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We are a national retailer of individual components and not all products depicted on this website are legal in every state. Shipping of various products found on this website are prohibited to some states (such as California, Connecticut, New Jersey, New York, Pennsylvania, and Washington). The information, pictures, text or products presented on this website are not a representation by us, and should not be understood by you, that any product or completed firearm is legal to assemble or own in your state of residence. We encourage each and every builder to perform their own research about the state and federal laws that apply to them. It is your responsibility to understand the law and we encourage you to consult with an attorney or your local ATF representative.

How to Complete an 80% Lower (Drill Press/Mill)

Posted by 80 Lower Jig, Co. on Sep 2nd 2020

How to Complete an 80% Lower (Drill Press/Mill)

Ready to build your own AR-15 rifle or pistol from scratch? This instructional guide's shows you how to complete an 80% lower using the Elite Builder 80% Jig and a basic drill press or mini-mill. This jig and these instructions work for any AR-15 (5.56, .223, 300 BLK) and AR9 (9mm) 80% lower. We're using this forged receiver with engraved "FIRE/SAFE" markings. Before getting started, we also need some hand tools.


Tool Checklist

  • Drill press or mill
  • Work-holding vise
  • Canned air or bush
  • Aluminum cutting fluid
  • Ear protection
  • Eye protection

Optional

  • Digital calipers
  • Drill stop collars

We recommend using at least a 4" vise to keep things stable. Cutting fluid is a must if you want to keep your bits cool and sharp while machining aluminum. A brush or canned air will help to keep your work area free of debris and aluminum chips. Although your drill press or milling machine comes with a depth gauge and stop, you can always double up with a digital caliper and stop collars. Both will help ensure you're cutting the receiver correctly with the end mill bits.


Four Steps to Complete

  1. Cut out the fire control cavity
  2. Cut the trigger slot in the floor
  3. Drill the hammer pin hole
  4. Drill the trigger pin hole
  5. Drill the safety lever hole

We're using a standard forged 80% lower. Lucky for you, this is one of the most basic jigs with a nearly universal design, and these machining steps apply for AR-15 (5.56, .223, 300 BLK, 6.5 Grendel), AR9 (9mm), and LR-308 (6.5 Creedmoor, .308 Win) 80% receivers.

Let's get started.


Step 1: Set the lower in the jig

Two polished steel pins located on the jig's plates will align with the pivot and takedown holes at the front and rear of the receiver. Set your receiver on one plate and align the pins. Align the other plate then set the assembly upright.

Step 2: Install the template holder

The template holder holds the template plates for cutting and drilling. Rest the holder atop the jig and receiver. Align the large holes on either side of the holder with the threaded holes drilled into the top of each side plate. Using the provided large Allen key and hex-head bolts, tighten the holder to the jig.

WARNING: Do not over-tighten! The template holder only needs to be hand-tight plus one-quarter turn. Over-tightening the bolts will cause the jig to warp.

Step 3: Install the pilot hole template

The pilot hole template is the only template with various holes drilled into it. Collect it with the small countersunk hex-head screws and Allen key. Don't over-tighten these countersunk screws; they can strip easily. With the entire jig assembly ready, it can be secured in your work-holding vise. Again, don't apply too much pressure to the jig or the plates will warp. Only tighten the vise enough to prevent the jig from sliding or moving while you work.

Step 4: Set up the 3/8" Drill Bit

Before cutting the receiver to make room for the parts kit, we're going to drill pilot holes using the kit's 3/8" drill bit. This is done to remove a large portion of the aluminum in the receiver and to provide various entry points when using the end mill bits.

As the pilot hole template indicates, we need to drill down 2.00" from the surface of the plate. Set your drill stop on your mill or drill press accordingly. If you're a new machinist or using a basic drill press, stop collars and calipers will help dial in your drill depth.

Step 5: Drill the pilot holes

With your bit fitted and your machine ready, begin drilling through each hole in the template to make pilot holes in the receiver. Take your time and drill down incrementally. Use plenty of cutting fluid and occasionally remove the bit from the work to clean the receiver and bit of aluminum debris.Drill down 2.00" from the face of the template until all six pilot holes have been made. Once all pilots are made, you may remove the template. Replace it with the next template, which is the first cutting template. It's labelled "3/8 EM 1.375 DEEP FROM THIS FACE." 

Step 6: Cut Receiver 1.375" Deep (Short 3/8" End Mill)

Remove the drill bit from your machine. Replace it with the short 3/8" end mill. The plate indicates we must cut to a depth of 1.375" from the surface of the plate. Calipers and a stop dollar can assist with the setup of your machine to prevent you from milling or plunge-cutting too deep.

Read below before machining.

Using the cutting template

The cutting templates will guide your end mill bits as they cut the receiver. The shank can safely ride along the inner edges of the template to act as a lateral stop, tracing the shape of the main cavity for the parts kit. Use plenty of cutting fluid to lubricate both surfaces and prevent wear to the bit or template. Both are reusable for additional builds.

Cutting with a drill press

Unlike a milling machine, the spindle and chuck in a drill press aren't designed to accept high lateral loads (side-to-side pressure). Still, many machinists safely mill their 80% lowers using a drill press. Light passes must be made in this case. We recommend removing 1/4" or less of material with each cutting pass. The EB Jig's end mills are also center-cut. These end mills can make vertical cuts like a drill bit, called a plunge cut. If you attempt to mill your receiver with a drill press and experience tool chatter or unsafe conditions, complete the 1.375"-deep cuts using plunge cuts. If plunge cuts are required, light milling can still be used to clean the receiver's walls of rough edges once the final depth has been reached.

Step 7: Cut Receiver 2.00" Deep (Long 3/8" End Mill)

Once the cavity has been cut to 1.375", remove the template and short end mill from the workspace. Inspect the receiver for debris and clean as needed. Reapply machining oil for the next cutting procedure. Install the third template labelled "3/8 EM 2.00 DEEP FROM THIS FACE." Install the long 3/8" end mill in your machine. We're now cutting the lower part of the cavity, where the trigger and hammer will install. Cut exactly 2" down from the face of the plate, repeating the best practices and methods in step 6.

Step 8: Cut the Trigger Slot

Once the 2" cutting depth has been reached, uninstall the long 3/8" end mill and remove the plate. Again clean the cavity of debris and reapply cutting fluid. Install the fourth template labelled "5/16 EM THRU BOTTOM." No depth measurement or stop collar's required here. Install the 5/16" end mill and cut through the receiver to make the trigger slot. Once the trigger slot has been made, uninstall the end mill and loosen the vise. 

Do not remove the template holder from the jig. It keeps the left and right jig plates secured and aligned with each other. Removing them could result in poorly drilled pin holes. Reposition the jig and receiver horizontally, so the right side plate's drill bushings are facing up.

Step 9: Drill the Pin Holes

Reinstall the 3/8" drill bit in your machine and apply some cutting fluid to the drill bushing. Drill through both sides of the receiver to create the hole for the safety selector lever. Take care not to drill too far, or you may damage your vise or table. Remove the 3/8" bit and replace it with the 5/32" bit. Apply cutting fluid to the remaining bushings, and drill through the receiver twice more to complete the trigger and hammer pin holes.


You're Finished!

Congratulations! With all the holes drilled, your 80% lower is no more. Now, you're the proud owner of a real AR-15 stripped lower receiver.

You're ready to install a lower parts kit, buffer, buttstock (or pistol brace) and upper, and hit the range. Your fire control cavity may not look perfect. That's okay. As long as you followed instructions and set your depths correctly, your lower parts kit and trigger will install just fine. Your jig is reusable, so grab another receiver blank and build another AR when you're ready to master your skills.

Check out our detailed lower parts kit assembly guide!

DISCLAIMER: If you are new to the world of DIY gun building, you likely have a lot of questions and rightfully so. It’s an area that has a lot of questions that, without the correct answers, could have some serious implications. At 80-lower.com, we are by no means providing this content on our website to serve as legal advice or legal counsel. We encourage each and every builder to perform their own research around their respective State laws as well as educating themselves on the Federal laws. When performing your own research, please be sure that you are getting your information from a reliable source.

We are a national retailer of individual components and not all products depicted on this website are legal in every state. Shipping of various products found on this website are prohibited to some states (such as California, Connecticut, District of Columbia, New Jersey, New York, Rhode Island, and Washington). The information, pictures, text or products presented on this website are not a representation by us, and should not be understood by you, that any product or completed firearm is legal to assemble or own in your state of residence. We encourage each and every builder to perform their own research about the state and federal laws that apply to them. It is your responsibility to understand the law and we encourage you to consult with an attorney or your local ATF representative.