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Z Axis - precision and accuracy

Discussion in 'Mods and Upgrades' started by Ziggy, Mar 25, 2014.

  1. Galaxius

    Galaxius Well-Known Member

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    What's the purpose of the purple guide/attachment on your z axis as it doesn't appear to be for auto leveling?
     
  2. nickster

    nickster Member

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    z nut isolator snaps over the brass Z acorn nut and keeps it from turning. Technique is similar to http://www.thingiverse.com/thing:20147, but the big diff is the round part of the brass acorn nut is in direct contact with the hole in the steel bolt above it, and forms a ball joint. Other thingiverse designs let the Z nut float on a teflon or nylon washer, allowing it to freely wobble with the Z screw. Idea with this design is to precisely constrain the lateral motion of the Z nut while still allowing it to absorb any Z screw axial wobble without creating any significant Z displacement. There were a number of criticisms that using an isolator arm like this that it introduces nut rotation error. There is a few thousandths of a inch clearance about the smooth rod and isolator which represents a Z displacement error of about 0.0005mm - so just not an issue. Also since the Z nut is constrained, there is no real Z displacement from the screw wobbling about its axis. On the redo, I would make the isolator sit lower on the nut exposing more of the rounded surface. That allows the hole with the mating beveled surface on the steel screw to be made larger. Having the contact point sit lower on the nut makes it more resistant to any lateral Z screw forces being able to lift the nut out of socket and lift the Z/X carriage.

    z nut isolator.jpg DSC02708.JPG
     
  3. Mike Kelly

    Mike Kelly Volunteer

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    Any way you could share a video of it in motion?
     
  4. Galaxius

    Galaxius Well-Known Member

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    So you've removed the standard z axis nut and the acorn nut is what's lifting your z axis? I'm just trying to see how it all fits together. I've been trying to find a brass m8 coupler nut to use for the z axis and auto level but can't, looks like i'll have to go with steel or stainless. I was thinking about using 2 normal brass nuts, one to lift the x carriage and one for the auto level switch held by a bracket like your purple one, but i'm not sure if its a good idea or if it'll be accurate enough.

    Excellent upgrades too by the way, and very nice prints.


    Sent from my iPad using Tapatalk
     
  5. nickster

    nickster Member

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    @Galaxius are you planning to go with a rigid coupler and let the Z nut float? Mechanical diagram for mine is two ball sockets.

    Yup Acorns lift the Z/X carriage. Tkx on the print quality. Still a ways to go. You don't want to see my first prints.

    Problem with my original upgrade was the aluminum Z couplers bend sideways as you put any tension on them so I had to add the aluminum ring with the 3 centering screws to bring the Z screw back close to center.

    Right now I am using the stock Z limit set up which lives on the other side of the purple nut isolator bars. The bars tend to ride up on the nuts over time. They are just press fitted on. There is supposed to be a gap between the bars and the drilled out hex bolt that mates to the acorn nuts. I need to reprint them and add a better snap in lip. How do you use the 2nd nut for the auto level switch?

    Really important to have the mating surface for the Z nuts be really flat if you are going to let them slide around. Stock Robo nuts had some rotation, and wiggling the Z screw side to side would result in Z displacement.

    Anybody else see cold flow issues with the black base plate?

    @Mike I can do a vid, but it would be pretty boring. You can't see any wobble in the Z screw between the stepper and Z nut. What are you looking for?
     
  6. Galaxius

    Galaxius Well-Known Member

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    I'm just throwing ideas around. I'm about to upgrade to metric z rods + Ziggy's z axis improvements and upgrade my y axis to Mike's smooth rod design. Also looking at doing Mike's auto bed leveling but i'm on the fence about weather it's necessary if i have my bed level. For the auto bed level you need a long z nut so there's room to attach the switch mount to but i'm wanting to use a brass z nut as brass should provide less resistance on the stainless threaded z rod but i can't find a long m8 brass nut so was thinking about having 1 for raising the x axis and 1 for the z switch but i don't think the nut for the z switch will stay at a consistent distance from the nut lifting the x axis. Think i'll just use a steel z nut for the time being.


    Sent from my iPad using Tapatalk
     
  7. nickster

    nickster Member

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    @Galaxius - Just read up on Mike's auto level stuff. Really well done. Need to understand the algorithms and error sources better, then hack mine up as well.

    Long Z nut does help to stabilize any rocking of the microswitch. After the nut unloads, but before the switch trips, the Z nut is floating. For double nuts, you only care that they track each other while the main Z nut is loaded. After that, you only care that the Z height nut is supported by the Z screw and doesn't rock.

    Coefficient of Friction:
    http://www.engineeringtoolbox.com/friction-coefficients-d_778.html
    Materials and Material CombinationsStatic Frictional Coefficient - μs
    Clean and Dry SurfacesLubricated and Greasy Surfaces
    Brass Steel 0.35 0.19
    Steel Steel 0.5 - 0.8 0.16
     
  8. Galaxius

    Galaxius Well-Known Member

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    Thanks nickster. I'm using mike's guide and now I don't have to worry about brass nuts :) thanks for the engineering info.
     
  9. warlocke

    warlocke Active Member

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    I use brass nuts as well and have been having issues finding brass couplers.

    I was considering reversing the switches to N/O and mounting them on the underside of the carriage itself with the switch levers touching the nuts so that when they unload they activate the switch.
     
  10. Mike Kelly

    Mike Kelly Volunteer

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    I recommend the NO legs

    It's generally better to use similar metals since he way they wear against each other is better. With dissimilar metals one tends to wear more than the other leading to premature degradation .
     
  11. warlocke

    warlocke Active Member

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    I tend to agree with dissimilar metals and degradation, but with proper lubrication I don't think z-axis wear is going to be an issue in the near future.

    Looking at nickster's guide, I may just get some stainless couplers.
    I am still see-sawing over switching from a z-stop to the autolevel as it is. :)
     
  12. Ziggy

    Ziggy Moderator
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    I've posted my notes and fixes for the Z Ribbing in the thread "Z Axis Artifacts - Know Issues and Fixes"

    Feedback from the early testers is good, so I am confident the fixes will be effective on other machines.
     
    2 people like this.
  13. Ziggy

    Ziggy Moderator
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    When I was looking at the Z Ribbing issues, I had a closer look at the Metric versus Imperial Z threaded rod differences and whether some Z artifacts could be caused by so called "rounding errors".

    I think there is a fair amount of inconsistent and incorrect information in various forums regarding the merits of metric v imperial. I did some tests earlier and have now written up a draft of what I hope is an accurate, fact based analysis of the differences and why metric is a better choice.

    I think the reasons for choosing metric are sound. But imperial "rounding error" causing Z artifacts is not justified as a reason to switch IMO.

    My draft doco is attached.

    Comments, corrections and suggestions are very welcome.

    Warning : This doco could be a cure for insomnia! :rolleyes:


    edit:fixed a formatting error in the appendix
     

    Attached Files:

    7 people like this.
  14. mark tomlinson

    mark tomlinson ༼ つ ◕_ ◕ ༽つ
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    Your conclusion is solid. Well done.
     
  15. Peter

    Peter Member

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    Excellent article!

    Is there any reason that the metric benefits (as limited as they are) would not be valid for a 2mm lead screw?
     
  16. SteveC

    SteveC Well-Known Member

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    Yes, it's a nice thorough analysis. I really like Ziggy's use of an Arduino C code sketch for the layer height emulator.
     
  17. SteveC

    SteveC Well-Known Member

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  18. SteveC

    SteveC Well-Known Member

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    Peter, 2mm pitch means 1600 steps/mm instead of 2650 for 1.25mm pitch. So 2mm pitch gives 60% of the Z resolution of a 1.25mm pitch. But if you choose a multiple of a 0.1mm layer height they both should be exact and the difference should not matter, right?
     
  19. Ziggy

    Ziggy Moderator
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    This whole debate about metric v imperial really is driven by the fact that Marlin, the various slicers and hosts use metric internally. If they were using imperial calculations internally, the argument would be reversed and probably more complex because of the imperial arithmetic calcs.

    But given 3d printing is a generally a metric affair...

    Another way of looking at this would be "What is the ideal screw?" ..... to match Marlin, slicers, hosts, motors etc etc

    Theoretically the ideal screw should have a pitch which gives a integer multiple of 3200 usteps/mm. For example, M8 fine has a pitch of 1mm - absolutely perfect. But maybe the friction is too high and the motors can't handle it? There's a bunch of factors to be considered.
     
  20. nickster

    nickster Member

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    hehehe:rolleyes: Ziggy thank you for the write up; it goes in my library!

    Put a dial indicator between the hotend and bed, and start jiggling stuff around; zscrews, nuts, belts, carriages... I won't monologue about all the sources of mechanical error. But I still claim I measured 0.005mm Z repeatability, many points, no hysteresis, with ball and socket setup over 3 screw revs w/ stock screw. Here is one to add to the list:

    What does Step Angle Accuracy +/- 5% mean relative to micro stepping? :confused::eek:

    http://hybridmo.en.china.cn/selling...Motor-With-4-6-Lead-Wires-For-3D-Printer.html

    .18A - 3A 1.8 Nema 17 Gearbox Stepper Motor With 4 / 6 Lead Wires For 3D Printer:
    Delivery Time:FOR SAMPLES , 7-10 days For batch . 15-25days.
    Certification:CE,ROHS ,ISO9001
    0.18A - 3A 1.8° Nema 17 Gearbox Stepper Motor With 4 / 6 Lead Wires For 3D Printer
    Specifications
    nema 17 series stepper motor
    42mm diameter
    stepper angle1.8
    phase current :0.18-3A
    2and4 phases, 4and6 lead wires
    42HS nema17 series stepper moter , usually used in 3d printer
    Items
    Spec.
    Step Angle
    1.8 °
    Step Angle Accuracy
    ±5%
    Resistance Accuracy
    ±10%
    (data truncated; lost formatting)
     

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