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It Starts With a Design

The first step in figuring out how to print a magazine on a 3D printer is deciding which magazine to make. Those of you who know the gun can probably identify the magazine showing here as being to an M11 submachine gun. We selected this magazine for a few reasons.

One, there’s a factory design that’s made from all plastic, which gives us a starting point. Two, Jason has a legally owned M11 submachine gun.Three, it would be really cool to make a magazine that can keep up with a full auto cyclic rate. Since we’re still prototyping, and have not moved into testing, this might prove yet to be rather cocky. In truth I’d be happy to have one that keeps up on semi-auto.

But there are some difficulties here. The plastic M11 magazine is made from a molded thermoset polymer. These types of polymers form rigid cross-linked polymer chains during the curing process that while less flexible and more brittle, also tend to be fairly strong. MakerBot’s Thing-o-Matic extrudes ABS, which is a thermoplastic. Thermoplastics are less brittle, but also not as strong, and certainly more prone to flex.  On top of that, there are issues with the extruded printing technique. All of these mean it’s probably going to be impossible for our printed magazine to have a capacity that’s identical to the factory magazine because of the need to have thicker walls due to the limitations of our material and printer.

To the left you can see a CAD drawing of our current prototype. Due to the size limitations on the printer’s build area, it requires the magazine to be broken down into three different sections to be printed, and then glued together at the end. In the drawing you’ll notice there are round disks over or under some of the parts. This indicates the parts in question are to be printed standing up, and the disks are intended to stabilize the part during the printing process. After the print run is finished, the disks can be cut away with an X-ACTO knife.

Showing above on the right (click to embiggen) a printout of an early prototype magazine. You will notice, if you look carefully, exactly how the printer extrudes. It builds layer on top of layer of strands of melted ABS that are fractions of a millimeter thick, resulting in the texture you see in the photograph. Unfortunately, these ridges provide a rough enough surface to encourage the magazine to hang up and jam. They can, however, be sanded out, but that’s painstaking work.

In further posts I will be talking about my follower design, and maybe I’ll see if Jason wants to come on and speak of some of the difficulties with doing the magazine body, and some of the steps that had to be taken to make it all work.

21 Responses to “It Starts With a Design”

  1. Yu-Ain Gonnano says:

    Well, if the texture is a problem, could you print the magazine laying down instead of standing up? The rings would then be parallel to the direction of motion instead of perpendicular (nothing to catch on).

  2. I posted this on the other thread, but I’ll post it here too. I would wager that the ABS is much stronger along the path it is deposited than across the different layers where you’re more likely to have microvoids, etc. So you probably want to print the mag lengthwise as much as possible instead of parallel to the baseplate like you’re doing. The problem is forming a closed mag body that way. You might need to insert a support or something.

    That or just make some simple molds and VARTM a glass/epoxy model out.

  3. Bryan S. says:

    Some of the sintered metal and other RP machines are getting into the same category as any home machining equipment, such as a good vertical knee mill.

    At that point, you could make just about anything you could design, and if not instantly use, cast and make stronger.

  4. Sebastian says:

    I’ll see if I can get Jason to address some of the issues with the printer, and it’s limitations. He knows more about that than I do.

  5. KCSteve says:

    Another possible solution to the texture problem – and an aid to getting the full length – would be to add a liner. Because the liner wouldn’t be providing any real structural support it can be something thin. The plastic used for milk jugs is the right kind of stuff – very thin, very smooth.

    Off the top of my head suggestion for getting them in place would be to make a holding fixture (something you can probably just print up) to hold all four panels in place. Coat the outside of each with epoxy, slide the magazine down over it and let it dry. The liner running across the seams should add strength / stability.

  6. aeronathan says:

    Bryan S. beat me to the point I was gonna make. If the ABS RP machines are now cheap enough to fit into the realm of personally owned equipment, we’re probably only a short time away from having the more advanced machines available to individuals….

  7. Bryan S. says:

    But just having the machine, or the software, does not make parts. Know how and design are 2 great assets. Heck, ive been doing CAD and 3d work for 10 years, and could probably muddle may way to get to the same point, but it would not be pretty.

  8. Robert says:

    Well, if the texture is a problem, could you print the magazine laying down instead of standing up? The rings would then be parallel to the direction of motion instead of perpendicular (nothing to catch on).

    The problem there would be building the side of the magazine that would be facing up. There’d be nothing for the printer to print on, so the plastic would just be deposited on the side of the magazine that’s on the bottom.

    I suppose you could print the “top” plate separately and glue the it to the rest of the magazine, though.

  9. “I suppose you could print the “top” plate separately and glue the it to the rest of the magazine, though.”

    Or you print the right and left sides and then glue the mag together in the middle like this []. You’d want to put in bonding surfaces at the seams where they join though. Or you print one side and middle portions, then add a support block down the center of the mag that would allow you to print the other side.

  10. Matthew Carberry says:

    KCsteve beat me to the liner idea, but why not light sheetmetal?

    It still follows the theme of “can be made with parts that can’t realistically be restricted” as they’d have to restrict every piece of HVAC venting in the US.

    Just get a flat sheet, cut to length (tin snips) and make 3 bends into a rectangle (there are table top benders but you can do it by hand) and as KC described epoxy it in place. Between it and the ABS the strength should be plenty and it should be smooth (and oilable) enough to allow the follower to move easily.

  11. Bryan S. says:

    And a metal liner would give you something to print on so it could be left sideways.

  12. Sebastian says:

    As soon as you start talking about metal liners, you’re getting to the point where making the magazine requires skill. The idea here is that you could print this with relatively minimal skills, and with only the printer, and maybe some sandpaper or common household items as tools.

  13. Matthew Carberry says:

    Sebastian,

    I’m talking go to home depot for a sheet of tin and a tube of epoxy, measure and make two cuts (length and approx width) with tin snips and use a piece of wood as a guide for three bends you can do with a set of vice grips.

    There’s no skill involved, even less if you set up a homemade jig for the bending or buy a small sheetmetal bender (again at home depot).

    I can train a child to run one of those, “bend on the lines honey”.

  14. Matthew Carberry says:

    If “they” restrict sheet tin I can use ductwork pieces, old or new. Or car bodies, or old filing cabinets.

    All with hand tools.

  15. illspirit says:

    This may have been mentioned in comments on the other post, but how about adding shallow vertical grooves to the inside of mag body, then glue heavy gauge steel wire into them? Both to act as rails for the follower, and for the extra support. Snipping a bit of wire to length doesn’t really require any skill.

  16. PhillipC says:

    This is absolutely fascinating, and I see my idea for using metal as stiffeners has already been brought up. I’ll admit, my thought was more along the lines of adding the metal in as parts of the sides and printing around it, though.

  17. aeronathan says:

    “But just having the machine, or the software, does not make parts. Know how and design are 2 great assets. Heck, ive been doing CAD and 3d work for 10 years, and could probably muddle may way to get to the same point, but it would not be pretty”

    Agree totally. I’m an engineer who happens to suck at CAD work so I know the challenges first hand.

    I will say its amazing what large crowds will figure out given enough time and resources though. And once one guy determines the right setup, design, tool paths, etc the knowledge will spread like a disease.

  18. Sigivald says:

    My impression from other people is that the texture issue can also be ameliorated with careful use of solvents.

    (Apart from the “neat!” factor of plastic printing, I still think that the mag body is best made from bent sheet metal, both in terms of operability and proving the point – the “anyone can do this” factor is more obvious and the price point is lower when you’re talking bent sheet.)

  19. >My impression from other people is that the texture issue can also be ameliorated with careful use of solvents.

    Acetone. You can also glue the body pieces together with acetone too. You can also make a goopy goo by chopping up your ABS rafts and chips and adding acetone to make a paste.

  20. dg13 says:

    FYI,

    You can use an ABS cement (much like PVC pipe glue) to actually chemically melt the parts together. This also works if you paint it onto the printed part, it seems to meld the printed layers together, increasing the strength. make sure to give it a whole day to dry though..

    Acetone works too, but is probably too stong for very thin walls.

  21. Simpler solution: design a 10 round magazine that can be disassembled–and then the bodies can be stacked vertically to make a 30 round magazine.

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