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rolling screw extruder

a concept extruder, using threaded rods rolling around the filament to extrude.

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Theoretically, this style of extruder could put lots more force on the filament due to its long contact area. However, there are several problems, like the possible twisting forces on the filament or the inconsistent steps/mm. The goal of this project is to create a prototype and play around with this style of extruder. Ultimately, I want to measure how much pulling force I can exert on the filament. It would be very cool if it could pull hard enough to snap the filament.

Many designs for this style of extruder that I've seen involve tilted rollers.  However, I don't have a lathe, so I need a different option.  I'm going for vertical M6 bolts instead of tilted grooved rollers, allowing me to fabricate it without a lathe.  Additionally, this means that the contact shape against the filament is linear, unlike the hyperbolic shape of the tilted rollers, allowing a more uniform interface.  

I'm also powering it with a brushless motor, because the screws mean I need high speed.  For 15 mm^3/s, the main body has to rotate about 1600 rpm.  Given that I want to use this extruder for high-speed printing, the main body has to be able to rotate at 6000+ rpm, which is way out of the range of okay speeds for stepper motors.

  • v3

    ReidDye01/13/2022 at 02:24 1 comment

    First log!  For context, here's v1 and v2 (sorry about table formatting, couldn't figure out side-by-side images):

    v1:

    V2, with housing and drive gear:

    V2, just the spinny bits:

    I just printed out all of the parts, and it's not going to work.  Here are my observations:

    1. the gear teeth are way too tiny.  They won't work.
    2. the sliding action of the parts that hold the screws is surprisingly smooth; the whole tensioning mechanism works better than expected
    3. the spring I bought (mcmaster 1561T43) is a little bit big.  It doesn't stay centered on the housing
    4. everything is really small.  Even with a jig, I still don't know how I'm going to drill a nice centered hole in the M6 screw.
    5. the hole in the center for the filament to go through came out way smaller than expected.  I designed it as 2mm, it came out 1mm.

    Taking those into consideration, I made V3:

    Here's a cross-sectional view.  I think this makes it easier to understand.  The small blue and yellow pieces on the left side of the middle are pushed apart by the spring (show compressed), which in turn push on the pink and purple wedge-shaped things.  These wedges hold on to the axles for the screws, so the spring maintains constant pressure on the filament.  

    Only the left side really makes sense here because the mechanism is repeated every 60 degrees.

    Here's what I changed:

    1. gear teeth are now twice as wide
    2. main housing is now 19mm diameter, to better fit the spring
    3. sliders are beefed up, and are >< shaped instead of <>.  This makes them more stable.

    I still need to design a v3 motor mount and housing, but that can wait until I know the base idea works.  Time to start printing!

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