You want to build an AR using an 80% lower. We're breaking down the required and recommended tools. AR9, AR-15, and LR-308 80% lowers use the same parts, like the trigger, hammer, and safety lever. So, they require the same tools when it comes to fabrication.
1. Safety Equipment
We're fabricating metal with cutting power tools, and safety equipment is a must. You should wear safety glasses and hearing protection while operating your power tools and working on your 80% lower. Even high-quality drill bits and end mill bits frequently chip or break. Broken bits and mistakes made can cause serious injury. Protect your eyes and ears while you work.
2. 80% Jig
The jig is the primary tool required for all lowers and frames. For AR-type lowers, jigs are configured to machine either AR-15 and AR9, or LR-308 receivers. The Elite Builder Jig above can fabricate the former two. The longer dimensions of the .308 lower dictates using a jig with longer plates. AR-15 and AR9 receivers share almost all the same dimensions and they use the same parts kit and pins, so their jigs are usually interchangeable.
Drill press/mill vs. router jigs
The 80% market has two types of jigs available for completing an AR receiver. Drill/mill jigs are favored by machinists and those who own a drill press or milling machine. Router jigs are typically bought by first-time builders who own no power tools and have no experience with any sort of fabrication. We prefer mill/drill jigs because they're more stable and they avoid direct tool-on-tool contact. Router jigs are inherently unstable and operating one causes wear-and-tear on both the router's base plate, and the jig itself.
3. Drill Bits and End Mill Bits
You need bits to machine your AR's lower. Most jigs come with the bits you need for the lower receiver. Since all the AR's calibers use the same parts kits and pin hole diameters, the bits required are universal:
- 3/8 Short End Mill: Required for milling the rear shelf portion of the receiver.
- 3/8 Long End Mill: Required for milling the main cavity for the parts kit install.
- 5/16 Long End Mill: Required for cutting the trigger slot in the floor of the receiver.
- 5/32 Drill Bit: Required for drilling the pin holes for the hammer and trigger/disconnector.
- 3/8 Drill Bit: Required for drilling the pin hole for the safety selector lever, and pilot hole for milling.
To ensure your bits work efficiently, it's best to use high-speed steel (HSS). You should stick with center-cut end mill bits which allow for plunge-cutting. This may be required if you're using a drill press. Using a short end mill for the rear shelf is typically required so the blades/flutes of the bit don't accidentally cut the jig's milling template. Technically, any diameter end mill can be used to mill the main cavity. The small diameter of the 3/8 end mill produces a clean cut and works great with drill presses, especially when using one as a makeshift mill.
4. Drill Press or Milling Machine
You need a drill press to drill your lower's pin holes. In addition to drilling the receiver, you'll need to mill the fire control cavity, and a milling machine is ideal for this step. Mills are expensive, so many builders choose to use their drill press to mill the receiver. Aluminum is a relatively soft metal, so this can be usually done safely if the right precautions and best practices are used.
Drill vs. Mill
A drill press uses a Jacobs chuck and tapered spindle/arbor to hold a drill bit in the machine. These parts rely on being press-fit together and are not designed to accept high lateral loads. After all, drill presses are made to go up and down, and they often have more lash than a rigid mill. A milling machine uses threaded parts and something called a drawbar to hold end mill bits in place when lateral loads are applied to the bit. Nonetheless, drill presses are often employed as mills, especially when it comes to cutting an 80% lower. If you choose to take this route, follow these best practices to ensure safety:
Making shallow cuts
You should only cut the receiver using shallow depths, 1/4" or less being ideal. This will also extend bit life, reduce chatter, and produce a cleaner finish.
Only use climb milling
Always cut with the direction of the feed. This is called climb milling. Cutting with the blade going into the feed makes the bit want to "climb" upward, rather than being pulled down. On a drill press, this helps maintain the connection of the chuck and spindle with the bit.
Feed the receiver through the bit slowly, and frequently clean and lubricate the bit and receiver.
Excess chatter? Do plunge cuts
If you try to mill with precautions taken but your drill press still fails to cut like a mill, stop. Attempting to go further could result in injury. If you bought a jig like the Elite Builder, you can still use the end mill bits with your drill press safely. Using the drill press, you can complete the cavity using plunge cuts with the end mill bits.
Plunge cutting with a drill press
Basically, you're using the end mill bits like drill bits: Plunging the bit straight down into the receiver to the final cutting depth. This eliminates the risk of using your drill press like a mill and allows you to complete your receiver even still. It's important to note that if you use plunge-cuts to complete your lower, the bits must be center-cut:
Non-center-cut end mills cannot cut vertically and trying to use one with your drill press will damage the receiver, bit, and/or press. Most jigs' included end mills are center-cut to accommodate this fabrication method.
A vise (like this 4.25" drill press vise) is required to secure the jig and 80% lower for machining. If your machine doesn't have an X-Y table installed, a cross-slide vise should be used to provide quick lateral movements while cutting, reducing the time required to complete your receiver. You should never attempt to hold and control the movement of the lower and jig by hand. The tooling will likely catch the receiver and cause kickback and/or extensive damage.
6. Cutting Fluid
When you machine metal, you need to frequently apply lubricating oil called cutting fluid to your bits and lower receiver. This prevents the bits from overheating and dulling or breaking. It also ensures the lower's milled with a smooth, polished finish. Jigs and bits are reusable if maintained, so cutting fluid is a great investment either way.
Drill Stop Collars
Drill stop collars can be attached to your drill bits and end mill bits. They're an extra layer of protection against drilling or milling your receiver too much, and they only cost a few dollars. If you're a first-time builder or you're new to using equipment like a drill press, these can help verify your measurements with each step.
Calipers do an excellent job of ensuring your receiver's been milled to precise depths. The calipers can be used with stop collars to manually check the drilling and cutting depths of your bits. It's always a good idea to measure twice and cut once.