DIY 80% AR-15: odd equation or gunsmithing fun? Part 3.
TR Enabling 80% AR-15 Lower Receiver and Completion Fixture
The next two receivers I worked were from a company with the odd name of TR Enabling (www.shop.lower80. com). I specifically chose this brand for its low cost. When I ordered these receivers, they were on sale for $79, which made them the lowest priced 80% receiver I could find on the Internet. Since I ordered my receivers the price has been increased.
After I got one receiver for this article, I was impressed enough that I ordered another one solely for a personal project (and yes I did pay for it myself so that should tell you that I was impressed with the receiver).
Part of the reason for its lower price was that it was. made from the lower-priced 6061 aluminum alloy. 7075 is the alloy normally used for AR-15 receivers, but 6061 is a good choice for a lower cost alternative material.
Considering that today they are successfully making AR receivers from something related to plastic milk jugs and potato chip can lids (the fancy name is polymer!), 6061 aluminum is certainly more than strong enough for an AR-15 receiver.
The TR receiver is a machined billet receiver. It is very attractive if you like the modernistic appearance. The receiver looks rather bulky compared to a forged GI type receiver, mostly due to the square styling of the fences and magazine well bottom.
I did, however, do some measuring and found that it actually was somewhat thicker than the other receivers I had. In the magazine well area and the fire control group area, it was about .040" thicker. This will result in thicker receiver walls once the cavity is machined out, and I like that idea even if the standard walls are adequate.
One of the most noticeable features of the TR receiver is the large trigger guard that is integral with the receiver rather than being a pinned in place part. While I liked the larger trigger guard, it could have been just a little smaller for my taste. The TR receiver looks to be well made. The finish was bare aluminum that had been vibratory polished to remove machining marks.
TR Enabling also offers its own completion fixture, and it's budget-friendly at $69. It is well-machined, but to keep cost down it does not have hardened drill bushings and only features drilled holes in the aluminum sideplates.
The sideplates completely surround the receiver and are somewhat thinner than those of the KE and ACT jigs. The plates are noticeably larger in area, even though they are thinner. The TR fixture is unique in that it does not have bolts through the sideplates to clamp the receiver in place.
To locate the receiver in the jig, TR uses guide pins that have been pressed into the plates. These pins fit in the front pivot pin hole and the rear takedown pin hole. The screws in the top guide plate hold the pieces together until they are solidly clamped into a vise.
Rather than supplying two separate top guide plates, TR uses one that features both cavity sections. Install the plate in one direction for machining one section and then reverse it to machine the remaining section. TR also offers replacement guide plates for those who accidentally machine into their top plates.
The jig I received contained two top plates, one made from aluminum and the other made from steel. This is a real nice plus on this jig. The TR jig looks a little different than the ACT and KE jigs, but it works the same and precisely locates the holes and cavities. I machined both of the receivers I got and it worked fine. It is well worth its budget-friendly price.
I set the TR fixture up in my mill to drill-holes through the sideplates. Since the jig has no hard drill bushings, be sure to have the drill centered over the holes. I used my method of using a hole sized dowel pin in the guide holes.
As usual, I drilled my holes undersize and only partially through the receiver. One thing I did differently was to remove the receiver and fixture from the milling machine and ream my holes on the drill press.
After the holes were done, I milled the cavity. Just as before, I only machined up close to the edge of the guide plates. This time, however, I gave myself a little more room. The way the top guide plates attach to the sideplates of the TR fixture means they are not adjustable for exact centering. After getting close, I removed the plates and measured to verify how much more material needed to be removed to obtain a centered cavity with equal wall thicknesses.
Although the TR fixture has no drill bushings, you will be able to use it over and over if you are careful and set it up correctly. Don't wallow out the guide holes. The receivers I machined with the TR fixtures were well within specs.
Before the fixture is clamped in a vise, the fit is rather loose, but once it's solidly clamped in a vise, everything is tight. I used my little supports made from nuts and bolts between the sideplates on the bottom of the fixture so I wouldn't have to worry about the vise smashing the receiver or warping the plates.
I really liked the TR receiver. I will be using it to rework my bull-barreled varmint AR-15 that features a very thick-walled DPMS target upper receiver. The thicker TR receiver will match up better with the thick target upper. I was very impressed with the TR receiver and low priced fixture. I would recommend it.
American Spirit Arms 80% Receiver
The last brand of AR receiver I worked was from American Spirit Arms, and they actually refer to it as a 95% receiver. It is a forged 7075 aluminum receiver in GI format. I believe they have added a machining operation that is not found on some GI-style receivers.
They have machined the portion where the rear upper receiver lug fits into the lower receiver. This is a nice feature (also found on some other brands of 80% receivers) since you don't have to worry about getting the rear cavity centered and cut to exact size.
For novice receiver builders, this ensures the upper will fit without any excessive clearance that will cause the upper receiver to fit loosely. The "95%" receiver is priced at $130, but this rises to just under $150 with shipping cost.
This receiver looks to be very well made. It is finished as smooth as any forged 80% receiver that I have seen. The good finish may not make it work any better, but it sure looks nice. I did like the looks well enough that I purchased one (at my expense) for an upcoming personal project where I wanted a GI-style receiver (even though at this point I sort of had receivers coming out my ears!).
American Spirit does not make or offer a completion fixture. They do recommend a certain brand of completion jig. Since I already had four brands of fixtures to choose from I just used my ACT Tactical fixture. I had no complaints towards the other brands of jigs I reviewed, but the ACT jig just fit my preferences and way of building better.
Since I already reviewed the process for using the ACT fixture (3/20 issue), I see little reason to cover it again with the ASA receiver. The ASA receiver fit the fixture just fine.
Once the machine work was done, I had a completed receiver that looked just like any other high quality newly manufactured receiver minus an anodized finish. If you are looking for a good 80% receiver that requires a little less work to complete, the American Spirit Arms 95% receiver is worth a look.
While I had no issues with the other brands of forged GI-style receivers, I really liked the ASA offering and would use it again if I needed a quality GI-format receiver. It looked good, it machined well, and the finished product looked and worked great, what more can I say?
The "Jig-less" Option
Up to this point, I used jigs and fixtures to complete these 80% AR-15 receivers. They all worked very well and I can recommend any of them. If you're resourceful, or just plain "cheap" like I am, there's the option of completing a receiver without a fixture. I saved one of my KE Arms billet receivers just for the purpose of showing how to go "jig-less."
When I first decided to make an 80% receiver for a single personal project, I had no intention of buying a completion fixture, but I decided to investigate them for SGN. Since I am a semi-skilled machinist I was confident that I could complete one of these 80% receivers without the cost of a fixture.
One of the reasons I easily completed these receivers with the fixtures is that I have more than 20 years' experience as a machinist and mechanic in an industrial facility. One of my jobs was building jigs and fixtures for use by production personnel.
Since I was the one building the fixtures and jigs, I had. nothing but blueprints to use in completing them. I fact, many times it was I who had to draw up the blueprints and decide just what dimensions would be used on a fixture. I am therefore very comfortable building projects with nothing more than prints or drawings. If you have some fabrication experience and can interpret drawings, you should have no trouble completing an 80% receiver without the aid of a jig.
While I liked using the jigs for locating the holes for the hammer pin, trigger pin and safety lever, I actually preferred to machine the cavities without the use of a guide plate. I just found that for me personally the guide plates just got in my way and made the job harder than it needed to be.
To go jig-less, the first thing you need to do is to apply some layout fluid (commonly known as Dykem) to the top and sides of the receiver. Then use a fine point scribe to draw out the approximate shape of the cavity and the hole locations, based on the dimensions derived from your blueprints.
Some AR-15 blueprints can be pretty difficult to decipher, with their huge numbers of dimensions fisted. Many dimensions are based on some other dimensions and that can get confusing.
The easiest way for the hobbyist machinist to locate specific locations is to base the dimensions from some fixed point from which it is easy to measure. To aid readers, I have tried to simplify the maze of dimensions on some of the more information-dense blueprints.
The most important dimension for completing a receiver is the spacing between the hammer and trigger pins. The hammer pin hole is .843" in front of the trigger pin hole and .314" above it. Be aware that the dimensions I will be supplying here are based on my specific blueprint from unknown sources for a forged receiver.
Receiver dimensions can vary considerably, all depending on where the designer wanted to place things. I measured several brands of receivers and fixtures, and I noticed that on some receivers and jigs, the hammer and trigger pin holes were as much as .050" forward or back of GI specs.
The spacing between the hammer and trigger pin holes does have to be very close to the same dimensions on all types of receivers, but the locations forward and back for the hammer and trigger subassembly can vary considerably and still work correctly, so long as the distance between the two parts remains within specifications.
If the hammer hole is too far from the trigger hole, the sear engagement may be insufficient. If the hammer hole is too close, the parts may bind. If the spacing is not within specs, the proper operation of the disconnector can also be affected.
I scribed the locations for the 5/32". (.156") diameter holes on the sides of the receiver. If you measure correctly and use a very fine-point scribe, you can lay out locations to within about .003"-.005", which is more than adequate.
To make the location of these holes easy, I based these supplied dimensions from the front edge of the receiver and the top of the receiver. The hammer pin hole is 3.367 inches from the front of the receiver and .625" down from the top. The trigger pin hole is 4.210 inches from the front and .938" down from the top edge of the receiver.
This should be very close to the ideal spacing of .843" and .314". The safety lever hole is 5.342 inches back from the front and .712" down from the top of the receiver. The location of the safety lever hole is not as critical as the hammer and trigger pin holes.
I checked these print-derived dimensions against a CMMG forged receiver I had, and they were all pretty close. Verify for yourself that these dimensions are correct before drilling, since you never know when I will make a mistake or there will be a number misprint in SGN. The pictures used to demonstrate this principle show very large lines, since fine point lines do not show up well in black and white newsprint magazine pictures.
In the real world your lines will only be a thousandth or two wide. Once you have the lines scribed, center-punch the location, followed by measuring again to verify that you have center-punched an accurate drilling point.
Drill in multiples steps to get precisely located holes. Drill bits tend to wander if you use a too-large bit. Use a small center drill to start the hammer and trigger pin holes (and the safety lever hole too, even if it isn't as critical) then drill to just under finished size,, followed up with reaming to final size.
For most accurate hole locating, you should mark and drill from both sides, just as I did when using the jigs. When you're not using a jig, you also must figure out on your own how to support the workpiece precisely square to your drill.
I found that a 3/8" piece of aluminum plate worked well if it was placed at the fire control group area. Just be sure it fits flush and is not held away from the sides by receiver protrusions. If you are not using a jig, there is' no drill bushing to help straighten out the drill if it's not square as it enters the receiver.
Going jig-less works-well if you're good at measuring, but remember it's all up to you to get things right. One thing I will warn builders about should be obvious, but I'm sure someone will try this anyway. Do not try to drill these holes with a hand drill, no matter how well you think you can handle a drill. The holes need to be drilled precisely square to the receiver, not running off at an angle, no matter how slight. A well set up drill press is the minimum tool for this job.
Once the holes were drilled, it was time to do the cavity. Prior to setting your work up in a mill, you need to scribe out a rough location for your cavity, just as I recommended when using the jigs. I scribed lines to represent a rough cavity location and then measured when I got close to the lines to get to final finished size.
I recommend that you size your rough cavity about .025" or so under final size. The first opening to scribe out is the large portion of the cavity. The finished size of this cavity is .695" wide by 2.050 inches long and 1.25 inches deep. This cavity starts TOO" back (finished location) from the slot in the top of the receiver for the bolt hold-open device.
Do not go any closer to the slot than .100" or you may mill into the spring pocket for the magazine release that is under the slot and end up with an unsightly hole in the front of your cavity. Locate your cavity so that you will have equal receiver wall thicknesses once the machine work is done. Once I had this cavity scribed out, I set my receiver up in my mill.
I don't recommend attempting a jig-less cavity job with anything less than a small milling machine. I know you can do it on a drill press, but the results are less than what I want on my projects. If you insist on doing the cavities with a drill press, you can figure it out on your own. Since you won't have a nice flat and square fixture to support the receiver, you will have to make something to hold the odd-shaped receiver in a vise.
I used two pieces of 2% by 27/s by 3/8" thick pieces of aluminum plate for my improvised holding fixture. I placed a 1/4" hole .200" in from the edge in one corner so I could bolt it to the receiver through the rear takedown pin hole. Since you don't have a nice straight jig to hold your workpiece square to the mill, you will have to set it up squarely by whatever methods work the best for you. I used a dial indicator to set it up square and parallel.
Once set up. I started milling with a long 1/2" end mill to rough out the cavity to a depth of 1.125". I then switched to a long 3/8" end mill to finish the cavity to its full size of .695" x 2.050 x 1.25 inches deep. You could just use the 1/2" mill, but a 3/8" mill will give tighter corners and look better.
Then I scribed out the rear portion of the cavity to rough size. Remember that the supplied dimensions listed here are finished sizes and your rough size will be different. This cavity will run from 1/16" to 3/32" in front of the buffer detent hole all the way to the large cavity. At its widest point, it will be .500" wide. Depth will be .630".
Keep this size very close to spec so the receiver will fit tight when the upper and lowers are closed. The .500" cavity needs to be centered so you end up with equal-sized walls that will be from .180" to .200" thick, depending on brand and type of receiver.
There is a protrusion over the right side of the safety lever hole that reduces the cavity width at that point to about .420" to .450". The length of the protrusion isn't really important; just make it so that it looks more or less centered over the safety lever hole. All it is for is to make installation of prohibited full-auto parts more difficult. For milling this portion I used a 3/8" end mill for both rough shaping and finishing.
Once you have the rear portion of the cavity done, there's one more operation for the center of the cavity. At its forward end, the narrow cavity will need to be deepened. This part will be deepened to match the depth of the large cavity.
The deep portion will run from roughly .100" to .125" behind the safety lever hole and extend all the way to the deep cavity. You will not be making this part any wider, you will just be following the existing shape and deepening it.
The purpose of this deepening is to create clearance for the rear of the trigger where it fits under the safety lever, so exact size isn't critical.
Now all that remains is to cut the trigger slot. This is done with a long 1/4" end mill. I prefer to cut my trigger slots undersize and then hand file the slot for exact width and length needed to allow passage of the trigger. Hand filing the slot to finish size also allows you to center the slot in reference to the underside of the receiver.
Sometimes it looks centered from the inside but when you turn the receiver over, it looks to be off slightly from that perspective. I scribed out the rough shape of the slot in the bottom of the cavity. For the front of the slot, I marked a line at .660" back from the front of the cavity. The line for the rear is scribed at 1.187".
The .275" to .300" width is scribed centered between the walls. Once it was scribed out, I plunged a 1/4" mill down from the top and milled out the slot. After milling I removed the receiver from the milling machine vise and turned the receiver over to view the slot. I measured it to see if it was exactly centered.
Then I filed whichever side was needed to get it centered. Once centered, I opened it up to allow the trigger to slide through without excessive clearance.
At this point the receiver is completed. Personally I find the jig-less method not much harder than the jig method. Using jigs really makes pinhole locations easy to form, since you don't need to measure them, but since I prefer to measure my cavities, the jig-less method suits me better. To me the top guide plate just seems to obstruct my view of what I want to cut out. I just like knowing I am cutting the cavities to exact size rather than depending on some plate to determine my cuts.
Finally, my work is done. I machined 10 receivers for this article. The procedures and dimensions are almost carved in my brain. All the various brands of receivers and fixtures worked well and I can recommend any of them.
Of all of them, I prefer the ACT Tactical fixture. I like the fact that the top is flush with the top of the receivers and that feature fits in well with my methods of measuring the cavities before machining to final size. I also like the hardened drill bushings in the sideplates; I just wish ACT had installed them on both sides.
I would have chosen the KE Arms fixture if the sides were flush with the top since I really liked the blue anodizing and stenciled dimensions on the plates. Both the ACT and KE Arms jigs are good choices for those planning on doing a lot of receiver completion jobs. For those only wanting to do two to five receivers, the minimal Stealth Arms jig is an inexpensive choice. It is cheap, yet will do acceptable work if you use it correctly.
My choice of receivers is almost a toss-up. For a GI-style forged 7075 receiver, I think I would choose the American Spirit Arms 95% receiver. It is very well made and finished plus the fact that it has the receiver lug area already done does make it slightly easier to complete.
I also like the fact that it was a basic GI-type receiver without any non-standard enhancements like pictograms or grooves machined into the exterior. In the billet category, I really liked the TR Enabling receiver. I liked its unique styling, plus I liked the fact that it was slightly thicker than standard. I especially liked its low price, although they raised its price recently.
All in all, the 80% receiver idea really appealed to me. While there was very little cost savings over buying a receiver completely machined, I liked the idea that I would be doing at least some of the receiver fabrication. Like many others, I get a lot of satisfaction from building my own projects and doing as much of the building and fabrication as possible.
While I have no trouble passing the required background check and having a government record of my purchases, I still like the convenience of just placing an order for a receiver and having it delivered to my door. If building your own 80% receiver sounds like your kind of project then why not give some of these products a try?
80% AR-15 RECEIVER AND FIXTURE SOURCES
ACT Tactical 16200 S. Garfield Ave, Dept. SGN, Paramount, CA, 90723, 562-602-0080, www.acttactical.com or www.80percentAR.com.
Stealth Arms 635 Streine Dr, Dept. SGN, New Bremen, OH, 45869, 406-478-5011, www.stealtharms.com.
KE Arms 4343 E. Magnolia St, Dept. SGN, Phoenix, AZ, 85034, 480-256-9745, www.kearms.com.
TR Enabling 34 Front ST, Dept. SGN, Indian Orchard, MA, 01151, 413-342-4005, www.tr-enabling.com
American Spirit Arms 16001 N Greenway Hayden Loop, Suite B, Dept. SGN, Scottsdale, AZ, 85260, 480-367-9540, www.AmericanSpiritArms.com.
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|Date:||May 20, 2014|
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