How to Complete an 80% Lower
Posted by Gun Builders Depot on Oct 3rd 2022
If you're here, congratulations, you made the right choice: You're building your AR un-serialized from an 80% lower receiver. This guide covers how to properly complete your receiver. We're using the popular Elite Builder jig for this project. It's a drill press/milling jig that doesn't require extra tools, like a router. You'll want to set aside at least an hour if this is your first time. Before we get started, make sure you have the right tools for the job.
For the jig we're using, you'll need at least a 4" vise. This ensures the jig and lower can be clamped horizontally to drill the pin holes. If you're using a drill press, it'll greatly reduce your machining time if you have a cross-slide vise.
TAP Magic Cutting Fluid is affordable -- it runs a few bucks -- and provides excellent cooling and lubrication for your bits and aluminum receiver while cutting and drilling. This will greatly extend the life of your bits.
Recommended: Digital Calipers
A simple set of calipers will help verify the cuts you make with your jig's end mill bits are made to the right depth in the receiver's fire control cavity.
Recommended: Drill Stop Collars
You'll need to drill through one side of the jig, through the receiver, and through the jig's opposite side plate to complete the various pin holes for the parts kit installation. It may also help to have a safety stop when milling the receiver's cavity so you don't cut too deep. So, a simple set of drill stop collars will be beneficial (though they're not required). You only need a 3/8" stop collar for the primary end mills, though a 5/16" stop collar can also be used with the EB Jig's smaller end mill when cutting the trigger slot.
Not Pictured: Eye and Ear Pro
Always wear eye and hearing protection when using power tools. We're machining aluminum, and aluminum powder is highly flammable. Aluminum shavings can also cause serious injury at velocity. Be smart, protect the only two eyeballs and ear drums you were given.
The 80% Jig's Components
Before getting started, it's important to get familiarized with the jig's components. Being confident in its design, construction, and use will allow you to focus on the only thing that matters with this process: Getting the right measurements with your cutting and drill depths.
1. Side Plates
The side plates secure the 80% lower receiver via the front pivot pin and rear take-down pin holes. These plates do not secure directly, but rather via a connecting plate which rests over top of the receiver. Look closely and you'll see three drill bushings on one side plate. These are guides for drilling the lower parts kit's pin holes.
2. Template Holder
That top plate is the template holder. Its job is to secure the receiver by bolting to the two side plates. More importantly, this holder acts as the host for securing the various template plates you'll use to cut and drill your receiver.
3. Pilot Hole Drilling Template
To make milling the fire control cavity easier, a series of large pilot holes are drilled into the cavity via this pilot hole drilling template. You'll use the 3/8" drill bit for this first step.
4. 1st Cavity Milling Template
The first milling template is provided for cutting the main fire control cavity and the shallow, stepped up rear shelf area, where the safety lever and rear take-down pin go.
5. 2nd Cavity Milling Template
The second milling template is provided for finishing up the bottom of the main cavity. This deeper template provides the final cutting depth for the floor of the receiver, where the trigger and trigger spring will rest. Both milling templates will use the two provided end mill bits.
6. Trigger Slot Template
The final cutting template is used with a provided end mill bit to cut the trigger slot. No precise measurements are required for this step, since you're simply cutting through the floor of the receiver to make a hole.
7. Fasteners and Wrenches
Four hex-head bolts secure everything together. The two larger bolts secure the template holder, and the smaller bolts with countersunk heads secure the various templates.
End Mills and Drill Bits
The 80% jig comes with the required bits for fabricating your receiver, too.
(In order, left to right):
3/8" End Mill Bits
Two end mill bits -- a long and short configuration -- are provided in 3/8" diameter for cutting the receiver itself. The shorter bit is used with the first milling template, and the longer bit with the second, deeper template.
5/16" End Mill Bit
A 5/16" end mill bit provides the appropriate diameter for completing the trigger slot.
3/8" Drill Bit
This bit provides the appropriate diameter for completing the safety selector lever pin hole and various pilot holes.
5/32" Drill Bit
This bit provides the appropriate diameter for completing the trigger, disconnector, and hammer pin holes.
Cutting and Drilling the Receiver
1.) Secure 80% Lower in Jig
Secure the 80% lower between both side plates. The two polished pins on the corners of each side plate align with the pivot and take-down pin holes on the lower itself.
2.) Secure Template Holder
With both side plates seated, grab the template holder and hex-head bolts. Orient the holder so the rounded edge is resting against the buffer tube housing. Tighten both hex-head bolts until hand-tight.
Do not over-tighten. There will not be any major force applied to the jig during machining, and a high amount of torque is not required on any bolts. Over-tightening can warp the jig ever so slightly, causing misalignment of the pin hole drilling guides on the side plates.
3.) Secure Pilot Hole Template
With the template holder tightened, the first template can be installed: Collect the countersunk hex bolts, small Allen wrench, and the pilot hole template. Tighten the template to the template holder using the small bolts. Then secure the jig and lower in your vise. Tighten the vise enough to prevent the assembly from shifting while working. Again, do not over-tighten. Cranking on the vise too much will also warp the side plates and cause misalignment.
4.) Prep Drill Press or Mill (3/8" Drill Bit)
Ensure your drill press or milling machine is properly trammed. Tramming or "squaring up" a machine ensures the head is centered on the Y axis, ensuring all vertical movements remain as such: Perfectly up and down. Secure the provided 3/8" drill bit and set your drill/depth stop so the drill's depth is precisely 2" from the top surface of the pilot hole template plate.
5.) Drill Pilot Holes, Install 1st Milling Template
Measurement: Drill 2.00" deep from face of template.
Complete each pilot hole by drilling into the receiver's fire control cavity through the template to the appropriate 2" depth. Take your time and apply some machining oil to the bit and receiver while you work. Once all holes are complete, remove the countersunk bolts from the template plate and replace it with the first milling template. That'd be the template which is labelled with a milling depth of 1.375".
6.) Set End Mill Depth, Mill the Cavity
Measurement: Mill receiver interior to 1.375" deep from face of template.
The first cutting/milling operation can now be made. Remove the 3/8" drill bit from your press or mill and replace it with the 3/8" short end mill bit. You'll be cutting to a final depth of exactly 1.375" from the top surface of the first milling template plate. Optionally, you can use a 3/8" drill stop collar to ensure you're not cutting too deep, and digital calipers can be used to verify the appropriate depth has been cut. This first template will complete more than half of the fire control cavity, including finishing the rear shelf area for the safety lever.
Using the Templates
When cutting, allow the shaft of the bit to gently ride along the interior edge of the template plate. This ensures the fire control cavity is correctly shaped for the parts kit. Apply plenty of machining oil to reduce heat and friction while cutting. Do not allow the blades or head of the end mill to touch the template plate. These bits are made from high-speed steel and will easily mar or cut the plates.
Using a Drill Press
If you're milling your receiver with a typical milling machine, you know what to do: Run the machine and cut the cavity to the shape of the template. If you're using only a drill press, there are two methods you should employ with both milling templates to cut the cavity safely, so as not to damage your machine:
A. Plunge Cutting
A drill press is not intended to laterally cut material (side-to-side) like a milling machine. It is only designed to provide vertical (up-down) cutting and drilling. To accommodate this, the end mill bits provided with this jig are center-cut: They can be used like a drill bit, being plunged into the work area to make a cut vertically. At 3/8" diameter, the end mills can cut almost the entire cavity using plunge cuts.
B. Light Milling
With the bulk of the cavity's aluminum removed via plunge cuts, the left-over bumps and burs can be lightly milled using your drill press. This is not a risk when applying very light force to the machine's spindle, chuck, and bearings. Aluminum is a soft metal and small burs, bumps, and areas of remaining alloy can be safely removed by cutting with your press laterally, using a cross-slide vise or table.
Can I mill the whole receiver with a drill press?
The risk of milling with a drill press comes with its internal components being placed under high load: While milling machines use heavy-duty chucks and spindles with threads and locks, most drill press spindles and chucks are held in place with tapered fits and simple compression. When heavy side-to-side force is applied to a tapered spindle or Jacobs chuck, both units can fall out of the quill while running. Even still, experienced machinists have successfully cut their 80% lowers using shallow lateral cutting passes (approximately 0.25" at a time), effectively using their drill press as a mill.
7.) Install Long End Mill, 2nd Milling Template
Measurement: Cut receiver interior to 2.000" deep from face of template.
Once the final cutting depth of 1.375" has been achieved with the short end mill and 1st template, it's time to remove the bit and template. Replace both with the longer 3/8" end mill bit and the second milling template. This template is labelled for a 2.000" cutting depth and has a smaller guide for the bit. Install your drill stop collar if being used, set your depth stop to 2" on your press or mill, and repeat your machining process in step 6 to complete the fire control cavity.
8.) Cut Trigger Slot, Prep for Pin Holes
Measurement: Cut through receiver floor. Avoid damaging trigger guard.
The final cut is easy; simply install the last milling template marked for the 5/16" end mill bit, install the 5/16" bit in your press or mill, and cut through the floor of the receiver to make the slot for the trigger. Be sure to avoid going too far; the bit could damage the trigger guard area. Remove the jig from the vise, open the jaws to accommodate the assembly horizontally, and secure the jig with the right side of the assembly facing up. This side contains drill bushings for aligning each pin hole.
Note: It is a best practice to keep the template holder installed on the jig and receiver while drilling the pin holes. This prevents the side plates from becoming loose and potentially failing to align the pin holes on both sides of the receiver.
9.) Drill Pin Holes
Measurement: Drill through both receiver walls from right side only.
With the jig fastened horizontally, it's time to switch back to drilling: Collect the 5/32" and 3/8" drill bits. The larger bit is used to complete the safety lever pin hole. The smaller bit completes the two hammer and trigger pin holes. Drill through both sides of the receiver via the drill bushing side.
Your 80% Lower is Complete!
With your pin holes drilled, you'r 80% lower is complete! You can now either paint the inside of the receiver with a coating of your choice, or simply leave it bare as a mark of your homemade gunsmithing success. The next step is to install the lower parts kit. Check out our detailed LPK install guide with pictures and instructions.
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