My New 80% AR Build

[bj139]

I just built a new AR lower. Went from 80% to 100%.
I drilled and milled about 4 hours yesterday and 4 hours today.
I broke the end mill with 10 seconds to completion. Arrh.
I will start assembling the Legos tomorrow. I hope I have all the parts.

[Kswhitetails]

Like.

[brian y]

nice, I have been using 80% lowers for all my builds

[bj139]

nice, I have been using 80% lowers for all my builds

Wow. Yours are nice. Are you a machinist with a real mill? I'm using a Harbor Freight drill press and HF calipers.
I bought the jig with an 80% lower for $100 on Gunbroker. The trigger group seems to work fine.
I built a Polymer 80 lower last year. This is my first aluminum lower. I used the Polymer 80 endmill.

Now I need a new endmill.

[bj139]

In doing this project I have come to appreciate the toughness of 7075 aluminum.
It is some really strong metal.

If anyone has milling experience, what type of endmill should I buy to do more 7075 aluminum milling?
The one I broke was 3 flute high speed steel, I think.

[Randy99CL]

If anyone has milling experience, what type of endmill should I buy to do more 7075 aluminum milling?
The one I broke was 3 flute high speed steel, I think.

What is the cutting diameter and length of the one you broke? It's weird that they gave you a 3-flute because you can't measure the diameter with a normal micrometer.

[Crusty]

Mine turn out like Brian Y's pic above. I use an EP Armory jig ($40), a Harbor Freight trim router and a Home Depot ¼" HSS straight router bit and plenty of transmission fluid but I take my time and don't hog out lots of material on each pass. End mills appear to be on the fragile side from what I've read, and I've found that HSS cuts aluminum fine without the brittleness of carbide.

[bj139]

What is the cutting diameter and length of the one you broke? It's weird that they gave you a 3-flute because you can't measure the diameter with a normal micrometer.

It's a 3/8" endmill, 3-1/4" long with 1-1/2 flute length.
From what I've seen 3 flute is recommended for aluminum.
I'm using a drill press at 350 RPM.

[bj139]

Mine turn out like Brian Y's pic above. I use an EP Armory jig ($40), a Harbor Freight trim router and a Home Depot ¼" HSS straight router bit and plenty of transmission fluid but I take my time and don't hog out lots of material on each pass. End mills appear to be on the fragile side from what I've read, and I've found that HSS cuts aluminum fine without the brittleness of carbide.

I think I will try my next one with a router with the jig I have. It appears similar to the EP Armory jig.
I may have to buy a trim router. Thanks for the idea.

[bj139]

Completed 16" Grendel pistol build.
Right side charging for inconvenient left handed charging.
Function of lower appears 100%.

[bj139]

This lower works fine with standard mil spec parts. I thought I would install one of the Larue triggers I bought several months ago in it.
The trigger went in fine. The hammer went in fine. The safety would not go in.
The rear of the trigger does not drop down far enough to clear the safety. If I remove the hammer the safety will slide through so the hammer is holding the rear of the trigger up just a bit. I thought about grinding a bit of the rear of the trigger but it is so nicely finished, I don't know.
I should try the Larue trigger in an Anderson lower I have. I can't imagine the jig maker could get the hammer, trigger and safety holes misplaced but who knows. Any suggestions?

[bj139]

I checked a few other websites and this problem is not uncommon with the Larue trigger due to it's being so much wider than a mil spec trigger.
Initially, I did have to enlarge the trigger hole through the receiver bottom to get the trigger to fit without binding but I didn't give that a second thought.
I may just have to install trigger, safety then hammer , in that order, to get all the parts installed.

[Frontier Gear]

That is pretty common. Install sequence is the key here as you already mentioned.

[bj139]

I tried the second Larue trigger in the 80% with the same result. I tried the Larue in an Anderson lower and it worked fine.
I compared the trigger, hammer and safety holes between the Anderson and 80% lower by holding them next to each other and sighting through and I can't see a difference. I removed a bit of the top of the safety hole and sanded the safety a little. It still does not work.
The Larue trigger works fine in the Anderson lower. The Delton trigger which was in the Anderson works fine in the 80%.
I left the Larue in the Anderson lower and the Delton in the 80%. The feel of each is alike.
I don't know why the Larue is fitted to such tight tolerances.

[Crusty]

I think I will try my next one with a router with the jig I have. It appears similar to the EP Armory jig.
I may have to buy a trim router. Thanks for the idea.

Use plenty of transmission fluid for cutting fluid - I use a squirt bottle. Don't try to go to full depth in one pass - cut it in stages but make sure that the smooth shank of the router bit is always set to ride against the jig or you've just destroyed your jig. I start out with one 5/16" hole drilled down on a drill press nearly into the trigger slot, place my ¼" router bit into that hole and then start it up with a firm grip on the router so that it doesn't move when it starts, and make that hole bigger in easy cuts. Router bits cut best on the sides, not so much in plunge mode. I clear the chips often and I use a combination square to check my depth of cut every time I clear chips. Taking too aggressive of a cut with those cheap routers can let the bit slip in the collet and the cut will get gradually deeper - so check that depth. Save the last .010" for a final pass to get a nice cut in the bottom (which matters zero to the rifle's functioning). Take your time (~3 hours or more) and you can produce a nice looking lower. Then go take a shower and change those oily clothes.

My jig didn't fit my blanks that well and I had to drive the alignment pin into the jig some so that it would close on the receiver like it's supposed to. Tape in contact spots can keep the jig from wearing through the oxide, but these days I don't worry about it and just touch up afterwards with Aluminum Black.

[bj139]

I was looking at how I could do this with a router.
How do you clear the buffer tube ring at the rear of the receiver with the router?
I saw the 5D tactical jig is extra high to do this.

[Crusty]

I forgot to mention that I cut out a wide V in the plastic HF trim router base to get all the way to the rear of the cavity. With a coupon you can get one for around $30 and if you burn the router up HF will send you another one, but take easy cuts and that shouldn't be a problem.