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Thread: Malum Gun Sub-thread

  1. #31
    Join Date
    Jun 2013
    Tucson, AZ
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    Re: Malum Gun Sub-thread

    Quote Originally Posted by jwatte View Post
    so I hope you don't feel like throwing water on a goose here ...
    No, I'm absolutely enjoying this...

    I took a perfectly good gun/hopper/loader design, that as giantflaw pointed out, one of our other club members spent countless time perfecting & that the rest of our club is using...

    Respun it in my own gravity fed design...using HEDs to eliminate the mechanical switches on the existing design & were a points of failure

    Respun it again in a different gravity fed design using a potentiometer to replace the HEDs

    Respun it again driving it open loop only and eliminated all feedback devices (but was still gravity fed)

    Decided I didn't like any of them them because they were gravity fed & slow

    Made the above screw loader design

    Decided I didn't like the screw loader design because it was larger than I liked and couldn't force feed the BBs and fire fast enough

    Made my current loader design but with a different hopper that sat next to the gun (actually had two separate versions) that eliminated any tubing that was needed to route the BBs. It fired fast and was reliable but...

    Respun the hopper to mount it in my body to lower c.g. and made the tubing hard and routed through my elevation rotation axis

    All of the previous designs worked well enough... but had occasional reliability issues... this last design seems to be working well with no issues (at least so far and the loader I've now used successfully for two years)

    I think I've finally got the bug out of me to tinker with the gun... glad to see your carrying on the tradition.

  2. #32

    Re: Malum Gun Sub-thread

    Also: I found these:
    Click image for larger version. 

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    Not as artisanal a source as Ace Hardware, I admit, but their CAD models are pretty nice!

    Cutting a 7.7mm diameter, 3mm pitch left-hand thread in a 6.5mm hole using a quarter-inch thread mill makes for a very nice channel to wind this spring into as a flexible outlet once the BBs are all in a nice channel, so my initial requirement for fixed connection between magazine and breech is now a bit more ... flexible.

  3. #33

    Re: Malum Gun Sub-thread

    Not giving up on this. Just too busy at work to make post-worthy progress.

    A dual-start worm screw, perhaps?

  4. #34

    Re: Malum Gun Sub-thread

    I 3D printed a slightly wider version of this, and that seemed to work surprisingly well.
    The draw-back is that I don't quite know how to make it using 3D milling.
    A lillipop cutter doesn't quite get enough undercut. I really need something with more of an oval profile -- like an oval thread mill ...

    Click image for larger version. 

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  5. #35

    Re: Malum Gun Sub-thread

    I printed a second variant on this idea, and it, too seems to run without catching, so I think I may be on to something.
    I also found an undercutting convex radius end mill that I can use to actually make the groove on the inside of the bore. It won't get here before 4th of Juli, so I won't be machining it on that day off, but this is getting exciting!

  6. #36

    Re: Malum Gun Sub-thread

    Turns out, the cutter did get here before today, so I've been cutting!

    I now have a bit of a manufacturability problem, though. The 3D printed version works great. However, the undercut right where the spiral groove turns into the exit tube opens up the wall more in the machined version than the 3D version, because of the 7/8" radius of the cutter.

    Here's the area:

    Here's a BB, jammed:

    Here's a BB, not jammed:

    In the 3D printed version, the opening has a thicker side wall, less bent towards the left from the inside, and thus always captures the BB into the tube, instead of jamming it in the "D" on the left side.

    There are some possibilities I know of to try to overcome this. What I'm looking for is possibilities I haven't thought of!

    1) I could buy a five-axis mill. A cool $200k for a basic Haas UM-750 is just a small additional mortgage on the house. (I don't need anything fancy like a Mori or anything.) Tempting as this option is, the 40's era concrete pad floor on my garage is not strong enough to host that machine, so this option is out.

    2) I could use the 3D printed version. In plastic. Totally ruining my "all aluminum construction" claim to fame and losing several milligrams of face in front of my friends and peers by taking the easy way out. This is, obviously, not an option to consider lightly.

    3) I could order a custom ground cutter with narrower outer diameter. Given that this is aluminum and I'm OK taking many passes, a 10mm outer radius (2x3.5mm scoops, 3mm shank diameter) would be achievable. It's a few hundred bucks for a custom grind, but that kind of sacrifice may be worth it in the quest for the most over-engineered and under-decorated robot on the battlefield! Definitely an option.

    4) I could sub this out. Some starving children in China would love to grind this out with hand files, I'm sure, or I'll just send it to a random garage in Georgia using Xometry. I technically still stay within "I designed it all" and "it's all made out of aluminum" but I no longer get to claim "and I cut it all in my garage." Loss of style points, not cool.

    5) Fabricobble some kind of tilted workholding that lets me cut the final 15 degrees of the bottom of the groove using a ball end mill. Dial it in using indicators and metric eyeballs, walk in the depth of cut, and hope it doesn't suddenly fling the part in my face. (There's a center hole in the bottom, intended for a shaft extension if needed, which I could use for fixturing, too.) This is probably my current favorite, because it includes both not spending a lot of money, AND getting to apply high-grade Jank Design (tm.)

    But I'm always interested in learning more!
    Last edited by jwatte; 07-04-2019 at 02:22 PM.

  7. #37
    Join Date
    Sep 2010
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    Re: Malum Gun Sub-thread

    Why does the concrete pad rule out option number one? You could probably get that repoured for a fraction of the cost of the mill!

  8. #38

    Re: Malum Gun Sub-thread

    @cire, that's a very good point! Thinking outside the box! I shall check with my boss, she probably doesn't need any retirement savings anyway!

  9. #39

    Re: Malum Gun Sub-thread

    My boss said her hair couldn't take a garage remodel right now, so re-pouring the pad is on hold.

    However, given that I have neither a sine bar nor a set of angle blocks, I can kind-of squint and align the almost-finished part sideways in the vise, clamp it up, and hope for the best. That's almost as good!

    (Nor gage blocks. Nor a tilting rotary table. Nor a surface plate. What kind of machinist am I, really?)

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