Fun, but won't do another one
A while back I bought a 22LR AR upper that was on sale and the little horned guy on my shoulder told me to do it. I've just not used it that much and I got the idea somehow that I'd use it more if it was on it's own lower. Well, since I have the cnc, I thought it might be more fun and easy to make one of the 80% polymer lowers (google eparmory or see the sticker below). So I ordered one of their lowers while it was on sale and it's been sitting on the shelf for a couple of months now as I've been bargain shopping parts for it.
The lower comes with some little stickers that one is supposed to stick on and remove the waste inside the opening. They also sell some jigs you can buy to aid you in the process of removing the waste a little more accurately. I don't think I'd recommend either method, the plastic is tough and hard to remove even in thin slices, much like trying to slice up fiberglass, maybe a little harder.
My method was to import some lower drawings as pdf into vectric and scale it to match the sizes show on the drawings. I then did an overlay for each of my operations and output those to gcode.
Before I could do those operations, I measured and created some jig plates to clamp the lower into, each plate was then used to hold the lower in place level for being able to work on the sides.
So I finished with the first operations and after it got so deep, the chips weren't being removed as well and the bit started to get hot and plastic started gumming up on the bit. What is interesting is that the blanks are made with the pocket in them, then filed with the same polymer to make the lower meet atf rules for 80% lowers. As you can see in the first pic, the fill plastic started coming away from the sides when it was hot. This also resulted in a little warping of the wall on one side. I worked this out a little with a heat gun later
For the drill holes on the sides, I simply made drill marks on the blank on each side and drilled those by hand on the drill press.
My router's hand wheel also got into the top a little, I've sanded that area off anyway as it was interfering with the charging handle (which is larger than a normal one for the upper I have). It's supposed to be built up to strengthen the buffer tube area for larger calibers, but this is for a .22, so should be OK. This was after a little hand cleanup of the pocket.
I finished assembly, but took a lot of fitting of the parts to get them moving smoothly. I was missing one of the detents for the rear take-down pin and the buffer tube and stock are coming today. I also have to deal with fitting the lower to the upper, the buffer tube area is just a bit tight, which is an issue on the upper or a combination of the two as it's tight on my other rifles too, just really tight on this one.
Like I said, it was a fun project, but I won't do another. I didn't save that much, maybe the transfer fees over buying one, and can't ever sell it unless I get licensed to manufacturer them according to the rules. Even then I'm not sure I can sell this one as it would have been made prior. I could however part it out and only be out the $40 for the blank, so no big deal. The biggest issue was the time to fit everything. I've built several using manufactured lowers and can knock one out in about 15 minutes and have very few things to tweak/fit for a plinker.
To a small child, the perfect granddad is unafraid of big dogs and fierce storms but absolutely terrified of the word “boo.” – Robert Brault