Machining Question

I'm close to milling my AR lower receiver, this "95%" machined receiver needs a pocket for the fire control group and 3 holes cross drilled for 2 pins and a selector/safety.

I found some instructions for machining the pocket using a DRO, they recommend drilling the selector switch hole, machining the pocket then drilling the trigger & hammer pin holes. Why not drill all 3 cross holes in that 1 setup and then machining the pocket? I would think the drill bits might be more likely to walk as they start through the 2nd side. I don't see a benefit to set up for drilling, turn turn the part for milling, then turn back to the first setup to drill 2 more holes, any idea why they would do, or recommend this?

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RogerN

Reply to
RogerN
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I drill all the holes in one setup long before milling the FCG cavity. I suppose one theory for drilling after the FCG cavity would be to reduce the chances of the drill getting off course, but that doesn't seem to be an issue in my experience. The Ray-Vin guide is the best reference IMO.

Reply to
Pete C.

That's more what I was thinking, drill the cross holes first in one setup and then mill the fire control group. I have the Ray-Vin guide and a blank lower also, the "95%" machined lower should get me shooting right away and the 0% won't be rushed. Do you know how long it takes to completely machine a lower using the Ray-Vin guide?

RogerN

Reply to
RogerN

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Nope, but close. I use the 30% lowers from TM, so that saves the magwell and decking steps. The 30s take most of a day to machine on a manual mill working at a careful pace with lunch and dinner breaks. A good chunk of the time is the 8 or so setups and getting each one indicated and edge located.

Reply to
Pete C.

I have one lower raw forging now and 4 more that have been on order since early February. I'm hoping the other 4 come in and I can machine them as a batch of 5. I would like to machine them all to the 80% stage so I could legally sell some if I wanted to. I could complete however many I think I wanted to keep. I now have one rifle and one carbine less lowers, would like to also get a 300 blackout, I hear barrel and ammo is the only difference from the .223/5.56. An area gun shop had a Bushmaster 450 upper, I'm not sure what cartridge they shoot.

RogerN

Reply to
RogerN

Know where a complete drawing is? Would 6016-t6 bar be a suitable peice to start with?

Remove 333 to reply. Randy

Reply to
Randy333

There are a bunch of AR prints on the 'net, just search around a bit.

7075-T6 is what you want.
Reply to
Pete C.

does anybody make plastic unfinished lowers, just for machining practice?

It would seem like a shame, and expensive to be using trashed lowers for the next aluminum casting project.

Reply to
Cydrome Leader

Real AR-15s and M-16s have 7075-grade uppers and lowers. You can't cast that with home hobby equipment.

I've read that some of the aftermarket is using 6061 for lowers. But the people making those comments probably don't really know. Yield strength of 6061 is roughly half that of 7075.

6061 is a wrought grade, but it can be cast without doing anything special.
Reply to
Ed Huntress

I meant once you bungle a partially made lower, you're finished, and it's just expensive scrap metal, of a useless shape probably only good for tossing into the aluminum scrap pile for some backyard casting. It seems like a shame.

Reply to
Cydrome Leader

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Well, it is a shame, but I wouldn't toss a high-zinc alloy, which also has around 1.5% copper, into a pile I was going to use for backyard aluminum casting. Zinc and copper will hake it hard to make a casting without internal shrinkage voids.

Reply to
Ed Huntress

As long as you follow the directions and have some ability they are not real hard to machine. The biggest thing involved is to KNOW the machine you will be using. If you know that your machine has .020 backlash on the X axis repeatably then you can correct for it. If you think it does but in reality it isn't repeatable then OOPS you get a scrap upper.

Reply to
Steve W.

The original ARs back in the Vietnam timeframe had 6061 forgings. They had some trouble with corrosion from guys' sweat, so 7075 was the next step. Some of the early clone lowers were castings, some so bad that they broke when the rifle fell over onto the floor. Stoner's genius was you could probably make a lower out of recycled bubblegum and have it work as long as it stood up to the fire control springs' pressure. So 6061 or 7075, doesn't make much difference except when anodizing. .223 recoil is about nil, the forgings/castings don't directly take chamber pressure so yield strength is irrelevant for most civilian uses. If you intend on beating up bunnies or whacking coyotes with the buttstock, it might make a difference.

Stan

Reply to
Stanley Schaefer

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So it's a total loss then.

How much might it cost to make some really crappy molds (we're talking

1970s Hong Kong grade stuff) to make these out of plastic?
Reply to
Cydrome Leader

Something is strange there. I'd have to dig out my ASM book to check, but 7075 has low corrosion resistance overall and is not recommended at all for marine environments (or salty sweat, one would assume).

6061 is somewhere in the middle.
Reply to
Ed Huntress

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Well, if you have enough aluminum to sell your scrap from time to time, you could toss it in there.

First off I'd look at the redesign of the plastic lower that those 3D printing guys have designed. Just duplicating the aluminum one in plastic led to a very short life, so they've redesigned it.

As for the molds, it depends on what plastic and how many. It also will depend somewhat on the configuration and drafts. For machined molds, aluminum (2024 or 6061) is used for injection-mold prototyping and very short runs but some plastics require a chrome plating. After that, it's mold steel, and big bucks unless you're machining it yourself.

If you're using a pourable resin, like epoxy or rigid polyurethane, you can make molds out of plaster of Paris. They'd be stronger than most injection-moldable plastics but they're much more expensive, too.

Reply to
Ed Huntress

It's a chicken and egg sort of deal to anybody (like me) that's new to machining, but familiar with what the finished product should be.

Do I right now have the ability to finish a lower receiver? No, I do not.

Could I learn? sure, but it will take lots of practice, which at this point would be futile and expensive if I bought a box of forgings and jumped right in. I'm sure I'd learn quite a bit trying though, while making a huge pile of scraps, but again, it costs to much to try.

Reply to
Cydrome Leader

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It depends on exactly what you bungle, but most mistakes in the machining can be remedied/worked around to make a perfectly serviceable lower.

On the 7075 vs. 6061 thing, operationally it won't make much difference since the AR receiver basically guides parts and have very very little stress placed on it due to the fact that the bolt locks into the barrel extension and no into any portion of the receiver. I believe the first M16s were 6061 and they only changed to 7075 due to corrosion issues in the 'Nam jungles.

With the lower forgings running in the $25-$80 range depending on completion state, the financial risk isn't really much more than the financial risk of breaking an end mill or reamer in the machining.

Reply to
Pete C.

Snipped - just to show it's technically possible...

I wondered about that too.

What's the story here?

Reply to
Richard

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hmm, I've not stumbled across the $25 ones, but that would be worth it just for the practice.

Reply to
Cydrome Leader

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