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Twisted Extrusion

Discussion in 'Using Alibre Design' started by Martinm, Nov 3, 2019.

  1. Martinm

    Martinm Member

    I'd like to make a herringbone gear like this: http://www.welter-zahnrad.de/wp-content/uploads/WELTER-Pfeilverzahnung.jpg
    Application is a 3D printed microscope focusing rack (like this one https://www.amazon.ca/KOPPACE-Microscope-Focusing-Accuracy-Aperture/dp/B07MDGSF76). So it need not be geometrically perfect, just good enough. I already have the linear part and am now struggling with the gear.
    The idea was: Use the gear generator python script to generate a 20 tooth gear sketch (20 mm pitch diameter) and make a twisted extrusion, rotating the profile by one tooth (360°/20). How do I do this?
    Using a loft, I'd probably have to split the 18° twist in several steps, making several sketches (on offset planes) rotated by like 6° each, but I'd say this does not give a geometrically correct result (not that it would matter) and seems quite unelegant.
    Somewhere in this forum I found an example of a helical boss with profile direction (advanced) set to normal. However I can't get this to work and documentation is zero here.
    So what would you recommend? (Of course I could use a "straight" gear. But what would be the fun in that?)

    Regards, Martin
     
  2. Lew_Merrick

    Lew_Merrick Alibre Super User

    Hi Martin -- I can see two ways to accomplish your "gear," but I would need more information to begin -- What is the "Herringbone Angle" needed and what is the Diametral Pitch (or Module) to which it is being "fut?" [Irrational minds want to know.] -- Lew
     
  3. bigseb

    bigseb Alibre Super User

    Helical boss tooth profile in one direction. Then repeat in the other direction with reverse twist. This makes one tooth. Circular pattern and fill the centre with a revolve/extruded boss.
     
  4. Martinm

    Martinm Member

    Stativ.png @seb I have not tried a helical boss with one tooth - is there a reason why the whole gear profile would not work?
    @Lew_Merrick : I'd design the herringbone so it overlaps one tooth over the full with. Tooth module is 1 (pi mm pitch, 20 mm dia, 20 teeth) and width of the gear 2*10 mm. I tried it with a loft, which works:
    zahn.png
    which works, but I think inner and outer "diameters" of this gear are not perfectly cylindrical, so this is just an approximation. Good enough for 3D printing, but I'd like to know how one could do this properly. I guess the helical boss would be the correct option?
     
  5. bigseb

    bigseb Alibre Super User

    A very basic version but you get the idea...
     

    Attached Files:

  6. idslk

    idslk Alibre Super User

    upload_2019-11-3_18-59-55.png
    Made one with the scripts: Gear Generator and CopySketch
    Made a sketch with Gear Generator with zero depth (it made an extrude error...ignored it because i only used the sketch...)
    The copied the sketch withCopySketch with rotation
    Then a Loft and finaly a mirror of the loft.
    The scripts are included in the part.
    Regards
    Stefan
     

    Attached Files:

  7. Martinm

    Martinm Member

    Hmmmm... is there any reason why the full profile helical boss does not work?
    error.png
     

    Attached Files:

  8. Martinm

    Martinm Member

    @Stefan: Basically I wanted the twist to be one full tooth, so first and last profile are identical. If you add just one intermediate sketch (rotated one half tooth), then the loft function can't possibly know whether to rotate left or right. I guess this could require a guide curve (?) or just as I did two intermediate sketches.
     
  9. bigseb

    bigseb Alibre Super User

    As with many things in AD there are limitations as to what can be done. Do one tooth and pattern.
     
  10. MilesH

    MilesH Alibre Super User

    The Helical Boss won't allow your Profile Sketch to bridge the helical axis. If you split the Profile leaving a one micron gap either side of the axis, it should work. If you want the tooth profile normal to the helix, you'll need to do a single tooth and pattern as above.
     
  11. Martinm

    Martinm Member

    Thanks everyone for the explanations!
    - Martin
     
  12. idslk

    idslk Alibre Super User

    I've appended a part with loft and helix in one part only to compare...;)
    yes, for one tooth shift you need more than one step. Loft takes the shortest way without guide, so with the known accuracy the result with a half shift won't be stable...
    If you use loft and copysketch with rotate and scale, you can create "gears" like this
    Regards
    Stefan
     

    Attached Files:

  13. Lew_Merrick

    Lew_Merrick Alibre Super User

    But what is the "contact angle" and the relative angle between the "herringbone rack" feature to which you are mating?
     
  14. Martinm

    Martinm Member

    I used a pressure angle of 20° (did you mean that with "contact angle") - seems to be common.
    The herringbone rack has an offset of 1 tooth (pi mm) over the half width of 10 mm. So angle would be atan(pi/10)= 17.44°.

    - Martin
     

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