RepRap: Blog

Flushed with success, I thought I'd try the new probe out over a large area - 2mm x 1.3mm - and this did not look so flash. But we learn from our mistakes so here is one. 15μm spacing but a stippled image (figure it out, heh):

Click for the full-size image

Careful examination shows that the probe contacts as a point nicely across the print area along X=0, and going up to I dunno 0.7mm or so? Then the probe starts smacking into the slide too much for the flexure to cope with and it starts skidding off sideways.

Something is off somewhere, because beyond that area there are some dots, and some skids. I do not know why.

The tip fitted to the current Touch/Probe head is far too fine for me to practically work with on the 10's of microns scale that my hands are capable of. But before I retired it, I just had to find out what it would do to "Hello World". We really are bumping into the limits of what one can do with a pointy thing, Sharpie marker, and a Konus binocular microscope here, and I had to drag out the Awful (but cleaned) Biolux to get better shots. So here's what I can see with an 8μm spacing using the Konus:

Few things to note. First off, that's 56μm tall. This about the size of the spacing between dots in previous Hello World's (50μm). Second off, the amount the probe skids sideways on contact is far, far smaller than previous attempts. I attribute this to the more precise probe height positioning possible with the Touch/Probe, and the fact that the Probe Tip moves vertically when pushed rather than diving off at an angle.

Time then to zoom in on a gory detail shot with the Biolux:

 

Woah. I think this illustrates pretty well that PIKA has sufficient control to work on single digit micron scales.

Which begs the question, what if I tried 4μm spacing? OK then.

At this point the debris kicked up by poking the point into the Sharpie is becoming a limiting factor. Here's a shot through the Biolux:

Yep, definitely pushing it there. Limits seem to be in the substrate/tip, not the μRepRap.

So, takeaways from all this. PIKA with the Touch/Probe is going to be up to positioning the probe as accurately as is needed for sub-10μm features. That means I need to get the Touch/Probe design put in the repository, and put a wider tip on it to begin resin deposition on the 10-20μm scale once more.

FYI This is the current probe configuration:

 

I took the pointy contact probe from the previous blog entry and smacked it into the glass slide a few times to Z-10μm and it seemed consistent, though the Z axis seemed to pick up unwanted vibration. Will have to look into that. Anyway, it didn't seem to be flattening so I put some resin on the slide and picked some up with the probe point.

Now this point is a scrap one I over-etched, and it looked a bit too pointy. I figured a finer point would be more likely to exhibit damage, and I didn't have a use for it, so it got sacrificed. I got confused because the contact indicator was showing contact with the slide, and as far as I could tell the Z axis wasn't going down any further, but I could not see any resin deposited through the USB microscope.

So I took the slide to the Konus trinocular and looked at it under 90x. Couldn't see any dots. But I had a feeling, and so I brought out the not-so-brilliant Biolux microscope and had a good look at 160x (10x objective, 16x eyepiece) with some lighting and focus plane tweaks that I can't do on the Konus:

This was taken through the eyepiece with my phone (5x optical zoom), and the Biolux optics are gritty with age*, but I think you can see a horizontal trail of dots across the middle. So I was depositing resin, and deposited in excess of 20 dots on one dip of the probe.

The dots are spaced 20μm apart, and measuring the image pixel by pixel in the enlarged section suggests a diameter of approximately 3 microns. That's smaller than half a human red blood cell.

That's pretty impractical for me to work with at this point, and that line is not as horizontal as I would like it to be. Not that the wavyness matters so much if I'm drawing lines 20μm wide. But it's nice to know that in theory I can make features that small.

Anyone want to chip in for a 2x Barlow lens for enhancing the Konus? 

 

* That image was so horrible it actually drove me to take the Biolux eyepiece apart and clean each lens individually with IPA. Not a huge improvement, but a bit better. Do I dare fiddle with the objective lens and risk ducking it all up?

Having made a Touch Probe that works give or take a micron, I wanted to start printing in resin again. But I became troubled by the issue of zeroing the probe's point on the slide to the stage where it was causing prevarication. Thus prevaricating, I decided to stick a Probe Tip on the Touch Probe:

I turned the probe beam section into an L-shaped trough and just glued the probe wire in with copious cyanoacrylate adhesive (wire was held in place with Blue Tape while that cured). The side wall of the L prevents adhesive from seeping into the flexures. There is no electrical probe connection - I'll be levelling it manually, so no need for a Z Touch capability right now.

The main concern is that I'll knacker the probe tip by banging it on the glass slide before I figure the height out. These tips seem fairly robust though, so I might get away with it. I'll test on aluminium first, which is a bit softer.

A minor concern is that the probe beam might vibrate too much. I didn't see that happening during touch tests. Limitation: The whole purpose of is is because I can't see micron movements... 

The main operational disadvantage is that I can no longer view the Stage from directly above. The Touch Probe flexures get in the way. The new microscope pole clamps are working pretty well, so I hope I'll still be able to get an informative angle on what's happening at the pointy end. But if I place a microscope to view from the "front" then I can't twist the pole to move both microscopes out of the way as easily. We'll see if that's an actual problem.

If the new touch/tip probe does anything useful I'll upload the CAD files. 

I've made a small test flag and printed it with a lug to mount it on the Z Axis with. The "flag" was flipped around so that it was near the contact point - much handier when you're trying to watch everything through a microscope.

I can certainly see movement that happens over a micron or less. This is done by bouncing the probe 10μm. If it twitches as it gets to the bottom of the movement, then you're contacting the slide. If it doesn't, drop it by a micron and bounce again. It makes for a boring video.

There's a bit of tidying up to do, making things align in the microscope's field of view, making sure there are more straight edges when you're looking for motion etc. But I might be on to something.

Options might include making it into a proper indicator gauge, and being clever with capacitors or inductors to give electrical feedback. For now, just watching the indicator arm twitch gets me +/- 1 micron.

Detecting when the probe contacts the surface is not easy. When it gets there, it just stops. This makes determining bed levelling difficult to do by just probing at it. Even using an electrical probe has problems, as that relies on a perfectly flat contact surface and a perfectly clean one at that, with no oxide build-up anywhere and minimal electrical field effects. This never happens for me.

So I wondered if it might be possible to make a mechanical surface probe. I have a DTI gauge on my lathe which is notionally accurate to 10 microns, after all. I came up with a "Scott-Russell" flexure system with a flag sticking out of it. This is basically a flag on a very unstable toggle joint. I made several sizes: 

The sensitivity to motion less than 10 microns seems to be roughly comparable to that of the DTI, but the flexures require far less force to actuate. By just watching for the flag to start to move, you can fairly easily pick up a 10μm tip motion with the naked eye. 

These examples are printed on a Prusa XL, but it would be interesting to try to print a smaller one on a μRepRap. Doing that with an improperly calibrated μRepRap might not be possible, but if I can keep the delicate parts arranged in one direction then you only have to calibrate reasonably accurately along one axis for a relatively short distance. The theory is that once printed, one of these could be stuck to a probe tip, and then - when observed through a microscope - be used to level the Stage and determine the height of printed structures accurately.

Lots of if's and but's, dubious rigidity of the UV resin,a desire for the thing not to swivel sideways on contact, concern over separating the thing from the Stage etc. Might work, might not. Won't know until we try, will we? 

 

["Do You Have A Flag?" reference is to a comedy sketch by  Eddie Izzard, go look it up.]

The objective of the test was originally to attempt to measure the height of deposited droplets by banging the probe into them and looking for bumpy motion using the 4K microscope. Unfortunately the probe was rather destructive, and I was unable to get anything out of it other than "well, it's 1-2 microns-ish". However, I happened to record the deposition process, and CC subtitled it for YouTube:

I doubt YouTube has done the original resolution justice, but even zoomed in on the original I could only just make out the droplets being deposited. This was all being controlled manually, and thus a wee bit haphazard. I set the CNC step size to 50μm and just slammed it around. Under software control, the probe is decelerated when it gets to within ~10μm and then is lowered gently, so those tend to be a bit more uniform.

As "measuring by braille" didn't work, I stuck the slide under the microscope horizontally and vertically. Here's the images but I couldn't determine much about the droplet height from it:

One of the big problems with imaging things at high magnification is that your depth of focus is about half a gnat's whisker. So even though there were 5 dots, I could only get two of them in focus at the same time. I could probably do a bit better if I made an improved swivelling slide holder, but that's a project for another day.