After years of cutting my hands on the exposed threads of my Prusa Mendel i2, it was time for a long overdue upgrade. I didn’t want to just buy a new printer because it’s no fun. So, I decided to buy a new frame for my printer. I settled on the P3Steel, a laser cut steel version of the Prusa i3. It doesn’t suffer from the potential squaring problems of the vanilla i3 and the steel makes it less wobbly than the acrylic or wood framed printers of similar designs.
I expected a huge increase in reliability and print quality from my new frame. I wanted less time fiddling with it and more time printing. My biggest hope was that switching to the M5 threaded screw instead of the M8 the i2 used would boost my z-layer accuracy. I got my old printer working just long enough to print out the parts for my new one, and gleefully assembled my new printer.
I didn’t wait until all the electronics were nicely mounted. No. It was time. I fired it up. I was expecting the squarest, quietest, and most accurate print with breathtakingly aligned z-layers. I did not get any of that. There was a definite and visible ripple all along my print. My first inclination was that I was over-extruding. Certainly my shiny new mechanics could not be at fault. Plus, I built this printer, and I am a good printer builder who knows what he’s doing. Over-extruding looks very much like a problem with the Z-axis. So, I tuned my extrusion until it was perfect.
It didn’t work. My next target was the threaded rods. I suspected them because the supplier I had purchased the kit from sent me really awful smooth rods. They were un-hardened, un-ground, un-straight, regular old steel rod stock. Useless as ways for bearings. I ended up having to cut the rods from my old printer up, cancelling a project I had in mind for them. So, logically, it followed that the threaded rod they sent me was garbage too.
I ran to the hardware store, purchased a brand new M5 rod, some new M5 nuts (with a real class rating on them this time), and went home. I spent an hour straightening the rods until they rolled on the surface of my desk with no obvious high spots. I installed the new rods in place of the ones that came with the printer. No result. In fact, the new rods were wobblier than the old ones.
I was getting a little frustrated at this point. I fired up a print and watched my printer as carefully as I could, noting any visible deviation. I was using a bit of rubber tubing as the coupler for my z-threaded rods and the stepper motor. While I was watching the printer run, I noticed that occasionally the threaded rods would slip in the coupler for a half turn and then return to their previous orientation the next time the motors reversed direction. Aha! I took apart another project, and robbed them of some spiral cut flexible couplings. I replaced the rubber tubing.
I was flabbergasted. The print quality barely improved. How? I had fixed an obvious visible flaw in the printer. Apparently the slipping was repeatable enough that it just worked itself out of the equation. Okay. Well, I noticed that since the inner diameter of my couplers were not exactly matched to the threaded rod, the rod was not perfectly aligned with the shaft of the stepper. That could cause some wobble. So I spent some time with kapton tape and got the threaded rods running as centered as I could without running out to use the lathe at the hackerspace.
Surely that’s it. I’ve replaced nearly every component on the Z at least once. Alas, still wobble. I was about to give up any claim to technician, engineer, hacker, etc and go study yodeling or consider vagrancy. I decided to look closely at the printer one more time and see if I could spy anything, anything at all, wrong with the Z.
That’s when I noticed, when looked at closely, the Z rod appeared to go into the lead nut at an angle of a degree. If the lead nut was going in at an angle that meant it had to bend each time it turned. Which would be 100% the cause of wobble, but there was the problem with the theory. How does a rigidly held rod and a precisely placed threaded rod even get out of alignment?
My first thought was to blame my old printer as this part was printed on it. The i2 had problems printing square. Things were often parallelograms, especially in the Z (entirely my fault). So perhaps the part that held the nut and the bearings weren’t square. If it was leaning one way or another it could certainly cause this problem. So I carefully printed out new parts and reassembled my printer exactly the way it was before. I know my new printer is square. I measured. So these new parts have to fix the problem. They have to.
Vagrancy, right? I mean, the fresh air, not having to do laundry, occasional free food, and just walking the land. Living day by day… Despite the new parts, it was still wobbling. What else could I possibly fix? So I sat and really thought about it for the fiftieth time or so, and to my surprise, I figured it out. It was time to get out my precision mechanism adjuster.
My logic went like this. One side of the mechanism holds the smooth rods for the X in place. The other side has holes that go all the way through the part. When you assemble the printer you’re supposed to slide the rods through the one part and seat them in the other. Well, me being clever, I reamed out the holes of both parts to a snug press fit. (For plastic you can chuck the smooth rod into a drill and run it through the hole, and it will actually cut a pretty good press-fit.)
This resulted in both sides being able to hold the rod very securely. The design of the toolson P3 is very clever. One side holds the rods stable. The other side can slide. It may seem like if the rods can slide the machine would be imprecise, but it’s not so. The rods hold the nozzle in its Z position only. The direction they can slide is the X. However, the nozzle’s X position is determined by the belt. The rods are not involved at all. The belt is tensioned through the rods to the part that holds the stepper. So, the X position of the right carriage doesn’t affect the X position of the nozzle at all.
That was a lot, but in theory it means that if one side can slide it should self align to the Z-rods. It should be theoretically impossible to get the Z-rods out of alignment if the rods are held in place properly. That is, unless you intentionally make the through holes a press fit — which is where I failed. So rather than the part sliding along the shaft until it rested comfortably at its equilibrium. I could pick a location on the shaft for it to sit. I was being OCD so I made sure my shafts lined up perfectly with the end of the plastic part, bending both rods inward. This caused the angle I was seeing.
I tapped it with my precision mechanism adjuster, which gave it enough force to get over the static friction and spring close to equilibrium. I fired up the printer. Lo and behold, my Z was dramatically improved. The title picture has a before and after.
I still have some issues (probably because I broke a lot of things trying to fix this). My Z isn’t perfect. My next steps are to order lengths of threaded rod from McMaster because their rods have a tolerance on the thread and will hopefully be straighter. I also want to replace the stainless steel nut with a brass one for a better fit.
I also would like to reprint the through-hole part and ream it to a nice sliding fit. Theoretically this will solve my Z issues, but hey, it may be just something else. Anyone else have a similar debugging story?
Fail of the Week is a Hackaday column which celebrates failure as a learning tool. Help keep the fun rolling by writing about your own failures and sending us a link to the story — or sending in links to fail write ups you find in your Internet travels.