After publishing yesterday’s post I observed a strange thing – the lid of my Elegoo Mars came off during printing. The encoder I mounted on top of the Z-axis bounced it off. That was strange, how? And then it hit me. I have never, ever measured backlash of the bearings of the lead screw. I have only measured the backlash of the screw itself. Well, I think a video is worth more than a thousand words:
There is a play roughly 2 mm in the housing of the lead screw! I disassembled the printer:
The cause is clear – even the motor has a lead screw as shaft and therefore, you would assume it is designed for axial load, it is not. There are ordinary ball bearings (no axial nor angle contact bearings) and most importantly – there is a spring washer tensioning the bearings – just like in an ordinary stepper designed for purely radial load. This is, in my opinion, a clear failure of the motor manufacturer MOCOC TECH. Also, there is another source of problems – the silencer – as the screw is mounted in the motor and the rubber silencer is soft. The silencer probably prevents from resonating with the top plate of the printer and also creates a flexible element which can compensate for the axial misalignment of the screw and the nut.
When you combine this flexible play in the screw with my observation about forces present during printing, you get imprecise print height – up to the size of the play of the screw. It can shrink or squeeze your layers arbitrarily.
What is the solution? There are three solutions in my mind:
- dirty & cheap – get two M8 washers, put them in the motor’s rotor, remove the spring washer and tightened motor body screws carefully to slightly tension the ball bearings. Also, remove the silent block. This is a solution for roughly 3 CZK. Warning: This is a dirty solution. Deep groove ball bearings are not designed for axial load nor tensioning. Also by removing the silent block, you remove flexible element which could compensate for misalignment of the screw and the nut. Your risk shorter life of the bearing, screw, and nut. On the other hand, the speeds and axial loads on Elegoo Mars are not that big, so you might be OK for years with this solution.
- better solution – get a pair of axial bearings F8-19G and use them instead of the ball bearing. Pretention them either as in the previous case or with a hard spring washer.
- The best solution – build separate housing for the screw with contact angle bearings and connect the motor via flexible shaft coupler. This solution provides noise reduction using the silencer. However, when you try this it might be worth it to rebuild the Z-axis to use a linear rail as the Elegoo solution of the Z-axis has a high effect-to-cost ratio, however, I can measure about 0.2 mm of play when I apply reasonable forces by hand.
As the G8-19G bearings are not available at my local store and I had to order them, I applied the dirty & cheap fix to find out what improvement can I get. Spoiler: a huge one.
The full dataset can be found in this table (new measurements are from sample 10).
Most notably what changed is that if an error is introduced in a layer, it is compensated by the others. Therefore absolute precision is preserved. See that all the test pieces got practically the same height.
If you look at test piece 11, you’ll see it is quite distorted. It is the sample surrounded by a full plate of material. There was noticeable distortion, however, it was different compared to the previous cases. The overall test piece height was preserved, but the layers surrounded by material were a little bit higher. Just like first layers of other pieces. This is probably due to the effect I described in volume 4 (recommend reading before continuing). The effect is that the resin is viscous and as the build plate sinks, it has to push away the resin. When I introduced the delay to allow the resin to flow away and to settle the build plate in place I got much more precise pieces. On the simple pieces, even the first layers got the correct height. On the pieces with extra material, the distortion is still there, however it is much less significant. I believe by introducing even longer delay, we can get much more precise (I plan to do this experiment).
What struggles me is that instead of 3 mm I got 2.9 mm – pretty constantly. Therefore, I printed another staircase – 0.5 mm steps, 15 mm in total height (sample 15). I also got less – 14.7 mm. Currently, I have no idea what is this caused by – it not a constant error (not coming from incorrect bed leveling) and it is too large for shrinkage (2 % – epoxy or polyurethane resins have shrinkage less than 0.5 % and I don’t expect printer resin to be that different). Maybe tensioned bearings with misaligned screws cause step loses on the stepper. I am also not sure the error is linear – I’ll have to run many more tests. Any ideas what could it be caused by?
On the topic of lost steps – before the first print I releveled the build plate. When the print started, the build platform started to move down as expected. The build plate touched the bottom of the VAT and the stepper still continued – by the sound it clearly lost some steps. I am sure I have leveled my bed correctly. I leveled it against empty VAT. Is it possible the printer FW moves the platform a little bit below the zero point to pretention the Z-Axis to mitigate the problem with flexible housing of the screw? I don’t know, but this also something I would like to explore in the future.
After all, even there are still open questions I consider my Elegoo Mars to be used as I intended when buying it – to produce precise functional mechanical components.