Machining an AR15 lower receiver

[Highpower]

This is a project that I did at the end of last year, just to see if I could do it – and try to learn a few things about machining along the way. I wanted to give myself a challenge and hopefully end up with a usable part when it was all finished. I made a few minor mistakes along the way, but I learned from them and I am happy for the education that I got while doing it.

You have to start somewhere, so the first thing to do was to find a set of drawings to work from. I managed to find these on the net.


Next was to obtain the raw materials. The 2 forgings are 7075-T6 aluminum which is what the mil-spec called for when these rifles were originally introduced. I also picked up a couple of pieces of 6061 flat stock to use for making fixture plates to hold the forgings while they were being machined.




This is one of the forgings being clamped to the mill table with a fixture plate below it to hold it level and off of the table surface.


After aligning the forging to the table travel with a DTI, and setting a datum point to work from - the first cuts are made to level the deck of the receiver and square up the extension housing lug in the rear. The radius on the lug is also cut, that mates with the upper receiver that holds the barrel and bolt carrier assemblies.


Here the forging is mounted to an angle block and being squared, before edge finding the datum surfaces to locate the next series of cuts to be made.




Drilling and reaming the take-down pin holes that align and hold the upper and lower receiver halves together.

[Highpower]

Drilling and reaming the hole for the selector (safety) lever, and milling the flats on the “stop” lugs to keep the lever from over-rotating. Here the lever is being checked for proper fit in the hole.


I guess I forgot about taking pics of a couple of steps, but here is a view after drilling and reaming the holes for the hammer and trigger pins / drilling holes for the bolt catch and magazine catch springs, and milling the necessary slots in the side to hold bolt and magazine catches themselves. Also the left side of the pistol grip tang has been milled to thickness.




Forging is flipped over and finding the edge to begin work on the right side.


Here the rear take-down pin hole has been counter sunk / the slot milled for the magazine release button / the detent spring housing milled for the front take-down pin head / the right side of the pistol grip lug milled to thickness / and finally… a bleed hole drilled into the detent spring housing. Whew!


This is where things started to get interesting. Standing the forging on end and trying to drill a tiny little hole (straight!) through these lugs which is extremely close to the side of the forging. A 13” aircraft drill is used to get the drill chuck out of the way of hitting the work.


I did a LOT of careful measuring to drill a hole through a short piece of ½” square steel stock. When butted up against the angle plate and the side of the forging, the hole was in just the right spot to guide the business end of the drill bit without wandering.


Boring out the rear lug for the receiver extension. (Buffer tube.)

[Highpower]

Tapping the threads for the receiver extension.


Milling the pistol grip tang to length.


I decided to stop there and take a break from the project, so that I could drop off the forgings at a local engraving company to have some identifying information put on them. The shop foreman there was intrigued by what I was doing and he took me back and gave me a tour of the shop. COOL! He showed me how he would use his old German made engraving machine (Deckel?) to make the templates for my info, and how he transfers it to the work piece. He showed me how well the machine was balanced and how he could control the cutter with the tip of a finger on the stylus. An interesting day.


To be continued:

[EdK]

Ooooh, a genuine Starrett pinky. Nice!

Ed

[doubleboost]

Great pictures
I like those clamps with the copper tips
If we start playing with guns we will get locked up
Even modifying a air rifle above 12 ftlb is a prison job
John

[EdK]

(06-20-2012, 04:48 PM)doubleboost Wrote: If we start playing with guns we will get locked up
Even modifying a air rifle above 12 ftlb is a prison job
John

That's ridiculous! Doesn't Sweden require every home to have firearms? You don't see that country going to hell in a hand basket. Oops, I better not go there.

Sorry.

Ed

[Highpower]

(06-20-2012, 04:47 PM)EdAK Wrote: Ooooh, a genuine Starrett pinky. Nice!

Ed

Shushhhhhhhhh........

Don't mention "pink" when Dave is around!

[Highpower]

(06-20-2012, 04:48 PM)doubleboost Wrote: I like those clamps with the copper tips

John

John, those are the bee's knee's for welding. Being copper - spatter doesn't stick to the pads or the threads, and you can actually REMOVE the clamps when you are done welding.

[EdK]

(06-20-2012, 04:58 PM)Highpower Wrote:

(06-20-2012, 04:47 PM)EdAK Wrote: Ooooh, a genuine Starrett pinky. Nice!

Ed

Shushhhhhhhhh........

Don't mention "pink" when Dave is around!

Oops! I forgot about that.

Ed

[TomG]

Great post Willie!

I bought a lower forging when you first started this project and have accumulated most of the parts for the rest of the rifle, but I haven't started machining it yet. Your pics and descriptions will help a lot when I find the time to get started.

Tom