Ray Brandes has written an excellent treatise on machining an AR-15 lower receiver from an inexpensive “0%” forging. What follows is a photographic expo of following those instructions (mostly) to create a new AR-15 lower receiver.
For the most part, I will let the captions do the talking.
To start you’ll want an AR-15 raw forging, and the tooling plates described in the instructions. A lower parts kit is handy for checking part fits.
In the first setup the lower is mounted to the table, using one of the tooling plates. The top ‘deck’ is aligned, as much as possible, with the X-axis of the milling machine.
The first cut is made across the deck, and on three sides of the buffer tube tower. The instructions detail using a small endmill to do this, but using a 1.5″ diameter end mill hits the right radius in one pass.
This hasty setup is used to machine a flat spot in the bottom of what will become the magazine well. This spot allows a surface for clamping and will be machined away when the magwell is cut.
The forging is then clamped to an angle plate, as shown, and the ‘butt face’ is used to indicate it square with the spindle.
After using the mag-release boss as an intermediate reference, the location of the forward take-down pin is determined then drilled and reamed to size.
The rear takedown hole gets the same treatment.
The safety hole is drilled and reamed.
Then the holes for the hammer and trigger are drilled.
The rear (pivot) hole for the trigger guard is then drilled.
The hole for the bolt catch spring is drilled.
And the first part of the bolt catch slot is machined. More machining will be necessary to complete this in another setup.
A quick pass across the safety stop nubs allows the safety lever to go into position. (Machining the forward nub isn’t really necessary, but is cosmetically preferred.
It’s always gratifying to see a part in place, so here’s a quick test fit.
Cutting the relief for the magazine release bar
…and a quick test fit of the magazine release bar.
It seems like we’ll machine the pistol grip mounting tang a little bit in every setup. I find it convenient to move a clamp out of the way for clearance.
One side of the grip tang complete.
Here’s a quick comparison of how much has been done in just that second setup.
Moving into the next setup, it’s easiest to get a quick alignment using a precision square. Note the light behind the square to discern if any light is visible between the square and the butt face.
Spot-facing the rear takedown pin in the third setup.
Then cut the clearance for the head of the takedown pin.
Make sure it fits before moving on!
Mill the mag release button slot and check the fit.
Drill the forward trigger guard hole. This time only half way – it doesn’t go all the way through.
It’s the small details that count, and the take-down pin detent vent hole is one of the smallest.
Again, I move a clamp to make clearance for machining the pistol grip tang.
The second side of the tang is machined.
On to the next setup. Here the lower is aligned vertically against the angle plate. Note the square at the bottom of the picture for course alignment and the dial indicator for fine tuning.
The bolt release pivot pin hole is a difficult thing to reach! An extra-long drill is used and a support block clamped to the side of the forging to give it rigidity close to where the hole goes.
A view of the drill going through the fixture block and ready to drill the bolt release pivot hole.
And the drilled hole.
Next is drilling the rear takedown pin detent hole. This is a deep hole for its diameter, so drill a little bit at a time, retracting the drill to clear chips frequently.
I’ll finish up the buffer tube tower in the lathe, so here I just cut a pilot hole and opened it up a bit.
With and endmill, cut the anti-rotation cavity for the stock.
And, of course there is machining to do to the pistol grip tang. Cut the rearmost surfaces.
…and the front rear-facing surfaces.
On to a new setup. Using the tooling plate, dial in the deck flat. Note only one clamp present while the setup is being dialed in. A second clamp is added, and the setup verified before any cutting is done.
With a gauge pin (a dowel pin should work as well) in the front takedown pin hole, the hole is located using an edgefinder.
Two tooling holes are drilled in material that will later be machined away.. These are only for fixturing.
The holes are tapped.
Then the lower is bolted to a square bar using two bolts threaded into the previously prepared holes.
The square bar allows the lower to be chucked in a 4-jaw chuck on the lathe. The chuck jaws are snugged up, then the sides of the lower are indicated parallel to the lathe bed before tightening the bolts.
Then the fixture must be centered up on the previously established hole. Using a center in the tailstock you can quickly get pretty close.
A dial indicator is then used to perfect the setup.
Next, the hole must be opened to the appropriate diameter for threading.
Boring completed.
Without disturbing the setup, change to a threading tool.
Set the thread box / gears and take a scratch pass. Verify the correct TPI before proceeding.
Thread the buffer tube tower.
Make sure the buffer tube threads on before you break the setup.
Setup vertically. The buffer tube tower can be used for course alignment.
Drill the pivot pin detent hole.
Then cut the clearance for the upper receiver lug. If you have an upper receiver handy, this is a good thing to test fit.
A bevel protractor or angle level is used to setup for the next step.
And yet another surface is cut on the pistol grip tang.
The pistol grip mounting hole is drilled and tapped.
The angle protractor is again used to set a new angle.
The buffer tube retainer hole is started with a spotting drill. Going into threads it’s important to spot this hole before drilling.
Then the buffer retainer hole can be completed.
Yet another angle to hit.
This time for the charging handle clearance.
The next setup is flat across, ready to cut into the magazine well.
Spotting the corners provides guides as well as providing finishing radii if you mill the mag well.
At the front of the mag well there’s a groove that provides clearance for rivets on the magazine. This is easily put in with a drill and reamer at this stage.
As the instructions point out, drilling is a great way to remove a bunch of metal relatively quickly. Here the center material is ‘chain drilled’ from the mag well.
The ridges are removed with a long roughing mill.
At this point, I’ll deviate a little from the instructions to cut the mag well on a shaper. Here is the shop-made shaper tool, next to the shaper lantern, with the forging in the background for scale.
It’s difficult to see the forging in this photo, but it’s attached to the angle plate on the shaper table.
Partially roughed out mag well. You can see the long cut marks from the shaper as well as the rough surface left by the ‘corn cob’ mill.
The magazine well close to completion.
Always test with a few magazines before breaking the setup.
Back in the milling machine, finish cutting the bolt release slot.
Finished bolt release slot. The Fire Control Group (FCG) section has also been spotted for more drilling to remove bulk material.
Material hogged out. Watch the depths here.
Milling the front part of the FCG pocket.
…and the narrower rear section.
With all that material removed, a couple of holes can be drilled to locate the trigger slot.
Then simply mill them together to complete the trigger slot.
At this point I switched to the vise. Using both tooling plates to avoid crushing the now hollow receiver.
Here, just to the left of the pistol grip tang, the hole for the safety detent is drilled.
Then machine the slot for the trigger guard, front and back.
That completes all the steps required to make a functional lower.
Of course there are still forging seams and sharp edges so some finish work is in order. I like Cratex type rubberized abrasives for making short work of details like that without risking giant uh-oh’s.