A complicated fantasy firearm, moving parts and lots of metal in a small apartment? What could go wrong?
Step 1: The Concept
As mentioned in my previous Instructable, this was a non-firing replica of a weapon that never was; the heroic and dangerous 45mm double-barreled break-open grenade launcher carried by 1930's pulp adventurer Mr. Fury.
The original concept was to make use of the growing technology of 3d printing; to model the complete weapon in 3d and print out the parts that linked several military surplus parts that looked cool for the project.
It ended up with me doing serious metal working in my apartment, with inadequate tools. The biggest tool I had was a budget, bench top drill press from Harbor Freight. Most of the serious cutting was done with a hacksaw!
I also made some serious errors, such as being misled by my experiences with welding steel to think I could braze multiple brass parts together in the same way. In the end, the thing was as much epoxy and other glues as it was metal, and in hindsight we really could have printed more of it, done a lot less metal work, and saved ourselves much time and money.
Step 2: Importing the Parts
The first step was to bring the existing parts in to the scale-accurate 3d model. To avoid lens distortion I placed the pistol grip on top of my scanner and SCANNED it in. Then I aligned and scaled it (as described in a previous Instructable).
The other parts were done by hand...calipers in hand, frequent measurements, lots of playing with scale tools and grid settings. Early on I set the project in metric...and then it turned out almost all of the important parts lined up better in Imperial units. Not only that, there is STILL a bug in Carrara Studio wherein if you set the workspace to metric it multiplies all numeric entry by 2.54 (the number of centimeters in an inch, natch).
Step 3: The First Attachment
It would have been simpler to epoxy or weld. It was just too tempting to make it possible to "field strip" the pistol grip. Thus, the pistol grip is secured to the rest of the weapon by two pins. And the pins of course had to be metal to be strong enough (I made an attempt to calculate sheer forces but my physics is rusty).
You can 3d print in a bronze-infused Stainless Steel that is as strong as mild steel. It also costs over ten dollars per cc of printed volume. So the decision very early on was to print the smallest shape possible that would attach the pistol grip firmly, and attach everything else to it.
As the design concept evolved, it become clear the barrels would hang from a pivot point that rode a buffer rod through the centerline of the weapon and all the way back to a spring in the butt stock. Since this was 90% of the serious loads on the weapon, the print was also designed to support this rod, thus becoming an integrated central skeleton of metal to support the weight and other forces involved.
Step 4: The First Fabrication
We started with several pieces of 2" acrylic. Unfortunately those couldn't even take the stresses of cutting. So I placed an order with Online Metals dot com and ordered a chunk of brass tube. At least the brass was a little easier to cut than steel would have been. But, really, I wish I could have used PVC.
A problem that plagued the project was that tube is measured by OD (outside diameter) but pipe by ID (inside diameter). It is easy to find "two inch" pipe in PVC, ABS, and other nice strong cheap plastics. But it isn't 45mm inside dimension, and it isn't 2" outside dimension (as needed to fit into the surplus butt stock.) So we had to purchase tube, which limited us to acrylic or metal as the ready choices.
The brass took a lot of sweat to work, and then I discovered the flaw in using a huge chunk of brass. Brass is a conductor. A great conductor. To braze to it, I would have to bring the entire lump up to brazing temperature. And my tiny portable oxy/butane kit wasn't going to do that.
Fortunately, JB Weld seems to be holding up. The design of the thing meant I could slather on a good layer of epoxy putties to keep the pieces together.
Oh, and after all of my care there was an error in the dimensioning somewhere, and I had to grind and file at the printed part to make if fit. Which meant going through several grinding and high-speed bits on my new Dremel, chewing my way into stainless steel!
Step 5: Extending
The next step was to build out towards the front of the weapon, putting down steel so the user could support the weight of the barrels on the wooden forend (a Winchester 93 wooden forend that we mail ordered).
The original concept was to braze a short pipe into the stainless steel print, then thread that into a necking adaptor down to 1" black pipe, which in turn slid inside the wooden shotgun forend with a little space to spare. As the concept evolved, a second 3d print -- this time in plastic -- was incorporated to fill the difference between the diameter of the wood and metal parts, and to provide a protective flange for the user's hands as the barrels "recoiled during firing."
When the wooden forend arrived, it was obviously too thin to directly support the surplus swivel clip the sling would be attached to, so I extended the 1" black pipe, filed a flat spot, and drilled and tapped it to bolt the swivel clip to it.
Also, it turned out there was no such neck-down fitting available. Not for those pipe sizes. So I laboriously cut threads into one end of a 3/8" brass rod and inserted that into the print instead. That went through a hole drilled in a brass pipe cap and a 3/8" coupler stood in for a large nut; because I filled the cap with JB Weld to keep the parts tight and aligned.
Step 6: The Buffer Assembly
So there is a 5/16" stainless steel rod running the length of the weapon. The barrels hang on one end, and the other goes into a spring. This (in the imaginary world in which this thing could actually chamber live rounds), takes some of the recoil shock.
I was looking at lots of enclosed spring and gas spring assemblies, and had built a closed spring assembly from 1" pipe, but when it finally came down to it I went simple; a couple of pieces of PVC glued together, filed down until it just fit inside the rear of the brass piece, and epoxied in place. If it looked too fragile, I could always drill through the side and run in a steel pin.
The spring is simplicity itself; a 3/8" t-nut is epoxied on to one end, and a 5/16" collar is locked on to the buffer rod with a set screw.
Step 7: Joinery
With the forend assemblies all epoxied into place, the time had come to smooth the join between the carved brass of the receiver and the printed plastic flanges that extended out along the forend.
Apoxie Sculpt, rough-carved with x-acto knife, then filed and sanded. Then a second pass to build up and smooth the shapes more (Apoxie also bonds very well to itself).
Step 8: The Front Fork
To hang the barrels I needed the part that connected the buffer rod to the barrels...or, technically, the trunnions (the pins in the barrels.) The original drawing had the pivot on the bottom but this seemed simpler and more elegant. In the final version a pair of holes was drilled and tapped and set screws inserted in them from the outside of the fork, and CA glued to keep them from backing out.
To take the weight it needed steel. To stay on schedule and under budget I couldn't have it printed. But I'd designed knowing that years back I had bent steel to make wrought-iron look scenery for the stage.
I cut a jig out of the only wood I had -- white pine -- and heated the 1/8" thick mild steel bar stock to red-hot. The first bend was nearly perfect, but carbonized the jig (it also set fire to the jig, but I had been expecting that.) Did I mention among my hardware store runs was a run to the welding supply store for gas goggles and new welding gloves? Or the fire extinguisher? Or that I did THIS part outdoors?
One jig and two more bends later, I took the two with the closest match and hammered and twisted at them until they looked like they matched.
Step 9: Welding
I couldn't finish the fork without finally attaching the barrels together.
First I tried with my little Worthington welding kit. No go. The tiny tank of oxygen gave out before I could get started on a braze.
Then I tried to have it done. Chased after a local welding company for days, getting a terrible run-around, before realizing I was better off dealing with it myself. I knew how to weld. Trouble is, I didn't have a shop any more.
At last I picked up the phone and called my brother. Visited his wonderful shop for one short trip -- used his MIG kit to make the ugliest weld that would ever leave his shop. Also used his cold saw to trim the barrels flush; I'd had them cut by the metal supplier but the tolerance was no more than 1/8" of an inch.
I knew not a lot of strength was needed on this weld, though. And even better; it would never be seen. Because the weld would be under a strip of aluminum bar stock. And to add to the amusement, a sculpted set of fake weld beads done in Apoxie Sculpt adorned the edges of the bar stock.
Step 10: Detailing the Fork
Such a pity...after all of this metal, right at the front of the prop where it was most likely to get dinged up I'd have fragile plastic and epoxy.
With the newly-bent fork pieces bolted to the buffer rod and anti-twist rod, they could be fleshed out to look properly beefy. First was a simple washer, filled out with Apoxie.
Next were strips of styrene half-round. The styrene was heated with a hot air gun until it could be bent easily, then CA glued in place. One trick I discovered is that the heat loosens the glue...I had to bend the whole thing then glue it all at once.
A few bits of sheet styrene cut out and also CA glued on to the washers completed the look, and more Apoxie smoothed out the joints and filled the gaps. I even put lumps of Apoxie over the exposed bolts -- they look sort of like rivets now.
Painted up, it looks pretty much like I wanted it to look; like a single rough metal casting. So nice that for this prop, a lack of fifteen coats of primer and wet-sanding with 800 grit in between was actually an advantage.
Step 11: Surgery
At this point I'd completed the interior mechanisms (see my other Instructable) and I could check latching and unlatching the barrels.
And failure. The plastic printed part that was transferring the force from the wooden forend to the presser rod couldn't take the stress. it was coming apart.
With the trusty razor saw I cut a slit in the smoothed, sanded, and painted receiver (sigh) and extracted the broken part. Then got a sheet of steel from the hardware store and over a long day of cutting and filing created a replacement.
There were more adjustments to make, more errors...the brazed connection from the sliding brass guide to the rod that pressed against the breech assembly didn't hold. And the rod was too small and was getting bent. Still later in the process, the fake weld detail I'd added in a fit of foolishness to the bottom of the barrels got caught on the presser rod as well and had to be carved away.
The basic concept of the mechanism worked, but it took a bit of adjustment! I know it all seems the obvious way to build it now (actually, the obvious way is with pivots instead of sliding joints...!) but it took two weeks and several dozen sheets of sketches on paper to work it out.
Step 12: Final Details
I did the last stages from final paint to the last little details in one great rush, not stopping to take pictures.
For a week I was moving half-painted parts back and forth. Multiple coats of auto primer, sprayed thin, with sanding and patching with Tamiya White between coats as well. Then several coats of a harder-wearing flat black epoxy. Then staining and wash with acrylic paints. A "chip and scratch" layer of silver paint dry-brush. And, finally, a hand-rubbed graphite treatment to give it a metallic luster.
I drilled and tapped for the surplus "ladder" sight. Created a front sight by chucking a piece of 1/8" brass rod in my drill press and holding a hand file against it. Kids, don't try this at home! The piece of stainless steel acting as a "dust cover" (mostly, hiding the springs and bolts and stuff) needed to be cut and drilled and that meant more ruining tools on hard steel.
The German butt stock was thick paint over rust and I chose to partially strip it before re-painting. I also stripped off some of the layers of grease from the furniture on the pistol grip, and painted it up with acrylics to match the nice new wood of the Winchester shotgun forend. I carefully wiped down the stainless pieces, and even buffed the larger one with buffing compound and a Dremel buffing wheel.
There is one thing I have not done yet, however.
And that is to take the final model out into the woods, with of course the appropriate costuming, and take some decent pictures of the completed prop!
Step 13: Complete!
Assembled, handed over to the client. A couple weeks later, we took it out into the woods to take some pictures.