RepRap: Blog

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.

I've just finished the first draft of the RepRapMicron Microscope Pole documentation https://github.com/VikOlliver/RepRapMicron/wiki/Microscope-Pole . As usual, comments and queries are welcome.

I used the probe-wriggling levelling technique to do the "Hello World" thin with pixels spaced at 30μm. As you can see, the more accurate probe height results in much smaller impacts on the left. The slide appears to not be correctly levelled on the right and the dots smear out a bit as the probe slides under downward pressure.

You can't see half the dots using the USB microscope. This shot was taken with the bench 'scope using strong underlighting. If I level the slide you can expect more fine dots of the size that appear in the 'H'. Those seem to be <10μm across. I'll do a bit more levelling and then use the wiggle technique to estimate the height of a deposited resin dot. That'll be hard data.

I've done a little bit of upgrading of the RepRapMicron poseable microscope clamps. The thumbscrews now have a bit more clearance, and the clamps actually have the diameter printed on them so you know which is which.

 

Files are on github in the PIKA directory and on Printables. Proper writeup in the PIKA assembly instructions to follow.