My kid got a bit too enthusiastic today while operating the microscope remotely, and the foot of the X-axis actuator detached from the stage linkage. Reprint time!
Is there a reason that linkage can’t be more robust? I would print this with ABS, but I don’t have very good ventilation in the room where my printer is, so I generally try and avoid it, especially for 8 hours. If I can make something less fragile and still use PLA that would be preferable.
I’m fine with modifying the STL myself, but I wondered if there’s some reason why it’s constructed the way it is (so small), and whether modifying it will impact the functionality.
@r.w.bowman maybe that semicircular part at the foot of the actuator could be more of a rectangle?
Also, I see in the sangaboard firmware some code for end stops - I don’t see any STL files or instructions for end stop mounting, so I’m wondering - how do I add end stops? I guess I’d have to use very sensitive and well calibrated bump switches for that. Has anyone constructed a microscope with functional end stops? I’d love to avoid breaking my microscope.
sorry that broke - it is a bit of a weakpoint (and your cross-section shows one of my attempts to improve it, which is the very thin hole through the small semicircular part - it adds some perimeter, which is stronger than infill).
The reason it’s semicircular is for print bed adhesion - a rectangle would have sharp corners, which are more inclined to peel up from the print bed. We could just make it taller, i.e. extend it upwards as you’ve got it in that picture. That would probably help a little, at the expense of changing the shape of the microscope feet, and thus the footprint of the microscope (which is a little annoying, as various folk have now designed accessories that rely on the footprint). It probably is something we should improve in a future version though.
Just to check, how long is the screw in your actuator? The easiest way to break it in the way you describe is using a screw that’s a little too long, as that will exert a huge force on the bottom of the hole down the inside of the column.
Ah, I was wondering why that tiny channel was there. That makes sense now.
The print bed adhesion in my case is a non-issue - I’m using a glass plate with PLA that has unusually good adhesion. Of course, this doesn’t help the folks that do have adhesion troubles. I completely understand why a semicircle is preferable given the context.
I’m using a 25mm pan head screw for the actuator, with a hex nut glued flush with the bottom of the head, so the exposed length of the screw is actually a bit shorter than the design. The other axis is fine, so maybe this was a combination of a really narrow linkage and too-fast print settings.
3 possibilities:
I originally printed with 2 perimeters. Printing the first 2mm of the part with 3 perimeters instead of 2 results in the base of the actuator being printed entirely with perimeters, which should in theory be stronger. I’m going to try this first.
Since there are about .8mm of height from the z origin to where the linkage connects to the base of the actuator, maybe the footprint could be kept the same, but the part could loft out a bit through those first 4 or so layers? That would make the cross-sectional distance from the inside face of the actuator to the outside point 5mm instead of 4.2, giving that section a bit more rigidity perpendicular to the line where the most stress is occurring.
There will still be an issue with repeated flex - PLA is not particularly ductile, so it’s likely that delamination will happen eventually, despite the larger cross-section at the base of the actuator. As long as the stress is occurring along those right-angle intersections, it will cause delamination. Shifting the stress to the midpoint of the linkage (as shown below) might slow that process down. Ideally the linkage should fracture due to repeated stress before the base of the actuator delaminates.
Same here! I tried gluing it but since the part is nested inside the frame, and because the layers pulled apart unevenly, it’s pretty impossible
As for fillets - yeah, maybe, but whatever glue I use would need to be as flexible as the filament itself, if not more flexible. And I don’t know a glue aside from rubber cement that would fit the bill…
@wschalle We are starting to do some FEA on the flexure to work out how best to limit the stress. The issue with the fillets is that by the time you slice it you have quite a different shape.
@j.stirling Yeah, I thought about that a bit too. The base of the fillet is 2x nozzle width, but depending on the xy resolution of the printer, it may be too subtle of a detail.
FEA on 3d printed parts seems incredibly complicated due to the wide variations in material properties and print settings. I’m really curious - how are you approaching it?
As a simple experiment, I could print out just this part of the linkage with fillets and without, on .2mm and .1mm settings. Then I could measure the moment of force at failure for each and compare.
Would that be useful?
I think regardless of what the stress analysis shows, because of the orientation of most FDM printers, joints on the xy plane will be weak unless they are designed with moment resisting features of some kind.
The part that breaks is not actually a part that is supposed to bend I think. In that case a multimaterial solution would be to print it with a small hole vertically and glue in something with the ‘grain’ the other way: a pin, or a small bolt or a matchstick (woodworking again)
The plan with the FEA is that if we are staying within the elastic limit then material properties are a scaling factor (Young’s modulus). Then we hope to be able to draw layered structures similar to the final sliced object, we can then look at shaping the layers to to smooth out the change in compliance. Not FEA experts here, at first there will be some delving into the literature.
I think it’s Z resolution that really hurts filleting here - even just the jump from 2-3 (or 3-4 depending on height) layers is quite a big one. Our thinking is that we might be able to be a bit cunning with the shape of the layers to get a stiffness gradient, and in fact one of our PhD students (who isn’t on this forum yet) is starting to model exactly that. As Julian says, though, we have quite a bit to learn about FEA first…
Have you thought about defining additional volumes which can be used as “modifiers” in PrusaSlicer to add more infill to certain, weak sections on the mainbody? Someone may look into if this could be done with 3mf rather than .stl output. But I suspect that the whole pipeline OpenScad -> PrusaSlicer + Infill Parameters + Modifier Volumes is just broken.
You’re right, the pipeline of CAD -> shape -> slicer doesn’t allow nearly as much information to be passed through as we’d like. To be honest, I think the weakpoints in the microscope are mostly not improvable by increasing the infill settings - depending on which parts you think are “weak”! The thin vertical legs will never be super strong, and the flexures will always break if dropped, but provided my printer is working reliably, I have usually found it to be surprisingly robust in normal operation.
I do have a very early prototype for v7 of the microscope that uses a totally redesigned (and much, much stronger) actuator - but that will be a while in the coming, I think!
I guess it broke at the upper travel limit, where the motor can exert a big force. Is that correct? At the other end, at least in mine, the actuator hits the hard stop and the nut comes loose, so it’s less likely to break. I’m thinking about a limit switch, but it is much easier to put it on the end with the actuator at the end with the foot sticking out, not where I think you want it.
Pete.
@siddons that is probably right; when the actuator hits the top of the travel it should just stop there, because the motor won’t have enough torque to pull it any higher. However, it’s possible that the torque applied to the column is high enough that the twisting motion breaks the bottom of the column. Understanding the exact failure mode there is something we’re looking into. Figuring out a nice way to get a top limit switch would be useful; I’ve previously wondered about putting one on the inside of the actuator casing, triggered by the horizontal lever there, but never figured out if it was feasible to fit something in there.
I am sorry for awakening this destructive topic without bringing any real solutions/answer with me, but this issue occurred to me on my recent builds of the openflexure microscope v7.0.0-beta1.
First thing I usually do after building and calibrating a microscope is to check movability. I start out in a centered position and push the x and y movement until they stop (one at the time). I usually get ~80,000 steps in each direction (+/-). I do this to make sure the actuators and o-rings work properly. But this time, while going in negative direction (foot getting pulled into the actuator), I had the y-actuator foot snap (detached). I re-assembled with a new main_body but unfortunately the same thing happened here. I tried going to the extremes on the x-axis as well, and it snapped too. All snaps happened in the negative direction (~70,000 steps in), so slightly before it usually would stop by it self. In all cases it was a clean layer separation between the 4th and 5th layer (which is the first layer of the two actuator columns using my print settings).
I did not get the feeling that the actuator had hit the top of the travel (meaning it should stop there) but maybe it is still possible that:
… even before it has hit the top, especially if the PLA is weak. Or is it more likely that it just snapped due to the perpendicular stress of being pulled?
All parts of the build were according to instruction except that I used some left over v6.1.3 feet, but looking into the change log that should not be an issue since only the “uneeded hole on sides of feet” were removed.
I have not experimented with the printer settings since I am out of that PLA and I have main_body’s of other color that works well. One exception I thought of though, with regards to these failed prints compared to all others that have not snapped, is with the PLA. With the two main_body’s that broke I used opaque-ish PLA for the first time, pearl white (https://www.prusa3d.com/product/prusament-pla-pearl-white-blend-970g/). On their page the PLA is tagged as “silk”, which a guy on the internet said can be more brittle. I am no PLA expert and it’s difficult to get good data on the strength/layer adhesion/ductility for each PLA, but could this be a thing? @wschalle maybe you used some opaque/silk variant too? It could also just have been a poor quality PLA independent of color/silkiness/opaqueness. Anyways, I thought it was worth sharing. At least I will avoid “silk” for the main_body-part in the future.
My issue also made me curious what other OFM users think of pushing their microscopes to the limits (x and y). Is it stupid practice to push the microscope like this (at least once)?
At some point in the future I will try to repair the main_body’s with glue and pins (good idea @WilliamW thanks) so hopefully no PLA was spilled in vain.
I also have no real data, but a feeling that printing in solid colours seems to feel more robust and clear colours are more brittle. It may well not be the actual colour - good quality control and keeping filament dry are very important, and different reels of the same thing can behave differently.
I’ve never heard anyone say anything good about silk PLA, except that it’s cheap! If interlayer adhesion isn’t good, I guess that is where it would snap. @JohemianKnapsody and one of our summer students were routinely hitting the limits of travel with no ill effects.
Sorry to check the obvious, but did you remember to remove the ties that stop the legs moving during the print? If these are left in place they might well generate enough force to snap the actuator on a big move… They should be documented in the “prepare the main body” step.
Okey, yes, all ties were removed. I should read up on PLA more. I might try and set up some simple procedure (print shapes and weights/force) to somewhat quantify the interlayer adhesion strength of the PLA, incase I want to go for some fancy color in the future.
If you figure out a nice test, let us know - it would be great to add that in the section on testing the printer. Adding “don’t use silk, we’ve heard it’s not great” doesn’t feel quantitative enough for the instructions, but it would be nice to specify the print material more precisely than just “PLA”…
The plot thickens….Silk PLA is obtained by mixing PLA with TPU. TPU is basically indestructible. So silk PLA should be more flexible and stronger than regular PLA alone. I printed one using red silk pla and lasted me for about 3 years with regular use (sunlu silk pla). But another one using bronze silk pla broke quickly. How to Test the Silk PLA.
