Lead screw nut - brass vs POM and radial clearance/backlash
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@akstrfn Agree with everything you've said there! I'm pretty confident my remaining issues are in the details of how I've put together my Z-axis and not the usual red herrings.
I am going to try aligning the motors/leadscrews/couplers better before spending more on it, but am definitely considering the rigid couplers. I had been trying to avoid putting the z-load through the motor bearings so went for the elastomeric couplers and flanged bearing approach, but the reality is over 3 motors the load is pretty small and I reckon the off axis loads from the thing being miss-aligned will do more to damage the stepper bearings anyway.
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Just stumbled upon this video. Informative beginning and proposes an interesting magnetic Z lead screw to bed DIY coupler design.
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@engikeneer said in Lead screw nut - brass vs POM and radial clearance/backlash:
Guessing that the multiple starts might cancel out any off-axis-ness or be ginerally less susceptible to pitch errors?
That's an interesting point. Never though of that. I wonder if having only 1 start contributed to the z banding I got.
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@zapta said in Lead screw nut - brass vs POM and radial clearance/backlash:
Just stumbled upon this video. Informative beginning and proposes an interesting magnetic Z lead screw to bed DIY coupler design.
Yikes.
That's a very complicated way to add backlash to a ballscrew!There have been a number of printed compliant joint designs kicking around for ages, constraining things so that the motion is only planar is not that hard.
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@zapta on second thought I think it might be less about the number of starts and more the different leads. For a given pitch error (caused by an amount of wobble, over constraint etc) which will typically occur once per rev, the impact on each layer of the print is 4x less with an 8mm lead than a 2mm lead, so each 'band' on your print is going to be 4x smaller. Also they'll be spread put more so I'm guessing they aren't as noticeable.
I have some rigid couplers arriving in the next couple of days so will try fitting them and ditching the flanged bearings. I think I also need to redesign the printed z motor mounts are they are a bit flimsy and I can see noticeable flex now I've watched them more in motion. This is where I wish I had my own cnc mill!
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@engikeneer I've been fighting some z banding myself. My motor mount was pretty flimsy. Replaced it with an aluminum 90 degree mount secured to an extrusion. Much more rigid. That seems to have helped quite a bit.
Your comment on 2mm vs 8mm lead makes sense. I use a 1mm lead and I also wondered if some of the pattern I was seeing was due to the large amount of rotation needed by the lead screws causing the wobble to repeat more frequently.
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@zapta said in Lead screw nut - brass vs POM and radial clearance/backlash:
Just stumbled upon this video. Informative beginning and proposes an interesting magnetic Z lead screw to bed DIY coupler design.
The video is uh oh and it hurts a bit... You gotta love when people play engineering and put "requirements" which in this case boil down to "my requirements are to use ballscrew" -> find justification hard. Steel leadscrews don't not last long enough is he like serious... Is it so hard to learn from e.g. prusa or creality?
Why buy a motor with integrated leadscrew from aliexpress for 10eur (15eur delivered) when I can pay 20eur for ballscrew that does not work? On occasions you can get these motors from amazon for 20-30eur prime (but mostly 8mm lead).
@engikeneer @Phaedrux too much flimsiness for sure does not help if you create lever like force on top however it might not be as bad as you think. Let me show you setup that works without a problem once I made sure that rotation is centric:
5mm ABS... As flimsy as gets, however no z-banding present... One body aluminum coupler is what ender uses and I can not recommend it enough, its very clever way of doing more with less.
The point is that if bed movement is well constrained you can not really expect the problem to come from lateral movement of the screw pushing it around so what needs to be happening is that there is repetitive non linear movement in z-axis i.e. z-axis is first being lifted more in one part of the rotation then less in the other part of the rotation which causes compression of certain set of layers and stretching of others.
While you might not notice this problem visually on 8mm lead do measure the height of different objects and if you have the problem described above you will get non linear change of heights that does not match desired print height. I had height precision issue for a very long time on my ender until fixing the leadscrew...
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Originally it was a plastic motor mount and tensioner like this.
Since changed to this.
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@Phaedrux very fancy setup although I would put pulleys below the bearing. Do you have some picture of the assembly of leadscrew bearing system? I guess you are using duet so I'm curious whats the reason for single motor drive?
For z-banding you mentioned did you test without z-hop?
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@akstrfn yes single motor for simplicity. No duex when I started with it. The bed is stable and has a 3 point leveling screws. So no problem there.
The lead screw mounts are a double bearing block with a collar above and below. It was a bit tricky getting it aligned with the bed mounts. It was designed on the fly rather than in CAD. Not recommended.
No z hop.
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@Phaedrux I'm thinking of doing 3Z similar to your setup but driven by 3 motors since I can use spare extruder motor on duet 2 for third motor. But as you mention the bed does not go out of level so often hence not really motivated to mess with it for now just for autoleveling
