RepRap: Blog

I've decided to bite the bullet and update PIKA now. Main differences are a tripod-style Stage levelling mount, and an integral microscope pole socket. These changes alone remove 24 parts from the design, believe it or not. So it's faster to print, quicker to assemble, and needs fewer purchased components:

 

The other big structural change is to the Z Tower. That now has bracing on the vertical Axis Driver supports, and the Driver is now held by two vertical M3 x 50mm screws that double as stiffeners for said supports. Jon can use M3 bolts instead of screws if he wants (if you know, you know).

Another relatively minor change is the elimination of a lot of unused holes. These were rightly criticised as taking up a lot of print time and introducing potential weak spots, but I didn't know which ones were needed. Now I do.

While the Base, Frame, and Stage have all changed, the extant PIKA V0.05 Axis Drivers still fit the same structure.

From an administrative perspective, I'm now going to separate the PIKA and MAUS directories. This means a bit of file duplication, but gives me the freedom to muck around with the designs a lot more and optimise print speed, hole location, etc. 

Personal note: We have a tropical cyclone coming through on Sunday, and I have to get ready for it. I should get this update printed and start testing before then, but expect some disruption to the blog, SuperHouse livestream, and git as I batten down the hatches and deal with the inevitable power cuts &c.

Been away for a bit. Back on bed levelling. I've updated the bed leveller a bit (it doesn't just crash if the probe is already touching the bed), and been playing around some. I can get the bed levelled to 1μm over a mm or so, but trying to stretch that to 4mm doesn't work quite so well.

The problem I think is that there are four adjustable fixing points on the Stage, and there should only be three. The three-legged stool level thing. Now Jon Oxer is going to hate me for this, but I'm going to design a new Stage attachment scheme, which means I'll need to make a new PIKA XY Table design. The change is trivial to make in the model, not that it'll make Jon feel happier :)

To make me unhappy, I want to see how Jon goes with his assembly, so I know what other changes I need to make, and change them all at the same time. So I am very tempted to go do other things with μRepRap and come back to the Stage issue later. Of course, I want it all changed at once so I can go and do the documentation with assembly pictures - which also has to wait if I go down that path. Your thoughts? 

(I didn't want 3-point anchoring because I'm worried that the Y Axis Driver might pull the Stage out of line. However, I can't see a way out of it.)

I switched back to a hypodermic needle probe. They're more rigid and have sharper edges than an ogival tip, giving better penetration of the oxide layer on aluminium. The improvement was marked:

The range of heights returned when repeatedly scanning a single point dropped to typically a micron or less. In this sample, only one point had a variation of 1.3 microns. So the scanned 2mm square was levelled to within the limits of the probe and foil surface.

Next step is to scan a larger square, then scan smaller squares within that one to see if the whole surface is generally level.

Practical takeaway is that ogival tips are not useful for bed levelling. This explains why I can't seem to zero a probe on the foil before deposition and invariably end up having to tweak it manually.

Something happened today, and I don't know if it's humidity, contamination, subtle probe damage, or what. But probing the aluminium foil touchplate got wildly inaccurate. I've put a microscope on it, and you can now not only see the probe pushing down and bending, but actually watch it push the entire XY Stage to the side when it contacts. The error spread in multiple contacts on the same point has gone from 4-6μm to as much as 50μm. I didn't believe this, so I've improved utils/levelling_probe.py with multiple point probing and a bunch of other command line parameters. Here's a sample couple of runs:

 

Damn, it seems to be happening. The probe is behaving OK, the movement is smooth and eyeballing suggests consistent with the numbers. It's just dodgy contacting. So, I'll make a new touchplate from different foil and see how that works out. Maybe try a different probe too, but another family event looms and I have little time.

At least it spurred me to improve the levelling utility. 

Fitting the new Flexure Coupling design (with a tapered motor shaft hole) to the X Axis Driver was extremely easy and needed no tools. It did mount 2-3mm above the old one though, so some faffing around with the spacers etc. was needed to line the Drive Screw height up. The result of the backlash test was impressive though:

As a reminder, in the photo above the top row is drawn right to left, the bottom left to right. The two left-hand lines are drawn at the same X coordinate, but in different directions on the Y axis. This allows me to spot any trouble caused by a misbehaving Y axis.

Gap between the top lines is 80μm. Subsequent gaps between the lower lines are increased by 0.5μm. I it's hard to tell if the 2nd or 3rd line from the left lines up best, which would be a maximum of 1μm difference. As backlash happens in both directions, the actual backlash distance would be half that, or 0.5μm max., compared with 1.25μm with the previous (cracked) Flexure Coupling. I'll take that.

If accidents like this keep happening, I may have to design the Flexure Coupling to be a bit more robust, but not having to fit the thing with a mallet may increase its working life... 

After seeing Jon struggle to fit the Flexure Coupling to his RepRapMicron  stepper motor, I've reached the conclusion that relying on precise printer dimensions for shaft fit is foolish. Coincidentally, I appear to have stressed my X Axis Flexure Coupling to the point where it cracked (see black mark):

This was caused by me tightening the Drive Screw down too much, but spurred me to modify the Flexure Coupling to have a conical hole in it (on github and Printables). The gentle taper makes it easier to start pushing the motor shaft in. The downside is that different printers and shafts will bottom out at different points. Still, there's a few mm adjustment in the design, and spacers in the print file for adjusting for gros shaft length.

The cracked flexure also explains why my X backlash increased after I fiddled with the Axis Driver. I'll fit the new one and see if that fixes it.

The main moderator of this blog has had to retire for personal reasons. I'd like to take the opportunity to publicly thank them for all the work they have done not just for the blog, but for The RepRap Project as a whole. You have my respect, and I wish you the best possible future.

Sincerely,

Vik Olliver 

The python script to create X & Y axis verniers for detecting backlash is up on github at https://github.com/VikOlliver/RepRapMicron/blob/main/utils/backlash_vernier.py

This has allowed more detailed examination of the backlash (line spacing is approx 80μm):

The first set of lines (upper or left) spacing increases by 0.5μm each time. The other set is at a constant spacing. Each set is drawn heading in the opposite direction to its partner. So the left set spaced on the Y axis aligns with the second marker (1μm) and the right set aligns at the 4th or 5th marker (2.5μm).

As they're headed in different directions, the relative backlash is doubled, so backlash on Y is 0.5μm and on X 1.25μm. I may need to increase the strength of the backlash bands on the X Axis Driver, but frankly for this stage of the game I'll only do that if I have to take it apart or realign it for some other reason.

FYI The slightly kinked line roughly in the middle of  the leftmost horizontal set is where I bumped the bench.