Month: March 2011

My goal was to try to come up with a design where if you had one of these low cost ABS printers, you could just download the file, print the parts, superglue them together, add the spring, and you’d have a working 32 round magazine.

Right now I’m at the point where you can download the file, print the parts, superglue them together, sand like crazy, add the spring, and you would have a somewhat finicky 20 to 25 round magazine.

The problem I’m having is a combination of ABS being a bit too flexible and software issues with tool path generation. The walls of the magazine needs to be at least a couple millimeters thick and solid in order to prevent excessive flexing. The tool path generating software (Skeinforge) doesn’t deal well with thin solid walls whose thickness is some uneven multiple of the print head width. For example, if the wall thickness is 3.5 times that of the print width, it will generate one pass along the outer surface of the wall, one pass along the inner surface, and then one more interior pass against either the outside or inside surface. But that leaves the remaining 0.5 of the wall thickness unfilled, and you end up with a hollow very flexible wall. What it really should be doing is a zig-zag like pass over the wall interior. I can trick Skeinforge into filling in the walls completely by lying to it about print head width, but the wall surfaces end up off a bit and then I have issues with the magazine fitting in the mag well or the rounds fitting in the magazine. So I’ve been tweaking the wall thickness and print head size setting in order to try to get Skeinforge to do what I want.

The right solution is probably to modify Skeinforge so that it will generate the correct tool paths from the correct model, rather than adjusting the model to try to make up for Skeinforge’s deficiencies.

Other than the flexing issue ABS seems to be strong enough. I have one magazine I printed which can hold the full 32 rounds and feeds fine, but it bulges so much when fully loaded that it won’t fit in the mag well. The one magazine rupture I had was due entirely to a mistake in the cad file that caused the printer not to correctly bond the insert pieces (where one section of the mag body fits into another) to the rest of the part.

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.