Building an AR 15 is probably one of the most rewarding feelings there is. Taking that chunk of aluminum we call an 80% lower and turning it into a rifle is on par with hitting your first 500 yard shot on steel. Instead of hearing a gong ring, you hear a power tool going to work. Which power tool depends on the option you decide to finish your 80% lower receiver with:
If you own a CNC Machine, or have easy access to one, and know how to use it, there is no reason to use any other method. CNC machines are by far the easiest, cleanest, and most precise method to finish an 80% receiver. CNC machines use computer aided design to give you what is basically a template to finish your 80% lower. If it’s programmed correctly there is no need to worry about depth, drilling too wide, or generally screwing up your lower receiver.
If you don’t have training on CAD, or a CNC machine this is probably not a realistic route to take. CNC machines are very expensive, and the training to properly use CAD and a CNC machine may not be a realistic option for most AR 15 builders.
This is probably the most common method of producing turning an 80% lower into a real lower. Most jigs out there are designed for drill presses, and a Jig is a must have for AR 15 builds. A drill press is mounted on a stand and often has a vice, or the ability to mount a vice. A vice will hold your lower as you drill. A drill press has the ability to drill in a perfect straight line, and allows the user to very easily control the depth of their drilling.
Drill presses are relatively inexpensive, and can be found in almost any hardware store. It lacks the finesse of a CNC machine, but is about 100 times easier to learn to use. Since the drill press is the most common method of making an AR 80% lower they are tons of instructional videos and articles out there.
Milling machines are somewhat like JR CNC machines. They require substantially less training than a CNC machine and are substantially cheaper. Milling machines vary in automation and some require more manual work that others. Milling machines are quite precise and are a good compromise on not having a CNC machine.
Milling machines are not plug and play tools though. They do require some training, and the user should probably have a tutor present at first. It’s easy to learn to use a milling machine, but it’s certainly a hard skill to master.
Routers are most commonly used for cutting wood, but a good metal cutting router can finish an AR 80% lower very quickly. Since there is now a Jig and detailed instructions out there on how to use a router to mill your 80% lower. With the jig in place this is a very easy task to accomplish, and does reduce a lot of risk associated with milling an 80% lower receiver.
The biggest problem is that routers move with speed and ferocity. You have to stress safety with a router and take serious precautions against harming yourself. There are custom bits designed to work with aluminum, always use those bits, not normal metal cutting bits. Aluminum may melt to a bit and render it useless.
Hand drills are a common option and the subject of many, many questions. Everyone has a hand drill, and when you complete an AR 15 80% lower you’d rather use something you already have right? Well, this is like playing a video game on that difficulty past hard mode.
You also need to have a powerful drill, with a very fast RPM. A drill is hard to use because it relies on the user to make perfectly straight axi downwards. Also depth settings are impossible, so a depth collar is needed, and they aren’t always reliable. Just be very careful if you go the hand drill route. I do not suggest the hand drill for anyone new to DIY projects do to the difficulty.
Regardless of which method you choose there are a few universal tools you need. First and foremost, a good set of drill bits, preferably a set designed for aluminum. Next, you need a good jig, one that can last and last. Lastly, gloves, long sleeves, safety glasses, and potentially ear protection is always required for safety reasons. Always be safe.