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Discussion Starter · #1 ·
I'm sure this has been debated ad nauseam but apparently I can't put together the right search string to find the threads and all my friend Mr Google can show me are links to companies and discussions with a vested interest in one or the other.

I'd like to jump into doing a couple of these 80% lowers because I like projects. I already have a drill press, a router is on my short list of tools to buy for household projects and I know how to use both.

Are there any significant differences between polymer and aluminum when it comes to ease of finishing and durability when it comes to the average 5.56 or 6.8 build? Not a superlight or BR/varmint gun but just a plain, average build. Pros and cons of each?

Any and all information is appreciated, especially from those who have worked with both.

Thanks!
 

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I've milled and built several aluminum lowers, and work in the plastics industry, but have never milled a poly lower. One plus for the Al lowers is as the fcg holes wear (which they will) you can lightly peen them to snug them up again. My understanding with the poly lowers is they have strengthened the receiver extension attachment area since they first came out, so that's a plus. Plastics can be difficult to work with, especially if bits start getting hot or dull. Not sure of the exact polymer being used in lowers, but often times the CtE of plastics is higher than aluminum which can be noticeable on hot days.


-Alexander
 

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If you have a cool workshop(temp wise, mine is in the basement) milling the plastic receivers goes fairly easy, it is essential to not use a high speed(creates heat) or a dull bit, machining time is easily half what is needed for completing a aluminum 80%.

I have several aluminum 80% lowers that see nearly daily use. Running normal milspec FCG pins will cause wear in the receiver from the pins rotating, BUT there are several excellent products on the market to eliminate that issue, I myself prefer JP oversized FCG pins.

The plastic 80% lowers are also usually about twice the price of a forged aluminum.
 

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Discussion Starter · #4 ·
Thanks for the answers so far!

The extension area was a concern for me on the polymer because I'm bad about giving things I'm working on or assembling that one more bit of oomph to make sure it's tight and every so often it bites me.:a21: Wearing of the FCG holes were also something I'd thought about with the polymer but it never really dawned on me the aluminum might run into the same issue. Thanks for the head's up on that point!

Right now I'm really leaning towards aluminum even though it looks to take longer and heat is a big reason. Texas summers aren't usually mild...:D I spent time in the garage (aka work area) Saturday working on the kitchen cabinet doors we redid and I spent as much time wiping sweat off my glasses as I did wiping router shavings off the doors.:a21:
 

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Aluminum...I won't ever do a poly 80% again...had 3 break in the tower area where no machining is done, they are just too thin in the thread area unless someone comes out with one that has metal reinforced buffer tower tbreads...did not over torque either
 

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I use a water mix coolant/lubricant, usually it is made with a ratio of 1oz of oil and 16oz of water. I made a hanging reservoir with a poly line and a simple needle valve(from a refrigerator water line kit) to adjust flow. You can use a catch pan under the jig and recycle the coolant, but on my set-up most of the coolant get's sucked up along with the aluminum debris with my shop vac.

Heat is your enemy when doing any machining, it can cause a shifting of the lower in the jig, it is best to keep the lower as cool(room temp, same as jig) as possible, go slowly and do the machining/routing in stages.
 

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...

The plastic 80% lowers are also usually about twice the price of a forged aluminum.
I have gotten polymer 80% for $40-$45 and not had a problem. However when drilling the FCG holes the inside can chip if you don't pay attention. I have done both and the polymer is a lot easier but I believe the aluminum will hold up much better over the long run.

Shop different sites for the lowers and the jig. Here are polymer 80% lowers at the best price I have seen http://www.eparmory.com/Polymer-80-Lower-Receiver-s/1881.htm and delivery was quick.
 

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Those poly 80% lowers have really come down in price.

Hmm, I think I may get one to try. With the goofy stock we have to use here in NY(that mounts in place of the evil feature pistol grip), the only outside pressure the buffer tube receives is as a cheek rest.
 

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Those poly 80% lowers have really come down in price.

Hmm, I think I may get one to try. With the goofy stock we have to use here in NY(that mounts in place of the evil feature pistol grip), the only outside pressure the buffer tube receives is as a cheek rest.
The ones I linked to need the pistol grip hole tapped. It is drilled but was not tapped on the one I did as you can't get the tap all the way in, at least the tap I have. Maybe, as I didn't try it it before I milled it as I didn't know I needed to tap it.
 

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The ones I linked to need the pistol grip hole tapped. It is drilled but was not tapped on the one I did as you can't get the tap all the way in, at least the tap I have. Maybe, as I didn't try it it before I milled it as I didn't know I needed to tap it.
Was that a bottoming tap you used, or a regular one?
 

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I have done a few of the poly EP lowers. I just use the grip screw to tap the holes. use the screw just like a tap. 3/4 clock wise back off 1/4 CC-wise. Go slow and take a break it will get Hot. I wet my screw before I start. I also had to do some hand file/sanding on the Safe selector hole. I think the poly stretches and retracts when you drill the hole.​
 

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I have gotten polymer 80% for $40-$45 and not had a problem. However when drilling the FCG holes the inside can chip if you don't pay attention. I have done both and the polymer is a lot easier but I believe the aluminum will hold up much better over the long run.

Shop different sites for the lowers and the jig. Here are polymer 80% lowers at the best price I have seen http://www.eparmory.com/Polymer-80-Lower-Receiver-s/1881.htm and delivery was quick.
I've got 2 of their polymer lowers on order. For $35 each, I figured they would be a cheap way to learn how to finish a lower.
 

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Was that a bottoming tap you used, or a regular one?
Used a regular one. Had to hit about four different places to even find that one.
 

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Used a regular one. Had to hit about four different places to even find that one.
Yup, when ya need one... nuttin' replaces a bottoming tap.
 
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