We've covered the mechanical steps you need to take to successfully cut and drill an 80% lower here. But there are some critically important tools and accessories you'll need to invest in, lest you wind up with a stripped receiver that simply doesn't work (or you ruin it in the machining process). Let's cover the tools required for finishing your 80 percent lower. We'll also cover recommended tools to make the job easier.
80% Jig with Bits
The single most important tool you need to complete your 80% lower is a jig. The jig is the tool -- or really, set of tools -- that provides the templates and bits for cutting and drilling your receiver. We cover the jig's individual components here, and we cover how it works with the included drill bits and end mill bits. Jigs are commonly categorized as drill press/milling jigs, or router jigs. We provide drill press/mill jigs (like the Elite Builder Jig shown above) because they typically provide a more stable platform for machining your receiver blank with the tools discussed below.
Drill Press or Mini-Mill
Utilizing the jig to complete your receiver means having either a drill press (good) or mini-milling machine (great) on hand. If you're new to machining, you should know that a mill is designed to drill vertically and cut laterally (side to side), while a drill press is intended to only drill holes vertically.
Using a Drill Press
When a commercially-fabricated receiver is made, a CNC milling machine is typically used to cut the receiver. But most of the material you need to remove inside the receiver can be taken care of with a drill press, while still providing a polished and clean finish.You will complete most of your 80% lower's fire control cavity by drilling pilot holes using the drill bits and a pilot hole template that come with your jig. This removes the bulk of the aluminum inside the fire control cavity, where the parts kit will install.
To produce straight, clean lines on the floor and interior walls of the receiver, the end mill bit can be used with the drill press like a drill bit, driving the bit vertically to the required final depth. This is called "plunge-cutting" and can only be done with a center-cut end mill bit, which we cover in the jig guide linked above. Once the floor of the receiver is cut clean, the interior walls can be smoothed over and made flush by performing very light milling with the end mill bit. Drill presses are not designed for lateral machining, but many builders are capable of safely performing this final "polish" given so little material remains. Aluminum a soft metal that cuts easily, so laterally cutting small amounts of the alloy in this fashion does not place a high load on the drill press chuck, which would otherwise cause damage. Performing these final "clean-up" steps is best performed with the receiver and jig secured in a cross-slide vise, which allows for precise lateral movements of the assembly around the bit.
Investment: A decent drill press capable of machining aluminum doesn't cost much. You'll likely spend between $150 and $250 for an adequate unit with good torque, weight/stability, and control. Central Machinery offers a popular line of drill presses through Harbor Freight.
Using a Milling Machine
Although more expensive, a milling machine is more efficient and easier to use than a drill press for this operation because it allows one to complete the fire control cavity using just a single pilot hole as a starting position for the end mill bit. Since the mill is designed to cut material laterally, the end mill bit can be used to complete the fire control cavity relatively quickly. Virtually all mills include a cross-slide platform that provides lateral adjustments via control knobs or adjustment wheels. Since the mill can also be used to make vertical holes with the included drill bits, it can be used to complete the various pin holes on the receiver walls that will secure the hammer, trigger, disconnector, and safety selector lever in the parts kit.
Investment: A mini-milling machine that's capable of cutting aluminum will run around $500 to $800. The Sieg X2D is a popular mini-mill that provides more than enough power at a relatively low cost. It is a popular machine among 80% builders and is also offered through Harbor Freight, branded under the Central Machinery line of tools.
You'll need to secure the 80% jig in a vise to provide a stable platform for drilling and cutting the receiver inside. If you're using a milling machine or a drill press with a cross-slide table, the vise can be stationary. If you're using a drill press with a stationary table, a cross-slide vise will make drilling and final polishing quicker. If using our Elite Builder Jig, you won't need a large vise.
Recommendation Specifications: The Elite Builder jig and receiver, when assembled, only measures 2.375" in width (for completing the cavity) and approximately 3" in height (for securing the jig horizontally to complete the pin holes). A vise with 4" of jaw clearance should be sufficient.
Machining Oil/Cutting Fluid
To ensure your drill bits and end mill bits remain sharp and capable, it's important to frequently lubricate your work area while you cut and drill your lower. Do not use WD-40; it is insufficient for providing adequate lubrication. Look for an aluminum-specific cutting fluid. A simple 4-ounce bottle is enough for completing a receiver or two. A-9 Aluminum Cutting Fluid is an affordable and popular choice.
Optional & Recommended Tools
Bit Stop Collars
Individually measuring each pilot hole or cutting depth with your drill bits and end mill bits can be time-consuming, even with a depth gauge or integrated stop mechanism on your drill press or mill. To make machining quicker and easier, you can secure a stop collar to the drill bits and end mill bits, which will provide a second physical stop when you reach the appropriate drilling and cutting depths on your receiver. These collars secure directly to the shank of each bit. MCS provides affordable steel stop collars for the various bits your jig comes with.
Drill press and milling machine depth gauges are pretty reliable and accurate, but if you want to double-check your work and ensure your receiver's fire control cavity was drilled and cut to the right depth, a simple set of calipers will suffice. Calipers can also be used for verifying correct placement of any stop collars you might use on your bits.
A router is the alternative to using a drill press or milling machine. Combined with the proper jig, the router provides a handheld (albeit less stable) method of completing the fire control cavity in your lower. A router's trade-off is that it's easy to store and may be more affordable than a drill press or mill. The Porter Cable model is one of the common choices used for finishing a lower.