I was given a project some months back. I said I'd know more about its feasibility when I'd had a chance to lay out the parts I have and stare at them for a while.
I stared at them today.
This is a de-militarized KP/-31 "Suomi," a Finnish-made submachine gun introduced in 1925 and used in World War II. The "de-militarization" consists of -- as you can see from the photograph -- using a torch to cut the receiver into little bits.
My task is to make a new receiver. Not, I hasten to add, a functional one. In fact, there's some nice little laws I'd run afoul of if I made more than 80% of a functional receiver and sold it or was otherwise recompensed for the work. Figuring out where that line lies, and making sure what I build can't accidentally be fired, is going to be part of the problem.
So how to it?
There are two basic strategies (both of which are also applied by gunsmiths who do intend to end up with a functional weapon at the end). The first is re-weld. This might be the fastest, especially as I have no need of the special jigs and resulting accuracy of the gunsmiths. I just need the parts to more-or-less fit.
The other is a new receiver. And where I part company with the gunsmiths is in materials. My first impulse was to sculpt the entire thing and cast it in resin. I might even be able to take some impressions from the existing scraps (or negatives from the parts that mate with them) in order to save on hand-sculpting.
The best argument for sculpting is that it is the most iterative process. At any point, I can add more material back in; there's no risk of cutting too deep and ruining the piece. The biggest downside is lack of strength; the necessary shapes include some very thin portions, and the weight of the barrel and the pressure of the trigger assembly are going to put a lot of stress on them.
Which leaves metalwork. The shape is too complex and the bounding box too large to make subtractive methods optimal, as much as it has a surface attractiveness to simply CAD the entire thing and CNC it out of a single block of aluminium.
And additive methods bring back some degree of iteration; you can work on a sub-assembly without risking the entire piece.
In the real weapon -- and also in the "80% kits" that used to be commercially available (they seem to have dried up), this is how it is done. The "tube" and "sidewalls" are machined separately, then welded together.
So that's my working assumption; take a tube that's as close as possible to the required ID, lathe it down, cut the rear threads, mill out the lugs, mill ejection port and a long slot along the bottom. Then cut into a slab (or several pieces) of flat stock to create the sidewalls, the part that hooks into the stock, and the magazine well.
Press-fit is not going to work here, though, so aluminium welding, and then clean-up of that. That's a lot of work. And a lot of the cuts and holes are necessary because so much of the receiver is exposed; is the look of the gun. And they have to be right to fit the existing parts that show externally, like that trigger group. There's also a question of how to treat aluminium so as to fit in with the look of the other metal.
There may be a third route. It might be possible to simulate the look of the receiver and support the other parts of the gun with rods and careful bits of water-soluble glue. I'll have to stare at it longer, but at the moment that isn't looking very plausible.
In any case, it doesn't seem the best "first project" for the mill. It might actually be a little easier to tackle this thing:
This is one of an excellent run in pewter-cast of the pistol used by the character Jubal Early on Firefly. Not my favorite design, but apparently there are enough people interested in one to justify looking at trying to build a few. It looks like a natural for CNC, with a little traditional machining to open some of the mating slots and tapped holes.
Well, I still have seven grenades to go to fill the current orders. That's at least two more days of machining, plus painting time. Pity I'm not making enough money at this yet to pay for next year's TechShop membership...