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Precision Z Coupler Upgrade

Discussion in 'Mods and Upgrades' started by nickster, Dec 20, 2013.

  1. nickster

    nickster Member

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    Goal is to remove the compressibility of the Z couplers between the steppers and Z lead screws by making the Z lead screw and stepper shaft into a very flexible ball joint.

    I used a metal lathe to make parts, but I am thinking a plastic version should be possible (have some ideas if anyone is interested).

    Way this works is to put a 3.16mm ball bearing (sitting on stepper shaft in pic) between the stepper shaft and Z linear screw and use the Z coupler provided by Robo 3D as a compressive spring to keep everything in contact with the Ball. Takes advantage of the hole in the end of the stepper. To do this, the aluminum spring Z coupler was put in a a 3 Jaw, and the 8mm side bored to a diameter of 0.371" and depth of 0.32". The split in the Z coupler spanned one of the jaws so the coupler did not compress as the 3 Jaw was tightened in place for the boring operation. Next, a 3/8 hex head shoulder bolt was drilled for a 5/16 tap drill and cut off to 0.3" length while being held in a collet. After the cut off, the bolt section was held in the same collet and tapped with a 5/16-18 (Robo lead screw dia/pitch) using the tail stock to hold the tap true. The threaded rod end cone divot where the ball sits was created in the lathe with a 1/16" ball end mill as a starter drill and then using a TIN coated drill bit. Either my threaded rod was hardened or I had an exceptionally dull center drill, so carbide ball end mill. Not sure what the final drill face angle was, but I used one that looked steeper than my normal drills (Harbor Freight special; could also grind a drill face to get a steeper cone angle; not sure it is necessary)

    Assembly: the 3/8 bolt gets inserted flush into the aluminum coupler. Grease ball and place on end of stepper. Attach coupler over stepper shaft. Place two nuts on threaded rod then insert into threaded 3/8 bolt head. Continue screwing in until ball is engaged and coupler starts to expand. I think I did 1-2 turns more. Tighten first nut against 3/8 bolt head. 2nd nut goes in X/Z carriage like before. Do for each side.

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  2. tonycstech

    tonycstech Active Member

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    Am not sure if its an upgrade or a downgrade.
    Its a good thing that it can compress.
    #1 u dont land as hard on the bed if your z switch is off the place
    #2 if rod is not perfectly centered, it will be able to wobble thus compensating for not being straight (as i am) yet keep it the extruder level.

    The inner cut of the coupler is too big. Making it dead center is nearly impossible unless you wrap some foil around it and put it back in.

    What benefit do you get making it sturdy like that with a locking nut at the top ?
     
  3. nickster

    nickster Member

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    Hi Tony. It is a total upgrade. Walk you thru it.

    Cone shaped indentation is cut while holding the lead screw in a lathe collet. It will be perfectly centered. The coupler only acts as a spring to keep the lead screw in intimate contact with the ball bearing that sits on the hole in the stepper shaft. Coupler has no effect on the position. Ball bearing locks the lead screw and stepper together while acting as a universal joint. The lead screw pivots on the stepper shaft.

    Lead screw screws into the threaded shoulder bolt. You continue screwing the lead screw in until it touches the ball bearing. As the lead screw is screwed in more and presses on the ball bearing, it starts to stretch out the Z coupler. This tension forces the lead screw cone hole to stay centered on the ball bearing and for the ball bearing to stay in the hole at the end of the stepper shaft.

    Springs are good if you don't care about print quality. Any slight tugging on the Z carriage compresses or unloads the springs which messes with your layer height. Things like pulling filament off the reel, compressing the X stepper/ heater cable and moving the X carriage from one side to the other, all change the Z height. Higher end printers and other CNC equipment use a fixed bearing to rigidly support the lead screw, and a coupler to compensate for axial stepper shaft misalignment. This setup gets you the same precision of a fixed bearing without the cost. And since the coupler has no impact on lead screw/stepper shaft alignment, the coupler can be low precision or even printed.

    Not sure how it is possible to have the extruder hit the bed with any force. It just can't go down that fast. As it contacts the bed the Z nuts automatically unload anyway, so there is no way to have the Z lead screws drive the extruder into the bed.

    The locking nut just keeps the lead screw from backing out of the threaded shoulder bolt.
     
  4. nickster

    nickster Member

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    There have been a number of floating Z nut designs on the web that attempt to let the threaded rod do its thing. Difference here is the driving and receiving end of the rod are constrained by ball joints.

    Work in progress update. It is not done yet, but with the latest mods, I saw agreement of Z height of extruder over bed to a dial indicator of 0.0002" over a 5mm Z span. If we want to hit 50um layer height, we are talking 0.00196" so even 0.0002" represents a 10% varianace. No evidence of hysteresis. Hits exactly the same spot moving up or moving down. This provides a strong counter example to other posts on the web saying a metric lead screw is required to get accurate repeatable results. Nut is (brass plated; solid brass will be better) acorn nut that was drilled thru. It forms a ball and socket with the drilled out 1/2"-13 bolt that is threaded into the plastic Robo X_Motor and X_Idler pieces. So there is a ball and socket between the servo shaft and the threaded rod, and a ball and socket between threaded rod to the X/Z carriage.

    There is a trade off as to how big the hole is in 1/2" bolt, as to where it contacts the acorn nut. Too small and it contacts near the top of the acorn nut which makes it easier for any off axis threaded rod wobble to lift the X/Z carriage off the nut. Too large, and you start to get in to the more non-rounded part of the acorn nut. I am probably too shallow right now, and decided to add the centering adjusters on top of the Z couplers. The adjusters attach solidly to the Z coupler, but allows the threaded rod to be more or less centered axially using the three adjustment screws. Plan is to replace the adjusters and couplers with a printed part in the future. Also investigated using a 1/2" threaded ball bearing instead of the acorn nut.

    On the stock Robo3D, I found that the Z nut wiggling in the X/Z housings could cause a 0.006 to 0.009" Z displacement. Part of the issue is that the thru hole in the injection molded X_Idler part is very oversize, so that only the edges of nut are supporting the carriage. Any lateral motion causes a Z displacement since the carriage rides up and down the beveled faces of the nut.

    I stuck some old test cubes in the first pic as a spacer to make the acorn nut visible. Very interesting -if you look closely, you see bumps on the edges of the test cubes. Turns out this is a Slic3r artifact caused by movement from inner to outer perimeters. Too much filament gets deposited on these transitions because flow does not turn off instantaneously on the moves. In this case, the pattern repeats every 4 0.3mm layers where the start point precesses one face one each layer. I have modified gcode where the start point is the same, and the faces are absolutely smooth. Vases are also smooth.

    This design retains the ability that the Z nuts unload in case the extruder is driven into the bed. On an eject, the nuts just keep going and eventually stall the steppers; not the end of the world. Will be adding some emergency stop hacks to Marlin at some point.

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  5. Peter Krska

    Peter Krska Active Member

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    This is great to know. Thanks for sharing.


    Sent from my iPhone using Tapatalk
     
  6. Bob64

    Bob64 Member

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    Mind explaining how you solved your Slic3r artifact with your gcode?
     
  7. Peter

    Peter Member

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    I had thought of throwing a bb in the coupler before, but never got around to it. If i do, i may take the effort to 'cone' the end of the thread as you did, but i'll just pack out the coupler spring with credit card plastic to give it a nominal preload, then assemble in vanilla fashio and pull out the packing. Voila, spring coupler in tension.

    Nice work though. any pics of the difference it made.
     
  8. SteveC

    SteveC Well-Known Member

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    Peter, The motor shafts already have a small hole at the end, at least mine do. I put a 3mm bearing on it, filed the threaded rod flat and clamped it in on with no compression. I can't get the Z axis to compress and it is really hard of pull up. There should be no significant forces pulling it up anyways. I think it works quite well without messing with a precisely matching hole in the rod. Any mismatch in the two holes will cause more problems than the bearing solves.

    Before this mod one of my rods was resting directly on the motor shaft and I could see both sides of the coupler expand slightly when I bent the rod sideways.
     
  9. nickster

    nickster Member

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    @Bob64 - eeek old thread. The artifact has to do with the starting/ending point of layer pattern. What looks like ribbing is actually the starting point precessing around the cube. I hand edited some GCODE so that each layer had the same starting point, and the "ribbing" disappeared. There was still a deformation where the filament for the perimeter ended. Haven't run a test cube recently, but slic3r now has an option to start in the same place or something. I am still debugging some end of layer issues. Could try doing a full retract and z wipe at the end of a layer. One thing I did note was that keeping the cooling fan on at all times to some min level reduced edge pulling when a layer finished.
     
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