bottom one lowered. This increases stability, and makes the lower adjustment screws easier to access.

The Microscope Clips will now have to be redesigned slightly - not quite enough clearance on the V0.04 ones. Once I've done that, and this massive commission job is done, I'll post to github and test. That won't happen this weekend as I'm away at a competition, but my life will settle down again after that.