Machining An AR-15 Lower Receiver From A Forging

First off, I need to say that I didn't design the item or the process I followed, and without the EXCELLENT instructions and drawings by Mr. Ray Brandes, of Ray-Vin, it would not have been possible for me to do this. I am a contributing member of a great forum called HomeGunsmith.Com, where Mr. Brandes' instructions are linked. This is a great board for those who like to do (some of) their own work, and is focused on home/hobby machine shop and firearms work almost exclusively.  

I also used some of the stock blueprints from over at BiggerHammer.Net, to verify a few measurements and clarify in my own mind some of the relationships of the dimensions. BiggerHammer is a great repository of technical information on all sorts of firearms.

As shown in the "ATF FAQs" link below, making *your own* firearm is legal to do, essentially with the caveats that:

• The firearm is otherwise legal to possess
• That you are otherwise not prohibited from owning firearms
• That it is not a semi-auto assembled from imported parts (922r stuff)

I began with a "0% 7075-T6 forging, as purchased from DSArms, Inc

I used a "hobby" mill/drill machine, a Taiwanese "benchtop" model that I bought used last winter, which is pictured below. A full size mill would be better, both with respect to more table room/travel, and better precision. As it is, the import mill worked fine, and tolerances are easily kept, but I did have to switch my setups for a few operations to allow for the less than adequate table travel. The table moves far enough to reach the entire forging, but, not with the necessary fixtures (4x5x6" angle block) in place.

Once a few items are made as per instructions to facilitate the work, such as side plates to allow the irregularly-shaped forging to lie flat and parallel to the table, and a small block to act as a drill guide to drill that otherwise extremely difficult bolt catch bore, we can start on the actual work.

Top Deck Operations

Once this is done, we need to mill the top deck. I used a 1" end mill long enough to cut across the entire surface, cut the flats at the top deck, buffer tower and rear face, and stepped around the rear radius as per a chart of coordinates by Mr. Brandes.

Amazingly, even being limited to only X & Y travel in steps, this (0.75") radius cut worked great with my 1" cutter and required only the smallest amount of finish sanding to blend smoothly and fit perfectly with the corresponding upper receiver radius.

Once the top deck is cut true and flat, I clamp the forging to an angle block over a couple of 1-2-3 blocks, and cut a small flat where the mag well will be, to provide a good spot to clamp to. It is very important throughout this operation, as with any machining operation, that the work be securely clamped and held solidly.

With pretty much the same setup, I clamp the forging so it's left side is up, locate it true to the table travel, and cut and/or bore the various features on the left side. The main difference in the second setup is that we must pay attention to what areas we will need to work on and avoid puttling clamps or blocks where they would obstruct the cutter or spindle.

I could reach everything with my mill but the rear trigger guard pin bore (done later), and did the front pivot pin, rear takedown pin, fire control pins, selector bore, bolt catch spring bore and slot and the mag catch bore and slot. I also faced off the lower half of the selector stop bumps and milled out the left half of the pocket where the pistol grip goes.

Once the left side is done, we flip it over, true it up and locate it again, and then we do everything we need to do on the right side. The mag catch button slot gets milled out for the button and counterbored for the spring, the small spot-face for the takedown pin gets done. We also face off the flat at the front pivot pin to allow for the anti-rotation flat on the side of the pivot pin head, and the detent bore area.

The right side of the pistol grip pocket gets done during this setup too.

Now, the operations get done at the rear. We complete the bolt catch pin bore, the rear pin detent bore, the butt stock's anti-rotation recess, and, the first scary part . . . the buffer tube boring and threading.

The forging is clamped (and trued & located) with the rear up, and these operations are done.

To bore for the buffer tube, I step-drilled it up to 1/2" diameter to reduce cutting pressure and to help keep the hole square to the rear flat and round, I then used a 1" end mill to open the hole to 1".

The 1" end mill is my largest milling cutter. The hole needs to be 1.125" to accept the 1-3/16" x 16 thread, so we need another 1/8" and I went to a boring head and carbide cutter to open it up a little at a time.

I cut a little, checked the size, cut a little more, checked, cut, checked, cut . . . until it was at 1.123", at which point I quit boring. I was afraid I would go oversize if I tried to get that last .002". With a hole this size in aluminum, that very small amount undersize is much preferable to ANY amount oversize.

Below, you can see the slight blur, as the boring head is in motion at the bottom of that pass.

Next step is to tap it. I used the 1-3/16" x 16 tap with lots of cutting fluid - note the paper towel to catch the excess. I used a small piece of blank drill rod in the mill's collet to guide the back end of the tap and give me a little down-pressure to get it started well.

Because of my "reach problem" with the little mill, I changed setups to cut the back and front of the pistol grip boss. This could have been done at the same time as the buffer tube bore on a (1/2"!!) larger mill.

While I had my angle bock turned to 90o, I also drilled my trigger guard pin holes while I could reach them.

Next up are the front operations, where the front pivot boss is slotted to accept the upper receiver's front lug (0.500" wide, -0.000", +0.004"), and the detent pin bore is drilled to accept the spring and detent pin that captivates the pivot pin.

Now we finish the pistol grip area work, where the bottom is cut to the right depth and proper angle, and the screw hole drilled and tapped for the 1/4" x 28 screw.

Now, I am ready to cut the mag well opening - scary part number two.

The top deck must be true to the quill (parallel to the table and perfectly level), and one can see the .0005" indicator setup used to check and set this alignment.

Once we're trued up and located, the 1/8" holes at the corners of the mag well pocket are drilled all the way through to define the corners and give us the correct radii.

We first center drill these holes to their exact location, then use a (very stiff) stub drill to extend the holes to about 1/2" deep.

Once we've got all of the corners bored as deeply as we can with a stub drill, we go to a long (taper-length) drill to get all the way through. While I'm doing this small drill work, I also put in pilot holes for the big drills I'll use to get started with the massive material removal required.

Now that our corners are defined and some pilot holes drilled, it's time to remove some meat. We start by drilling at intervals as closely as possible, in progressively larger steps up to 1/2", all the way through.

After drilling out as much material as possible, I went to a pair of 3/4" end mills and roughed it out to the limits possible with that size cutter. I used a ball-end mill to plunge through the drilled holes, since it follows the large pilot holes better and cuts smoother on the plunge. I then used a square-end mill to get most of the material out, since the one I have is sharper than my ball-end. Once I had 99% of the material out, I popped the longer ball-end mill back in and was able to smooth the sides with one pass. Were it new, I could have done the whole deal with the 3/4" ball-end mill, but it's a little dull and wants to chatter on the heavy "side cuts".

Any chatter, especially with a cutter that big, wants to make the setup move no matter how hard it's clamped. Since any movement is going to create big problems, both with alignment to the mill, with locating dimensions relative to the centerline and zero points, and since it's not always obvious something has moved, we need to avoid the risk.

This whole pile of chips is from the mag well cutout with the 3/4" mills.

There are internal features that require more detailing, so a 3/8" end mill is put in the collet and we do those next.

Below are a few shots of the lower in the vise with the mag well showing. The detailing possible, just using 1/8" drills, some 3/4" roughing cutters and a 3/8" end mill is actually surprising, and very little hand work is required. The corner fillets get cleaned up with some careful filing, finished with a light sanding and magazines are used to ensure a "drop free" fit.

After doing a little hand work on the mag well, and test fitting (all of) my magazines, I'm ready to move on to the next steps . . . the top portion of the bolt stop slot and the fire control pocket.

The lower needs to be clamped to the angle block and trued to the table again, so we use the .0005" dial test indicator to help set the top deck level.

Once we're trued to the table, we need to locate the forging relative to the spindle, so I use an electronic edge finder to locate the end and side of the forging.

First spot to find is the rear flat, so we have a relative position front-to-back. This surface is machined relative to the front pin and is a known quantity, so it's one simple operation.

Then we need a good centerline and will use the side of the forging to start. I've already measured the thickness at this point, so I subtract half of the edge finder's diameter (to get the center of the spindle) and add half of my forging thickness. When I move the table to that point, I reset this as my centerline zero and double-check it by going to both sides of the forging with the edge finder to see that it reads the same to "zero". I will work from that reference to mill the bolt catch and fire control pocket stuff.

Now we use a 1/8" end mill to cut the 5/32" wide slot for the bolt catch.

I use the smaller mill to avoid binding, since a 5/32" mill would be cutting both sides and the bottom simultaneously and there is no clearance for the ever-present minute movement due to slop in the system.

Next thing is to set the depth to zero when the mill contacts the top surface. This must be done with each/any cutter change and gives us a reference point to measure down from.

Now to start cutting, taking light passes both wider and deeper, to provide a smooth, accurate cut.

The bolt catch slot is completed.

Then we test fit the part to make sure it moves freely and does not bind. Seems to work fine. My clamping pad prevents it from fully entering the slot, but the portion it cannot reach was already checked when the side cut was made.

Onward to the fire control pocket. . . As with the magazine well, where a large amount of metal must come out, drilling is the most efficient way to remove it. For precision, we'll always want to start with center drilling our exact locations. Here, a 3/16" center drill is ready to go.

Once the hole locations are spotted with the center drill, I use a 3/16" drill to create pilot holes for the progressively larger drills. As before, using pilot drills and stepping up drill sizes reduces cutting pressure and helps keep the bores in the correct place.

Once the 3/16" drill has done it's work, I step up to a 3/8" drill to remove more metal. This is as big as they get in the rear area, but I'll go ahead and use the 3/8" drill to open the forward holes for the next size larger drill.

The four holes closer to the mag well get drilled to 1/2". Though it's difficult to see, there is only a very small distance between the 1/2" holes, which is why getting and keeping our locations correct from the beginning is so important.

I now go to a 1/2" end mill to cut the area out between the drilled holes, and take them to near the finished depth.

The rear area is smaller and only 7/16" wide in the narrowest spot, so I use a 3/8" cutter to clean out that area. Then we move to a 7/16" end mill to get the correct corner radii and to bring it all to the specified size and depth.

That was it for the top operations, and we're almost done.

The last few operations are on the bottom, so we flip the forging (technically now a "lower receiver" ) over.

In this setup, I use a pair of 1-2-3 blocks to support the lower, which at the same time provides a surface true and parallel to the table to make the lower sit level.

My centerline is still at the same position for this operation, so I edge find the butt end and proceed with my trigger guard cuts. This cut is 7/16" wide, but as with the bolt stop slot, multiple passes with a smaller (3/8") cutter avoids binding and provides a very smooth finish.

Our last drilling operation is for the detent that controls the selector switch. We move to the correct coordinates, and first spot the location with our center drill.

The detent hole is then drilled through with a 1/8" drill to meet the selector bore, which provides clearance for the detent's body. The detent's "head" is larger and will not enter the 1/8" bore.

The detent hole is then drilled through with a 1/8" drill to meet the selector bore, which provides clearance for the detent's body. The detent's "head" is larger and will not enter the 1/8" bore.

Sidebar: Although good machine shop practice dictates keeping drills as short as possible for best accuracy and tightest bores, I am working with a bench top mill, with it's limited spindle travel and round-column. Whenever the head of a round column mill is moved, the entire locating operation must be repeated, and the preceding operations were already done with the head as low as possible to clear the work with the cutters when the spindle is in the up position. Having center drilled this relatively non-critical hole first, and then keeping very light feed pressure on a nearly new drill, the long drill will accomplish this relatively non-critical work satisfactorily. I have compromised and used a long 1/8" drill.

Since the detent has a head to limit it's penetration into the bore, and thus it's engagement with the selector, a larger diameter drill is used to create a step in the bore. Here, I use a 5/32" drill to counterbore a step to the specified depth. If we did not create a step to limit the detent's penetration, it would put too much pressure on the selector and make it nearly impossible to rotate.

OK, that was "it" for the machining portion of the work.

As I've gone from step to step, I've used a tool to de-burr the sharp edges of each cut and bore, but at this time, I go back over the entire lower to make sure there are no unfriendly places.

Now we need to clean up the remaining forging seams. Many of these areas were cut away during the machining process, but we do have a few spots left to deal with. The inside of the trigger guard area, the rear of the grip area and the front of the mag well need to be smoothed out. Various files, followed by sandpaper, will do the trick. I was able to cut some of the front forging seams away with the end mill when doing the slot in the pivot pin boss, but the rest is pretty much hand work.

Here are several views of the finished lower prior to filing.

To do the file work, the lower is clamped in the vise. My vise has nylon jaws I had made to provide a non-marring surface, but the rag helps keep embedded grit in the soft jaws from giving me any more scratches to work out.

Just for fun, here are some views of the nearly complete lower, next to an unaltered forging. In the last pic, one can see where I have begun to smooth out the front of the mag well with the files.

This is about the end of the useful pictures, but there is still more work to do.

Once the filing and sanding is complete, I will give it a light going over with a Scotch-Brite pad to smooth out any small dings, then give it a light bead blast. The bead blast with both give it a uniform surface texture and a matte finish. Once beaded, I'll be putting my name, approximate addy and a serial number on it.

To provide a harder, wear-resistant surface, as well as the opportunity to provide permanent color, I will anodize, dye and seal the lower prior to using it. I am hoping not to damage the lower in this next "learning experience", but, as they say: "nothing ventured, nothing gained. . .".

Anodizing of these lowers, as well as other small parts, will be on this Anodizing Page - INACTIVE AT THIS TIME.