80% lowers, 7075-T6 vs. 6061-T6

My Toy · Apr 24, 2018

How much harder is it to machine a forged 7075-T6 80% lower than a billet 6061-T6 80% lower with a router jig? I've read that the 7075 aluminum is harder and stronger than the aluminum in the 6061 billet. Is there any practical difference between the strength of each different material in the finished product?

Neutron · Apr 24, 2018

I'd like to hear a knowledgeable answer on this too. Even the polymer lowers seem to work fine. I'm thinking the difference if any is in the long-term use and the holes for the pins working loose. Just my guess though.

kohburn · Apr 24, 2018

they all work. hard and stronger and forged is just better

Rab1515 · Apr 24, 2018

Don't know how much harder it is, but it's not that hard.

Bolts Rock · Apr 24, 2018

7075 machines a little cleaner. Just take lighter cuts and use a good cutting fluid like Tap Magic for aluminum. Blow the chips out and off of everything every pass. I just recently did 8 with no problems. I did find drilling the FCG holes 1/4" deep before routing the recess eliminated burrs inside. Also a V bit for the selector works better than a 3/8" bit (.377 vs. .375). Drill the holes in one steady pass with a drill press for best results, use lots of cutting fluid.

PS- 5d Tactial pro jig and a DeWalt DWP 611 the first took an hour, the 8th took 35 minutes.

My Toy · Apr 24, 2018

Bolts Rock said:
7075 machines a little cleaner. Just take lighter cuts and use a good cutting fluid like Tap Magic for aluminum. Blow the chips out and off of everything every pass. I just recently did 8 with no problems. I did find drilling the FCG holes 1/4" deep before routing the recess eliminated burrs inside. Also a V bit for the selector works better than a 3/8" bit (.377 vs. .375). Drill the holes in one steady pass with a drill press for best results, use lots of cutting fluid.

PS- 5d Tactial pro jig and a DeWalt DWP 611 the first took an hour, the 8th took 35 minutes.

Great information. Thanks.

JoeRinMD · Apr 26, 2018

Bolts Rock said:
7075 machines a little cleaner. Just take lighter cuts and use a good cutting fluid like Tap Magic for aluminum. Blow the chips out and off of everything every pass. I just recently did 8 with no problems. I did find drilling the FCG holes 1/4" deep before routing the recess eliminated burrs inside. Also a V bit for the selector works better than a 3/8" bit (.377 vs. .375). Drill the holes in one steady pass with a drill press for best results, use lots of cutting fluid.

PS- 5d Tactial pro jig and a DeWalt DWP 611 the first took an hour, the 8th took 35 minutes.

Bolts, I have the same jig/router setup you do. John in MD gave me a gallon of Synkool. (Thanks again, John!) Do you think there's any difference between a cutting fluid like Tap Magic and Synkool? I've now done 4 lowers with a single mill and it's still cutting well. BTW, drilling the trigger pin holes in my drill press, I didn't have any issues with burrs on the inside. I used the same technique you noted...one pass with lots of fluid.

Joe

Bolts Rock · Apr 26, 2018

My Toy said:
Great information. Thanks.

You're welcome.

JoeRinMD said:
Bolts, I have the same jig/router setup you do. John in MD gave me a gallon of Synkool. (Thanks again, John!) Do you think there's any difference between a cutting fluid like Tap Magic and Synkool? I've now done 4 lowers with a single mill and it's still cutting well. BTW, drilling the trigger pin holes in my drill press, I didn't have any issues with burrs on the inside. I used the same technique you noted...one pass with lots of fluid.

Joe

Free Synkool is just as good, my local Fastenal had Tap Magic in stock. There was a slight burr when I drilled after machining, no burr if I drilled before machining. I'm debating reaming the FCG holes slightly larger before hard anodize since it supposedly adds .002" to the surface.

Pinecone · Apr 27, 2018

IIRC, the wrong alloy is NOT a good thing on lowers.

There was the YouTube video about the issues with the M-16 in SEA. And one major issue was corrosion due changing the alloy used. And I think the change was to 7057 from 6061, but could have been the other way.

But enough that you want to pick the proper one.

Bolts Rock · Apr 27, 2018

Pinecone said:
IIRC, the wrong alloy is NOT a good thing on lowers.

There was the YouTube video about the issues with the M-16 in SEA. And one major issue was corrosion due changing the alloy used. And I think the change was to 7057 from 6061, but could have been the other way.

But enough that you want to pick the proper one.

That matters less outside of constantly hot humid places like SEA but the change was from 6061 to 7075.

Z_Man · Apr 27, 2018

6061 and 7075 are both "aircraft" and "military grade" alloys.

the mil spec for M16 is 7075 T6. 7075 is the strongest aluminum you can get your hands on. 6061 is more corrosion resistant, welds better and is easier to work with (but you can cold work 7075 just fine, just need more steps/annealing). 7075 is stronger and harder than 6061.

6061 – Mechanical Properties
•Ultimate Tensile Strength 45000 psi
•Tensile Yield Strength 40000 psi
•Fatigue Strength 14000 psi
•Shear Strength 30000 psi
•Hardness, Rockwell 40

7075 – Mechanical Properties
•Ultimate Tensile Strength 83000 psi
•Tensile Yield Strength 73000 psi
•Fatigue Strength 23000 psi
•Shear Strength 48000 psi
•Hardness, Rockwell 53.5

now assuming that your lower is what the manufacturer says it is, 7075 is the way to go for numerous reasons. harder steels than 7075 are machinable enough, and it conducts heat better than steel so its a bit easier to keep cool than steel is.

imo the savings you would have using a 6061 80% isn't worth it, and although 7075 is more difficult to machine than 6061, it is still relatively easy to machine compared to a lot of steels. 7075 is almost twice the strength, and is substantially harder, which will keep it in spec longer.

now, if you are going to keep your 80% naked, then 6061 might be for you, as the corrosion resistance is extremely good. we could go into the design of the AR and talk about the highest stress points of the reciever, then compare that to both materials and get a rough factor of safety and determine if 6061's FOS would be acceptable, and then we could talk about fatigue stress (aluminum's is unlimited, unlike steel which hits a lower base and doesn't decrease beyond it), and how many more thousands of rounds (cyclic stress, such as the loading and unloading every time you fire the firearm) 7075 will be above the stress limit compared to 6061 for your deemed acceptable factor of safety. someone somewhere has most definietly done that analysis, and i would wager (assuming you don't knock off the trigger guard tabs) that the point most likely to experience that failure mode is the receiver extension area, but we are talking thousands and thousands of cycles.

80% lowers, 7075-T6 vs. 6061-T6

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