80% Lowers

[ScrapMetal]

I really have nothing against anyone who uses one of these jigs but here is a copy of a rant I wrote else where of my experience/opinion of the jigs I've seen/worked with...

Here are what I see as problems with these jigs:

First of all, when the 80% lower is completely enclosed within the jig there is no way to tell if the lower's orientation within the jig is correct, "square", etc. If it's not then everything done from that point on can be skewed.

Next, when you use the drilling guides you will never have the drill perfectly in line with the holes. Even if by some miracle it was there would still be run out on the drill bit, drill chuck, or drill press table that would cause it to be "off". Granted, some of these issues apply to the mill as well so we try and minimize them the best we can. As the jig is made from the same material you are cutting (or even softer), each time you use that hole it will become more oblong and less accurate. Drill bushings would be great for the side holes (trigger group pins, safety, etc.) but if the drill is not started in straight there would still be enough slop to make a difference.

This also applies to when you trying to mill out the fire control cavity. You will need multiply passes with an end mill and each time you hit the side of the jig you will be taking a chunk out of it. Since we are not using a DRO to tell us when to "stop" all we can go on is visual and audio clues, both of which will result in buggering up the jig "guide" a little bit more each time.

Then there are problems in just using the jig. The drill/route guides covering the top of the fire control cavity created a couple of issues in itself. The first of which is that any drill or end mill had to be an extra half inch long to reach the bottom of the cavity so instead of needing a drill/end mill with a cutting depth of 2.5" you will need one with 3" of depth, a costlier tool with less rigidity.

I also found that the jig made it more difficult to clear out the chips as the "guide" acted like a lid over the cavity and kept everything right there making it more difficult to see what the cutter was doing and making the cut less clean.

Another issue was when reconfiguring the jig as every time you take it apart to put in a different guide there was no guarantee that once it was re-assembled that it would be "zero" with itself or the lowers orientation within it would be the same.

I could keep going but I've gotten across the general experience I had with a jig in this case. I am sure that there are some AR lower jigs that would perform better than the one I had in hand but even then some of these issues will just not go away. In my opinion, if you have access to a mill with an accurate DRO and practice proper cutting, measuring, and mounting techniques you would be better off without trying to use one of the lower jigs on the market.

JMHO

-Ron

 

[MattM]

I'll

Keith gives us all some very good advice. In that these receivers are very unforgiving to errors. I've done several of these now, and used two different jigs; one from CNC Gun Smithing and one from KE Arms. Both are very fine tools. All drill guide holes are spot-on to mil-specs. I liked the KE better, it has the drill guide holes on both sides, and has hardened bushings on the guide holes. I know some folks don't like them. I'm not sure why. They completely surround your receiver, keep your work square on your table and make set-up a lot easier. After 40 yrs. and eyes not as great, easy is good!! As I've mentioned before drill your holes undersize and finish them off with a reamer, all drill bitts not being equal. Most bitts are for general hole drilling, fine for woodworking, but not for precision gun smithing.

I'll go along with that I like to ream to final fit. My friend Bullitt Bob a machinist with about 50 years in his wake uses jigs. But only to trace the out lines on the top jig then he mills to the lines. He can complete a 80% upper in about 30 minutes. I've seen his work.

He said it might take me a little longer on my first one.

 

[Daddy Dave]

Thanx Ron and Matt. As in all things precise set-up is the key to a good job. When I drill my holes I use a dowel pin in the mill chuck for set-up, making sure it moves up and down without binding and get a nice hole every time. As long as your not drilling all the way through I've never had a problem with bitt deflection. I tried using the top guide the first time for the fire control pocket and didn't like either. As I said in an earlier post, I used dykem and used the guide for a template and milled right up to the scribe marks and made it a lot easier to see what your doing, and using a shop vac helped with the chips. Whether you choose to use a jig or not you still need to clamp your receiver into your vice with wood or alum. blocks so all the rules of set-up and squaring are the same. Thanx! Dave.....

 

[MattM]

Now I'm glad I haven't yet received my forging and jigs. It has given me time to partake of all this new (t0 me) knowledge. Thank you all.

 

[MontanaAardvark]

I wish I had dropped by and seen this thread earlier. I built an AR from an 80% lower back in 2010. The lower I used was from Colfax. Their fixture didn't touch the mill bit in any way, and they provided lots of good documentation. It doesn't have bearings where the holes are drilled, and that would be a nice improvement. I serialized my AR and put my own logo on it, figuring it had to be easier to do that than to explain to some nice local officer why it didn't need to have one. My logo:

Well, it's just an engraving; no pink nose. I used a DPMS upper and a DPMS lower kit to finish it, so I figured a little parody of their fierce looking panther would work. A "puddy tat" (as Tweety would say) instead of a panther. (My avatar is my cat Mojo - any resemblance is purely coincidental)

The Colfax fixture came with a drill plate to use to drill out most of the FCG pocket. I drilled those holes with the drill press, then removed the scrap with the mill. I did all the milling on my CNC mill, a hybrid A2Z CNC and Sherline. The Sherline won't take off a lot of material in a pass, but that's the nice thing about CNC. It doesn't care if you tell it to take a thousand passes. I didn't go through those all those tests. Just verified it by dry firing several times at home, then went to the range and fired a single round in a magazine, then two in a magazine, then four, while looking for strange behavior.

Funny thing is that after about a couple of hundred rounds (maybe 500?), it started to double fire. It would fire, chamber a second round then fire again all on one trigger pull. That stopped me in a hurry. Spent a while trying to figure out what could cause it, and then bought another trigger group, replacing the DPMS trigger with a two stage Rock River Arms. That fixed it and it never happened again. I think I have over another thousand rounds on the new trigger.

I have three of the Tactical Machining 80% lowers that I got when they had a 3 for $90 sale and two 0% forgings. I think I'm going to build another 80% as a different caliber, and then try a 0% forging.


Bob

 

[ScrapMetal]

Cool logo Bob. I picked up an old Engravograph machine that needs a little TLC in order so that I can do a bit of engraving and put some numbers on the next couple of AR builds that I do.

-Ron

 

[cs696]

My understanding is that you can sell a homemade firearm, you just can't make it with the intent to sell. It's all about intent. Now if you were selling several a year it would be hard to convince BATF that you are not manufacturing. It is very grey.

That is my understanding also. If you finished an 80% and five years later wanted to sell it you would not have a problem. If you made one a year then sold it you might meet Bubba.
I just finished an 80% and am getting ready to anodize it. I did finish a 0% a few years ago using Ray Brandies guide (50+ pages). I don't think I will be making a second.

 

[cs696]

I really have nothing against anyone who uses one of these jigs but here is a copy of a rant I wrote else where of my experience/opinion of the jigs I've seen/worked with...



-Ron

I did my first Ar Lower out of a 0% forging without a jig. I just made up side plates as described in Ray Brandies book.
All they were for was to hold the lower securely, no holes or guides.
I did not keep track of how long it took but there are 50 pages to his guide.
The first set of side plates I made were out of Corian because I didn't have 1/2" Aluminum plate handy.
They worked well until I dropped one and it broke.
A few months ago I ordered some 1/2" plate and made a set of new ones.
I changed the design some form Rays plans, his set only fit between the pivot hole and take down hole.
Since all of his measurements are zeroed from the center of the pivot pin I made the side plates with two holes.
One for the pivot and one for take down.
I then drilled holes for the hammer,trigger and safety.
I don't use a guide for the FCG pocket or safety areas.
It is tiresome to mill out the FCG and safety areas because of having to return to zero point every time you change where you are milling.
That is why I am starting to install a DRO. I have finished the Z axis and have started on the X That will make things much easier.
I have only done one receiver so far and know the side holes will get enlarged so I want to put hardened bushings in the side plates.
What should the dimensions be for a press fit of the bushings

 

[Rick Leslie]

It's funny how much 'expert' legal advice you get from the internet. I wouldn't listen to anyone or anything concerning the legality of your hobby. Go straight to the AFT website and research any issues. ATF agents are actually very helpful.

Ditto on the Ray-Vin (Ray Brandes) video and tutorial. They are great. I've built 0%, 80% and 100% lowers. Money never entered into the equation. If you want to save money, buy a complete rifle. One completes an unfinished casting/forging for the experience and fun. With that being said, I do prefer the 80% lowers from 80% Arms and Tactical Machining.
http://www.80percentarms.com/
http://www.tacticalmachining.com/80-products.html
I've used both and dealt with both proprietors and you won't find better people.

I also prefer to use a jig, not so much for machining, but for ease of handling and repeatability. You will find it necessary to remove the work between ops from time to time and a jig and work stop make setting everything back up much easier.

The only real advice I have is go slow, take your time, check fit often and have fun.

 

[Jeeper]

Matt,

I have some great AR lower drawings that are oriented towards completing an 80% lower (only those dimensions are included, with the pivot and takedown pin holes as datums). Let me know if you would like them. I will just need to convert them from .igs or .dwg into a PDF or something, if I can figure out how to do that (I know machining better than I know computers ).