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Thread: Malum Gun Sub-thread

  1. #1

    Malum Gun Sub-thread

    Strapping a barrel to a gearbox and aiming it at the enemy isn't particularly hard, mechanically speaking. (Or even electrically -- either use a 10A motor driver, or use a MOSFET driven by your microcontroller.)

    The challenge is avoiding jams/clogs, where the flow of BBs restrict down to a single channel, from which it can be "packed" into the breech and propelled by the gearbox.

    My initial design was gravity-fed from above, with a simple wiper that "stirred" the BBs around a plain hole where the dropped into a channel that gravity-fed into the breech. This was surprisingly effective, and worked pretty well! (Still works well; I put the files up on Shapeways and I've seen people still use the design.) There are two things I've been trying to improve off of this design, though:
    1. The center of gravity is high, which means that the gun (and robot) "sways" more than necessary when walking around.
    2. The gravity feed is somewhat slow and somewhat unreliable in timing -- just "fire" doesn't quite feed fast enough, so I need to pace timing, and ideally use a sensor.

    I've seen some good gun feeder designs, with Dhon and R-team leading the pack and Jim not far behind! Immortal also seemed to have a good system, but I didn't get close-up pictures of the actual mechanism.

    I'm trying to replicate the features that make those designs work. Specifically, the main challenge is funneling the BBs from a main "hopper" into a single channel.

    This turns out to be a science that mechanical engineers have worked on for hundreds of years, with scientific papers still published yearly. The best we've come up with as a species seems to be "add vibration, or shoot clogs with a second, smaller, air gun!" (There's also the concept of "mass flow" versus "funnel flow," where you get "mass flow" by having steep, slick walls in your hopper.) Consensus seems to be that particles will build "bridges" (self-supporting arcs causing a blockage) if the outlet width is less than 5x the particle width. Unfortunately, in this application, we strictly need it to be exactly 1x.

    All approaches that work well seem to have a carousel-style ("sprocket") agitator/feeder at the bottom, pushing BBs into a narrow channel to create the necessary tension to force them into the breech.

    Dhon's feeder collects BBs from above the carousel, all around it, and feeds them out a hole lateral to the agitator. (Dhon has shared CAD files, too, which are quite helpful.) My BAMF magazine design used this approach, but had a much narrower area of BB collection (essentially a 1.5" aluminum tube, with the motor and agitator mounted on the inside.) The restriction led to too many narrow channels and the BBs would jam somewhat frequently. I tried channeling BBs with 3D printed channels, but still ran into the bridging/jamming problem, because at that point, it's all gravity feed not agitated by the mechanical movement of the feeder itself.

    Jim's feeder collects BBs from an arc of about 3.5 BB diameters, and steep walls, and then feeds them out an axial hole using a slanted roof in the carousel channel (I was delighted when I first saw this! How clever!)

    The problems I've made for myself come from trying to keep the robot as compact as possible. Even though I use bigger servos, Malum has a slightly smaller body than most of the other robots. This means I can have longer leg reach and still stay within the avenues of the arena (assuming I can see the buildings on the side -- more about that in another thread!)
    By aiming with the legs, it also gets a significantly lower profile. We don't yet have overhangs in the arena, but I imagine it'll only be a matter of time until we get there! (And embankments! And formation warfare! ;-)

    This leaves an area of about 50x90mm on the side of the gun, and the breech feeder hole is horizontal about 27 millimeters above the top deck of the robot. This turns out to be a quite awkward configuration -- the hole is not high enough that I can use a sideways mounted feeder wheel, but the 50mm width is narrow enough that I don't have a lot of clearance for tube-style feeder assemblies. (Even worse, the gun is mounted on a bracket that encroaches at the bottom of this 50mm area by another 10mm or so.)

    Click image for larger version. 

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    So, with that background out of the way, this is the current feeder I've iterated towards:

    Click image for larger version. 

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    The BB collector area is about a 3.5xD arc. The wedge-shaped flat on the right is the key to flow, because it has an angle that lets BBs decide to either go out (which leads to an empty spot in the feeder) or in (where there's only room for one BB per slot in the carousel.) The roof on top is there to avoid jamming against a flat wall, which would otherwise happen at some point (worst case, at the outlet.) There's still a risk that the BB will jam in the angle between the wedge and the roof (which has a slope that matches the slope into the carousel) but I'm hoping the angles are shallow enough that BBs will go "in" or "out" and not get stuck.

    This is the end point of a number of iterations, and if this still has an actual pinch point, I'll start over from blank slate. I'll let you know how it goes once the mill is done cutting this iteration!
    Last edited by jwatte; 06-02-2019 at 02:27 PM.

  2. #2
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    Re: Malum Gun Sub-thread

    Most of the designs I've pondered involve single-file magazines to completely eliminate the need for sorting BBs during combat as that has been the greatest source of issues people have been having since the very beginning.

    One of the many designs I've really wanted to try involves printing/milling a helical channel (basically a threaded rod) magazine around the length of the barrel and surrounding that with a thin-walled tube to hold the BBs in the channel. Numerous methods can be utilized to propel the BBs through the channel to the breech. Currently thinking a narrow axial slot/recess in the outer tube to allow a pin/nut/sled to thread along the channel as the outer tube rotates due to a motor and/or spring.

    Edit: actually, just making the channel 3~4mm deep and adding a 1~2mm tall axial ridge along the inside of the outer tube should work with fewer moving parts than a sled.
    Last edited by tician; 06-02-2019 at 04:07 PM.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    bleh

  3. #3

    Re: Malum Gun Sub-thread

    Preloading 300 BBs in a single helix does sound interesting :-) And storing it around the barrel is a novel idea. Could look really cool!

    Meanwhile, there was still a pinch point between the "wedge" on the bottom and the "roof" angle (and, of course, the driving "sprocket.") I hand-filed and then cut a channel to let BBs that decided to take the "out" path in the "in or out" priority step on the wedge free escape. As a second benefit, it lets me see what's going on. Then I tested, and ... now I got a pinch between the wedge and the sprocket, no roof involved.

    I had never seen a pinch between wedge and sprocket before, when testing without roof, so this is annoying. I thought the wedge "priority" design was solid. Back to the drawing board. I may have to give myself more room somehow, to fit what's needed...


    Click image for larger version. 

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  4. #4

    Re: Malum Gun Sub-thread

    What worked well for me before?
    A simple wiper, with BBs falling into a plain opening.
    I wonder if I could re-use this thought somehow?

  5. #5

    Re: Malum Gun Sub-thread

    A rotating wiper and a stationary hole is equivalent to a stationary wiper and a rotating hole, right?
    Because if the hole rotates, then some kind of agitator/sprocket can rotate below it without using a second motor.

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    Re: Malum Gun Sub-thread

    Only equivalent if the hopper is also rotating relative to the fixed wiper. Otherwise, the fixed 'wiper' is just a blockage helping keep BBs away from the hole/sprocket.

    Perhaps something like this crap drawing...
    Click image for larger version. 

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    Orange is shaft of brush (and reference 6mm BBs), red is soft bristles (and/or shorter at very bottom/output), dark blue is 3D printed screw/ramp/guideway, white is ~1.5"-ID tube, light blue-green is cutout in bottom of tube for BB output. The only time the blue printed ramp somewhat prevents the bristles touching the BB is at the very bottom where it has a protrusion to force the BB out of the hole in the bottom or a cutout in the side of the tube.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    bleh

  7. #7

    Re: Malum Gun Sub-thread

    Interesting!

    My current thoughts are something like this: (Seen from above; the U shapes are where BBs fall down, and the "spiral" on the bottom feeds the BBs into the up-bent channel)

    Click image for larger version. 

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    I would bolt a shallow funnel on top of this to actually get the BBs to the rotating wiper/collector in the middle, and the funnel would need an exclusion area for the egress feeder tube where I'd probably use a 1/4" ID spring to create a channel to the breech. (Again, thanks to R-team for that idea!)

    The wiper/agitator/feeder looks something like this from the underside. Note that the rotation is clockwise when seen from this underside; the part that pushes the BB is the curved-away spiral on the left, intended to "wedge" the BB into the feeder tube. There's a groove at the bottom of the main housing that turns out to the egress; the groove occupies the area from 0 to 2 mm, and the "wiper" sits at 2.5mm up to 6.5mm, and also creates the "roof" to create constrained motion for the BB.

    Click image for larger version. 

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  8. #8
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    Re: Malum Gun Sub-thread

    Quote Originally Posted by jwatte View Post
    The wiper/agitator/feeder looks something like this from the underside. Note that the rotation is clockwise when seen from this underside; the part that pushes the BB is the curved-away spiral on the left, intended to "wedge" the BB into the feeder tube.
    Congrats, you just reinvented the one-way roller bearing. ;-) Hope your motor can handle repeated, sustained stalling.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    bleh

  9. #9

    Re: Malum Gun Sub-thread

    Congrats, you just reinvented the one-way roller bearing. ;-)
    Ha! :-)

    More like a linear bearing with cycling bearing balls, maybe? (In fact, going looking in that area might give me some more ideas?)

    Hope your motor can handle repeated, sustained stalling
    Yes, that's a given in this application. I'm using a 6V/500mA motor and I only need to stall it at about 2.5V/100mA, and I have an encoder so I know when it stalls and can turn it off. The stall is basically what puts the feed pressure on the BBs, and all the R-team assemblies seem to use that same basic construct.

    That being said, I still have a pinch point. However, I think I can smooth this out with a much flatter turn-out race/channel on the bottom.

    Click image for larger version. 

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    ... or a similar swoopy turn on the trailing ("pushing") edge of the agitator's roof. And some kind of downwards funnel on the entrance to the egress channel?
    Or maybe just make the channel shallower, so that the BB can jump the race if it's jammed in place.
    Last edited by jwatte; 06-03-2019 at 05:02 PM.

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    Re: Malum Gun Sub-thread

    That thing is the essence of a one-way bearing: shaft/wiper spins one direction and it removes the roller/pin/BB from contact with the housing completely decoupling motion of shaft and housing; spin the other way and the roller/pin/BB gets thrown into the housing and jammed between the housing and shaft/wiper to lock them together.

    If you get the curvature of the convex edge of the spiral wrong, it will jam with the slightest amount of friction between the BB and housing. Basically, the closer the convex edge is to a simple radial line, the less likely to jam. If the angle between tangential lines at the two points of contact on the BB (BB+housing and BB+spiral) is less than ~45 degrees, it will jam.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    bleh

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