What is RepRap & RapMan Z-Axis Wobble?

I'm fairly confident that any issues with the Y-Axis alignment and the Bed Leveling alignment on my RapMan has been successfully solved using the new tools that I designed to make the process more precise.

But, when printing at .125mm, it is apparent that one more issue may need to be tackled.  And, that is probably some issue with some relatively minor Z-Axis wobble.

This video isn't all that great.  The lighting is poor and the image is 640x480 because it was shot at 240fps so we could analyze the image in slow motion.  I'll try to get better images of all four threaded rods this evening.
However, it is still useful enough to use right now.

What this video shows is that if there is any wobble at all, it is very. very minor.  Yet, because we are working with such tiny tolerances, it is still noticeable in the printed output.  As the following samples show.

Here is the test object.  It's a tower having 3 equal sides.  This shape was chosen because it results in 3 sharp edges.  It was printed at .125mm resolution.  And, as you can see, there are some visible ridges that probably indicate Z-Axis wobble.

Test Tower

Looking at a crop of the image, we can see the ridges a bit more distinctly.

Test Tower - Crop showing ridges more clearly

By laying the tower on its side and angling the light to create shadows, the ridges become more identifiable.

Lighting to accentuate the ridges

While we can see an interesting pattern in the above picture, it's not so clear as to how many layers there are in the repeats of that pattern.  For that we need a microscope.  Again, the following picture was created using the worst microscope I have in my collection and the camera was hand held to the eyepiece.  So, it's not the best we can do.  Even so, it is helpful to see for this article.

Ridge pattern through a 20x microscope

The most important thing about this image, and something with which I want to follow up, is that the ridge pattern looks to be about 12 rows.  If 12 (or so) rows equates to a full turn of the Z-Axis threaded rods, we have evidence of Z-Axis wobble.  Yes, it's on a small scale.  The wobble appears to be less than .5mm.  But, it is there.  And, theoretically, we should be able to get that down to virtually zero.

In subsequent posts, we'll explore some of the solutions that other 3D printer users have come up with to deal with this issue.  But, before we leave this subject, it might be interesting to see what we are up against in dealing with the threads of the threaded rods.

Threaded Rob Up Close and Personal

If you click on the above image, you will see that threaded rods aren't always as cleanly threaded as we would like.  In fact, I don't believe I have ever seen a perfectly clean threaded rob.  This one is as good as any I've seen.  Yet, it clearly has some defects and/or debris that might affect the print.

While looking at the screw, it dawned on me that I should be able to place the test tower next to the threaded rob and take some macro images to see if the threads match the pattern of ridges.  I'm guessing they do.  :)