Well, how do you? Finishing an 80 percent lower is the first step in building an AR-15 from scratch. The process can sound overwhelming: You need to set up machinery and tooling, prep your 80% lower and get it seated in your jig, and you need to set your drilling and milling depths. Yes, machining metal sounds like an advanced task that you can’t complete – trust us, here, you can. How do you finish an 80 lower? Let’s find out:
#1: Select Your 80 Lower and 80 Lower Jig
You’ll need to consider what type of 80% lower you want to finish: AR-9, AR-15, AR-10, or DPMS .308? Most 80 lower jigs are compatible with most lowers but you’ll need to confirm your combination is right, first. We offer convenient 80% Lower and Jig Kits to get you started. Check ‘em out here!
#2: Set up Your 80 Lower and Jig
Once you receive your lower and jig, you’ll need to get the lower seated for drilling and machining. Most jigs function the same way: Rest the 80 lower in one guide plate, press the opposite guide plate against the lower, secure the assembly together with a set screw, and verify alignment of your drilling points.
All 80% jigs pre-measure your drilling points for you – all you need is the right bit, described in the instructions. We strongly recommend having a tabletop vise handy to aid in securing your lower and jig. You’ll need a stable, immovable surface to work on safely and precisely. A drill press vise like the one we offer is perfect, simple, and affordable.
#3: Machine your Fire Control Group
You’ll need to machine the fire control area of your 80% lower using an end mill but but first, you’ll need to drill a pilot hole for your end mill bit’s starting point. Most jigs include a pilot hole guide. Once your pilot hole is drilled, you’ll switch to your end mill bit.
Your 80% lower jig will accommodate measuring your end mill bit’s depth with three pre-measured depth guides, often found on the top plate of the jig. Simply set the depth and begin milling.
#4: Drill your Fire Control Group Pin Holes
You’ll need to drill numerous pin holes in the side of your 80% lower. Again, your jig’s guide plates will set those pin hole locations – all you’ll need to do is drill each marked hole with the appropriate drill bit.
Tips While Machining
You’ll be removing a fair amount of metal from your 80 lower, and if you’re working with a forged 7075 T6 lower, the process of machining will be longer and more difficult. To make sure you’re drilling and milling without slowing down or wearing out any tooling, we strongly recommend investing in some machining lubricant.
Lubricate all cutting and drilling surfaces, and don’t try going too fast. Allow your router and drill to dictate the speed at which you work. Stop frequently during drilling and cutting passes to remove debris and wasted alloy. This will keep your cutting area free of debris that could harm your lower, and it’ll reduce tool wear.