It looks like my problem with the waviness was software rather than hardware. Specifically, Creation Workshop Host (now Photonic3D) running on a Raspberry Pi, which is what I had driving my printer.
Looking closely at the waviness I realized that it was due to some layers being thicker than others, not layers being displaced back and forth. The images of each layer (generated by Creation Workshop) were fine, so I recorded video of the printer in operation and used it to measure the exposure time of each layer. The print I did this for didn't have the consistent waviness like in the photo shown above, but it did have some funny layers. It turns out the exposure times were inconsistent -- most layers were fine, but every ~5-6 layers one might be slightly underexposed or overexposed by almost a second.
I moved the printer back to running directly from my PC using Creation Workshop, and the waviness went away. (Of course, now I'm back to prints being ruined by screen savers, programs opening windows on the projector's screen, someone else using the computer and switching to their account, etc. Which is why I wanted the printer running on a dedicated Raspberry Pi in the first place.)
I might try a Raspberry Pi 3 and see if it does better; the one I have now is a version 1, and it might be underpowered for running a Java application in X Windows that has specific timing needs.
Statistics: Posted by Mattasmack — Tue Jun 14, 2016 1:22 pm
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(Full disclosure, this isn't quite a standard LittleRP. I started with the design docs on the website but printed my own FDM printed parts, and first attempted to make my own z axis from parts I had on-hand from an earlier printer. (I was working on the cheap!) The z axis didn't work out and piece by piece I ended up going back to the LittleRP design, and I think I'm now back to the standard design except for a RAMPS board for the controller. But it's possible I've done something else goofy without realizing it that is unique to this machine.)
Statistics: Posted by Mattasmack — Thu May 26, 2016 8:51 pm
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Here are a couple of photos of recent prints in B9 Emerald.
The first shows the waviness I was mainly referring to. The scale at left is in millimeters, so there are 4-5 waves per millimeter of height. (It's tough to say exactly, since the scale isn't exactly adjacent to the print or perfectly vertical.) Thing is, I'm printing at 40 um layers, so those aren't individual layers; each wave is five or six layers. If they weren't present the print would be much smoother, so I'd really like to eliminate them! I saw a similar features on prints from my FDM printer; there they were caused by a wobbly z axis and the pitch of the waves matched the pitch of the z-axis screw. Here, I don't think there's anything mechanical that has a pitch of 200-250 um. I'm using 40 um layers because that matches the distance travelled by a full step of the z-axis stepper motor.
The second photo shows two jumps, one about 2/3 of the way up the print and the other about 4/5 of the way up. They tend to occur when the area of the layers is growing quickly, but can happen randomly too.
If you have any suggestions or theories I'd love to hear them!
Statistics: Posted by Mattasmack — Thu May 12, 2016 6:53 pm
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