2020-10-21 (W) 80PercentArRifle

I stopped installing parts into the gray receiver because I was confident I learned enough to start with a fresh one and finish it correctly. The biggest problem I had was the thin plastic around the trigger, and the remedy for that was to adjust the milling depth properly the first time. When I started this project, I couldn't find a decent picture that showed how deep to cut, so I included a picture here. This post and tomorrow's will be an account of completing a polymer 80% receiver on a drill press properly and what I learned from doing this improperly.

The first thing to do is clamp a 1x4 piece of lumber to the ejector side of the jig. The board should be large enough to cover the whole face. You should clip a smaller scrap of wood on the safety side because there is a bump-out where you shouldn't clamp. Keep the trigger pin, hammer pin, and safety holes exposed so you can drill them to the required 5/32" and 3/8" diameters. Some people suggest cutting 1/4" into the receiver, but I recommend cutting half-way through the receiver, so your holes meet in the middle.

While you drill the larger 3/8" hole, be mindful that you will encounter a hidden spring channel that could snag your drill bit. I recommend that you don't tighten your drill bit fully into your drill chuck. It is better to slip a bit than bite harder into the plastic. When my 3/8" holes met in the middle, the large bit tried to jerk the jig out of my hand as it tore into thin plastic, so I stopped drilling. After you cut all the holes, remove your clamps, and take the receiver out of the jig so you can clean any debris that might distort the plastic.

It is time to clamp your receiver into the slide-vise. Use the wooden blocks like before, so your jig presses together evenly. There shouldn't be any gaps.

Adjust your drill so that your holes are 1.25" deep. The black ring on the milling bit is deceptive, and it should not plunge fully into the receiver. The picture below, as promised, shows how deep the holes should cut.
Proper depth for milling bit as compared to the jig

Drill presses are built to withstand force in line with the bit, and when we use it as a milling machine, it is hard on the bearings. Even the aluminum jigs instruct you to start by drilling an array of holes with a drill bit or the milling bit.
Swiss cheese receiver

I went a step further and drilled down to remove most of the material. In addition to making things easier on the drill press, this is also a safe direction for a cheap slide vise, which doesn't have to move under tension.
After many presses on the drill

I lowered the drill press and held it in place with a bungee cord, and made a pass at the perimeter. The edges are rough right now, but that will get cleaned up later. Take your time moving the receiver around, and I recommend stabilizing the jig with your off-hand. Keep your hand a safe distance from the milling bit. Always cut so the material goes into the bit, not away, which will keep it from catching and jostling the work. I took the picture below after two or three passes because I didn't want to remove too much plastic.
Roughly milled fire control area

I pushed drill bits through the holes I cut earlier to clear them and show that they should be aligned with one another. When you assemble the gun, the smaller holes will host the trigger pin and hammer pin. The wider hole is where the safety goes. I cleaned up the edges with a razor by sliding it along the corners, but I angled the blade away so it would not bite in. For the receiver's bottom, I held a long drill bit and jiggled loose plastic out, and used the corners to remove burrs. I still have to drill out the trigger port and smooth the sides.
Wavering sides but clear pin holes

The rest of the posts for this project have been arranged by date. First time here?

Completed projects from year 1
Completed projects from year 2
Completed projects from year 3
Completed projects from year 4
Completed projects from year 5
Completed projects from year 6
Completed projects from year 7

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