On Tue, 9 Apr 2013 12:23:13 -0500, "Terry Coombs"
wrote:

"Ed Huntress" wrote in message
.. .
On Mon, 8 Apr 2013 22:04:03 -0500, "Terry Coombs"
wrote:

"Ed Huntress" wrote in message
news On Mon, 8 Apr 2013 19:24:51 +0000 (UTC), Cydrome Leader
wrote:

Ed Huntress wrote:
On Mon, 8 Apr 2013 17:14:22 +0000 (UTC), Cydrome Leader
wrote:

RogerN wrote:

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?

https://colfaxtactical.com/docs/Fire...%20a%20DRO.pdf

RogerN

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.

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.

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.

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.


--
Ed Huntress

Neither would I . I'd toss it into the pile for ZA alloy casting . Ed ,
where did you acquire your knowledge about casting and alloys ? Some of
the
stuff you've posted is directly opposite what experience and hobbycasting
yahoogroup has taught me .

Mostly as Materials Editor at _American Machinist_. I had to study
basic metallurgy for a couple of years to be able to handle the tech
papers coming in from Alcoa, U.S. Steel, and so on.

I'm hardly an expert but I know the sources. I'm very curious about
what the hobbycasting group is saying that's directly opposite. Does
it concern 7075? It's notorious for having lousy corrosion resistance.
There are some 7xxxx alloys that are much better, but not 7075.

As for copper in aluminum, it makes it extremely hot-short and prone
to porosity. That's why welding 2024 is not recommended except by some
specialized techniques. It cracks like crazy, and if it happens not to
crack today, it may in a couple of weeks, as the HAZ age-hardens.

Anyway, what is opposite?

--
Ed Humtress

Not that 7075 is corrosion-resistant , just that it can indeed be cast by
a home shop foundry . I've cast a couple of pieces with no problem - in fact
it casts nicely and machines well if you chill it in water straight out of
the mold then let it age for a couple of weeks . 6061 is very gummy
especially when freshly cast , and 356 is sweet to work with .
As far as copper in aluminum , we try to avoid it . It does indeed cause
problems , mostly in the machinibility of the items . A trace is no problem
, but get too much in the melt and you'll find that it's nearly impossible
to machine , even with carbide tooling . Some of the best bronzes are mostly
copper with small amounts of al , ampco 45 for example my bike rides on
swingarm bushings made of it ... and they too are difficult to machine .
I'm not set up to weld aluminum , and cannot speak to the weldability of any
of the alloys . When I need that knowledge I'll be looking for info !
--
Snag

Ok, a little deeper digging: 7075 develops brittle microstructure and
intermetallics when cast. There have been many professional
experiments with it, including rapid chilling and squeeze-casting (not
really full-melt casting), but they have been abandoned as far as I
can tell.

An application that requires wrought 7075 requires yield strength on
the order of 73,000 psi or higher. The best figure I've found for cast
alloys in the 7xxx series is 30,000 psi.

I have some more checking to do when I get a chance. But what I think
you're getting with your castings is an alloy that may machine well,
but which has low strength compared to any wrought 7075, plus 7075's
limitations: poor corrosion resistance and low ductility.
Intermetallics frequently aid machining. They also destroy strength.

The alloy was developed by Sumitomo for making fighter aircraft in the
1930s, similar to our earlier development of 4130 steel. Since then,
other aluminum alloys in the 7xxx series (and 7xx.x for casting
alloys) have been developed, and they deal better with individual
limitations of 7075.

That's not to say you can't make useful castings with it. If it works
for you, great. The copper level is not high, so it may avoid the
hot-short issue and internal voids that make 2024 so difficult.
Porosity is an open question. But it seems unlikely that you're
getting a result with any more strength than 6061, and with less
ductility.

I'll let you know if I find anything else. Not today, though. The
trouble with the information available is that 7075 is not a casting
alloy so you aren't going to find a lot of scientific information on
it. And I don't think most hobby casters do enough testing to know
what they have.

--
Ed Huntress

On 04/09/2013 06:09 AM, Randy333 wrote:
On Mon, 08 Apr 2013 21:56:51 -0700, Gunner Asch
wrote:

Id not mind having a lower someday. Though given the prices of the
upper Stuff..it would likely remain a door stop.

ARs are quasi legal here in California..but they have to be neutered
with a 10 rd mag and a bullet button etc etc


Bullet button?

California prohibits (most) detachable magazines, so people work around
this by using a "fixed" magazine that requires a "tool" to detach the
magazine.

It so happens that the mechanism some people use fix the magazine is
with a fastener that requires a tool that is of the same shape as a
typical bullet, ergo a "bullet button".

Jon

On Tue, 09 Apr 2013 12:35:48 -0400, "Steve W."
wrote:

Richard wrote:
On 4/8/2013 11:19 PM, Steve W. wrote:
Could you machine one out of a stainless steel block?


Already been done. There are a few companies that already sell stainless
lowers.

I did one out of 416 just to see what it took. It will be the only one I
ever do as well. It machined very well but even when I thinned areas
down to reduce weight it's still pretty hefty. Does make the .308 a lot
nicer to shoot though. I have been tossing around the idea of making one
out of steel though. Thinking of areas like the sides of the mag well
for skeletonizing...

Anyone with a mid sized mill can handle the job. If you buy some guides
you can even use a good drill press!



What does that do to the weight???

Adds about 8 oz. to the bare lower compared to a "normal" lower.

Half a pound isnt ****.