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Thread: Malum Gun Sub-thread

  1. #11

    Re: Malum Gun Sub-thread

    That's good to know! (And matches what I've seen -- the BBs slide much less than I would have hoped ...)

    Separately, I built a model of the funnel, and of course ... there's another pinch point.
    I'll leave it up as an exercise for the reader to mark it on the drawing, but it should be pretty obvious where this will jam ...

    (And to fix it, I will have to move the agitator backwards a bit, and extend the feed tube to create enough space to avoid it!)

    Click image for larger version. 

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  2. #12

    Re: Malum Gun Sub-thread

    You know what's great? Physical simulation is great!

    Click image for larger version. 

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  3. #13

    Re: Malum Gun Sub-thread

    In engineering in general, there's always a point of "undecidability."

    In any electronic system (even a digital one,) there is SOME signal that will lead the input buffer being in an unstable/intermediate state, and that SOME signal can propagate through the rest of the system. The probability of that signal appearing on the inputs is essentially zero, but it always EXISTS. Any attempt to "jitter" the signal or "break the tie" just pushes the problem around, or alters the shape that the "undecidable" input signal must have. There still exists such a signal.

    I think this mechanical "force the BBs to arrange themselves into a single file" challenge is an exercise in the same problem. There will always be a pinch point. The goal in the system design has to be to minimize the risk of BBs actually finding that configuration to as close to zero as possible. Then, I can use the encoder to detect a jam and wind the agitator backwards a little bit (hoping that it won't be in a reverse-jam configuration at the same time!)

    No matter what I do to remove the final pinch point, it's like pushing spaghetti around on the plate. It's still there, just in another configuration.

  4. #14
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    Re: Malum Gun Sub-thread

    Stop thinking rigidly and things will stop breaking.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    bleh

  5. #15
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    Re: Malum Gun Sub-thread

    Years back one of the RTEAM guys (Jeff, does the MW PA announcing) built over a dozen hopper prototypes and performed jam studies. This was back when we knew little about this. Over the months he would call me to video a "good" hopper in action. He would say this one never jams. I would come over and he would show me. Then while I was recording the video of the action it would jam. 7 bbs forming a stable arc holding everything up. Or a single bb jamming the feeder at the indecision point...etc. Pretty funny. Jeff said the bbs were evil.

  6. #16

    Re: Malum Gun Sub-thread

    @giantflaw Yes, I'm going to have to live with some indecision point mitigation. Also some bristles/velcro pads, perhaps.

    @tician I agree that toughness comes from flexing, but that's not the particular problem I have here :-) Jamming a plastic BB into a plastic race can happen just as easily as jamming it into an aluminum race. Although perhaps adding bristles to the actual point of indecision (great name!) would be helpful?

    I'm also looking at worm screws as an option. Main problem (for me) is that those aren't easy to make on a 3 axis mill. I may have to make at least some part in 3D printed plastic instead ...

  7. #17
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    Re: Malum Gun Sub-thread

    My hopper is the oldest (2012) design in our club that doesn't jam. It is a gravity fed with a rotating wheel with 8mm holes to collect 10 bbs at a time and drop one at a time down a tube. When a bb goes over the drop tube it first travels under a hole cover that shadows the bb from all the other bbs in the hopper. It has an indecision point between two surfaces coming together. In this design the probability of a jammed bb increases as the hopper has less bbs in it. Originally it did sometimes jam when the hopper was low. To fix it I put a brush on one of the indecision point surfaces. It pushes the bb away from the indecision point. If the bb goes in all the way fine. If it does not the brush pushes it out. Never had a jam again.

  8. #18
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    Re: Malum Gun Sub-thread

    Quote Originally Posted by jwatte View Post
    I'm also looking at worm screws as an option. Main problem (for me) is that those aren't easy to make on a 3 axis mill. I may have to make at least some part in 3D printed plastic instead ...
    I made a screw loader for one of my gun variants. Think I used it at one of the Robogames.

    It was actually a lot more challenging to get to work w/o jamming than the wheel loaders. You have to keep the velocities of the BBs constant as they go thought screw and exit. What this means as the screw radius decreases the pitch needs to increase. Actually had to do some math to get that one right.

    I abandoned it for a couple reasons:
    1) It wasn't as compact as my wheel loader
    2) It can only apply a limited force on the BBs as it relies on friction on the stack of (round) BBs behind it to generate force. It had limited capability to force BBs through tubing or force feed the BBs into the gun breach at high rates.

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    Last edited by GhengisDhon; 1 Week Ago at 08:59 PM.

  9. #19
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    Re: Malum Gun Sub-thread

    Screw Loader in action. Notice how I cleverly bounced all the BBs off the edge of the cup prior to entering.

  10. #20

    Re: Malum Gun Sub-thread

    @GhengisDon That's pretty fast :-)

    @giantflaw Yes, my best design so far was a gravity loader, too. (It's the one available on Shapeways.) The thing I'm trying to get away from is storing all the BBs above the gun, with the high center of gravity I get there.

    In fact, I don't quite understand how GhengisDhon's design in the available CAD files (wheel based, in the bottom) can work, because it has a point of indecision / pinch point similar to my previous designs ... There's got to be something special about the specifics there.

    Here's the equivalent pinch point in Dhon's magazine:

    Click image for larger version. 

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    I wonder if the fact that there will "usually" be BBs pusing down from above, will help avoid this particular jam in practice?

    (It's also not escaped my attention that the "mysterious through hole" goes right through where the BB would be lodged here ...)

    A loader with two motors might be the right thing to do. Or a motor and a solenoid, or something like that.
    Last edited by jwatte; 1 Week Ago at 01:31 AM.

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